Merge from rustc

289 files changed, 3398 insertions, 3300 deletions

diff --git a/compiler/rustc_abi/src/layout.rs b/compiler/rustc_abi/src/layout.rs
index 7bffeaf4cc9..42250aa173b 100644
--- a/compiler/rustc_abi/src/layout.rs
+++ b/compiler/rustc_abi/src/layout.rs
@@ -315,7 +315,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
         repr: &ReprOptions,
         variants: &IndexSlice<VariantIdx, IndexVec<FieldIdx, F>>,
         is_enum: bool,
-        is_unsafe_cell: bool,
+        is_special_no_niche: bool,
         scalar_valid_range: (Bound<u128>, Bound<u128>),
         discr_range_of_repr: impl Fn(i128, i128) -> (Integer, bool),
         discriminants: impl Iterator<Item = (VariantIdx, i128)>,
@@ -348,7 +348,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
                 repr,
                 variants,
                 is_enum,
-                is_unsafe_cell,
+                is_special_no_niche,
                 scalar_valid_range,
                 always_sized,
                 present_first,
@@ -505,7 +505,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
         repr: &ReprOptions,
         variants: &IndexSlice<VariantIdx, IndexVec<FieldIdx, F>>,
         is_enum: bool,
-        is_unsafe_cell: bool,
+        is_special_no_niche: bool,
         scalar_valid_range: (Bound<u128>, Bound<u128>),
         always_sized: bool,
         present_first: VariantIdx,
@@ -524,7 +524,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
         let mut st = self.univariant(&variants[v], repr, kind)?;
         st.variants = Variants::Single { index: v };
 
-        if is_unsafe_cell {
+        if is_special_no_niche {
             let hide_niches = |scalar: &mut _| match scalar {
                 Scalar::Initialized { value, valid_range } => {
                     *valid_range = WrappingRange::full(value.size(dl))
diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index 3fb88c9599d..1532ca77f71 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -120,6 +120,17 @@ impl Path {
         Path { segments: thin_vec![PathSegment::from_ident(ident)], span: ident.span, tokens: None }
     }
 
+    pub fn is_ident(&self, name: Symbol) -> bool {
+        if let [segment] = self.segments.as_ref()
+            && segment.args.is_none()
+            && segment.ident.name == name
+        {
+            true
+        } else {
+            false
+        }
+    }
+
     pub fn is_global(&self) -> bool {
         self.segments.first().is_some_and(|segment| segment.ident.name == kw::PathRoot)
     }
diff --git a/compiler/rustc_ast/src/ast_traits.rs b/compiler/rustc_ast/src/ast_traits.rs
index c9e2e9911ef..7f98e7ba8a6 100644
--- a/compiler/rustc_ast/src/ast_traits.rs
+++ b/compiler/rustc_ast/src/ast_traits.rs
@@ -6,7 +6,6 @@ use std::fmt;
 use std::marker::PhantomData;
 
 use crate::ptr::P;
-use crate::token::Nonterminal;
 use crate::tokenstream::LazyAttrTokenStream;
 use crate::{
     Arm, AssocItem, AttrItem, AttrKind, AttrVec, Attribute, Block, Crate, Expr, ExprField,
@@ -206,19 +205,6 @@ impl HasTokens for Attribute {
     }
 }
 
-impl HasTokens for Nonterminal {
-    fn tokens(&self) -> Option<&LazyAttrTokenStream> {
-        match self {
-            Nonterminal::NtBlock(block) => block.tokens(),
-        }
-    }
-    fn tokens_mut(&mut self) -> Option<&mut Option<LazyAttrTokenStream>> {
-        match self {
-            Nonterminal::NtBlock(block) => block.tokens_mut(),
-        }
-    }
-}
-
 /// A trait for AST nodes having (or not having) attributes.
 pub trait HasAttrs {
     /// This is `true` if this `HasAttrs` might support 'custom' (proc-macro) inner
diff --git a/compiler/rustc_ast/src/attr/mod.rs b/compiler/rustc_ast/src/attr/mod.rs
index 0b65246693d..d656d9b0b8a 100644
--- a/compiler/rustc_ast/src/attr/mod.rs
+++ b/compiler/rustc_ast/src/attr/mod.rs
@@ -627,7 +627,7 @@ pub fn mk_doc_comment(
     Attribute { kind: AttrKind::DocComment(comment_kind, data), id: g.mk_attr_id(), style, span }
 }
 
-pub fn mk_attr(
+fn mk_attr(
     g: &AttrIdGenerator,
     style: AttrStyle,
     unsafety: Safety,
diff --git a/compiler/rustc_ast/src/lib.rs b/compiler/rustc_ast/src/lib.rs
index da510e4967d..294c6c9ba7a 100644
--- a/compiler/rustc_ast/src/lib.rs
+++ b/compiler/rustc_ast/src/lib.rs
@@ -6,7 +6,6 @@
 
 // tidy-alphabetical-start
 #![allow(internal_features)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(
     html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/",
     test(attr(deny(warnings)))
diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
index b083e878905..6aae2e481a5 100644
--- a/compiler/rustc_ast/src/mut_visit.rs
+++ b/compiler/rustc_ast/src/mut_visit.rs
@@ -844,9 +844,9 @@ fn visit_lazy_tts<T: MutVisitor>(vis: &mut T, lazy_tts: &mut Option<LazyAttrToke
     visit_lazy_tts_opt_mut(vis, lazy_tts.as_mut());
 }
 
-/// Applies ident visitor if it's an ident; applies other visits to interpolated nodes.
-/// In practice the ident part is not actually used by specific visitors right now,
-/// but there's a test below checking that it works.
+/// Applies ident visitor if it's an ident. In practice this is not actually
+/// used by specific visitors right now, but there's a test below checking that
+/// it works.
 // No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
 pub fn visit_token<T: MutVisitor>(vis: &mut T, t: &mut Token) {
     let Token { kind, span } = t;
@@ -864,46 +864,12 @@ pub fn visit_token<T: MutVisitor>(vis: &mut T, t: &mut Token) {
         token::NtLifetime(ident, _is_raw) => {
             vis.visit_ident(ident);
         }
-        token::Interpolated(nt) => {
-            let nt = Arc::make_mut(nt);
-            visit_nonterminal(vis, nt);
-        }
         _ => {}
     }
     vis.visit_span(span);
 }
 
 // No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
-/// Applies the visitor to elements of interpolated nodes.
-//
-// N.B., this can occur only when applying a visitor to partially expanded
-// code, where parsed pieces have gotten implanted ito *other* macro
-// invocations. This is relevant for macro hygiene, but possibly not elsewhere.
-//
-// One problem here occurs because the types for flat_map_item, flat_map_stmt,
-// etc., allow the visitor to return *multiple* items; this is a problem for the
-// nodes here, because they insist on having exactly one piece. One solution
-// would be to mangle the MutVisitor trait to include one-to-many and
-// one-to-one versions of these entry points, but that would probably confuse a
-// lot of people and help very few. Instead, I'm just going to put in dynamic
-// checks. I think the performance impact of this will be pretty much
-// nonexistent. The danger is that someone will apply a `MutVisitor` to a
-// partially expanded node, and will be confused by the fact that their
-// `flat_map_item` or `flat_map_stmt` isn't getting called on `NtItem` or `NtStmt`
-// nodes. Hopefully they'll wind up reading this comment, and doing something
-// appropriate.
-//
-// BTW, design choice: I considered just changing the type of, e.g., `NtItem` to
-// contain multiple items, but decided against it when I looked at
-// `parse_item_or_view_item` and tried to figure out what I would do with
-// multiple items there....
-fn visit_nonterminal<T: MutVisitor>(vis: &mut T, nt: &mut token::Nonterminal) {
-    match nt {
-        token::NtBlock(block) => vis.visit_block(block),
-    }
-}
-
-// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
 fn visit_defaultness<T: MutVisitor>(vis: &mut T, defaultness: &mut Defaultness) {
     match defaultness {
         Defaultness::Default(span) => vis.visit_span(span),
diff --git a/compiler/rustc_ast/src/token.rs b/compiler/rustc_ast/src/token.rs
index d57a369eebf..055481f5d87 100644
--- a/compiler/rustc_ast/src/token.rs
+++ b/compiler/rustc_ast/src/token.rs
@@ -1,13 +1,10 @@
 use std::borrow::Cow;
 use std::fmt;
-use std::sync::Arc;
 
 pub use LitKind::*;
-pub use Nonterminal::*;
 pub use NtExprKind::*;
 pub use NtPatKind::*;
 pub use TokenKind::*;
-use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_macros::{Decodable, Encodable, HashStable_Generic};
 use rustc_span::edition::Edition;
 use rustc_span::{DUMMY_SP, ErrorGuaranteed, Span, kw, sym};
@@ -16,7 +13,6 @@ use rustc_span::{DUMMY_SP, ErrorGuaranteed, Span, kw, sym};
 use rustc_span::{Ident, Symbol};
 
 use crate::ast;
-use crate::ptr::P;
 use crate::util::case::Case;
 
 #[derive(Clone, Copy, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
@@ -34,10 +30,6 @@ pub enum InvisibleOrigin {
     // Converted from `proc_macro::Delimiter` in
     // `proc_macro::Delimiter::to_internal`, i.e. returned by a proc macro.
     ProcMacro,
-
-    // Converted from `TokenKind::Interpolated` in
-    // `TokenStream::flatten_token`. Treated similarly to `ProcMacro`.
-    FlattenToken,
 }
 
 impl PartialEq for InvisibleOrigin {
@@ -134,9 +126,7 @@ impl Delimiter {
         match self {
             Delimiter::Parenthesis | Delimiter::Bracket | Delimiter::Brace => false,
             Delimiter::Invisible(InvisibleOrigin::MetaVar(_)) => false,
-            Delimiter::Invisible(InvisibleOrigin::FlattenToken | InvisibleOrigin::ProcMacro) => {
-                true
-            }
+            Delimiter::Invisible(InvisibleOrigin::ProcMacro) => true,
         }
     }
 
@@ -337,9 +327,7 @@ impl From<IdentIsRaw> for bool {
     }
 }
 
-// SAFETY: due to the `Clone` impl below, all fields of all variants other than
-// `Interpolated` must impl `Copy`.
-#[derive(PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+#[derive(Clone, Copy, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
 pub enum TokenKind {
     /* Expression-operator symbols. */
     /// `=`
@@ -468,21 +456,6 @@ pub enum TokenKind {
     /// the `lifetime` metavariable in the macro's RHS.
     NtLifetime(Ident, IdentIsRaw),
 
-    /// An embedded AST node, as produced by a macro. This only exists for
-    /// historical reasons. We'd like to get rid of it, for multiple reasons.
-    /// - It's conceptually very strange. Saying a token can contain an AST
-    ///   node is like saying, in natural language, that a word can contain a
-    ///   sentence.
-    /// - It requires special handling in a bunch of places in the parser.
-    /// - It prevents `Token` from implementing `Copy`.
-    /// It adds complexity and likely slows things down. Please don't add new
-    /// occurrences of this token kind!
-    ///
-    /// The span in the surrounding `Token` is that of the metavariable in the
-    /// macro's RHS. The span within the Nonterminal is that of the fragment
-    /// passed to the macro at the call site.
-    Interpolated(Arc<Nonterminal>),
-
     /// A doc comment token.
     /// `Symbol` is the doc comment's data excluding its "quotes" (`///`, `/**`, etc)
     /// similarly to symbols in string literal tokens.
@@ -492,20 +465,7 @@ pub enum TokenKind {
     Eof,
 }
 
-impl Clone for TokenKind {
-    fn clone(&self) -> Self {
-        // `TokenKind` would impl `Copy` if it weren't for `Interpolated`. So
-        // for all other variants, this implementation of `clone` is just like
-        // a copy. This is faster than the `derive(Clone)` version which has a
-        // separate path for every variant.
-        match self {
-            Interpolated(nt) => Interpolated(Arc::clone(nt)),
-            _ => unsafe { std::ptr::read(self) },
-        }
-    }
-}
-
-#[derive(Clone, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+#[derive(Clone, Copy, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
 pub struct Token {
     pub kind: TokenKind,
     pub span: Span,
@@ -600,7 +560,7 @@ impl Token {
             | FatArrow | Pound | Dollar | Question | SingleQuote => true,
 
             OpenDelim(..) | CloseDelim(..) | Literal(..) | DocComment(..) | Ident(..)
-            | NtIdent(..) | Lifetime(..) | NtLifetime(..) | Interpolated(..) | Eof => false,
+            | NtIdent(..) | Lifetime(..) | NtLifetime(..) | Eof => false,
         }
     }
 
@@ -631,7 +591,6 @@ impl Token {
             PathSep                           | // global path
             Lifetime(..)                      | // labeled loop
             Pound                             => true, // expression attributes
-            Interpolated(ref nt) => matches!(&**nt, NtBlock(..)),
             OpenDelim(Delimiter::Invisible(InvisibleOrigin::MetaVar(
                 MetaVarKind::Block |
                 MetaVarKind::Expr { .. } |
@@ -703,7 +662,6 @@ impl Token {
         match self.kind {
             OpenDelim(Delimiter::Brace) | Literal(..) | Minus => true,
             Ident(name, IdentIsRaw::No) if name.is_bool_lit() => true,
-            Interpolated(ref nt) => matches!(&**nt, NtBlock(..)),
             OpenDelim(Delimiter::Invisible(InvisibleOrigin::MetaVar(
                 MetaVarKind::Expr { .. } | MetaVarKind::Block | MetaVarKind::Literal,
             ))) => true,
@@ -831,31 +789,20 @@ impl Token {
     /// Is this a pre-parsed expression dropped into the token stream
     /// (which happens while parsing the result of macro expansion)?
     pub fn is_metavar_expr(&self) -> bool {
-        #[allow(irrefutable_let_patterns)] // FIXME: temporary
-        if let Interpolated(nt) = &self.kind
-            && let NtBlock(_) = &**nt
-        {
-            true
-        } else if matches!(
+        matches!(
             self.is_metavar_seq(),
-            Some(MetaVarKind::Expr { .. } | MetaVarKind::Literal | MetaVarKind::Path)
-        ) {
-            true
-        } else {
-            matches!(self.is_metavar_seq(), Some(MetaVarKind::Path))
-        }
+            Some(
+                MetaVarKind::Expr { .. }
+                    | MetaVarKind::Literal
+                    | MetaVarKind::Path
+                    | MetaVarKind::Block
+            )
+        )
     }
 
-    /// Is the token an interpolated block (`$b:block`)?
-    pub fn is_whole_block(&self) -> bool {
-        #[allow(irrefutable_let_patterns)] // FIXME: temporary
-        if let Interpolated(nt) = &self.kind
-            && let NtBlock(..) = &**nt
-        {
-            return true;
-        }
-
-        false
+    /// Are we at a block from a metavar (`$b:block`)?
+    pub fn is_metavar_block(&self) -> bool {
+        matches!(self.is_metavar_seq(), Some(MetaVarKind::Block))
     }
 
     /// Returns `true` if the token is either the `mut` or `const` keyword.
@@ -1024,7 +971,7 @@ impl Token {
                 | PercentEq | CaretEq | AndEq | OrEq | ShlEq | ShrEq | At | DotDotDot | DotDotEq
                 | Comma | Semi | PathSep | RArrow | LArrow | FatArrow | Pound | Dollar | Question
                 | OpenDelim(..) | CloseDelim(..) | Literal(..) | Ident(..) | NtIdent(..)
-                | Lifetime(..) | NtLifetime(..) | Interpolated(..) | DocComment(..) | Eof,
+                | Lifetime(..) | NtLifetime(..) | DocComment(..) | Eof,
                 _,
             ) => {
                 return None;
@@ -1063,12 +1010,6 @@ pub enum NtExprKind {
     Expr2021 { inferred: bool },
 }
 
-#[derive(Clone, Encodable, Decodable)]
-/// For interpolation during macro expansion.
-pub enum Nonterminal {
-    NtBlock(P<ast::Block>),
-}
-
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Encodable, Decodable, Hash, HashStable_Generic)]
 pub enum NonterminalKind {
     Item,
@@ -1152,47 +1093,6 @@ impl fmt::Display for NonterminalKind {
     }
 }
 
-impl Nonterminal {
-    pub fn use_span(&self) -> Span {
-        match self {
-            NtBlock(block) => block.span,
-        }
-    }
-
-    pub fn descr(&self) -> &'static str {
-        match self {
-            NtBlock(..) => "block",
-        }
-    }
-}
-
-impl PartialEq for Nonterminal {
-    fn eq(&self, _rhs: &Self) -> bool {
-        // FIXME: Assume that all nonterminals are not equal, we can't compare them
-        // correctly based on data from AST. This will prevent them from matching each other
-        // in macros. The comparison will become possible only when each nonterminal has an
-        // attached token stream from which it was parsed.
-        false
-    }
-}
-
-impl fmt::Debug for Nonterminal {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        match *self {
-            NtBlock(..) => f.pad("NtBlock(..)"),
-        }
-    }
-}
-
-impl<CTX> HashStable<CTX> for Nonterminal
-where
-    CTX: crate::HashStableContext,
-{
-    fn hash_stable(&self, _hcx: &mut CTX, _hasher: &mut StableHasher) {
-        panic!("interpolated tokens should not be present in the HIR")
-    }
-}
-
 // Some types are used a lot. Make sure they don't unintentionally get bigger.
 #[cfg(target_pointer_width = "64")]
 mod size_asserts {
@@ -1202,7 +1102,6 @@ mod size_asserts {
     // tidy-alphabetical-start
     static_assert_size!(Lit, 12);
     static_assert_size!(LitKind, 2);
-    static_assert_size!(Nonterminal, 8);
     static_assert_size!(Token, 24);
     static_assert_size!(TokenKind, 16);
     // tidy-alphabetical-end
diff --git a/compiler/rustc_ast/src/tokenstream.rs b/compiler/rustc_ast/src/tokenstream.rs
index fb331e74aeb..43d25d18075 100644
--- a/compiler/rustc_ast/src/tokenstream.rs
+++ b/compiler/rustc_ast/src/tokenstream.rs
@@ -25,7 +25,7 @@ use rustc_span::{DUMMY_SP, Span, SpanDecoder, SpanEncoder, Symbol, sym};
 
 use crate::ast::AttrStyle;
 use crate::ast_traits::{HasAttrs, HasTokens};
-use crate::token::{self, Delimiter, InvisibleOrigin, Nonterminal, Token, TokenKind};
+use crate::token::{self, Delimiter, Token, TokenKind};
 use crate::{AttrVec, Attribute};
 
 /// Part of a `TokenStream`.
@@ -305,11 +305,6 @@ pub struct AttrsTarget {
 }
 
 /// A `TokenStream` is an abstract sequence of tokens, organized into [`TokenTree`]s.
-///
-/// The goal is for procedural macros to work with `TokenStream`s and `TokenTree`s
-/// instead of a representation of the abstract syntax tree.
-/// Today's `TokenTree`s can still contain AST via `token::Interpolated` for
-/// backwards compatibility.
 #[derive(Clone, Debug, Default, Encodable, Decodable)]
 pub struct TokenStream(pub(crate) Arc<Vec<TokenTree>>);
 
@@ -476,61 +471,6 @@ impl TokenStream {
         TokenStream::new(tts)
     }
 
-    pub fn from_nonterminal_ast(nt: &Nonterminal) -> TokenStream {
-        match nt {
-            Nonterminal::NtBlock(block) => TokenStream::from_ast(block),
-        }
-    }
-
-    fn flatten_token(token: &Token, spacing: Spacing) -> TokenTree {
-        match token.kind {
-            token::NtIdent(ident, is_raw) => {
-                TokenTree::Token(Token::new(token::Ident(ident.name, is_raw), ident.span), spacing)
-            }
-            token::NtLifetime(ident, is_raw) => TokenTree::Delimited(
-                DelimSpan::from_single(token.span),
-                DelimSpacing::new(Spacing::JointHidden, spacing),
-                Delimiter::Invisible(InvisibleOrigin::FlattenToken),
-                TokenStream::token_alone(token::Lifetime(ident.name, is_raw), ident.span),
-            ),
-            token::Interpolated(ref nt) => TokenTree::Delimited(
-                DelimSpan::from_single(token.span),
-                DelimSpacing::new(Spacing::JointHidden, spacing),
-                Delimiter::Invisible(InvisibleOrigin::FlattenToken),
-                TokenStream::from_nonterminal_ast(&nt).flattened(),
-            ),
-            _ => TokenTree::Token(token.clone(), spacing),
-        }
-    }
-
-    fn flatten_token_tree(tree: &TokenTree) -> TokenTree {
-        match tree {
-            TokenTree::Token(token, spacing) => TokenStream::flatten_token(token, *spacing),
-            TokenTree::Delimited(span, spacing, delim, tts) => {
-                TokenTree::Delimited(*span, *spacing, *delim, tts.flattened())
-            }
-        }
-    }
-
-    #[must_use]
-    pub fn flattened(&self) -> TokenStream {
-        fn can_skip(stream: &TokenStream) -> bool {
-            stream.iter().all(|tree| match tree {
-                TokenTree::Token(token, _) => !matches!(
-                    token.kind,
-                    token::NtIdent(..) | token::NtLifetime(..) | token::Interpolated(..)
-                ),
-                TokenTree::Delimited(.., inner) => can_skip(inner),
-            })
-        }
-
-        if can_skip(self) {
-            return self.clone();
-        }
-
-        self.iter().map(|tree| TokenStream::flatten_token_tree(tree)).collect()
-    }
-
     // If `vec` is not empty, try to glue `tt` onto its last token. The return
     // value indicates if gluing took place.
     fn try_glue_to_last(vec: &mut Vec<TokenTree>, tt: &TokenTree) -> bool {
diff --git a/compiler/rustc_ast_lowering/src/delegation.rs b/compiler/rustc_ast_lowering/src/delegation.rs
index 2296b05f69b..93c627f64c9 100644
--- a/compiler/rustc_ast_lowering/src/delegation.rs
+++ b/compiler/rustc_ast_lowering/src/delegation.rs
@@ -85,7 +85,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     .delegation_fn_sigs
                     .get(&local_def_id)
                     .is_some_and(|sig| sig.has_self),
-                None => self.tcx.associated_item(def_id).fn_has_self_parameter,
+                None => self.tcx.associated_item(def_id).is_method(),
             },
             _ => span_bug!(span, "unexpected DefKind for delegation item"),
         }
diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
index df4fd2ef52f..8e1a3cd1435 100644
--- a/compiler/rustc_ast_lowering/src/expr.rs
+++ b/compiler/rustc_ast_lowering/src/expr.rs
@@ -399,12 +399,16 @@ impl<'hir> LoweringContext<'_, 'hir> {
         &mut self,
         expr: &'hir hir::Expr<'hir>,
         span: Span,
-        check_ident: Ident,
-        check_hir_id: HirId,
+        cond_ident: Ident,
+        cond_hir_id: HirId,
     ) -> &'hir hir::Expr<'hir> {
-        let checker_fn = self.expr_ident(span, check_ident, check_hir_id);
-        let span = self.mark_span_with_reason(DesugaringKind::Contract, span, None);
-        self.expr_call(span, checker_fn, std::slice::from_ref(expr))
+        let cond_fn = self.expr_ident(span, cond_ident, cond_hir_id);
+        let call_expr = self.expr_call_lang_item_fn_mut(
+            span,
+            hir::LangItem::ContractCheckEnsures,
+            arena_vec![self; *cond_fn, *expr],
+        );
+        self.arena.alloc(call_expr)
     }
 
     pub(crate) fn lower_const_block(&mut self, c: &AnonConst) -> hir::ConstBlock {
diff --git a/compiler/rustc_ast_lowering/src/item.rs b/compiler/rustc_ast_lowering/src/item.rs
index 59099e5a554..d8b55bea3d7 100644
--- a/compiler/rustc_ast_lowering/src/item.rs
+++ b/compiler/rustc_ast_lowering/src/item.rs
@@ -1206,8 +1206,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let precond = if let Some(req) = &contract.requires {
                     // Lower the precondition check intrinsic.
                     let lowered_req = this.lower_expr_mut(&req);
+                    let req_span = this.mark_span_with_reason(
+                        DesugaringKind::Contract,
+                        lowered_req.span,
+                        None,
+                    );
                     let precond = this.expr_call_lang_item_fn_mut(
-                        req.span,
+                        req_span,
                         hir::LangItem::ContractCheckRequires,
                         &*arena_vec![this; lowered_req],
                     );
@@ -1217,6 +1222,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 };
                 let (postcond, body) = if let Some(ens) = &contract.ensures {
                     let ens_span = this.lower_span(ens.span);
+                    let ens_span =
+                        this.mark_span_with_reason(DesugaringKind::Contract, ens_span, None);
                     // Set up the postcondition `let` statement.
                     let check_ident: Ident =
                         Ident::from_str_and_span("__ensures_checker", ens_span);
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index 22a293d9b50..c40987541f4 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -32,7 +32,6 @@
 
 // tidy-alphabetical-start
 #![allow(internal_features)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(rust_logo)]
 #![feature(assert_matches)]
 #![feature(box_patterns)]
@@ -917,7 +916,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     }
 
     fn lower_delim_args(&self, args: &DelimArgs) -> DelimArgs {
-        DelimArgs { dspan: args.dspan, delim: args.delim, tokens: args.tokens.flattened() }
+        DelimArgs { dspan: args.dspan, delim: args.delim, tokens: args.tokens.clone() }
     }
 
     /// Lower an associated item constraint.
@@ -1766,24 +1765,23 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         ident: Ident,
         is_anon_in_path: IsAnonInPath,
     ) -> &'hir hir::Lifetime {
-        debug_assert_ne!(ident.name, kw::Empty);
         let res = self.resolver.get_lifetime_res(id).unwrap_or(LifetimeRes::Error);
         let res = match res {
-            LifetimeRes::Param { param, .. } => hir::LifetimeName::Param(param),
+            LifetimeRes::Param { param, .. } => hir::LifetimeKind::Param(param),
             LifetimeRes::Fresh { param, .. } => {
                 debug_assert_eq!(ident.name, kw::UnderscoreLifetime);
                 let param = self.local_def_id(param);
-                hir::LifetimeName::Param(param)
+                hir::LifetimeKind::Param(param)
             }
             LifetimeRes::Infer => {
                 debug_assert_eq!(ident.name, kw::UnderscoreLifetime);
-                hir::LifetimeName::Infer
+                hir::LifetimeKind::Infer
             }
             LifetimeRes::Static { .. } => {
                 debug_assert!(matches!(ident.name, kw::StaticLifetime | kw::UnderscoreLifetime));
-                hir::LifetimeName::Static
+                hir::LifetimeKind::Static
             }
-            LifetimeRes::Error => hir::LifetimeName::Error,
+            LifetimeRes::Error => hir::LifetimeKind::Error,
             LifetimeRes::ElidedAnchor { .. } => {
                 panic!("Unexpected `ElidedAnchar` {:?} at {:?}", ident, ident.span);
             }
@@ -2390,7 +2388,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         let r = hir::Lifetime::new(
             self.next_id(),
             Ident::new(kw::UnderscoreLifetime, self.lower_span(span)),
-            hir::LifetimeName::ImplicitObjectLifetimeDefault,
+            hir::LifetimeKind::ImplicitObjectLifetimeDefault,
             IsAnonInPath::No,
         );
         debug!("elided_dyn_bound: r={:?}", r);
diff --git a/compiler/rustc_ast_lowering/src/path.rs b/compiler/rustc_ast_lowering/src/path.rs
index c464c159c34..0bd65aec10f 100644
--- a/compiler/rustc_ast_lowering/src/path.rs
+++ b/compiler/rustc_ast_lowering/src/path.rs
@@ -448,8 +448,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
         generic_args.args.insert_many(
             0,
-            (start.as_u32()..end.as_u32()).map(|i| {
-                let id = NodeId::from_u32(i);
+            (start..end).map(|id| {
                 let l = self.lower_lifetime_anon_in_path(id, elided_lifetime_span);
                 GenericArg::Lifetime(l)
             }),
diff --git a/compiler/rustc_ast_passes/messages.ftl b/compiler/rustc_ast_passes/messages.ftl
index 25944392a52..80754a8f65a 100644
--- a/compiler/rustc_ast_passes/messages.ftl
+++ b/compiler/rustc_ast_passes/messages.ftl
@@ -79,6 +79,10 @@ ast_passes_extern_types_cannot = `type`s inside `extern` blocks cannot have {$de
     .suggestion = remove the {$remove_descr}
     .label = `extern` block begins here
 
+ast_passes_extern_without_abi = `extern` declarations without an explicit ABI are disallowed
+    .suggestion = specify an ABI
+    .help = prior to Rust 2024, a default ABI was inferred
+
 ast_passes_feature_on_non_nightly = `#![feature]` may not be used on the {$channel} release channel
     .suggestion = remove the attribute
     .stable_since = the feature `{$name}` has been stable since `{$since}` and no longer requires an attribute to enable
diff --git a/compiler/rustc_ast_passes/src/ast_validation.rs b/compiler/rustc_ast_passes/src/ast_validation.rs
index dc77e7b2834..9a7b7daabbf 100644
--- a/compiler/rustc_ast_passes/src/ast_validation.rs
+++ b/compiler/rustc_ast_passes/src/ast_validation.rs
@@ -684,7 +684,7 @@ impl<'a> AstValidator<'a> {
                     self.dcx().emit_err(errors::PatternFnPointer { span });
                 });
                 if let Extern::Implicit(extern_span) = bfty.ext {
-                    self.maybe_lint_missing_abi(extern_span, ty.id);
+                    self.handle_missing_abi(extern_span, ty.id);
                 }
             }
             TyKind::TraitObject(bounds, ..) => {
@@ -717,10 +717,12 @@ impl<'a> AstValidator<'a> {
         }
     }
 
-    fn maybe_lint_missing_abi(&mut self, span: Span, id: NodeId) {
+    fn handle_missing_abi(&mut self, span: Span, id: NodeId) {
         // FIXME(davidtwco): This is a hack to detect macros which produce spans of the
         // call site which do not have a macro backtrace. See #61963.
-        if self
+        if span.edition().at_least_edition_future() && self.features.explicit_extern_abis() {
+            self.dcx().emit_err(errors::MissingAbi { span });
+        } else if self
             .sess
             .source_map()
             .span_to_snippet(span)
@@ -996,7 +998,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 }
 
                 if abi.is_none() {
-                    self.maybe_lint_missing_abi(*extern_span, item.id);
+                    self.handle_missing_abi(*extern_span, item.id);
                 }
                 self.with_in_extern_mod(*safety, |this| {
                     visit::walk_item(this, item);
@@ -1370,7 +1372,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             },
         ) = fk
         {
-            self.maybe_lint_missing_abi(*extern_span, id);
+            self.handle_missing_abi(*extern_span, id);
         }
 
         // Functions without bodies cannot have patterns.
diff --git a/compiler/rustc_ast_passes/src/errors.rs b/compiler/rustc_ast_passes/src/errors.rs
index 8e53e600f7a..6f9737e0831 100644
--- a/compiler/rustc_ast_passes/src/errors.rs
+++ b/compiler/rustc_ast_passes/src/errors.rs
@@ -2,7 +2,7 @@
 
 use rustc_ast::ParamKindOrd;
 use rustc_errors::codes::*;
-use rustc_errors::{Applicability, Diag, EmissionGuarantee, SubdiagMessageOp, Subdiagnostic};
+use rustc_errors::{Applicability, Diag, EmissionGuarantee, Subdiagnostic};
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_span::{Ident, Span, Symbol};
 
@@ -394,11 +394,7 @@ pub(crate) struct EmptyLabelManySpans(pub Vec<Span>);
 
 // The derive for `Vec<Span>` does multiple calls to `span_label`, adding commas between each
 impl Subdiagnostic for EmptyLabelManySpans {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         diag.span_labels(self.0, "");
     }
 }
@@ -749,11 +745,7 @@ pub(crate) struct StableFeature {
 }
 
 impl Subdiagnostic for StableFeature {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         diag.arg("name", self.name);
         diag.arg("since", self.since);
         diag.help(fluent::ast_passes_stable_since);
@@ -823,3 +815,12 @@ pub(crate) struct DuplicatePreciseCapturing {
     #[label]
     pub bound2: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(ast_passes_extern_without_abi)]
+#[help]
+pub(crate) struct MissingAbi {
+    #[primary_span]
+    #[suggestion(code = "extern \"<abi>\"", applicability = "has-placeholders")]
+    pub span: Span,
+}
diff --git a/compiler/rustc_ast_pretty/src/pprust/mod.rs b/compiler/rustc_ast_pretty/src/pprust/mod.rs
index 97cb6e52d56..551506f2aef 100644
--- a/compiler/rustc_ast_pretty/src/pprust/mod.rs
+++ b/compiler/rustc_ast_pretty/src/pprust/mod.rs
@@ -5,14 +5,10 @@ pub mod state;
 use std::borrow::Cow;
 
 use rustc_ast as ast;
-use rustc_ast::token::{Nonterminal, Token, TokenKind};
+use rustc_ast::token::{Token, TokenKind};
 use rustc_ast::tokenstream::{TokenStream, TokenTree};
 pub use state::{AnnNode, Comments, PpAnn, PrintState, State, print_crate};
 
-pub fn nonterminal_to_string(nt: &Nonterminal) -> String {
-    State::new().nonterminal_to_string(nt)
-}
-
 /// Print the token kind precisely, without converting `$crate` into its respective crate name.
 pub fn token_kind_to_string(tok: &TokenKind) -> Cow<'static, str> {
     State::new().token_kind_to_string(tok)
diff --git a/compiler/rustc_ast_pretty/src/pprust/state.rs b/compiler/rustc_ast_pretty/src/pprust/state.rs
index 90caad1c845..0985ebf945b 100644
--- a/compiler/rustc_ast_pretty/src/pprust/state.rs
+++ b/compiler/rustc_ast_pretty/src/pprust/state.rs
@@ -11,7 +11,7 @@ use std::sync::Arc;
 
 use rustc_ast::attr::AttrIdGenerator;
 use rustc_ast::ptr::P;
-use rustc_ast::token::{self, CommentKind, Delimiter, IdentIsRaw, Nonterminal, Token, TokenKind};
+use rustc_ast::token::{self, CommentKind, Delimiter, IdentIsRaw, Token, TokenKind};
 use rustc_ast::tokenstream::{Spacing, TokenStream, TokenTree};
 use rustc_ast::util::classify;
 use rustc_ast::util::comments::{Comment, CommentStyle};
@@ -876,14 +876,6 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
         }
     }
 
-    fn nonterminal_to_string(&self, nt: &Nonterminal) -> String {
-        // We extract the token stream from the AST fragment and pretty print
-        // it, rather than using AST pretty printing, because `Nonterminal` is
-        // slated for removal in #124141. (This method will also then be
-        // removed.)
-        self.tts_to_string(&TokenStream::from_nonterminal_ast(nt))
-    }
-
     /// Print the token kind precisely, without converting `$crate` into its respective crate name.
     fn token_kind_to_string(&self, tok: &TokenKind) -> Cow<'static, str> {
         self.token_kind_to_string_ext(tok, None)
@@ -976,8 +968,6 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
                 doc_comment_to_string(comment_kind, attr_style, data).into()
             }
             token::Eof => "<eof>".into(),
-
-            token::Interpolated(ref nt) => self.nonterminal_to_string(&nt).into(),
         }
     }
 
diff --git a/compiler/rustc_attr_parsing/src/attributes/cfg.rs b/compiler/rustc_attr_parsing/src/attributes/cfg.rs
index 0d6d521b40c..48297b2ebd8 100644
--- a/compiler/rustc_attr_parsing/src/attributes/cfg.rs
+++ b/compiler/rustc_attr_parsing/src/attributes/cfg.rs
@@ -7,7 +7,6 @@ use rustc_session::config::ExpectedValues;
 use rustc_session::lint::BuiltinLintDiag;
 use rustc_session::lint::builtin::UNEXPECTED_CFGS;
 use rustc_session::parse::feature_err;
-use rustc_span::symbol::kw;
 use rustc_span::{Span, Symbol, sym};
 
 use crate::session_diagnostics::{self, UnsupportedLiteralReason};
@@ -89,20 +88,6 @@ pub fn eval_condition(
     let cfg = match cfg {
         MetaItemInner::MetaItem(meta_item) => meta_item,
         MetaItemInner::Lit(MetaItemLit { kind: LitKind::Bool(b), .. }) => {
-            if let Some(features) = features {
-                // we can't use `try_gate_cfg` as symbols don't differentiate between `r#true`
-                // and `true`, and we want to keep the former working without feature gate
-                gate_cfg(
-                    &(
-                        if *b { kw::True } else { kw::False },
-                        sym::cfg_boolean_literals,
-                        |features: &Features| features.cfg_boolean_literals(),
-                    ),
-                    cfg.span(),
-                    sess,
-                    features,
-                );
-            }
             return *b;
         }
         _ => {
diff --git a/compiler/rustc_attr_parsing/src/attributes/transparency.rs b/compiler/rustc_attr_parsing/src/attributes/transparency.rs
index ad83a1f7af8..ce42b0507ed 100644
--- a/compiler/rustc_attr_parsing/src/attributes/transparency.rs
+++ b/compiler/rustc_attr_parsing/src/attributes/transparency.rs
@@ -20,7 +20,7 @@ impl SingleAttributeParser for TransparencyParser {
     fn convert(cx: &AcceptContext<'_>, args: &ArgParser<'_>) -> Option<AttributeKind> {
         match args.name_value().and_then(|nv| nv.value_as_str()) {
             Some(sym::transparent) => Some(Transparency::Transparent),
-            Some(sym::semitransparent) => Some(Transparency::SemiTransparent),
+            Some(sym::semiopaque | sym::semitransparent) => Some(Transparency::SemiOpaque),
             Some(sym::opaque) => Some(Transparency::Opaque),
             Some(other) => {
                 cx.dcx().span_err(cx.attr_span, format!("unknown macro transparency: `{other}`"));
diff --git a/compiler/rustc_attr_parsing/src/context.rs b/compiler/rustc_attr_parsing/src/context.rs
index a68d4578b40..3bf03f84ce8 100644
--- a/compiler/rustc_attr_parsing/src/context.rs
+++ b/compiler/rustc_attr_parsing/src/context.rs
@@ -320,7 +320,7 @@ impl<'sess> AttributeParser<'sess> {
             ast::AttrArgs::Delimited(args) => AttrArgs::Delimited(DelimArgs {
                 dspan: args.dspan,
                 delim: args.delim,
-                tokens: args.tokens.flattened(),
+                tokens: args.tokens.clone(),
             }),
             // This is an inert key-value attribute - it will never be visible to macros
             // after it gets lowered to HIR. Therefore, we can extract literals to handle
diff --git a/compiler/rustc_attr_parsing/src/lib.rs b/compiler/rustc_attr_parsing/src/lib.rs
index a7465847e18..249e71ef70d 100644
--- a/compiler/rustc_attr_parsing/src/lib.rs
+++ b/compiler/rustc_attr_parsing/src/lib.rs
@@ -77,7 +77,6 @@
 
 // tidy-alphabetical-start
 #![allow(internal_features)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(rust_logo)]
 #![feature(let_chains)]
 #![feature(rustdoc_internals)]
diff --git a/compiler/rustc_attr_parsing/src/parser.rs b/compiler/rustc_attr_parsing/src/parser.rs
index a8a1460591c..384fae59873 100644
--- a/compiler/rustc_attr_parsing/src/parser.rs
+++ b/compiler/rustc_attr_parsing/src/parser.rs
@@ -485,25 +485,7 @@ impl<'a> MetaItemListParserContext<'a> {
         }
 
         // or a path.
-        let path =
-            if let Some(TokenTree::Token(Token { kind: token::Interpolated(_), span, .. }, _)) =
-                self.inside_delimiters.peek()
-            {
-                self.inside_delimiters.next();
-                // We go into this path if an expr ended up in an attribute that
-                // expansion did not turn into a literal. Say, `#[repr(align(macro!()))]`
-                // where the macro didn't expand to a literal. An error is already given
-                // for this at this point, and then we do continue. This makes this path
-                // reachable...
-                let e = self.dcx.span_delayed_bug(
-                    *span,
-                    "expr in place where literal is expected (builtin attr parsing)",
-                );
-
-                return Some(MetaItemOrLitParser::Err(*span, e));
-            } else {
-                self.next_path()?
-            };
+        let path = self.next_path()?;
 
         // Paths can be followed by:
         // - `(more meta items)` (another list)
diff --git a/compiler/rustc_borrowck/src/def_use.rs b/compiler/rustc_borrowck/src/def_use.rs
index 263f68d6a3d..b9ced81c46c 100644
--- a/compiler/rustc_borrowck/src/def_use.rs
+++ b/compiler/rustc_borrowck/src/def_use.rs
@@ -77,6 +77,9 @@ pub(crate) fn categorize(context: PlaceContext) -> Option<DefUse> {
         // Debug info is neither def nor use.
         PlaceContext::NonUse(NonUseContext::VarDebugInfo) => None,
 
+        // Backwards incompatible drop hint is not a use, just a marker for linting.
+        PlaceContext::NonUse(NonUseContext::BackwardIncompatibleDropHint) => None,
+
         PlaceContext::MutatingUse(MutatingUseContext::Deinit | MutatingUseContext::SetDiscriminant) => {
             bug!("These statements are not allowed in this MIR phase")
         }
diff --git a/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs b/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
index 8a8ecc3b96e..959cf9fa513 100644
--- a/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
@@ -647,7 +647,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                     && tc.polarity() == ty::PredicatePolarity::Positive
                     && supertrait_def_ids(tcx, tc.def_id())
                         .flat_map(|trait_did| tcx.associated_items(trait_did).in_definition_order())
-                        .any(|item| item.fn_has_self_parameter)
+                        .any(|item| item.is_method())
             })
         }) {
             return None;
@@ -3376,10 +3376,15 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
 
         let (sugg_span, suggestion) = match tcx.sess.source_map().span_to_snippet(args_span) {
             Ok(string) => {
-                let coro_prefix = if string.starts_with("async") {
-                    // `async` is 5 chars long. Not using `.len()` to avoid the cast from `usize`
-                    // to `u32`.
-                    Some(5)
+                let coro_prefix = if let Some(sub) = string.strip_prefix("async") {
+                    let trimmed_sub = sub.trim_end();
+                    if trimmed_sub.ends_with("gen") {
+                        // `async` is 5 chars long.
+                        Some((trimmed_sub.len() + 5) as _)
+                    } else {
+                        // `async` is 5 chars long.
+                        Some(5)
+                    }
                 } else if string.starts_with("gen") {
                     // `gen` is 3 chars long
                     Some(3)
diff --git a/compiler/rustc_borrowck/src/diagnostics/region_errors.rs b/compiler/rustc_borrowck/src/diagnostics/region_errors.rs
index 8d530b51636..4423edb0605 100644
--- a/compiler/rustc_borrowck/src/diagnostics/region_errors.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/region_errors.rs
@@ -888,7 +888,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                     // Skip `async` desugaring `impl Future`.
                 }
                 if let TyKind::TraitObject(_, lt) = alias_ty.kind {
-                    if lt.res == hir::LifetimeName::ImplicitObjectLifetimeDefault {
+                    if lt.kind == hir::LifetimeKind::ImplicitObjectLifetimeDefault {
                         spans_suggs.push((lt.ident.span.shrink_to_hi(), " + 'a".to_string()));
                     } else {
                         spans_suggs.push((lt.ident.span, "'a".to_string()));
diff --git a/compiler/rustc_borrowck/src/lib.rs b/compiler/rustc_borrowck/src/lib.rs
index 64ad1c96856..3b66142eb2c 100644
--- a/compiler/rustc_borrowck/src/lib.rs
+++ b/compiler/rustc_borrowck/src/lib.rs
@@ -2,7 +2,6 @@
 
 // tidy-alphabetical-start
 #![allow(internal_features)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(rust_logo)]
 #![feature(assert_matches)]
 #![feature(box_patterns)]
@@ -22,6 +21,7 @@ use std::cell::RefCell;
 use std::marker::PhantomData;
 use std::ops::{ControlFlow, Deref};
 
+use borrow_set::LocalsStateAtExit;
 use root_cx::BorrowCheckRootCtxt;
 use rustc_abi::FieldIdx;
 use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
@@ -304,33 +304,13 @@ fn do_mir_borrowck<'tcx>(
         root_cx.set_tainted_by_errors(e);
     }
 
-    let mut local_names = IndexVec::from_elem(None, &input_body.local_decls);
-    for var_debug_info in &input_body.var_debug_info {
-        if let VarDebugInfoContents::Place(place) = var_debug_info.value {
-            if let Some(local) = place.as_local() {
-                if let Some(prev_name) = local_names[local]
-                    && var_debug_info.name != prev_name
-                {
-                    span_bug!(
-                        var_debug_info.source_info.span,
-                        "local {:?} has many names (`{}` vs `{}`)",
-                        local,
-                        prev_name,
-                        var_debug_info.name
-                    );
-                }
-                local_names[local] = Some(var_debug_info.name);
-            }
-        }
-    }
-
     // Replace all regions with fresh inference variables. This
     // requires first making our own copy of the MIR. This copy will
     // be modified (in place) to contain non-lexical lifetimes. It
     // will have a lifetime tied to the inference context.
     let mut body_owned = input_body.clone();
     let mut promoted = input_promoted.to_owned();
-    let free_regions = nll::replace_regions_in_mir(&infcx, &mut body_owned, &mut promoted);
+    let universal_regions = nll::replace_regions_in_mir(&infcx, &mut body_owned, &mut promoted);
     let body = &body_owned; // no further changes
 
     let location_table = PoloniusLocationTable::new(body);
@@ -355,7 +335,7 @@ fn do_mir_borrowck<'tcx>(
     } = nll::compute_regions(
         root_cx,
         &infcx,
-        free_regions,
+        universal_regions,
         body,
         &promoted,
         &location_table,
@@ -368,24 +348,23 @@ fn do_mir_borrowck<'tcx>(
     // Dump MIR results into a file, if that is enabled. This lets us
     // write unit-tests, as well as helping with debugging.
     nll::dump_nll_mir(&infcx, body, &regioncx, &opt_closure_req, &borrow_set);
+    polonius::dump_polonius_mir(
+        &infcx,
+        body,
+        &regioncx,
+        &opt_closure_req,
+        &borrow_set,
+        polonius_diagnostics.as_ref(),
+    );
 
     // We also have a `#[rustc_regions]` annotation that causes us to dump
     // information.
-    let diags_buffer = &mut BorrowckDiagnosticsBuffer::default();
-    nll::dump_annotation(&infcx, body, &regioncx, &opt_closure_req, diags_buffer);
-
-    let movable_coroutine =
-    // The first argument is the coroutine type passed by value
-    if let Some(local) = body.local_decls.raw.get(1)
-    // Get the interior types and args which typeck computed
-    && let ty::Coroutine(def_id, _) = *local.ty.kind()
-    && tcx.coroutine_movability(def_id) == hir::Movability::Movable
-{
-    true
-} else {
-    false
-};
+    nll::dump_annotation(&infcx, body, &regioncx, &opt_closure_req);
 
+    let movable_coroutine = body.coroutine.is_some()
+        && tcx.coroutine_movability(def.to_def_id()) == hir::Movability::Movable;
+
+    let diags_buffer = &mut BorrowckDiagnosticsBuffer::default();
     // While promoteds should mostly be correct by construction, we need to check them for
     // invalid moves to detect moving out of arrays:`struct S; fn main() { &([S][0]); }`.
     for promoted_body in &promoted {
@@ -403,7 +382,6 @@ fn do_mir_borrowck<'tcx>(
             location_table: &location_table,
             movable_coroutine,
             fn_self_span_reported: Default::default(),
-            locals_are_invalidated_at_exit,
             access_place_error_reported: Default::default(),
             reservation_error_reported: Default::default(),
             uninitialized_error_reported: Default::default(),
@@ -435,6 +413,26 @@ fn do_mir_borrowck<'tcx>(
         promoted_mbcx.report_move_errors();
     }
 
+    let mut local_names = IndexVec::from_elem(None, &body.local_decls);
+    for var_debug_info in &body.var_debug_info {
+        if let VarDebugInfoContents::Place(place) = var_debug_info.value {
+            if let Some(local) = place.as_local() {
+                if let Some(prev_name) = local_names[local]
+                    && var_debug_info.name != prev_name
+                {
+                    span_bug!(
+                        var_debug_info.source_info.span,
+                        "local {:?} has many names (`{}` vs `{}`)",
+                        local,
+                        prev_name,
+                        var_debug_info.name
+                    );
+                }
+                local_names[local] = Some(var_debug_info.name);
+            }
+        }
+    }
+
     let mut mbcx = MirBorrowckCtxt {
         root_cx,
         infcx: &infcx,
@@ -442,7 +440,6 @@ fn do_mir_borrowck<'tcx>(
         move_data: &move_data,
         location_table: &location_table,
         movable_coroutine,
-        locals_are_invalidated_at_exit,
         fn_self_span_reported: Default::default(),
         access_place_error_reported: Default::default(),
         reservation_error_reported: Default::default(),
@@ -455,9 +452,9 @@ fn do_mir_borrowck<'tcx>(
         local_names,
         region_names: RefCell::default(),
         next_region_name: RefCell::new(1),
-        polonius_output,
         move_errors: Vec::new(),
         diags_buffer,
+        polonius_output: polonius_output.as_deref(),
         polonius_diagnostics: polonius_diagnostics.as_ref(),
     };
 
@@ -474,16 +471,6 @@ fn do_mir_borrowck<'tcx>(
 
     mbcx.report_move_errors();
 
-    // If requested, dump polonius MIR.
-    polonius::dump_polonius_mir(
-        &infcx,
-        body,
-        &regioncx,
-        &borrow_set,
-        polonius_diagnostics.as_ref(),
-        &opt_closure_req,
-    );
-
     // For each non-user used mutable variable, check if it's been assigned from
     // a user-declared local. If so, then put that local into the used_mut set.
     // Note that this set is expected to be small - only upvars from closures
@@ -514,7 +501,6 @@ fn do_mir_borrowck<'tcx>(
     };
 
     let body_with_facts = if consumer_options.is_some() {
-        let output_facts = mbcx.polonius_output;
         Some(Box::new(BodyWithBorrowckFacts {
             body: body_owned,
             promoted,
@@ -522,7 +508,7 @@ fn do_mir_borrowck<'tcx>(
             region_inference_context: regioncx,
             location_table: polonius_input.as_ref().map(|_| location_table),
             input_facts: polonius_input,
-            output_facts,
+            output_facts: polonius_output,
         }))
     } else {
         None
@@ -655,13 +641,6 @@ struct MirBorrowckCtxt<'a, 'infcx, 'tcx> {
     location_table: &'a PoloniusLocationTable,
 
     movable_coroutine: bool,
-    /// This keeps track of whether local variables are free-ed when the function
-    /// exits even without a `StorageDead`, which appears to be the case for
-    /// constants.
-    ///
-    /// I'm not sure this is the right approach - @eddyb could you try and
-    /// figure this out?
-    locals_are_invalidated_at_exit: bool,
     /// This field keeps track of when borrow errors are reported in the access_place function
     /// so that there is no duplicate reporting. This field cannot also be used for the conflicting
     /// borrow errors that is handled by the `reservation_error_reported` field as the inclusion
@@ -709,12 +688,11 @@ struct MirBorrowckCtxt<'a, 'infcx, 'tcx> {
     /// The counter for generating new region names.
     next_region_name: RefCell<usize>,
 
-    /// Results of Polonius analysis.
-    polonius_output: Option<Box<PoloniusOutput>>,
-
     diags_buffer: &'a mut BorrowckDiagnosticsBuffer<'infcx, 'tcx>,
     move_errors: Vec<MoveError<'tcx>>,
 
+    /// Results of Polonius analysis.
+    polonius_output: Option<&'a PoloniusOutput>,
     /// When using `-Zpolonius=next`: the data used to compute errors and diagnostics.
     polonius_diagnostics: Option<&'a PoloniusDiagnosticsContext>,
 }
@@ -938,13 +916,20 @@ impl<'a, 'tcx> ResultsVisitor<'a, 'tcx, Borrowck<'a, 'tcx>> for MirBorrowckCtxt<
             | TerminatorKind::Return
             | TerminatorKind::TailCall { .. }
             | TerminatorKind::CoroutineDrop => {
-                // Returning from the function implicitly kills storage for all locals and statics.
-                // Often, the storage will already have been killed by an explicit
-                // StorageDead, but we don't always emit those (notably on unwind paths),
-                // so this "extra check" serves as a kind of backup.
-                for i in state.borrows.iter() {
-                    let borrow = &self.borrow_set[i];
-                    self.check_for_invalidation_at_exit(loc, borrow, span);
+                match self.borrow_set.locals_state_at_exit() {
+                    LocalsStateAtExit::AllAreInvalidated => {
+                        // Returning from the function implicitly kills storage for all locals and statics.
+                        // Often, the storage will already have been killed by an explicit
+                        // StorageDead, but we don't always emit those (notably on unwind paths),
+                        // so this "extra check" serves as a kind of backup.
+                        for i in state.borrows.iter() {
+                            let borrow = &self.borrow_set[i];
+                            self.check_for_invalidation_at_exit(loc, borrow, span);
+                        }
+                    }
+                    // If we do not implicitly invalidate all locals on exit,
+                    // we check for conflicts when dropping or moving this local.
+                    LocalsStateAtExit::SomeAreInvalidated { has_storage_dead_or_moved: _ } => {}
                 }
             }
 
@@ -1301,7 +1286,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
         error_reported
     }
 
-    /// Through #123739, backward incompatible drops (BIDs) are introduced.
+    /// Through #123739, `BackwardIncompatibleDropHint`s (BIDs) are introduced.
     /// We would like to emit lints whether borrow checking fails at these future drop locations.
     #[instrument(level = "debug", skip(self, state))]
     fn check_backward_incompatible_drop(
@@ -1716,22 +1701,15 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
         // we'll have a memory leak) and assume that all statics have a destructor.
         //
         // FIXME: allow thread-locals to borrow other thread locals?
-
-        let (might_be_alive, will_be_dropped) =
-            if self.body.local_decls[root_place.local].is_ref_to_thread_local() {
-                // Thread-locals might be dropped after the function exits
-                // We have to dereference the outer reference because
-                // borrows don't conflict behind shared references.
-                root_place.projection = TyCtxtConsts::DEREF_PROJECTION;
-                (true, true)
-            } else {
-                (false, self.locals_are_invalidated_at_exit)
-            };
-
-        if !will_be_dropped {
-            debug!("place_is_invalidated_at_exit({:?}) - won't be dropped", place);
-            return;
-        }
+        let might_be_alive = if self.body.local_decls[root_place.local].is_ref_to_thread_local() {
+            // Thread-locals might be dropped after the function exits
+            // We have to dereference the outer reference because
+            // borrows don't conflict behind shared references.
+            root_place.projection = TyCtxtConsts::DEREF_PROJECTION;
+            true
+        } else {
+            false
+        };
 
         let sd = if might_be_alive { Deep } else { Shallow(None) };
 
diff --git a/compiler/rustc_borrowck/src/nll.rs b/compiler/rustc_borrowck/src/nll.rs
index 8a2a34f207a..fe899bb054f 100644
--- a/compiler/rustc_borrowck/src/nll.rs
+++ b/compiler/rustc_borrowck/src/nll.rs
@@ -21,7 +21,7 @@ use tracing::{debug, instrument};
 
 use crate::borrow_set::BorrowSet;
 use crate::consumers::ConsumerOptions;
-use crate::diagnostics::{BorrowckDiagnosticsBuffer, RegionErrors};
+use crate::diagnostics::RegionErrors;
 use crate::polonius::PoloniusDiagnosticsContext;
 use crate::polonius::legacy::{
     PoloniusFacts, PoloniusFactsExt, PoloniusLocationTable, PoloniusOutput,
@@ -117,11 +117,6 @@ pub(crate) fn compute_regions<'a, 'tcx>(
         Rc::clone(&location_map),
     );
 
-    // Create the region inference context, taking ownership of the
-    // region inference data that was contained in `infcx`, and the
-    // base constraints generated by the type-check.
-    let var_infos = infcx.get_region_var_infos();
-
     // If requested, emit legacy polonius facts.
     polonius::legacy::emit_facts(
         &mut polonius_facts,
@@ -134,13 +129,8 @@ pub(crate) fn compute_regions<'a, 'tcx>(
         &constraints,
     );
 
-    let mut regioncx = RegionInferenceContext::new(
-        infcx,
-        var_infos,
-        constraints,
-        universal_region_relations,
-        location_map,
-    );
+    let mut regioncx =
+        RegionInferenceContext::new(infcx, constraints, universal_region_relations, location_map);
 
     // If requested for `-Zpolonius=next`, convert NLL constraints to localized outlives constraints
     // and use them to compute loan liveness.
@@ -297,7 +287,6 @@ pub(super) fn dump_annotation<'tcx, 'infcx>(
     body: &Body<'tcx>,
     regioncx: &RegionInferenceContext<'tcx>,
     closure_region_requirements: &Option<ClosureRegionRequirements<'tcx>>,
-    diagnostics_buffer: &mut BorrowckDiagnosticsBuffer<'infcx, 'tcx>,
 ) {
     let tcx = infcx.tcx;
     let base_def_id = tcx.typeck_root_def_id(body.source.def_id());
@@ -335,13 +324,11 @@ pub(super) fn dump_annotation<'tcx, 'infcx>(
     } else {
         let mut err = infcx.dcx().struct_span_note(def_span, "no external requirements");
         regioncx.annotate(tcx, &mut err);
-
         err
     };
 
     // FIXME(@lcnr): We currently don't dump the inferred hidden types here.
-
-    diagnostics_buffer.buffer_non_error(err);
+    err.emit();
 }
 
 fn for_each_region_constraint<'tcx>(
diff --git a/compiler/rustc_borrowck/src/polonius/dump.rs b/compiler/rustc_borrowck/src/polonius/dump.rs
index eb53a98832c..6a943e19208 100644
--- a/compiler/rustc_borrowck/src/polonius/dump.rs
+++ b/compiler/rustc_borrowck/src/polonius/dump.rs
@@ -24,9 +24,9 @@ pub(crate) fn dump_polonius_mir<'tcx>(
     infcx: &BorrowckInferCtxt<'tcx>,
     body: &Body<'tcx>,
     regioncx: &RegionInferenceContext<'tcx>,
+    closure_region_requirements: &Option<ClosureRegionRequirements<'tcx>>,
     borrow_set: &BorrowSet<'tcx>,
     polonius_diagnostics: Option<&PoloniusDiagnosticsContext>,
-    closure_region_requirements: &Option<ClosureRegionRequirements<'tcx>>,
 ) {
     let tcx = infcx.tcx;
     if !tcx.sess.opts.unstable_opts.polonius.is_next_enabled() {
diff --git a/compiler/rustc_borrowck/src/region_infer/mod.rs b/compiler/rustc_borrowck/src/region_infer/mod.rs
index 569c46e6403..f8af9e59f63 100644
--- a/compiler/rustc_borrowck/src/region_infer/mod.rs
+++ b/compiler/rustc_borrowck/src/region_infer/mod.rs
@@ -9,7 +9,7 @@ use rustc_errors::Diag;
 use rustc_hir::def_id::CRATE_DEF_ID;
 use rustc_index::IndexVec;
 use rustc_infer::infer::outlives::test_type_match;
-use rustc_infer::infer::region_constraints::{GenericKind, VarInfos, VerifyBound, VerifyIfEq};
+use rustc_infer::infer::region_constraints::{GenericKind, VerifyBound, VerifyIfEq};
 use rustc_infer::infer::{InferCtxt, NllRegionVariableOrigin, RegionVariableOrigin};
 use rustc_middle::bug;
 use rustc_middle::mir::{
@@ -145,7 +145,7 @@ pub struct RegionInferenceContext<'tcx> {
     /// variables are identified by their index (`RegionVid`). The
     /// definition contains information about where the region came
     /// from as well as its final inferred value.
-    pub(crate) definitions: IndexVec<RegionVid, RegionDefinition<'tcx>>,
+    pub(crate) definitions: Frozen<IndexVec<RegionVid, RegionDefinition<'tcx>>>,
 
     /// The liveness constraints added to each region. For most
     /// regions, these start out empty and steadily grow, though for
@@ -338,8 +338,7 @@ fn sccs_info<'tcx>(infcx: &BorrowckInferCtxt<'tcx>, sccs: &ConstraintSccs) {
     let num_components = sccs.num_sccs();
     let mut components = vec![FxIndexSet::default(); num_components];
 
-    for (reg_var_idx, scc_idx) in sccs.scc_indices().iter().enumerate() {
-        let reg_var = ty::RegionVid::from_usize(reg_var_idx);
+    for (reg_var, scc_idx) in sccs.scc_indices().iter_enumerated() {
         let origin = var_to_origin.get(&reg_var).unwrap_or(&RegionCtxt::Unknown);
         components[scc_idx.as_usize()].insert((reg_var, *origin));
     }
@@ -385,6 +384,26 @@ fn sccs_info<'tcx>(infcx: &BorrowckInferCtxt<'tcx>, sccs: &ConstraintSccs) {
     debug!("SCC edges {:#?}", scc_node_to_edges);
 }
 
+fn create_definitions<'tcx>(
+    infcx: &BorrowckInferCtxt<'tcx>,
+    universal_regions: &UniversalRegions<'tcx>,
+) -> Frozen<IndexVec<RegionVid, RegionDefinition<'tcx>>> {
+    // Create a RegionDefinition for each inference variable.
+    let mut definitions: IndexVec<_, _> = infcx
+        .get_region_var_infos()
+        .iter()
+        .map(|info| RegionDefinition::new(info.universe, info.origin))
+        .collect();
+
+    // Add the external name for all universal regions.
+    for (external_name, variable) in universal_regions.named_universal_regions_iter() {
+        debug!("region {variable:?} has external name {external_name:?}");
+        definitions[variable].external_name = Some(external_name);
+    }
+
+    Frozen::freeze(definitions)
+}
+
 impl<'tcx> RegionInferenceContext<'tcx> {
     /// Creates a new region inference context with a total of
     /// `num_region_variables` valid inference variables; the first N
@@ -395,7 +414,6 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     /// of constraints produced by the MIR type check.
     pub(crate) fn new(
         infcx: &BorrowckInferCtxt<'tcx>,
-        var_infos: VarInfos,
         constraints: MirTypeckRegionConstraints<'tcx>,
         universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
         location_map: Rc<DenseLocationMap>,
@@ -426,11 +444,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             infcx.set_tainted_by_errors(guar);
         }
 
-        // Create a RegionDefinition for each inference variable.
-        let definitions: IndexVec<_, _> = var_infos
-            .iter()
-            .map(|info| RegionDefinition::new(info.universe, info.origin))
-            .collect();
+        let definitions = create_definitions(infcx, &universal_regions);
 
         let constraint_sccs =
             outlives_constraints.add_outlives_static(&universal_regions, &definitions);
@@ -526,18 +540,6 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     /// means that the `R1: !1` constraint here will cause
     /// `R1` to become `'static`.
     fn init_free_and_bound_regions(&mut self) {
-        // Update the names (if any)
-        // This iterator has unstable order but we collect it all into an IndexVec
-        for (external_name, variable) in
-            self.universal_region_relations.universal_regions.named_universal_regions_iter()
-        {
-            debug!(
-                "init_free_and_bound_regions: region {:?} has external name {:?}",
-                variable, external_name
-            );
-            self.definitions[variable].external_name = Some(external_name);
-        }
-
         for variable in self.definitions.indices() {
             let scc = self.constraint_sccs.scc(variable);
 
diff --git a/compiler/rustc_borrowck/src/type_check/constraint_conversion.rs b/compiler/rustc_borrowck/src/type_check/constraint_conversion.rs
index ccb257ae093..57516565147 100644
--- a/compiler/rustc_borrowck/src/type_check/constraint_conversion.rs
+++ b/compiler/rustc_borrowck/src/type_check/constraint_conversion.rs
@@ -21,7 +21,6 @@ use crate::{ClosureOutlivesSubject, ClosureRegionRequirements, ConstraintCategor
 
 pub(crate) struct ConstraintConversion<'a, 'tcx> {
     infcx: &'a InferCtxt<'tcx>,
-    tcx: TyCtxt<'tcx>,
     universal_regions: &'a UniversalRegions<'tcx>,
     /// Each RBP `GK: 'a` is assumed to be true. These encode
     /// relationships like `T: 'a` that are added via implicit bounds
@@ -34,7 +33,6 @@ pub(crate) struct ConstraintConversion<'a, 'tcx> {
     /// logic expecting to see (e.g.) `ReStatic`, and if we supplied
     /// our special inference variable there, we would mess that up.
     region_bound_pairs: &'a RegionBoundPairs<'tcx>,
-    implicit_region_bound: ty::Region<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     known_type_outlives_obligations: &'a [ty::PolyTypeOutlivesPredicate<'tcx>],
     locations: Locations,
@@ -49,7 +47,6 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
         infcx: &'a InferCtxt<'tcx>,
         universal_regions: &'a UniversalRegions<'tcx>,
         region_bound_pairs: &'a RegionBoundPairs<'tcx>,
-        implicit_region_bound: ty::Region<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
         known_type_outlives_obligations: &'a [ty::PolyTypeOutlivesPredicate<'tcx>],
         locations: Locations,
@@ -59,10 +56,8 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
     ) -> Self {
         Self {
             infcx,
-            tcx: infcx.tcx,
             universal_regions,
             region_bound_pairs,
-            implicit_region_bound,
             param_env,
             known_type_outlives_obligations,
             locations,
@@ -96,7 +91,7 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
         // into a vector. These are the regions that we will be
         // relating to one another.
         let closure_mapping = &UniversalRegions::closure_mapping(
-            self.tcx,
+            self.infcx.tcx,
             closure_args,
             closure_requirements.num_external_vids,
             closure_def_id,
@@ -111,7 +106,7 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
             let subject = match outlives_requirement.subject {
                 ClosureOutlivesSubject::Region(re) => closure_mapping[re].into(),
                 ClosureOutlivesSubject::Ty(subject_ty) => {
-                    subject_ty.instantiate(self.tcx, |vid| closure_mapping[vid]).into()
+                    subject_ty.instantiate(self.infcx.tcx, |vid| closure_mapping[vid]).into()
                 }
             };
 
@@ -127,14 +122,14 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
         predicate: ty::OutlivesPredicate<'tcx, ty::GenericArg<'tcx>>,
         constraint_category: ConstraintCategory<'tcx>,
     ) {
+        let tcx = self.infcx.tcx;
         debug!("generate: constraints at: {:#?}", self.locations);
 
         // Extract out various useful fields we'll need below.
         let ConstraintConversion {
-            tcx,
             infcx,
+            universal_regions,
             region_bound_pairs,
-            implicit_region_bound,
             known_type_outlives_obligations,
             ..
         } = *self;
@@ -145,7 +140,7 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
                 break;
             }
 
-            if !self.tcx.recursion_limit().value_within_limit(iteration) {
+            if !tcx.recursion_limit().value_within_limit(iteration) {
                 bug!(
                     "FIXME(-Znext-solver): Overflowed when processing region obligations: {outlives_predicates:#?}"
                 );
@@ -170,10 +165,11 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
                             );
                         }
 
+                        let implicit_region_bound =
+                            ty::Region::new_var(tcx, universal_regions.implicit_region_bound());
                         // we don't actually use this for anything, but
                         // the `TypeOutlives` code needs an origin.
                         let origin = infer::RelateParamBound(self.span, t1, None);
-
                         TypeOutlives::new(
                             &mut *self,
                             tcx,
@@ -205,7 +201,7 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
     /// are dealt with during trait solving.
     fn replace_placeholders_with_nll<T: TypeFoldable<TyCtxt<'tcx>>>(&mut self, value: T) -> T {
         if value.has_placeholders() {
-            fold_regions(self.tcx, value, |r, _| match r.kind() {
+            fold_regions(self.infcx.tcx, value, |r, _| match r.kind() {
                 ty::RePlaceholder(placeholder) => {
                     self.constraints.placeholder_region(self.infcx, placeholder)
                 }
diff --git a/compiler/rustc_borrowck/src/type_check/free_region_relations.rs b/compiler/rustc_borrowck/src/type_check/free_region_relations.rs
index eaac633b512..536a27763d2 100644
--- a/compiler/rustc_borrowck/src/type_check/free_region_relations.rs
+++ b/compiler/rustc_borrowck/src/type_check/free_region_relations.rs
@@ -49,14 +49,12 @@ pub(crate) struct CreateResult<'tcx> {
 pub(crate) fn create<'tcx>(
     infcx: &InferCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
-    implicit_region_bound: ty::Region<'tcx>,
     universal_regions: UniversalRegions<'tcx>,
     constraints: &mut MirTypeckRegionConstraints<'tcx>,
 ) -> CreateResult<'tcx> {
     UniversalRegionRelationsBuilder {
         infcx,
         param_env,
-        implicit_region_bound,
         constraints,
         universal_regions,
         region_bound_pairs: Default::default(),
@@ -181,7 +179,6 @@ struct UniversalRegionRelationsBuilder<'a, 'tcx> {
     infcx: &'a InferCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     universal_regions: UniversalRegions<'tcx>,
-    implicit_region_bound: ty::Region<'tcx>,
     constraints: &'a mut MirTypeckRegionConstraints<'tcx>,
 
     // outputs:
@@ -320,7 +317,6 @@ impl<'tcx> UniversalRegionRelationsBuilder<'_, 'tcx> {
                 self.infcx,
                 &self.universal_regions,
                 &self.region_bound_pairs,
-                self.implicit_region_bound,
                 param_env,
                 &known_type_outlives_obligations,
                 Locations::All(span),
diff --git a/compiler/rustc_borrowck/src/type_check/mod.rs b/compiler/rustc_borrowck/src/type_check/mod.rs
index a17dff5d271..3c00b819a96 100644
--- a/compiler/rustc_borrowck/src/type_check/mod.rs
+++ b/compiler/rustc_borrowck/src/type_check/mod.rs
@@ -113,7 +113,6 @@ pub(crate) fn type_check<'a, 'tcx>(
     move_data: &MoveData<'tcx>,
     location_map: Rc<DenseLocationMap>,
 ) -> MirTypeckResults<'tcx> {
-    let implicit_region_bound = ty::Region::new_var(infcx.tcx, universal_regions.fr_fn_body);
     let mut constraints = MirTypeckRegionConstraints {
         placeholder_indices: PlaceholderIndices::default(),
         placeholder_index_to_region: IndexVec::default(),
@@ -129,13 +128,7 @@ pub(crate) fn type_check<'a, 'tcx>(
         region_bound_pairs,
         normalized_inputs_and_output,
         known_type_outlives_obligations,
-    } = free_region_relations::create(
-        infcx,
-        infcx.param_env,
-        implicit_region_bound,
-        universal_regions,
-        &mut constraints,
-    );
+    } = free_region_relations::create(infcx, infcx.param_env, universal_regions, &mut constraints);
 
     let pre_obligations = infcx.take_registered_region_obligations();
     assert!(
@@ -160,7 +153,6 @@ pub(crate) fn type_check<'a, 'tcx>(
         user_type_annotations: &body.user_type_annotations,
         region_bound_pairs,
         known_type_outlives_obligations,
-        implicit_region_bound,
         reported_errors: Default::default(),
         universal_regions: &universal_region_relations.universal_regions,
         location_table,
@@ -226,7 +218,6 @@ struct TypeChecker<'a, 'tcx> {
     user_type_annotations: &'a CanonicalUserTypeAnnotations<'tcx>,
     region_bound_pairs: RegionBoundPairs<'tcx>,
     known_type_outlives_obligations: Vec<ty::PolyTypeOutlivesPredicate<'tcx>>,
-    implicit_region_bound: ty::Region<'tcx>,
     reported_errors: FxIndexSet<(Ty<'tcx>, Span)>,
     universal_regions: &'a UniversalRegions<'tcx>,
     location_table: &'a PoloniusLocationTable,
@@ -422,7 +413,6 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             self.infcx,
             self.universal_regions,
             &self.region_bound_pairs,
-            self.implicit_region_bound,
             self.infcx.param_env,
             &self.known_type_outlives_obligations,
             locations,
@@ -1567,11 +1557,15 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                         }
                     }
                     CastKind::Transmute => {
-                        span_mirbug!(
-                            self,
-                            rvalue,
-                            "Unexpected CastKind::Transmute, which is not permitted in Analysis MIR",
-                        );
+                        let ty_from = op.ty(self.body, tcx);
+                        match ty_from.kind() {
+                            ty::Pat(base, _) if base == ty => {}
+                            _ => span_mirbug!(
+                                self,
+                                rvalue,
+                                "Unexpected CastKind::Transmute {ty_from:?} -> {ty:?}, which is not permitted in Analysis MIR",
+                            ),
+                        }
                     }
                 }
             }
@@ -2507,7 +2501,6 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                 self.infcx,
                 self.universal_regions,
                 &self.region_bound_pairs,
-                self.implicit_region_bound,
                 self.infcx.param_env,
                 &self.known_type_outlives_obligations,
                 locations,
diff --git a/compiler/rustc_borrowck/src/universal_regions.rs b/compiler/rustc_borrowck/src/universal_regions.rs
index 5c57ab99a85..c11e14d214c 100644
--- a/compiler/rustc_borrowck/src/universal_regions.rs
+++ b/compiler/rustc_borrowck/src/universal_regions.rs
@@ -438,6 +438,10 @@ impl<'tcx> UniversalRegions<'tcx> {
         }
     }
 
+    pub(crate) fn implicit_region_bound(&self) -> RegionVid {
+        self.fr_fn_body
+    }
+
     pub(crate) fn tainted_by_errors(&self) -> Option<ErrorGuaranteed> {
         self.indices.tainted_by_errors.get()
     }
diff --git a/compiler/rustc_builtin_macros/messages.ftl b/compiler/rustc_builtin_macros/messages.ftl
index 603dc90bafc..5316e90847a 100644
--- a/compiler/rustc_builtin_macros/messages.ftl
+++ b/compiler/rustc_builtin_macros/messages.ftl
@@ -231,8 +231,6 @@ builtin_macros_format_unused_args = multiple unused formatting arguments
 
 builtin_macros_format_use_positional = consider using a positional formatting argument instead
 
-builtin_macros_invalid_crate_attribute = invalid crate attribute
-
 builtin_macros_multiple_default_attrs = multiple `#[default]` attributes
     .note = only one `#[default]` attribute is needed
     .label = `#[default]` used here
diff --git a/compiler/rustc_builtin_macros/src/cfg_eval.rs b/compiler/rustc_builtin_macros/src/cfg_eval.rs
index b3ba9073118..da01e3e9607 100644
--- a/compiler/rustc_builtin_macros/src/cfg_eval.rs
+++ b/compiler/rustc_builtin_macros/src/cfg_eval.rs
@@ -92,11 +92,7 @@ impl CfgEval<'_> {
         // the location of `#[cfg]` and `#[cfg_attr]` in the token stream. The tokenization
         // process is lossless, so this process is invisible to proc-macros.
 
-        // 'Flatten' all nonterminals (i.e. `TokenKind::Interpolated`)
-        // to `None`-delimited groups containing the corresponding tokens. This
-        // is normally delayed until the proc-macro server actually needs to
-        // provide a `TokenKind::Interpolated` to a proc-macro. We do this earlier,
-        // so that we can handle cases like:
+        // Interesting cases:
         //
         // ```rust
         // #[cfg_eval] #[cfg] $item
@@ -104,8 +100,8 @@ impl CfgEval<'_> {
         //
         // where `$item` is `#[cfg_attr] struct Foo {}`. We want to make
         // sure to evaluate *all* `#[cfg]` and `#[cfg_attr]` attributes - the simplest
-        // way to do this is to do a single parse of a stream without any nonterminals.
-        let orig_tokens = annotatable.to_tokens().flattened();
+        // way to do this is to do a single parse of the token stream.
+        let orig_tokens = annotatable.to_tokens();
 
         // Re-parse the tokens, setting the `capture_cfg` flag to save extra information
         // to the captured `AttrTokenStream` (specifically, we capture
diff --git a/compiler/rustc_builtin_macros/src/cmdline_attrs.rs b/compiler/rustc_builtin_macros/src/cmdline_attrs.rs
index 6afd8c4b43b..423b6a15b64 100644
--- a/compiler/rustc_builtin_macros/src/cmdline_attrs.rs
+++ b/compiler/rustc_builtin_macros/src/cmdline_attrs.rs
@@ -1,44 +1,37 @@
 //! Attributes injected into the crate root from command line using `-Z crate-attr`.
 
-use rustc_ast::attr::mk_attr;
-use rustc_ast::{self as ast, AttrItem, AttrStyle, token};
-use rustc_parse::parser::ForceCollect;
-use rustc_parse::{new_parser_from_source_str, unwrap_or_emit_fatal};
+use rustc_ast::{self as ast};
+use rustc_errors::Diag;
+use rustc_parse::parser::attr::InnerAttrPolicy;
+use rustc_parse::{parse_in, source_str_to_stream};
 use rustc_session::parse::ParseSess;
 use rustc_span::FileName;
 
-use crate::errors;
-
 pub fn inject(krate: &mut ast::Crate, psess: &ParseSess, attrs: &[String]) {
     for raw_attr in attrs {
-        let mut parser = unwrap_or_emit_fatal(new_parser_from_source_str(
-            psess,
-            FileName::cli_crate_attr_source_code(raw_attr),
-            raw_attr.clone(),
-        ));
-
-        let start_span = parser.token.span;
-        let AttrItem { unsafety, path, args, tokens: _ } =
-            match parser.parse_attr_item(ForceCollect::No) {
-                Ok(ai) => ai,
-                Err(err) => {
+        let source = format!("#![{raw_attr}]");
+        let parse = || -> Result<ast::Attribute, Vec<Diag<'_>>> {
+            let tokens = source_str_to_stream(
+                psess,
+                FileName::cli_crate_attr_source_code(raw_attr),
+                source,
+                None,
+            )?;
+            parse_in(psess, tokens, "<crate attribute>", |p| {
+                p.parse_attribute(InnerAttrPolicy::Permitted)
+            })
+            .map_err(|e| vec![e])
+        };
+        let meta = match parse() {
+            Ok(meta) => meta,
+            Err(errs) => {
+                for err in errs {
                     err.emit();
-                    continue;
                 }
-            };
-        let end_span = parser.token.span;
-        if parser.token != token::Eof {
-            psess.dcx().emit_err(errors::InvalidCrateAttr { span: start_span.to(end_span) });
-            continue;
-        }
+                continue;
+            }
+        };
 
-        krate.attrs.push(mk_attr(
-            &psess.attr_id_generator,
-            AttrStyle::Inner,
-            unsafety,
-            path,
-            args,
-            start_span.to(end_span),
-        ));
+        krate.attrs.push(meta);
     }
 }
diff --git a/compiler/rustc_builtin_macros/src/errors.rs b/compiler/rustc_builtin_macros/src/errors.rs
index 4bbe212f429..d14ad8f4014 100644
--- a/compiler/rustc_builtin_macros/src/errors.rs
+++ b/compiler/rustc_builtin_macros/src/errors.rs
@@ -1,7 +1,7 @@
 use rustc_errors::codes::*;
 use rustc_errors::{
     Diag, DiagCtxtHandle, Diagnostic, EmissionGuarantee, Level, MultiSpan, SingleLabelManySpans,
-    SubdiagMessageOp, Subdiagnostic,
+    Subdiagnostic,
 };
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_span::{Ident, Span, Symbol};
@@ -110,13 +110,6 @@ pub(crate) struct ProcMacro {
 }
 
 #[derive(Diagnostic)]
-#[diag(builtin_macros_invalid_crate_attribute)]
-pub(crate) struct InvalidCrateAttr {
-    #[primary_span]
-    pub(crate) span: Span,
-}
-
-#[derive(Diagnostic)]
 #[diag(builtin_macros_non_abi)]
 pub(crate) struct NonABI {
     #[primary_span]
@@ -691,13 +684,9 @@ pub(crate) struct FormatUnusedArg {
 // Allow the singular form to be a subdiagnostic of the multiple-unused
 // form of diagnostic.
 impl Subdiagnostic for FormatUnusedArg {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         diag.arg("named", self.named);
-        let msg = f(diag, crate::fluent_generated::builtin_macros_format_unused_arg.into());
+        let msg = diag.eagerly_translate(crate::fluent_generated::builtin_macros_format_unused_arg);
         diag.span_label(self.span, msg);
     }
 }
diff --git a/compiler/rustc_builtin_macros/src/lib.rs b/compiler/rustc_builtin_macros/src/lib.rs
index 606e85577f7..bcd40f980e6 100644
--- a/compiler/rustc_builtin_macros/src/lib.rs
+++ b/compiler/rustc_builtin_macros/src/lib.rs
@@ -5,7 +5,6 @@
 #![allow(internal_features)]
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(assert_matches)]
diff --git a/compiler/rustc_codegen_cranelift/src/main_shim.rs b/compiler/rustc_codegen_cranelift/src/main_shim.rs
index 3b48adb7e91..6eef97c14dd 100644
--- a/compiler/rustc_codegen_cranelift/src/main_shim.rs
+++ b/compiler/rustc_codegen_cranelift/src/main_shim.rs
@@ -1,6 +1,6 @@
 use cranelift_frontend::{FunctionBuilder, FunctionBuilderContext};
 use rustc_hir::LangItem;
-use rustc_middle::ty::{AssocKind, GenericArg};
+use rustc_middle::ty::{AssocTag, GenericArg};
 use rustc_session::config::EntryFnType;
 use rustc_span::{DUMMY_SP, Ident};
 
@@ -107,7 +107,7 @@ pub(crate) fn maybe_create_entry_wrapper(
                     .find_by_ident_and_kind(
                         tcx,
                         Ident::from_str("report"),
-                        AssocKind::Fn,
+                        AssocTag::Fn,
                         termination_trait,
                     )
                     .unwrap();
diff --git a/compiler/rustc_codegen_llvm/src/debuginfo/create_scope_map.rs b/compiler/rustc_codegen_llvm/src/debuginfo/create_scope_map.rs
index f52991b3697..d2591139d6e 100644
--- a/compiler/rustc_codegen_llvm/src/debuginfo/create_scope_map.rs
+++ b/compiler/rustc_codegen_llvm/src/debuginfo/create_scope_map.rs
@@ -3,7 +3,6 @@ use std::collections::hash_map::Entry;
 use rustc_codegen_ssa::mir::debuginfo::{DebugScope, FunctionDebugContext};
 use rustc_codegen_ssa::traits::*;
 use rustc_data_structures::fx::FxHashMap;
-use rustc_index::Idx;
 use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::{Body, SourceScope};
 use rustc_middle::ty::layout::{FnAbiOf, HasTypingEnv};
@@ -43,8 +42,7 @@ pub(crate) fn compute_mir_scopes<'ll, 'tcx>(
     let mut instantiated = DenseBitSet::new_empty(mir.source_scopes.len());
     let mut discriminators = FxHashMap::default();
     // Instantiate all scopes.
-    for idx in 0..mir.source_scopes.len() {
-        let scope = SourceScope::new(idx);
+    for scope in mir.source_scopes.indices() {
         make_mir_scope(
             cx,
             instance,
diff --git a/compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs b/compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs
index 1eb8f367c54..7f3e486ca31 100644
--- a/compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs
+++ b/compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs
@@ -1315,31 +1315,21 @@ fn build_generic_type_param_di_nodes<'ll, 'tcx>(
     ty: Ty<'tcx>,
 ) -> SmallVec<Option<&'ll DIType>> {
     if let ty::Adt(def, args) = *ty.kind() {
-        let generics = cx.tcx.generics_of(def.did());
-        return get_template_parameters(cx, generics, args);
-    }
-
-    return smallvec![];
-}
-
-pub(super) fn get_template_parameters<'ll, 'tcx>(
-    cx: &CodegenCx<'ll, 'tcx>,
-    generics: &ty::Generics,
-    args: ty::GenericArgsRef<'tcx>,
-) -> SmallVec<Option<&'ll DIType>> {
-    if args.types().next().is_some() {
-        let names = get_parameter_names(cx, generics);
-        let template_params: SmallVec<_> = iter::zip(args, names)
-            .filter_map(|(kind, name)| {
-                kind.as_type().map(|ty| {
-                    let actual_type = cx.tcx.normalize_erasing_regions(cx.typing_env(), ty);
-                    let actual_type_di_node = type_di_node(cx, actual_type);
-                    Some(cx.create_template_type_parameter(name.as_str(), actual_type_di_node))
+        if args.types().next().is_some() {
+            let generics = cx.tcx.generics_of(def.did());
+            let names = get_parameter_names(cx, generics);
+            let template_params: SmallVec<_> = iter::zip(args, names)
+                .filter_map(|(kind, name)| {
+                    kind.as_type().map(|ty| {
+                        let actual_type = cx.tcx.normalize_erasing_regions(cx.typing_env(), ty);
+                        let actual_type_di_node = type_di_node(cx, actual_type);
+                        Some(cx.create_template_type_parameter(name.as_str(), actual_type_di_node))
+                    })
                 })
-            })
-            .collect();
+                .collect();
 
-        return template_params;
+            return template_params;
+        }
     }
 
     return smallvec![];
diff --git a/compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/mod.rs b/compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/mod.rs
index 6792c307fdc..7c701926d2c 100644
--- a/compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/mod.rs
+++ b/compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/mod.rs
@@ -363,7 +363,6 @@ fn build_coroutine_variant_struct_type_di_node<'ll, 'tcx>(
 
             state_specific_fields.into_iter().chain(common_fields).collect()
         },
-        // FIXME: this is a no-op. `build_generic_type_param_di_nodes` only works for Adts.
         |cx| build_generic_type_param_di_nodes(cx, coroutine_type_and_layout.ty),
     )
     .di_node
diff --git a/compiler/rustc_codegen_llvm/src/debuginfo/mod.rs b/compiler/rustc_codegen_llvm/src/debuginfo/mod.rs
index ae7d080db66..0f94a1dbb0d 100644
--- a/compiler/rustc_codegen_llvm/src/debuginfo/mod.rs
+++ b/compiler/rustc_codegen_llvm/src/debuginfo/mod.rs
@@ -2,8 +2,8 @@
 
 use std::cell::{OnceCell, RefCell};
 use std::ops::Range;
-use std::ptr;
 use std::sync::Arc;
+use std::{iter, ptr};
 
 use libc::c_uint;
 use metadata::create_subroutine_type;
@@ -486,10 +486,40 @@ impl<'ll, 'tcx> DebugInfoCodegenMethods<'tcx> for CodegenCx<'ll, 'tcx> {
             generics: &ty::Generics,
             args: GenericArgsRef<'tcx>,
         ) -> &'ll DIArray {
-            let template_params = metadata::get_template_parameters(cx, generics, args);
+            if args.types().next().is_none() {
+                return create_DIArray(DIB(cx), &[]);
+            }
+
+            // Again, only create type information if full debuginfo is enabled
+            let template_params: Vec<_> = if cx.sess().opts.debuginfo == DebugInfo::Full {
+                let names = get_parameter_names(cx, generics);
+                iter::zip(args, names)
+                    .filter_map(|(kind, name)| {
+                        kind.as_type().map(|ty| {
+                            let actual_type = cx.tcx.normalize_erasing_regions(cx.typing_env(), ty);
+                            let actual_type_metadata = type_di_node(cx, actual_type);
+                            Some(cx.create_template_type_parameter(
+                                name.as_str(),
+                                actual_type_metadata,
+                            ))
+                        })
+                    })
+                    .collect()
+            } else {
+                vec![]
+            };
+
             create_DIArray(DIB(cx), &template_params)
         }
 
+        fn get_parameter_names(cx: &CodegenCx<'_, '_>, generics: &ty::Generics) -> Vec<Symbol> {
+            let mut names = generics.parent.map_or_else(Vec::new, |def_id| {
+                get_parameter_names(cx, cx.tcx.generics_of(def_id))
+            });
+            names.extend(generics.own_params.iter().map(|param| param.name));
+            names
+        }
+
         /// Returns a scope, plus `true` if that's a type scope for "class" methods,
         /// otherwise `false` for plain namespace scopes.
         fn get_containing_scope<'ll, 'tcx>(
diff --git a/compiler/rustc_codegen_ssa/src/codegen_attrs.rs b/compiler/rustc_codegen_ssa/src/codegen_attrs.rs
index ddb61188983..8f23a5f21cd 100644
--- a/compiler/rustc_codegen_ssa/src/codegen_attrs.rs
+++ b/compiler/rustc_codegen_ssa/src/codegen_attrs.rs
@@ -114,7 +114,8 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
                 AttributeKind::Repr(reprs) => {
                     codegen_fn_attrs.alignment = reprs
                         .iter()
-                        .find_map(|(r, _)| if let ReprAlign(x) = r { Some(*x) } else { None });
+                        .filter_map(|(r, _)| if let ReprAlign(x) = r { Some(*x) } else { None })
+                        .max();
                 }
 
                 _ => {}
diff --git a/compiler/rustc_codegen_ssa/src/lib.rs b/compiler/rustc_codegen_ssa/src/lib.rs
index 153935f2531..5b2ed69535b 100644
--- a/compiler/rustc_codegen_ssa/src/lib.rs
+++ b/compiler/rustc_codegen_ssa/src/lib.rs
@@ -2,7 +2,6 @@
 #![allow(internal_features)]
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(assert_matches)]
diff --git a/compiler/rustc_const_eval/src/errors.rs b/compiler/rustc_const_eval/src/errors.rs
index e2675e2f4c9..6472aaa5758 100644
--- a/compiler/rustc_const_eval/src/errors.rs
+++ b/compiler/rustc_const_eval/src/errors.rs
@@ -6,7 +6,7 @@ use rustc_abi::WrappingRange;
 use rustc_errors::codes::*;
 use rustc_errors::{
     Diag, DiagArgValue, DiagCtxtHandle, DiagMessage, Diagnostic, EmissionGuarantee, Level,
-    MultiSpan, SubdiagMessageOp, Subdiagnostic,
+    MultiSpan, Subdiagnostic,
 };
 use rustc_hir::ConstContext;
 use rustc_macros::{Diagnostic, LintDiagnostic, Subdiagnostic};
@@ -290,11 +290,7 @@ pub struct FrameNote {
 }
 
 impl Subdiagnostic for FrameNote {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         diag.arg("times", self.times);
         diag.arg("where_", self.where_);
         diag.arg("instance", self.instance);
@@ -302,7 +298,7 @@ impl Subdiagnostic for FrameNote {
         if self.has_label && !self.span.is_dummy() {
             span.push_span_label(self.span, fluent::const_eval_frame_note_last);
         }
-        let msg = f(diag, fluent::const_eval_frame_note.into());
+        let msg = diag.eagerly_translate(fluent::const_eval_frame_note);
         diag.span_note(span, msg);
     }
 }
diff --git a/compiler/rustc_const_eval/src/lib.rs b/compiler/rustc_const_eval/src/lib.rs
index e03849c32f9..da52d60ae59 100644
--- a/compiler/rustc_const_eval/src/lib.rs
+++ b/compiler/rustc_const_eval/src/lib.rs
@@ -1,7 +1,6 @@
 // tidy-alphabetical-start
 #![allow(internal_features)]
 #![allow(rustc::diagnostic_outside_of_impl)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(rust_logo)]
 #![feature(assert_matches)]
 #![feature(box_patterns)]
diff --git a/compiler/rustc_data_structures/src/marker.rs b/compiler/rustc_data_structures/src/marker.rs
index 744ae9b6fe2..5f07cfef133 100644
--- a/compiler/rustc_data_structures/src/marker.rs
+++ b/compiler/rustc_data_structures/src/marker.rs
@@ -1,13 +1,13 @@
 use std::alloc::Allocator;
 
-#[rustc_on_unimplemented(message = "`{Self}` doesn't implement `DynSend`. \
+#[diagnostic::on_unimplemented(message = "`{Self}` doesn't implement `DynSend`. \
             Add it to `rustc_data_structures::marker` or use `IntoDynSyncSend` if it's already `Send`")]
 // This is an auto trait for types which can be sent across threads if `sync::is_dyn_thread_safe()`
 // is true. These types can be wrapped in a `FromDyn` to get a `Send` type. Wrapping a
 // `Send` type in `IntoDynSyncSend` will create a `DynSend` type.
 pub unsafe auto trait DynSend {}
 
-#[rustc_on_unimplemented(message = "`{Self}` doesn't implement `DynSync`. \
+#[diagnostic::on_unimplemented(message = "`{Self}` doesn't implement `DynSync`. \
             Add it to `rustc_data_structures::marker` or use `IntoDynSyncSend` if it's already `Sync`")]
 // This is an auto trait for types which can be shared across threads if `sync::is_dyn_thread_safe()`
 // is true. These types can be wrapped in a `FromDyn` to get a `Sync` type. Wrapping a
diff --git a/compiler/rustc_driver/src/lib.rs b/compiler/rustc_driver/src/lib.rs
index 381309f83b2..a03834c519d 100644
--- a/compiler/rustc_driver/src/lib.rs
+++ b/compiler/rustc_driver/src/lib.rs
@@ -3,7 +3,6 @@
 
 // tidy-alphabetical-start
 #![allow(internal_features)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(rust_logo)]
 #![feature(rustdoc_internals)]
 // tidy-alphabetical-end
diff --git a/compiler/rustc_driver_impl/Cargo.toml b/compiler/rustc_driver_impl/Cargo.toml
index de643355f5f..c823d11126e 100644
--- a/compiler/rustc_driver_impl/Cargo.toml
+++ b/compiler/rustc_driver_impl/Cargo.toml
@@ -4,6 +4,7 @@ version = "0.0.0"
 edition = "2024"
 
 [dependencies]
+jiff = { version = "0.2.5", default-features = false, features = ["std"] }
 # tidy-alphabetical-start
 rustc_abi = { path = "../rustc_abi" }
 rustc_ast = { path = "../rustc_ast" }
@@ -50,7 +51,6 @@ rustc_trait_selection = { path = "../rustc_trait_selection" }
 rustc_ty_utils = { path = "../rustc_ty_utils" }
 serde_json = "1.0.59"
 shlex = "1.0"
-time = { version = "0.3.36", default-features = false, features = ["alloc", "formatting", "macros"] }
 tracing = { version = "0.1.35" }
 # tidy-alphabetical-end
 
diff --git a/compiler/rustc_driver_impl/src/lib.rs b/compiler/rustc_driver_impl/src/lib.rs
index 7d9560ab4c5..40cc82727a5 100644
--- a/compiler/rustc_driver_impl/src/lib.rs
+++ b/compiler/rustc_driver_impl/src/lib.rs
@@ -7,7 +7,6 @@
 // tidy-alphabetical-start
 #![allow(internal_features)]
 #![allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(decl_macro)]
@@ -30,7 +29,7 @@ use std::path::{Path, PathBuf};
 use std::process::{self, Command, Stdio};
 use std::sync::OnceLock;
 use std::sync::atomic::{AtomicBool, Ordering};
-use std::time::{Instant, SystemTime};
+use std::time::Instant;
 use std::{env, str};
 
 use rustc_ast as ast;
@@ -66,8 +65,6 @@ use rustc_span::FileName;
 use rustc_span::def_id::LOCAL_CRATE;
 use rustc_target::json::ToJson;
 use rustc_target::spec::{Target, TargetTuple};
-use time::OffsetDateTime;
-use time::macros::format_description;
 use tracing::trace;
 
 #[allow(unused_macros)]
@@ -1301,13 +1298,8 @@ fn ice_path_with_config(config: Option<&UnstableOptions>) -> &'static Option<Pat
                 .or_else(|| std::env::current_dir().ok())
                 .unwrap_or_default(),
         };
-        let now: OffsetDateTime = SystemTime::now().into();
-        let file_now = now
-            .format(
-                // Don't use a standard datetime format because Windows doesn't support `:` in paths
-                &format_description!("[year]-[month]-[day]T[hour]_[minute]_[second]"),
-            )
-            .unwrap_or_default();
+        // Don't use a standard datetime format because Windows doesn't support `:` in paths
+        let file_now = jiff::Zoned::now().strftime("%Y-%m-%dT%H_%M_%S");
         let pid = std::process::id();
         path.push(format!("rustc-ice-{file_now}-{pid}.txt"));
         Some(path)
diff --git a/compiler/rustc_error_codes/src/error_codes/E0755.md b/compiler/rustc_error_codes/src/error_codes/E0755.md
index 88b7f484969..b67f078c78e 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0755.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0755.md
@@ -5,7 +5,7 @@ Erroneous code example:
 ```compile_fail,E0755
 #![feature(ffi_pure)]
 
-#[ffi_pure] // error!
+#[unsafe(ffi_pure)] // error!
 pub fn foo() {}
 # fn main() {}
 ```
@@ -17,7 +17,7 @@ side effects or infinite loops:
 #![feature(ffi_pure)]
 
 extern "C" {
-    #[ffi_pure] // ok!
+    #[unsafe(ffi_pure)] // ok!
     pub fn strlen(s: *const i8) -> isize;
 }
 # fn main() {}
diff --git a/compiler/rustc_error_codes/src/error_codes/E0756.md b/compiler/rustc_error_codes/src/error_codes/E0756.md
index ffdc421aab5..aadde038d12 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0756.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0756.md
@@ -6,7 +6,7 @@ Erroneous code example:
 ```compile_fail,E0756
 #![feature(ffi_const)]
 
-#[ffi_const] // error!
+#[unsafe(ffi_const)] // error!
 pub fn foo() {}
 # fn main() {}
 ```
@@ -18,7 +18,7 @@ which have no side effects except for their return value:
 #![feature(ffi_const)]
 
 extern "C" {
-    #[ffi_const] // ok!
+    #[unsafe(ffi_const)] // ok!
     pub fn strlen(s: *const i8) -> i32;
 }
 # fn main() {}
diff --git a/compiler/rustc_error_codes/src/error_codes/E0757.md b/compiler/rustc_error_codes/src/error_codes/E0757.md
index 41b06b23c4f..fb75b028f45 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0757.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0757.md
@@ -6,8 +6,9 @@ Erroneous code example:
 #![feature(ffi_const, ffi_pure)]
 
 extern "C" {
-    #[ffi_const]
-    #[ffi_pure] // error: `#[ffi_const]` function cannot be `#[ffi_pure]`
+    #[unsafe(ffi_const)]
+    #[unsafe(ffi_pure)]
+    //~^ ERROR `#[ffi_const]` function cannot be `#[ffi_pure]`
     pub fn square(num: i32) -> i32;
 }
 ```
@@ -19,7 +20,7 @@ As `ffi_const` provides stronger guarantees than `ffi_pure`, remove the
 #![feature(ffi_const)]
 
 extern "C" {
-    #[ffi_const]
+    #[unsafe(ffi_const)]
     pub fn square(num: i32) -> i32;
 }
 ```
diff --git a/compiler/rustc_errors/src/diagnostic.rs b/compiler/rustc_errors/src/diagnostic.rs
index bd13c413a4d..539ecfbb42e 100644
--- a/compiler/rustc_errors/src/diagnostic.rs
+++ b/compiler/rustc_errors/src/diagnostic.rs
@@ -181,22 +181,9 @@ where
     Self: Sized,
 {
     /// Add a subdiagnostic to an existing diagnostic.
-    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
-        self.add_to_diag_with(diag, &|_, m| m);
-    }
-
-    /// Add a subdiagnostic to an existing diagnostic where `f` is invoked on every message used
-    /// (to optionally perform eager translation).
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        f: &F,
-    );
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>);
 }
 
-pub trait SubdiagMessageOp<G: EmissionGuarantee> =
-    Fn(&mut Diag<'_, G>, SubdiagMessage) -> SubdiagMessage;
-
 /// Trait implemented by lint types. This should not be implemented manually. Instead, use
 /// `#[derive(LintDiagnostic)]` -- see [rustc_macros::LintDiagnostic].
 #[rustc_diagnostic_item = "LintDiagnostic"]
@@ -1227,15 +1214,21 @@ impl<'a, G: EmissionGuarantee> Diag<'a, G> {
     /// interpolated variables).
     #[rustc_lint_diagnostics]
     pub fn subdiagnostic(&mut self, subdiagnostic: impl Subdiagnostic) -> &mut Self {
-        let dcx = self.dcx;
-        subdiagnostic.add_to_diag_with(self, &|diag, msg| {
-            let args = diag.args.iter();
-            let msg = diag.subdiagnostic_message_to_diagnostic_message(msg);
-            dcx.eagerly_translate(msg, args)
-        });
+        subdiagnostic.add_to_diag(self);
         self
     }
 
+    /// Fluent variables are not namespaced from each other, so when
+    /// `Diagnostic`s and `Subdiagnostic`s use the same variable name,
+    /// one value will clobber the other. Eagerly translating the
+    /// diagnostic uses the variables defined right then, before the
+    /// clobbering occurs.
+    pub fn eagerly_translate(&self, msg: impl Into<SubdiagMessage>) -> SubdiagMessage {
+        let args = self.args.iter();
+        let msg = self.subdiagnostic_message_to_diagnostic_message(msg.into());
+        self.dcx.eagerly_translate(msg, args)
+    }
+
     with_fn! { with_span,
     /// Add a span.
     #[rustc_lint_diagnostics]
diff --git a/compiler/rustc_errors/src/diagnostic_impls.rs b/compiler/rustc_errors/src/diagnostic_impls.rs
index cb2e1769fa1..8b59ba9984c 100644
--- a/compiler/rustc_errors/src/diagnostic_impls.rs
+++ b/compiler/rustc_errors/src/diagnostic_impls.rs
@@ -19,7 +19,7 @@ use {rustc_ast as ast, rustc_hir as hir};
 use crate::diagnostic::DiagLocation;
 use crate::{
     Diag, DiagArgValue, DiagCtxtHandle, Diagnostic, EmissionGuarantee, ErrCode, IntoDiagArg, Level,
-    SubdiagMessageOp, Subdiagnostic, fluent_generated as fluent,
+    Subdiagnostic, fluent_generated as fluent,
 };
 
 pub struct DiagArgFromDisplay<'a>(pub &'a dyn fmt::Display);
@@ -384,11 +384,7 @@ pub struct SingleLabelManySpans {
     pub label: &'static str,
 }
 impl Subdiagnostic for SingleLabelManySpans {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         diag.span_labels(self.spans, self.label);
     }
 }
diff --git a/compiler/rustc_errors/src/lib.rs b/compiler/rustc_errors/src/lib.rs
index 75bb0e8e7b4..c0c5dba4677 100644
--- a/compiler/rustc_errors/src/lib.rs
+++ b/compiler/rustc_errors/src/lib.rs
@@ -47,7 +47,7 @@ pub use codes::*;
 pub use diagnostic::{
     BugAbort, Diag, DiagArg, DiagArgMap, DiagArgName, DiagArgValue, DiagInner, DiagStyledString,
     Diagnostic, EmissionGuarantee, FatalAbort, IntoDiagArg, LintDiagnostic, StringPart, Subdiag,
-    SubdiagMessageOp, Subdiagnostic,
+    Subdiagnostic,
 };
 pub use diagnostic_impls::{
     DiagArgFromDisplay, DiagSymbolList, ElidedLifetimeInPathSubdiag, ExpectedLifetimeParameter,
diff --git a/compiler/rustc_expand/src/config.rs b/compiler/rustc_expand/src/config.rs
index bcc2703c39b..c70e259b2cd 100644
--- a/compiler/rustc_expand/src/config.rs
+++ b/compiler/rustc_expand/src/config.rs
@@ -238,18 +238,6 @@ impl<'a> StripUnconfigured<'a> {
                     Some(AttrTokenTree::Delimited(sp, spacing, delim, inner))
                 }
                 AttrTokenTree::Token(
-                    Token {
-                        kind:
-                            TokenKind::NtIdent(..)
-                            | TokenKind::NtLifetime(..)
-                            | TokenKind::Interpolated(..),
-                        ..
-                    },
-                    _,
-                ) => {
-                    panic!("Nonterminal should have been flattened: {:?}", tree);
-                }
-                AttrTokenTree::Token(
                     Token { kind: TokenKind::OpenDelim(_) | TokenKind::CloseDelim(_), .. },
                     _,
                 ) => {
diff --git a/compiler/rustc_expand/src/mbe/diagnostics.rs b/compiler/rustc_expand/src/mbe/diagnostics.rs
index e60a9e83184..b663e959744 100644
--- a/compiler/rustc_expand/src/mbe/diagnostics.rs
+++ b/compiler/rustc_expand/src/mbe/diagnostics.rs
@@ -66,9 +66,7 @@ pub(super) fn failed_to_match_macro(
     }
 
     if let MatcherLoc::Token { token: expected_token } = &remaining_matcher
-        && (matches!(expected_token.kind, TokenKind::Interpolated(_))
-            || matches!(token.kind, TokenKind::Interpolated(_))
-            || matches!(expected_token.kind, TokenKind::OpenDelim(Delimiter::Invisible(_)))
+        && (matches!(expected_token.kind, TokenKind::OpenDelim(Delimiter::Invisible(_)))
             || matches!(token.kind, TokenKind::OpenDelim(Delimiter::Invisible(_))))
     {
         err.note("captured metavariables except for `:tt`, `:ident` and `:lifetime` cannot be compared to other tokens");
@@ -162,7 +160,7 @@ impl<'dcx, 'matcher> Tracker<'matcher> for CollectTrackerAndEmitter<'dcx, 'match
                     .is_none_or(|failure| failure.is_better_position(*approx_position))
                 {
                     self.best_failure = Some(BestFailure {
-                        token: token.clone(),
+                        token: *token,
                         position_in_tokenstream: *approx_position,
                         msg,
                         remaining_matcher: self
diff --git a/compiler/rustc_expand/src/mbe/macro_parser.rs b/compiler/rustc_expand/src/mbe/macro_parser.rs
index d709fd79281..0065f83eb4e 100644
--- a/compiler/rustc_expand/src/mbe/macro_parser.rs
+++ b/compiler/rustc_expand/src/mbe/macro_parser.rs
@@ -179,7 +179,7 @@ pub(super) fn compute_locs(matcher: &[TokenTree]) -> Vec<MatcherLoc> {
         for tt in tts {
             match tt {
                 TokenTree::Token(token) => {
-                    locs.push(MatcherLoc::Token { token: token.clone() });
+                    locs.push(MatcherLoc::Token { token: *token });
                 }
                 TokenTree::Delimited(span, _, delimited) => {
                     let open_token = Token::new(token::OpenDelim(delimited.delim), span.open);
@@ -648,7 +648,7 @@ impl TtParser {
                     // There are no possible next positions AND we aren't waiting for the black-box
                     // parser: syntax error.
                     return Failure(T::build_failure(
-                        parser.token.clone(),
+                        parser.token,
                         parser.approx_token_stream_pos(),
                         "no rules expected this token in macro call",
                     ));
diff --git a/compiler/rustc_expand/src/mbe/macro_rules.rs b/compiler/rustc_expand/src/mbe/macro_rules.rs
index 77ec598e62a..c138b090877 100644
--- a/compiler/rustc_expand/src/mbe/macro_rules.rs
+++ b/compiler/rustc_expand/src/mbe/macro_rules.rs
@@ -12,7 +12,7 @@ use rustc_ast::{self as ast, DUMMY_NODE_ID, NodeId};
 use rustc_ast_pretty::pprust;
 use rustc_attr_parsing::{AttributeKind, find_attr};
 use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
-use rustc_errors::{Applicability, ErrorGuaranteed};
+use rustc_errors::{Applicability, Diag, ErrorGuaranteed};
 use rustc_feature::Features;
 use rustc_hir as hir;
 use rustc_lint_defs::BuiltinLintDiag;
@@ -27,19 +27,18 @@ use rustc_span::hygiene::Transparency;
 use rustc_span::{Ident, MacroRulesNormalizedIdent, Span, kw, sym};
 use tracing::{debug, instrument, trace, trace_span};
 
-use super::diagnostics;
 use super::macro_parser::{NamedMatches, NamedParseResult};
+use super::{SequenceRepetition, diagnostics};
 use crate::base::{
     DummyResult, ExpandResult, ExtCtxt, MacResult, MacroExpanderResult, SyntaxExtension,
     SyntaxExtensionKind, TTMacroExpander,
 };
 use crate::expand::{AstFragment, AstFragmentKind, ensure_complete_parse, parse_ast_fragment};
-use crate::mbe;
 use crate::mbe::diagnostics::{annotate_doc_comment, parse_failure_msg};
-use crate::mbe::macro_check;
 use crate::mbe::macro_parser::NamedMatch::*;
 use crate::mbe::macro_parser::{Error, ErrorReported, Failure, MatcherLoc, Success, TtParser};
 use crate::mbe::transcribe::transcribe;
+use crate::mbe::{self, KleeneOp, macro_check};
 
 pub(crate) struct ParserAnyMacro<'a> {
     parser: Parser<'a>,
@@ -640,6 +639,37 @@ fn is_empty_token_tree(sess: &Session, seq: &mbe::SequenceRepetition) -> bool {
     }
 }
 
+/// Checks if a `vis` nonterminal fragment is unnecessarily wrapped in an optional repetition.
+///
+/// When a `vis` fragment (which can already be empty) is wrapped in `$(...)?`,
+/// this suggests removing the redundant repetition syntax since it provides no additional benefit.
+fn check_redundant_vis_repetition(
+    err: &mut Diag<'_>,
+    sess: &Session,
+    seq: &SequenceRepetition,
+    span: &DelimSpan,
+) {
+    let is_zero_or_one: bool = seq.kleene.op == KleeneOp::ZeroOrOne;
+    let is_vis = seq.tts.first().map_or(false, |tt| {
+        matches!(tt, mbe::TokenTree::MetaVarDecl(_, _, Some(NonterminalKind::Vis)))
+    });
+
+    if is_vis && is_zero_or_one {
+        err.note("a `vis` fragment can already be empty");
+        err.multipart_suggestion(
+            "remove the `$(` and `)?`",
+            vec![
+                (
+                    sess.source_map().span_extend_to_prev_char_before(span.open, '$', true),
+                    "".to_string(),
+                ),
+                (span.close.with_hi(seq.kleene.span.hi()), "".to_string()),
+            ],
+            Applicability::MaybeIncorrect,
+        );
+    }
+}
+
 /// Checks that the lhs contains no repetition which could match an empty token
 /// tree, because then the matcher would hang indefinitely.
 fn check_lhs_no_empty_seq(sess: &Session, tts: &[mbe::TokenTree]) -> Result<(), ErrorGuaranteed> {
@@ -654,8 +684,10 @@ fn check_lhs_no_empty_seq(sess: &Session, tts: &[mbe::TokenTree]) -> Result<(),
             TokenTree::Sequence(span, seq) => {
                 if is_empty_token_tree(sess, seq) {
                     let sp = span.entire();
-                    let guar = sess.dcx().span_err(sp, "repetition matches empty token tree");
-                    return Err(guar);
+                    let mut err =
+                        sess.dcx().struct_span_err(sp, "repetition matches empty token tree");
+                    check_redundant_vis_repetition(&mut err, sess, seq, span);
+                    return Err(err.emit());
                 }
                 check_lhs_no_empty_seq(sess, &seq.tts)?
             }
@@ -778,7 +810,7 @@ impl<'tt> FirstSets<'tt> {
                         // token could be the separator token itself.
 
                         if let (Some(sep), true) = (&seq_rep.separator, subfirst.maybe_empty) {
-                            first.add_one_maybe(TtHandle::from_token(sep.clone()));
+                            first.add_one_maybe(TtHandle::from_token(*sep));
                         }
 
                         // Reverse scan: Sequence comes before `first`.
@@ -841,7 +873,7 @@ impl<'tt> FirstSets<'tt> {
                     // If the sequence contents can be empty, then the first
                     // token could be the separator token itself.
                     if let (Some(sep), true) = (&seq_rep.separator, subfirst.maybe_empty) {
-                        first.add_one_maybe(TtHandle::from_token(sep.clone()));
+                        first.add_one_maybe(TtHandle::from_token(*sep));
                     }
 
                     assert!(first.maybe_empty);
@@ -917,7 +949,7 @@ impl<'tt> Clone for TtHandle<'tt> {
             // This variant *must* contain a `mbe::TokenTree::Token`, and not
             // any other variant of `mbe::TokenTree`.
             TtHandle::Token(mbe::TokenTree::Token(tok)) => {
-                TtHandle::Token(mbe::TokenTree::Token(tok.clone()))
+                TtHandle::Token(mbe::TokenTree::Token(*tok))
             }
 
             _ => unreachable!(),
@@ -1093,7 +1125,7 @@ fn check_matcher_core<'tt>(
                 let mut new;
                 let my_suffix = if let Some(sep) = &seq_rep.separator {
                     new = suffix_first.clone();
-                    new.add_one_maybe(TtHandle::from_token(sep.clone()));
+                    new.add_one_maybe(TtHandle::from_token(*sep));
                     &new
                 } else {
                     &suffix_first
diff --git a/compiler/rustc_expand/src/mbe/quoted.rs b/compiler/rustc_expand/src/mbe/quoted.rs
index 0ea53627fe7..3f037259956 100644
--- a/compiler/rustc_expand/src/mbe/quoted.rs
+++ b/compiler/rustc_expand/src/mbe/quoted.rs
@@ -283,7 +283,7 @@ fn parse_tree<'a>(
         }
 
         // `tree` is an arbitrary token. Keep it.
-        tokenstream::TokenTree::Token(token, _) => TokenTree::Token(token.clone()),
+        tokenstream::TokenTree::Token(token, _) => TokenTree::Token(*token),
 
         // `tree` is the beginning of a delimited set of tokens (e.g., `(` or `{`). We need to
         // descend into the delimited set and further parse it.
@@ -321,7 +321,7 @@ fn parse_kleene_op(
     match iter.next() {
         Some(tokenstream::TokenTree::Token(token, _)) => match kleene_op(token) {
             Some(op) => Ok(Ok((op, token.span))),
-            None => Ok(Err(token.clone())),
+            None => Ok(Err(*token)),
         },
         tree => Err(tree.map_or(span, tokenstream::TokenTree::span)),
     }
diff --git a/compiler/rustc_expand/src/mbe/transcribe.rs b/compiler/rustc_expand/src/mbe/transcribe.rs
index 6e47ed6eb67..39186319b1c 100644
--- a/compiler/rustc_expand/src/mbe/transcribe.rs
+++ b/compiler/rustc_expand/src/mbe/transcribe.rs
@@ -1,5 +1,4 @@
 use std::mem;
-use std::sync::Arc;
 
 use rustc_ast::mut_visit::{self, MutVisitor};
 use rustc_ast::token::{
@@ -165,7 +164,7 @@ pub(super) fn transcribe<'a>(
                 if repeat_idx < repeat_len {
                     frame.idx = 0;
                     if let Some(sep) = sep {
-                        result.push(TokenTree::Token(sep.clone(), Spacing::Alone));
+                        result.push(TokenTree::Token(*sep, Spacing::Alone));
                     }
                     continue;
                 }
@@ -307,7 +306,9 @@ pub(super) fn transcribe<'a>(
                     let tt = match cur_matched {
                         MatchedSingle(ParseNtResult::Tt(tt)) => {
                             // `tt`s are emitted into the output stream directly as "raw tokens",
-                            // without wrapping them into groups.
+                            // without wrapping them into groups. Other variables are emitted into
+                            // the output stream as groups with `Delimiter::Invisible` to maintain
+                            // parsing priorities.
                             maybe_use_metavar_location(psess, &stack, sp, tt, &mut marker)
                         }
                         MatchedSingle(ParseNtResult::Ident(ident, is_raw)) => {
@@ -325,6 +326,11 @@ pub(super) fn transcribe<'a>(
                         MatchedSingle(ParseNtResult::Item(item)) => {
                             mk_delimited(item.span, MetaVarKind::Item, TokenStream::from_ast(item))
                         }
+                        MatchedSingle(ParseNtResult::Block(block)) => mk_delimited(
+                            block.span,
+                            MetaVarKind::Block,
+                            TokenStream::from_ast(block),
+                        ),
                         MatchedSingle(ParseNtResult::Stmt(stmt)) => {
                             let stream = if let StmtKind::Empty = stmt.kind {
                                 // FIXME: Properly collect tokens for empty statements.
@@ -385,15 +391,6 @@ pub(super) fn transcribe<'a>(
                         MatchedSingle(ParseNtResult::Vis(vis)) => {
                             mk_delimited(vis.span, MetaVarKind::Vis, TokenStream::from_ast(vis))
                         }
-                        MatchedSingle(ParseNtResult::Nt(nt)) => {
-                            // Other variables are emitted into the output stream as groups with
-                            // `Delimiter::Invisible` to maintain parsing priorities.
-                            // `Interpolated` is currently used for such groups in rustc parser.
-                            marker.visit_span(&mut sp);
-                            let use_span = nt.use_span();
-                            with_metavar_spans(|mspans| mspans.insert(use_span, sp));
-                            TokenTree::token_alone(token::Interpolated(Arc::clone(nt)), sp)
-                        }
                         MatchedSeq(..) => {
                             // We were unable to descend far enough. This is an error.
                             return Err(dcx.create_err(VarStillRepeating { span: sp, ident }));
@@ -441,7 +438,7 @@ pub(super) fn transcribe<'a>(
             // Nothing much to do here. Just push the token to the result, being careful to
             // preserve syntax context.
             mbe::TokenTree::Token(token) => {
-                let mut token = token.clone();
+                let mut token = *token;
                 mut_visit::visit_token(&mut marker, &mut token);
                 let tt = TokenTree::Token(token, Spacing::Alone);
                 result.push(tt);
diff --git a/compiler/rustc_expand/src/proc_macro_server.rs b/compiler/rustc_expand/src/proc_macro_server.rs
index ee6306e3961..4edaf68c89a 100644
--- a/compiler/rustc_expand/src/proc_macro_server.rs
+++ b/compiler/rustc_expand/src/proc_macro_server.rs
@@ -1,5 +1,4 @@
 use std::ops::{Bound, Range};
-use std::sync::Arc;
 
 use ast::token::IdentIsRaw;
 use pm::bridge::{
@@ -18,7 +17,7 @@ use rustc_parse::parser::Parser;
 use rustc_parse::{exp, new_parser_from_source_str, source_str_to_stream, unwrap_or_emit_fatal};
 use rustc_session::parse::ParseSess;
 use rustc_span::def_id::CrateNum;
-use rustc_span::{BytePos, FileName, Pos, SourceFile, Span, Symbol, sym};
+use rustc_span::{BytePos, FileName, Pos, Span, Symbol, sym};
 use smallvec::{SmallVec, smallvec};
 
 use crate::base::ExtCtxt;
@@ -309,15 +308,6 @@ impl FromInternal<(TokenStream, &mut Rustc<'_, '_>)> for Vec<TokenTree<TokenStre
                     }));
                 }
 
-                Interpolated(nt) => {
-                    let stream = TokenStream::from_nonterminal_ast(&nt);
-                    trees.push(TokenTree::Group(Group {
-                        delimiter: pm::Delimiter::None,
-                        stream: Some(stream),
-                        span: DelimSpan::from_single(span),
-                    }))
-                }
-
                 OpenDelim(..) | CloseDelim(..) => unreachable!(),
                 Eof => unreachable!(),
             }
@@ -467,7 +457,6 @@ impl<'a, 'b> Rustc<'a, 'b> {
 impl server::Types for Rustc<'_, '_> {
     type FreeFunctions = FreeFunctions;
     type TokenStream = TokenStream;
-    type SourceFile = Arc<SourceFile>;
     type Span = Span;
     type Symbol = Symbol;
 }
@@ -673,28 +662,6 @@ impl server::TokenStream for Rustc<'_, '_> {
     }
 }
 
-impl server::SourceFile for Rustc<'_, '_> {
-    fn eq(&mut self, file1: &Self::SourceFile, file2: &Self::SourceFile) -> bool {
-        Arc::ptr_eq(file1, file2)
-    }
-
-    fn path(&mut self, file: &Self::SourceFile) -> String {
-        match &file.name {
-            FileName::Real(name) => name
-                .local_path()
-                .expect("attempting to get a file path in an imported file in `proc_macro::SourceFile::path`")
-                .to_str()
-                .expect("non-UTF8 file path in `proc_macro::SourceFile::path`")
-                .to_string(),
-            _ => file.name.prefer_local().to_string(),
-        }
-    }
-
-    fn is_real(&mut self, file: &Self::SourceFile) -> bool {
-        file.is_real_file()
-    }
-}
-
 impl server::Span for Rustc<'_, '_> {
     fn debug(&mut self, span: Self::Span) -> String {
         if self.ecx.ecfg.span_debug {
@@ -704,8 +671,29 @@ impl server::Span for Rustc<'_, '_> {
         }
     }
 
-    fn source_file(&mut self, span: Self::Span) -> Self::SourceFile {
-        self.psess().source_map().lookup_char_pos(span.lo()).file
+    fn file(&mut self, span: Self::Span) -> String {
+        self.psess()
+            .source_map()
+            .lookup_char_pos(span.lo())
+            .file
+            .name
+            .prefer_remapped_unconditionaly()
+            .to_string()
+    }
+
+    fn local_file(&mut self, span: Self::Span) -> Option<String> {
+        self.psess()
+            .source_map()
+            .lookup_char_pos(span.lo())
+            .file
+            .name
+            .clone()
+            .into_local_path()
+            .map(|p| {
+                p.to_str()
+                    .expect("non-UTF8 file path in `proc_macro::SourceFile::path`")
+                    .to_string()
+            })
     }
 
     fn parent(&mut self, span: Self::Span) -> Option<Self::Span> {
diff --git a/compiler/rustc_feature/src/accepted.rs b/compiler/rustc_feature/src/accepted.rs
index 8968c1771e4..fcc11dd3c1f 100644
--- a/compiler/rustc_feature/src/accepted.rs
+++ b/compiler/rustc_feature/src/accepted.rs
@@ -95,6 +95,8 @@ declare_features! (
     (accepted, c_unwind, "1.81.0", Some(74990)),
     /// Allows `#[cfg_attr(predicate, multiple, attributes, here)]`.
     (accepted, cfg_attr_multi, "1.33.0", Some(54881)),
+    /// Allows the use of `#[cfg(<true/false>)]`.
+    (accepted, cfg_boolean_literals, "CURRENT_RUSTC_VERSION", Some(131204)),
     /// Allows the use of `#[cfg(doctest)]`, set when rustdoc is collecting doctests.
     (accepted, cfg_doctest, "1.40.0", Some(62210)),
     /// Enables `#[cfg(panic = "...")]` config key.
diff --git a/compiler/rustc_feature/src/builtin_attrs.rs b/compiler/rustc_feature/src/builtin_attrs.rs
index 1e33e2e9393..3b441729d75 100644
--- a/compiler/rustc_feature/src/builtin_attrs.rs
+++ b/compiler/rustc_feature/src/builtin_attrs.rs
@@ -6,6 +6,7 @@ use AttributeDuplicates::*;
 use AttributeGate::*;
 use AttributeType::*;
 use rustc_data_structures::fx::FxHashMap;
+use rustc_span::edition::Edition;
 use rustc_span::{Symbol, sym};
 
 use crate::{Features, Stability};
@@ -65,9 +66,12 @@ pub enum AttributeSafety {
     /// Normal attribute that does not need `#[unsafe(...)]`
     Normal,
 
-    /// Unsafe attribute that requires safety obligations
-    /// to be discharged
-    Unsafe,
+    /// Unsafe attribute that requires safety obligations to be discharged.
+    ///
+    /// An error is emitted when `#[unsafe(...)]` is omitted, except when the attribute's edition
+    /// is less than the one stored in `unsafe_since`. This handles attributes that were safe in
+    /// earlier editions, but become unsafe in later ones.
+    Unsafe { unsafe_since: Option<Edition> },
 }
 
 #[derive(Clone, Copy)]
@@ -187,12 +191,23 @@ macro_rules! template {
 }
 
 macro_rules! ungated {
+    (unsafe($edition:ident) $attr:ident, $typ:expr, $tpl:expr, $duplicates:expr, $encode_cross_crate:expr $(,)?) => {
+        BuiltinAttribute {
+            name: sym::$attr,
+            encode_cross_crate: $encode_cross_crate,
+            type_: $typ,
+            safety: AttributeSafety::Unsafe { unsafe_since: Some(Edition::$edition) },
+            template: $tpl,
+            gate: Ungated,
+            duplicates: $duplicates,
+        }
+    };
     (unsafe $attr:ident, $typ:expr, $tpl:expr, $duplicates:expr, $encode_cross_crate:expr $(,)?) => {
         BuiltinAttribute {
             name: sym::$attr,
             encode_cross_crate: $encode_cross_crate,
             type_: $typ,
-            safety: AttributeSafety::Unsafe,
+            safety: AttributeSafety::Unsafe { unsafe_since: None },
             template: $tpl,
             gate: Ungated,
             duplicates: $duplicates,
@@ -217,7 +232,7 @@ macro_rules! gated {
             name: sym::$attr,
             encode_cross_crate: $encode_cross_crate,
             type_: $typ,
-            safety: AttributeSafety::Unsafe,
+            safety: AttributeSafety::Unsafe { unsafe_since: None },
             template: $tpl,
             duplicates: $duplicates,
             gate: Gated(Stability::Unstable, sym::$gate, $msg, Features::$gate),
@@ -228,7 +243,7 @@ macro_rules! gated {
             name: sym::$attr,
             encode_cross_crate: $encode_cross_crate,
             type_: $typ,
-            safety: AttributeSafety::Unsafe,
+            safety: AttributeSafety::Unsafe { unsafe_since: None },
             template: $tpl,
             duplicates: $duplicates,
             gate: Gated(Stability::Unstable, sym::$attr, $msg, Features::$attr),
@@ -423,9 +438,9 @@ pub static BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
     ungated!(no_link, Normal, template!(Word), WarnFollowing, EncodeCrossCrate::No),
     ungated!(repr, Normal, template!(List: "C"), DuplicatesOk, EncodeCrossCrate::No),
-    ungated!(unsafe export_name, Normal, template!(NameValueStr: "name"), FutureWarnPreceding, EncodeCrossCrate::No),
-    ungated!(unsafe link_section, Normal, template!(NameValueStr: "name"), FutureWarnPreceding, EncodeCrossCrate::No),
-    ungated!(unsafe no_mangle, Normal, template!(Word), WarnFollowing, EncodeCrossCrate::No),
+    ungated!(unsafe(Edition2024) export_name, Normal, template!(NameValueStr: "name"), FutureWarnPreceding, EncodeCrossCrate::No),
+    ungated!(unsafe(Edition2024) link_section, Normal, template!(NameValueStr: "name"), FutureWarnPreceding, EncodeCrossCrate::No),
+    ungated!(unsafe(Edition2024) no_mangle, Normal, template!(Word), WarnFollowing, EncodeCrossCrate::No),
     ungated!(used, Normal, template!(Word, List: "compiler|linker"), WarnFollowing, EncodeCrossCrate::No),
     ungated!(link_ordinal, Normal, template!(List: "ordinal"), ErrorPreceding, EncodeCrossCrate::Yes),
 
@@ -752,7 +767,7 @@ pub static BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
     rustc_attr!(
         rustc_macro_transparency, Normal,
-        template!(NameValueStr: "transparent|semitransparent|opaque"), ErrorFollowing,
+        template!(NameValueStr: "transparent|semiopaque|opaque"), ErrorFollowing,
         EncodeCrossCrate::Yes, "used internally for testing macro hygiene",
     ),
     rustc_attr!(
diff --git a/compiler/rustc_feature/src/unstable.rs b/compiler/rustc_feature/src/unstable.rs
index 98213affc5b..e09ae3c1239 100644
--- a/compiler/rustc_feature/src/unstable.rs
+++ b/compiler/rustc_feature/src/unstable.rs
@@ -391,8 +391,6 @@ declare_features! (
     (unstable, async_trait_bounds, "1.85.0", Some(62290)),
     /// Allows using C-variadics.
     (unstable, c_variadic, "1.34.0", Some(44930)),
-    /// Allows the use of `#[cfg(<true/false>)]`.
-    (unstable, cfg_boolean_literals, "1.83.0", Some(131204)),
     /// Allows the use of `#[cfg(contract_checks)` to check if contract checks are enabled.
     (unstable, cfg_contract_checks, "1.86.0", Some(128044)),
     /// Allows the use of `#[cfg(overflow_checks)` to check if integer overflow behaviour.
@@ -477,6 +475,8 @@ declare_features! (
     (incomplete, ergonomic_clones, "1.87.0", Some(132290)),
     /// Allows exhaustive pattern matching on types that contain uninhabited types.
     (unstable, exhaustive_patterns, "1.13.0", Some(51085)),
+    /// Disallows `extern` without an explicit ABI.
+    (unstable, explicit_extern_abis, "CURRENT_RUSTC_VERSION", Some(134986)),
     /// Allows explicit tail calls via `become` expression.
     (incomplete, explicit_tail_calls, "1.72.0", Some(112788)),
     /// Allows using `aapcs`, `efiapi`, `sysv64` and `win64` as calling conventions
@@ -565,6 +565,8 @@ declare_features! (
     (incomplete, mut_ref, "1.79.0", Some(123076)),
     /// Allows using `#[naked]` on functions.
     (unstable, naked_functions, "1.9.0", Some(90957)),
+    /// Allows using `#[naked]` on `extern "Rust"` functions.
+    (unstable, naked_functions_rustic_abi, "CURRENT_RUSTC_VERSION", Some(138997)),
     /// Allows using `#[target_feature(enable = "...")]` on `#[naked]` on functions.
     (unstable, naked_functions_target_feature, "1.86.0", Some(138568)),
     /// Allows specifying the as-needed link modifier
diff --git a/compiler/rustc_fluent_macro/src/fluent.rs b/compiler/rustc_fluent_macro/src/fluent.rs
index b04fd1b48f7..c96bb48a036 100644
--- a/compiler/rustc_fluent_macro/src/fluent.rs
+++ b/compiler/rustc_fluent_macro/src/fluent.rs
@@ -25,7 +25,10 @@ fn invocation_relative_path_to_absolute(span: Span, path: &str) -> PathBuf {
         path.to_path_buf()
     } else {
         // `/a/b/c/foo/bar.rs` contains the current macro invocation
+        #[cfg(bootstrap)]
         let mut source_file_path = span.source_file().path();
+        #[cfg(not(bootstrap))]
+        let mut source_file_path = span.local_file().unwrap();
         // `/a/b/c/foo/`
         source_file_path.pop();
         // `/a/b/c/foo/../locales/en-US/example.ftl`
diff --git a/compiler/rustc_hir/src/definitions.rs b/compiler/rustc_hir/src/definitions.rs
index 34f3c169505..bf7b1eefcf6 100644
--- a/compiler/rustc_hir/src/definitions.rs
+++ b/compiler/rustc_hir/src/definitions.rs
@@ -47,12 +47,9 @@ impl DefPathTable {
         debug_assert_eq!(self.stable_crate_id, def_path_hash.stable_crate_id());
         let local_hash = def_path_hash.local_hash();
 
-        let index = {
-            let index = DefIndex::from(self.index_to_key.len());
-            debug!("DefPathTable::insert() - {:?} <-> {:?}", key, index);
-            self.index_to_key.push(key);
-            index
-        };
+        let index = self.index_to_key.push(key);
+        debug!("DefPathTable::insert() - {key:?} <-> {index:?}");
+
         self.def_path_hashes.push(local_hash);
         debug_assert!(self.def_path_hashes.len() == self.index_to_key.len());
 
diff --git a/compiler/rustc_hir/src/hir.rs b/compiler/rustc_hir/src/hir.rs
index c9c4936c156..3f5269eeb9b 100644
--- a/compiler/rustc_hir/src/hir.rs
+++ b/compiler/rustc_hir/src/hir.rs
@@ -42,7 +42,7 @@ pub enum IsAnonInPath {
 }
 
 /// A lifetime. The valid field combinations are non-obvious. The following
-/// example shows some of them. See also the comments on `LifetimeName`.
+/// example shows some of them. See also the comments on `LifetimeKind`.
 /// ```
 /// #[repr(C)]
 /// struct S<'a>(&'a u32);       // res=Param, name='a, IsAnonInPath::No
@@ -84,7 +84,7 @@ pub struct Lifetime {
     pub ident: Ident,
 
     /// Semantics of this lifetime.
-    pub res: LifetimeName,
+    pub kind: LifetimeKind,
 
     /// Is the lifetime anonymous and in a path? Used only for error
     /// suggestions. See `Lifetime::suggestion` for example use.
@@ -130,7 +130,7 @@ impl ParamName {
 }
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq, HashStable_Generic)]
-pub enum LifetimeName {
+pub enum LifetimeKind {
     /// User-given names or fresh (synthetic) names.
     Param(LocalDefId),
 
@@ -160,16 +160,16 @@ pub enum LifetimeName {
     Static,
 }
 
-impl LifetimeName {
+impl LifetimeKind {
     fn is_elided(&self) -> bool {
         match self {
-            LifetimeName::ImplicitObjectLifetimeDefault | LifetimeName::Infer => true,
+            LifetimeKind::ImplicitObjectLifetimeDefault | LifetimeKind::Infer => true,
 
             // It might seem surprising that `Fresh` counts as not *elided*
             // -- but this is because, as far as the code in the compiler is
             // concerned -- `Fresh` variants act equivalently to "some fresh name".
             // They correspond to early-bound regions on an impl, in other words.
-            LifetimeName::Error | LifetimeName::Param(..) | LifetimeName::Static => false,
+            LifetimeKind::Error | LifetimeKind::Param(..) | LifetimeKind::Static => false,
         }
     }
 }
@@ -184,10 +184,10 @@ impl Lifetime {
     pub fn new(
         hir_id: HirId,
         ident: Ident,
-        res: LifetimeName,
+        kind: LifetimeKind,
         is_anon_in_path: IsAnonInPath,
     ) -> Lifetime {
-        let lifetime = Lifetime { hir_id, ident, res, is_anon_in_path };
+        let lifetime = Lifetime { hir_id, ident, kind, is_anon_in_path };
 
         // Sanity check: elided lifetimes form a strict subset of anonymous lifetimes.
         #[cfg(debug_assertions)]
@@ -202,7 +202,7 @@ impl Lifetime {
     }
 
     pub fn is_elided(&self) -> bool {
-        self.res.is_elided()
+        self.kind.is_elided()
     }
 
     pub fn is_anonymous(&self) -> bool {
@@ -1014,7 +1014,7 @@ pub struct WhereRegionPredicate<'hir> {
 impl<'hir> WhereRegionPredicate<'hir> {
     /// Returns `true` if `param_def_id` matches the `lifetime` of this predicate.
     fn is_param_bound(&self, param_def_id: LocalDefId) -> bool {
-        self.lifetime.res == LifetimeName::Param(param_def_id)
+        self.lifetime.kind == LifetimeKind::Param(param_def_id)
     }
 }
 
@@ -1756,7 +1756,7 @@ pub enum PatKind<'hir> {
     Never,
 
     /// A tuple pattern (e.g., `(a, b)`).
-    /// If the `..` pattern fragment is present, then `Option<usize>` denotes its position.
+    /// If the `..` pattern fragment is present, then `DotDotPos` denotes its position.
     /// `0 <= position <= subpats.len()`
     Tuple(&'hir [Pat<'hir>], DotDotPos),
 
diff --git a/compiler/rustc_hir/src/hir/tests.rs b/compiler/rustc_hir/src/hir/tests.rs
index 62ef02d2f50..fcd0eafa461 100644
--- a/compiler/rustc_hir/src/hir/tests.rs
+++ b/compiler/rustc_hir/src/hir/tests.rs
@@ -57,7 +57,7 @@ fn trait_object_roundtrips_impl(syntax: TraitObjectSyntax) {
                 Lifetime {
                     hir_id: HirId::INVALID,
                     ident: Ident::new(sym::name, DUMMY_SP),
-                    res: LifetimeName::Static,
+                    kind: LifetimeKind::Static,
                     is_anon_in_path: IsAnonInPath::No,
                 }
             },
diff --git a/compiler/rustc_hir/src/intravisit.rs b/compiler/rustc_hir/src/intravisit.rs
index 93d20dfa799..3c2897ef1d9 100644
--- a/compiler/rustc_hir/src/intravisit.rs
+++ b/compiler/rustc_hir/src/intravisit.rs
@@ -6,7 +6,7 @@
 //! 1. **Shallow visit**: Get a simple callback for every item (or item-like thing) in the HIR.
 //!    - Example: find all items with a `#[foo]` attribute on them.
 //!    - How: Use the `hir_crate_items` or `hir_module_items` query to traverse over item-like ids
-//!       (ItemId, TraitItemId, etc.) and use tcx.def_kind and `tcx.hir().item*(id)` to filter and
+//!       (ItemId, TraitItemId, etc.) and use tcx.def_kind and `tcx.hir_item*(id)` to filter and
 //!       access actual item-like thing, respectively.
 //!    - Pro: Efficient; just walks the lists of item ids and gives users control whether to access
 //!       the hir_owners themselves or not.
diff --git a/compiler/rustc_hir/src/lang_items.rs b/compiler/rustc_hir/src/lang_items.rs
index 90fab01ba2d..b9f4a8cd165 100644
--- a/compiler/rustc_hir/src/lang_items.rs
+++ b/compiler/rustc_hir/src/lang_items.rs
@@ -182,6 +182,7 @@ language_item_table! {
     DynMetadata,             sym::dyn_metadata,        dyn_metadata,               Target::Struct,         GenericRequirement::None;
 
     Freeze,                  sym::freeze,              freeze_trait,               Target::Trait,          GenericRequirement::Exact(0);
+    UnsafeUnpin,             sym::unsafe_unpin,        unsafe_unpin_trait,         Target::Trait,          GenericRequirement::Exact(0);
 
     FnPtrTrait,              sym::fn_ptr_trait,        fn_ptr_trait,               Target::Trait,          GenericRequirement::Exact(0);
     FnPtrAddr,               sym::fn_ptr_addr,         fn_ptr_addr,                Target::Method(MethodKind::Trait { body: false }), GenericRequirement::None;
@@ -235,6 +236,8 @@ language_item_table! {
     IndexMut,                sym::index_mut,           index_mut_trait,            Target::Trait,          GenericRequirement::Exact(1);
 
     UnsafeCell,              sym::unsafe_cell,         unsafe_cell_type,           Target::Struct,         GenericRequirement::None;
+    UnsafePinned,            sym::unsafe_pinned,       unsafe_pinned_type,         Target::Struct,         GenericRequirement::None;
+
     VaList,                  sym::va_list,             va_list,                    Target::Struct,         GenericRequirement::None;
 
     Deref,                   sym::deref,               deref_trait,                Target::Trait,          GenericRequirement::Exact(0);
@@ -439,6 +442,8 @@ language_item_table! {
     DefaultTrait3,           sym::default_trait3,      default_trait3_trait,       Target::Trait,          GenericRequirement::None;
     DefaultTrait2,           sym::default_trait2,      default_trait2_trait,       Target::Trait,          GenericRequirement::None;
     DefaultTrait1,           sym::default_trait1,      default_trait1_trait,       Target::Trait,          GenericRequirement::None;
+
+    ContractCheckEnsures,     sym::contract_check_ensures,      contract_check_ensures_fn,      Target::Fn, GenericRequirement::None;
 }
 
 /// The requirement imposed on the generics of a lang item
diff --git a/compiler/rustc_hir/src/lib.rs b/compiler/rustc_hir/src/lib.rs
index 6bc0f797cca..a84857e3597 100644
--- a/compiler/rustc_hir/src/lib.rs
+++ b/compiler/rustc_hir/src/lib.rs
@@ -4,7 +4,6 @@
 
 // tidy-alphabetical-start
 #![allow(internal_features)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![feature(associated_type_defaults)]
 #![feature(box_patterns)]
 #![feature(closure_track_caller)]
diff --git a/compiler/rustc_hir_analysis/src/check/check.rs b/compiler/rustc_hir_analysis/src/check/check.rs
index f50746dd18d..4f338c6a19e 100644
--- a/compiler/rustc_hir_analysis/src/check/check.rs
+++ b/compiler/rustc_hir_analysis/src/check/check.rs
@@ -443,13 +443,13 @@ fn best_definition_site_of_opaque<'tcx>(
             let impl_def_id = tcx.local_parent(parent);
             for assoc in tcx.associated_items(impl_def_id).in_definition_order() {
                 match assoc.kind {
-                    ty::AssocKind::Const | ty::AssocKind::Fn => {
+                    ty::AssocKind::Const { .. } | ty::AssocKind::Fn { .. } => {
                         if let ControlFlow::Break(span) = locator.check(assoc.def_id.expect_local())
                         {
                             return Some(span);
                         }
                     }
-                    ty::AssocKind::Type => {}
+                    ty::AssocKind::Type { .. } => {}
                 }
             }
 
@@ -740,7 +740,7 @@ pub(crate) fn check_item_type(tcx: TyCtxt<'_>, def_id: LocalDefId) {
 
             for &assoc_item in assoc_items.in_definition_order() {
                 match assoc_item.kind {
-                    ty::AssocKind::Type if assoc_item.defaultness(tcx).has_value() => {
+                    ty::AssocKind::Type { .. } if assoc_item.defaultness(tcx).has_value() => {
                         let trait_args = GenericArgs::identity_for_item(tcx, def_id);
                         let _: Result<_, rustc_errors::ErrorGuaranteed> = check_type_bounds(
                             tcx,
@@ -942,7 +942,7 @@ fn check_impl_items_against_trait<'tcx>(
 
         if res.is_ok() {
             match ty_impl_item.kind {
-                ty::AssocKind::Fn => {
+                ty::AssocKind::Fn { .. } => {
                     compare_impl_item::refine::check_refining_return_position_impl_trait_in_trait(
                         tcx,
                         ty_impl_item,
@@ -952,8 +952,8 @@ fn check_impl_items_against_trait<'tcx>(
                             .instantiate_identity(),
                     );
                 }
-                ty::AssocKind::Const => {}
-                ty::AssocKind::Type => {}
+                ty::AssocKind::Const { .. } => {}
+                ty::AssocKind::Type { .. } => {}
             }
         }
 
diff --git a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
index 29a9931696f..06a7b1f6c29 100644
--- a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
+++ b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
@@ -43,9 +43,11 @@ pub(super) fn compare_impl_item(
     debug!(?impl_trait_ref);
 
     match impl_item.kind {
-        ty::AssocKind::Fn => compare_impl_method(tcx, impl_item, trait_item, impl_trait_ref),
-        ty::AssocKind::Type => compare_impl_ty(tcx, impl_item, trait_item, impl_trait_ref),
-        ty::AssocKind::Const => compare_impl_const(tcx, impl_item, trait_item, impl_trait_ref),
+        ty::AssocKind::Fn { .. } => compare_impl_method(tcx, impl_item, trait_item, impl_trait_ref),
+        ty::AssocKind::Type { .. } => compare_impl_ty(tcx, impl_item, trait_item, impl_trait_ref),
+        ty::AssocKind::Const { .. } => {
+            compare_impl_const(tcx, impl_item, trait_item, impl_trait_ref)
+        }
     }
 }
 
@@ -606,7 +608,7 @@ pub(super) fn collect_return_position_impl_trait_in_trait_tys<'tcx>(
     // with placeholders, which imply nothing about outlives bounds, and then
     // prove below that the hidden types are well formed.
     let universe = infcx.create_next_universe();
-    let mut idx = 0;
+    let mut idx = ty::BoundVar::ZERO;
     let mapping: FxIndexMap<_, _> = collector
         .types
         .iter()
@@ -623,10 +625,7 @@ pub(super) fn collect_return_position_impl_trait_in_trait_tys<'tcx>(
                     tcx,
                     ty::Placeholder {
                         universe,
-                        bound: ty::BoundTy {
-                            var: ty::BoundVar::from_usize(idx),
-                            kind: ty::BoundTyKind::Anon,
-                        },
+                        bound: ty::BoundTy { var: idx, kind: ty::BoundTyKind::Anon },
                     },
                 ),
             )
@@ -654,7 +653,7 @@ pub(super) fn collect_return_position_impl_trait_in_trait_tys<'tcx>(
                 cause.span,
                 E0053,
                 "method `{}` has an incompatible return type for trait",
-                trait_m.name
+                trait_m.name()
             );
             infcx.err_ctxt().note_type_err(
                 &mut diag,
@@ -1032,11 +1031,11 @@ fn report_trait_method_mismatch<'tcx>(
         impl_err_span,
         E0053,
         "method `{}` has an incompatible type for trait",
-        trait_m.name
+        trait_m.name()
     );
     match &terr {
         TypeError::ArgumentMutability(0) | TypeError::ArgumentSorts(_, 0)
-            if trait_m.fn_has_self_parameter =>
+            if trait_m.is_method() =>
         {
             let ty = trait_sig.inputs()[0];
             let sugg = get_self_string(ty, |ty| ty == impl_trait_ref.self_ty());
@@ -1255,7 +1254,7 @@ fn compare_self_type<'tcx>(
         get_self_string(self_arg_ty, can_eq_self)
     };
 
-    match (trait_m.fn_has_self_parameter, impl_m.fn_has_self_parameter) {
+    match (trait_m.is_method(), impl_m.is_method()) {
         (false, false) | (true, true) => {}
 
         (false, true) => {
@@ -1266,14 +1265,14 @@ fn compare_self_type<'tcx>(
                 impl_m_span,
                 E0185,
                 "method `{}` has a `{}` declaration in the impl, but not in the trait",
-                trait_m.name,
+                trait_m.name(),
                 self_descr
             );
             err.span_label(impl_m_span, format!("`{self_descr}` used in impl"));
             if let Some(span) = tcx.hir_span_if_local(trait_m.def_id) {
                 err.span_label(span, format!("trait method declared without `{self_descr}`"));
             } else {
-                err.note_trait_signature(trait_m.name, trait_m.signature(tcx));
+                err.note_trait_signature(trait_m.name(), trait_m.signature(tcx));
             }
             return Err(err.emit_unless(delay));
         }
@@ -1286,14 +1285,14 @@ fn compare_self_type<'tcx>(
                 impl_m_span,
                 E0186,
                 "method `{}` has a `{}` declaration in the trait, but not in the impl",
-                trait_m.name,
+                trait_m.name(),
                 self_descr
             );
             err.span_label(impl_m_span, format!("expected `{self_descr}` in impl"));
             if let Some(span) = tcx.hir_span_if_local(trait_m.def_id) {
                 err.span_label(span, format!("`{self_descr}` used in trait"));
             } else {
-                err.note_trait_signature(trait_m.name, trait_m.signature(tcx));
+                err.note_trait_signature(trait_m.name(), trait_m.signature(tcx));
             }
 
             return Err(err.emit_unless(delay));
@@ -1363,7 +1362,7 @@ fn compare_number_of_generics<'tcx>(
     let mut err_occurred = None;
     for (kind, trait_count, impl_count) in matchings {
         if impl_count != trait_count {
-            let arg_spans = |kind: ty::AssocKind, generics: &hir::Generics<'_>| {
+            let arg_spans = |item: &ty::AssocItem, generics: &hir::Generics<'_>| {
                 let mut spans = generics
                     .params
                     .iter()
@@ -1373,7 +1372,7 @@ fn compare_number_of_generics<'tcx>(
                         } => {
                             // A fn can have an arbitrary number of extra elided lifetimes for the
                             // same signature.
-                            !matches!(kind, ty::AssocKind::Fn)
+                            !item.is_fn()
                         }
                         _ => true,
                     })
@@ -1386,7 +1385,7 @@ fn compare_number_of_generics<'tcx>(
             };
             let (trait_spans, impl_trait_spans) = if let Some(def_id) = trait_.def_id.as_local() {
                 let trait_item = tcx.hir_expect_trait_item(def_id);
-                let arg_spans: Vec<Span> = arg_spans(trait_.kind, trait_item.generics);
+                let arg_spans: Vec<Span> = arg_spans(&trait_, trait_item.generics);
                 let impl_trait_spans: Vec<Span> = trait_item
                     .generics
                     .params
@@ -1412,7 +1411,7 @@ fn compare_number_of_generics<'tcx>(
                     _ => None,
                 })
                 .collect();
-            let spans = arg_spans(impl_.kind, impl_item.generics);
+            let spans = arg_spans(&impl_, impl_item.generics);
             let span = spans.first().copied();
 
             let mut err = tcx.dcx().struct_span_err(
@@ -1421,7 +1420,7 @@ fn compare_number_of_generics<'tcx>(
                     "{} `{}` has {} {kind} parameter{} but its trait \
                      declaration has {} {kind} parameter{}",
                     item_kind,
-                    trait_.name,
+                    trait_.name(),
                     impl_count,
                     pluralize!(impl_count),
                     trait_count,
@@ -1512,7 +1511,7 @@ fn compare_number_of_method_arguments<'tcx>(
             impl_span,
             E0050,
             "method `{}` has {} but the declaration in trait `{}` has {}",
-            trait_m.name,
+            trait_m.name(),
             potentially_plural_count(impl_number_args, "parameter"),
             tcx.def_path_str(trait_m.def_id),
             trait_number_args
@@ -1527,7 +1526,7 @@ fn compare_number_of_method_arguments<'tcx>(
                 ),
             );
         } else {
-            err.note_trait_signature(trait_m.name, trait_m.signature(tcx));
+            err.note_trait_signature(trait_m.name(), trait_m.signature(tcx));
         }
 
         err.span_label(
@@ -1581,7 +1580,7 @@ fn compare_synthetic_generics<'tcx>(
                 impl_span,
                 E0643,
                 "method `{}` has incompatible signature for trait",
-                trait_m.name
+                trait_m.name()
             );
             err.span_label(trait_span, "declaration in trait here");
             if impl_synthetic {
@@ -1703,7 +1702,7 @@ fn compare_generic_param_kinds<'tcx>(
     trait_item: ty::AssocItem,
     delay: bool,
 ) -> Result<(), ErrorGuaranteed> {
-    assert_eq!(impl_item.kind, trait_item.kind);
+    assert_eq!(impl_item.as_tag(), trait_item.as_tag());
 
     let ty_const_params_of = |def_id| {
         tcx.generics_of(def_id).own_params.iter().filter(|param| {
@@ -1741,7 +1740,7 @@ fn compare_generic_param_kinds<'tcx>(
                 E0053,
                 "{} `{}` has an incompatible generic parameter for trait `{}`",
                 impl_item.descr(),
-                trait_item.name,
+                trait_item.name(),
                 &tcx.def_path_str(tcx.parent(trait_item.def_id))
             );
 
@@ -1877,7 +1876,7 @@ fn compare_const_predicate_entailment<'tcx>(
             cause.span,
             E0326,
             "implemented const `{}` has an incompatible type for trait",
-            trait_ct.name
+            trait_ct.name()
         );
 
         let trait_c_span = trait_ct.def_id.as_local().map(|trait_ct_def_id| {
@@ -2235,16 +2234,19 @@ fn param_env_with_gat_bounds<'tcx>(
     // of the RPITITs associated with the same body. This is because checking
     // the item bounds of RPITITs often involves nested RPITITs having to prove
     // bounds about themselves.
-    let impl_tys_to_install = match impl_ty.opt_rpitit_info {
-        None => vec![impl_ty],
-        Some(
-            ty::ImplTraitInTraitData::Impl { fn_def_id }
-            | ty::ImplTraitInTraitData::Trait { fn_def_id, .. },
-        ) => tcx
+    let impl_tys_to_install = match impl_ty.kind {
+        ty::AssocKind::Type {
+            data:
+                ty::AssocTypeData::Rpitit(
+                    ty::ImplTraitInTraitData::Impl { fn_def_id }
+                    | ty::ImplTraitInTraitData::Trait { fn_def_id, .. },
+                ),
+        } => tcx
             .associated_types_for_impl_traits_in_associated_fn(fn_def_id)
             .iter()
             .map(|def_id| tcx.associated_item(*def_id))
             .collect(),
+        _ => vec![impl_ty],
     };
 
     for impl_ty in impl_tys_to_install {
diff --git a/compiler/rustc_hir_analysis/src/check/intrinsic.rs b/compiler/rustc_hir_analysis/src/check/intrinsic.rs
index ed8ca27d217..89fb35154b7 100644
--- a/compiler/rustc_hir_analysis/src/check/intrinsic.rs
+++ b/compiler/rustc_hir_analysis/src/check/intrinsic.rs
@@ -217,15 +217,11 @@ pub(crate) fn check_intrinsic_type(
         };
         (n_tps, 0, 0, inputs, output, hir::Safety::Unsafe)
     } else if intrinsic_name == sym::contract_check_ensures {
-        // contract_check_ensures::<'a, Ret, C>(&'a Ret, C)
-        // where C: impl Fn(&'a Ret) -> bool,
+        // contract_check_ensures::<Ret, C>(Ret, C) -> Ret
+        // where C: for<'a> Fn(&'a Ret) -> bool,
         //
-        // so: two type params, one lifetime param, 0 const params, two inputs, no return
-
-        let p = generics.param_at(0, tcx);
-        let r = ty::Region::new_early_param(tcx, p.to_early_bound_region_data());
-        let ref_ret = Ty::new_imm_ref(tcx, r, param(1));
-        (2, 1, 0, vec![ref_ret, param(2)], tcx.types.unit, hir::Safety::Safe)
+        // so: two type params, 0 lifetime param, 0 const params, two inputs, no return
+        (2, 0, 0, vec![param(0), param(1)], param(1), hir::Safety::Safe)
     } else {
         let safety = intrinsic_operation_unsafety(tcx, intrinsic_id);
         let (n_tps, n_cts, inputs, output) = match intrinsic_name {
diff --git a/compiler/rustc_hir_analysis/src/check/mod.rs b/compiler/rustc_hir_analysis/src/check/mod.rs
index 30921b6f055..5fbd771976b 100644
--- a/compiler/rustc_hir_analysis/src/check/mod.rs
+++ b/compiler/rustc_hir_analysis/src/check/mod.rs
@@ -205,7 +205,7 @@ fn missing_items_err(
 
     let missing_items_msg = missing_items
         .clone()
-        .map(|trait_item| trait_item.name.to_string())
+        .map(|trait_item| trait_item.name().to_string())
         .collect::<Vec<_>>()
         .join("`, `");
 
@@ -236,7 +236,7 @@ fn missing_items_err(
         let code = format!("{padding}{snippet}\n{padding}");
         if let Some(span) = tcx.hir_span_if_local(trait_item.def_id) {
             missing_trait_item_label
-                .push(errors::MissingTraitItemLabel { span, item: trait_item.name });
+                .push(errors::MissingTraitItemLabel { span, item: trait_item.name() });
             missing_trait_item.push(errors::MissingTraitItemSuggestion {
                 span: sugg_sp,
                 code,
@@ -407,14 +407,14 @@ fn fn_sig_suggestion<'tcx>(
         .enumerate()
         .map(|(i, ty)| {
             Some(match ty.kind() {
-                ty::Param(_) if assoc.fn_has_self_parameter && i == 0 => "self".to_string(),
+                ty::Param(_) if assoc.is_method() && i == 0 => "self".to_string(),
                 ty::Ref(reg, ref_ty, mutability) if i == 0 => {
                     let reg = format!("{reg} ");
                     let reg = match &reg[..] {
                         "'_ " | " " => "",
                         reg => reg,
                     };
-                    if assoc.fn_has_self_parameter {
+                    if assoc.is_method() {
                         match ref_ty.kind() {
                             ty::Param(param) if param.name == kw::SelfUpper => {
                                 format!("&{}{}self", reg, mutability.prefix_str())
@@ -427,7 +427,7 @@ fn fn_sig_suggestion<'tcx>(
                     }
                 }
                 _ => {
-                    if assoc.fn_has_self_parameter && i == 0 {
+                    if assoc.is_method() && i == 0 {
                         format!("self: {ty}")
                     } else {
                         format!("_: {ty}")
@@ -489,7 +489,7 @@ fn suggestion_signature<'tcx>(
     );
 
     match assoc.kind {
-        ty::AssocKind::Fn => fn_sig_suggestion(
+        ty::AssocKind::Fn { .. } => fn_sig_suggestion(
             tcx,
             tcx.liberate_late_bound_regions(
                 assoc.def_id,
@@ -499,14 +499,14 @@ fn suggestion_signature<'tcx>(
             tcx.predicates_of(assoc.def_id).instantiate_own(tcx, args),
             assoc,
         ),
-        ty::AssocKind::Type => {
+        ty::AssocKind::Type { .. } => {
             let (generics, where_clauses) = bounds_from_generic_predicates(
                 tcx,
                 tcx.predicates_of(assoc.def_id).instantiate_own(tcx, args),
             );
-            format!("type {}{generics} = /* Type */{where_clauses};", assoc.name)
+            format!("type {}{generics} = /* Type */{where_clauses};", assoc.name())
         }
-        ty::AssocKind::Const => {
+        ty::AssocKind::Const { name } => {
             let ty = tcx.type_of(assoc.def_id).instantiate_identity();
             let val = tcx
                 .infer_ctxt()
@@ -514,7 +514,7 @@ fn suggestion_signature<'tcx>(
                 .err_ctxt()
                 .ty_kind_suggestion(tcx.param_env(assoc.def_id), ty)
                 .unwrap_or_else(|| "value".to_string());
-            format!("const {}: {} = {};", assoc.name, ty, val)
+            format!("const {}: {} = {};", name, ty, val)
         }
     }
 }
diff --git a/compiler/rustc_hir_analysis/src/check/wfcheck.rs b/compiler/rustc_hir_analysis/src/check/wfcheck.rs
index 6292d03bf6a..33d5a86beb3 100644
--- a/compiler/rustc_hir_analysis/src/check/wfcheck.rs
+++ b/compiler/rustc_hir_analysis/src/check/wfcheck.rs
@@ -408,7 +408,7 @@ fn check_gat_where_clauses(tcx: TyCtxt<'_>, trait_def_id: LocalDefId) {
             let gat_def_id = gat_item.def_id.expect_local();
             let gat_item = tcx.associated_item(gat_def_id);
             // If this item is not an assoc ty, or has no args, then it's not a GAT
-            if gat_item.kind != ty::AssocKind::Type {
+            if !gat_item.is_type() {
                 continue;
             }
             let gat_generics = tcx.generics_of(gat_def_id);
@@ -432,7 +432,7 @@ fn check_gat_where_clauses(tcx: TyCtxt<'_>, trait_def_id: LocalDefId) {
 
                 let item_required_bounds = match tcx.associated_item(item_def_id).kind {
                     // In our example, this corresponds to `into_iter` method
-                    ty::AssocKind::Fn => {
+                    ty::AssocKind::Fn { .. } => {
                         // For methods, we check the function signature's return type for any GATs
                         // to constrain. In the `into_iter` case, we see that the return type
                         // `Self::Iter<'a>` is a GAT we want to gather any potential missing bounds from.
@@ -453,7 +453,7 @@ fn check_gat_where_clauses(tcx: TyCtxt<'_>, trait_def_id: LocalDefId) {
                         )
                     }
                     // In our example, this corresponds to the `Iter` and `Item` associated types
-                    ty::AssocKind::Type => {
+                    ty::AssocKind::Type { .. } => {
                         // If our associated item is a GAT with missing bounds, add them to
                         // the param-env here. This allows this GAT to propagate missing bounds
                         // to other GATs.
@@ -474,7 +474,7 @@ fn check_gat_where_clauses(tcx: TyCtxt<'_>, trait_def_id: LocalDefId) {
                             gat_generics,
                         )
                     }
-                    ty::AssocKind::Const => None,
+                    ty::AssocKind::Const { .. } => None,
                 };
 
                 if let Some(item_required_bounds) = item_required_bounds {
@@ -1076,7 +1076,7 @@ fn check_associated_item(
         };
 
         match item.kind {
-            ty::AssocKind::Const => {
+            ty::AssocKind::Const { .. } => {
                 let ty = tcx.type_of(item.def_id).instantiate_identity();
                 let ty = wfcx.normalize(span, Some(WellFormedLoc::Ty(item_id)), ty);
                 wfcx.register_wf_obligation(span, loc, ty.into());
@@ -1089,7 +1089,7 @@ fn check_associated_item(
                 );
                 Ok(())
             }
-            ty::AssocKind::Fn => {
+            ty::AssocKind::Fn { .. } => {
                 let sig = tcx.fn_sig(item.def_id).instantiate_identity();
                 let hir_sig = sig_if_method.expect("bad signature for method");
                 check_fn_or_method(
@@ -1101,7 +1101,7 @@ fn check_associated_item(
                 );
                 check_method_receiver(wfcx, hir_sig, item, self_ty)
             }
-            ty::AssocKind::Type => {
+            ty::AssocKind::Type { .. } => {
                 if let ty::AssocItemContainer::Trait = item.container {
                     check_associated_type_bounds(wfcx, item, span)
                 }
@@ -1716,7 +1716,7 @@ fn check_method_receiver<'tcx>(
 ) -> Result<(), ErrorGuaranteed> {
     let tcx = wfcx.tcx();
 
-    if !method.fn_has_self_parameter {
+    if !method.is_method() {
         return Ok(());
     }
 
diff --git a/compiler/rustc_hir_analysis/src/coherence/inherent_impls_overlap.rs b/compiler/rustc_hir_analysis/src/coherence/inherent_impls_overlap.rs
index dc616576c9c..242639125b1 100644
--- a/compiler/rustc_hir_analysis/src/coherence/inherent_impls_overlap.rs
+++ b/compiler/rustc_hir_analysis/src/coherence/inherent_impls_overlap.rs
@@ -51,7 +51,7 @@ impl<'tcx> InherentOverlapChecker<'tcx> {
 
         for &item1 in impl_items1.in_definition_order() {
             let collision = impl_items2
-                .filter_by_name_unhygienic(item1.name)
+                .filter_by_name_unhygienic(item1.name())
                 .any(|&item2| self.compare_hygienically(item1, item2));
 
             if collision {
@@ -64,7 +64,7 @@ impl<'tcx> InherentOverlapChecker<'tcx> {
 
     fn compare_hygienically(&self, item1: ty::AssocItem, item2: ty::AssocItem) -> bool {
         // Symbols and namespace match, compare hygienically.
-        item1.kind.namespace() == item2.kind.namespace()
+        item1.namespace() == item2.namespace()
             && item1.ident(self.tcx).normalize_to_macros_2_0()
                 == item2.ident(self.tcx).normalize_to_macros_2_0()
     }
@@ -113,7 +113,7 @@ impl<'tcx> InherentOverlapChecker<'tcx> {
         let mut res = Ok(());
         for &item1 in impl_items1.in_definition_order() {
             let collision = impl_items2
-                .filter_by_name_unhygienic(item1.name)
+                .filter_by_name_unhygienic(item1.name())
                 .find(|&&item2| self.compare_hygienically(item1, item2));
 
             if let Some(item2) = collision {
@@ -230,11 +230,11 @@ impl<'tcx> InherentOverlapChecker<'tcx> {
                 let mut ids = impl_items
                     .in_definition_order()
                     .filter_map(|item| {
-                        let entry = connected_region_ids.entry(item.name);
+                        let entry = connected_region_ids.entry(item.name());
                         if let IndexEntry::Occupied(e) = &entry {
                             Some(*e.get())
                         } else {
-                            idents_to_add.push(item.name);
+                            idents_to_add.push(item.name());
                             None
                         }
                     })
diff --git a/compiler/rustc_hir_analysis/src/collect.rs b/compiler/rustc_hir_analysis/src/collect.rs
index deded6904d4..4520fbe352c 100644
--- a/compiler/rustc_hir_analysis/src/collect.rs
+++ b/compiler/rustc_hir_analysis/src/collect.rs
@@ -44,7 +44,7 @@ use rustc_trait_selection::traits::ObligationCtxt;
 use tracing::{debug, instrument};
 
 use crate::errors;
-use crate::hir_ty_lowering::errors::assoc_kind_str;
+use crate::hir_ty_lowering::errors::assoc_tag_str;
 use crate::hir_ty_lowering::{FeedConstTy, HirTyLowerer, RegionInferReason};
 
 pub(crate) mod dump;
@@ -450,7 +450,7 @@ impl<'tcx> HirTyLowerer<'tcx> for ItemCtxt<'tcx> {
         item_def_id: DefId,
         item_segment: &rustc_hir::PathSegment<'tcx>,
         poly_trait_ref: ty::PolyTraitRef<'tcx>,
-        kind: ty::AssocKind,
+        assoc_tag: ty::AssocTag,
     ) -> Result<(DefId, ty::GenericArgsRef<'tcx>), ErrorGuaranteed> {
         if let Some(trait_ref) = poly_trait_ref.no_bound_vars() {
             let item_args = self.lowerer().lower_generic_args_of_assoc_item(
@@ -525,7 +525,7 @@ impl<'tcx> HirTyLowerer<'tcx> for ItemCtxt<'tcx> {
                 inferred_sugg,
                 bound,
                 mpart_sugg,
-                what: assoc_kind_str(kind),
+                what: assoc_tag_str(assoc_tag),
             }))
         }
     }
diff --git a/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs b/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs
index 9bcda35ee87..59ab36d98fd 100644
--- a/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs
+++ b/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs
@@ -16,7 +16,7 @@ use rustc_errors::ErrorGuaranteed;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::intravisit::{self, InferKind, Visitor, VisitorExt};
 use rustc_hir::{
-    self as hir, AmbigArg, GenericArg, GenericParam, GenericParamKind, HirId, LifetimeName, Node,
+    self as hir, AmbigArg, GenericArg, GenericParam, GenericParamKind, HirId, LifetimeKind, Node,
 };
 use rustc_macros::extension;
 use rustc_middle::hir::nested_filter;
@@ -646,14 +646,14 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
         arg: &'tcx hir::PreciseCapturingArg<'tcx>,
     ) -> Self::Result {
         match *arg {
-            hir::PreciseCapturingArg::Lifetime(lt) => match lt.res {
-                LifetimeName::Param(def_id) => {
+            hir::PreciseCapturingArg::Lifetime(lt) => match lt.kind {
+                LifetimeKind::Param(def_id) => {
                     self.resolve_lifetime_ref(def_id, lt);
                 }
-                LifetimeName::Error => {}
-                LifetimeName::ImplicitObjectLifetimeDefault
-                | LifetimeName::Infer
-                | LifetimeName::Static => {
+                LifetimeKind::Error => {}
+                LifetimeKind::ImplicitObjectLifetimeDefault
+                | LifetimeKind::Infer
+                | LifetimeKind::Static => {
                     self.tcx.dcx().emit_err(errors::BadPreciseCapture {
                         span: lt.ident.span,
                         kind: "lifetime",
@@ -774,26 +774,26 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
                         );
                     }
                 });
-                match lifetime.res {
-                    LifetimeName::ImplicitObjectLifetimeDefault => {
+                match lifetime.kind {
+                    LifetimeKind::ImplicitObjectLifetimeDefault => {
                         // If the user does not write *anything*, we
                         // use the object lifetime defaulting
                         // rules. So e.g., `Box<dyn Debug>` becomes
                         // `Box<dyn Debug + 'static>`.
                         self.resolve_object_lifetime_default(&*lifetime)
                     }
-                    LifetimeName::Infer => {
+                    LifetimeKind::Infer => {
                         // If the user writes `'_`, we use the *ordinary* elision
                         // rules. So the `'_` in e.g., `Box<dyn Debug + '_>` will be
                         // resolved the same as the `'_` in `&'_ Foo`.
                         //
                         // cc #48468
                     }
-                    LifetimeName::Param(..) | LifetimeName::Static => {
+                    LifetimeKind::Param(..) | LifetimeKind::Static => {
                         // If the user wrote an explicit name, use that.
                         self.visit_lifetime(&*lifetime);
                     }
-                    LifetimeName::Error => {}
+                    LifetimeKind::Error => {}
                 }
             }
             hir::TyKind::Ref(lifetime_ref, ref mt) => {
@@ -873,17 +873,17 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
 
     #[instrument(level = "debug", skip(self))]
     fn visit_lifetime(&mut self, lifetime_ref: &'tcx hir::Lifetime) {
-        match lifetime_ref.res {
-            hir::LifetimeName::Static => {
+        match lifetime_ref.kind {
+            hir::LifetimeKind::Static => {
                 self.insert_lifetime(lifetime_ref, ResolvedArg::StaticLifetime)
             }
-            hir::LifetimeName::Param(param_def_id) => {
+            hir::LifetimeKind::Param(param_def_id) => {
                 self.resolve_lifetime_ref(param_def_id, lifetime_ref)
             }
             // If we've already reported an error, just ignore `lifetime_ref`.
-            hir::LifetimeName::Error => {}
+            hir::LifetimeKind::Error => {}
             // Those will be resolved by typechecking.
-            hir::LifetimeName::ImplicitObjectLifetimeDefault | hir::LifetimeName::Infer => {}
+            hir::LifetimeKind::ImplicitObjectLifetimeDefault | hir::LifetimeKind::Infer => {}
         }
     }
 
@@ -1063,15 +1063,15 @@ fn object_lifetime_default(tcx: TyCtxt<'_>, param_def_id: LocalDefId) -> ObjectL
 
                         for bound in bound.bounds {
                             if let hir::GenericBound::Outlives(lifetime) = bound {
-                                set.insert(lifetime.res);
+                                set.insert(lifetime.kind);
                             }
                         }
                     }
 
                     match set {
                         Set1::Empty => ObjectLifetimeDefault::Empty,
-                        Set1::One(hir::LifetimeName::Static) => ObjectLifetimeDefault::Static,
-                        Set1::One(hir::LifetimeName::Param(param_def_id)) => {
+                        Set1::One(hir::LifetimeKind::Static) => ObjectLifetimeDefault::Static,
+                        Set1::One(hir::LifetimeKind::Param(param_def_id)) => {
                             ObjectLifetimeDefault::Param(param_def_id.to_def_id())
                         }
                         _ => ObjectLifetimeDefault::Ambiguous,
@@ -1241,7 +1241,7 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
                     // Fresh lifetimes in APIT used to be allowed in async fns and forbidden in
                     // regular fns.
                     if let Some(hir::PredicateOrigin::ImplTrait) = where_bound_origin
-                        && let hir::LifetimeName::Param(param_id) = lifetime_ref.res
+                        && let hir::LifetimeKind::Param(param_id) = lifetime_ref.kind
                         && let Some(generics) =
                             self.tcx.hir_get_generics(self.tcx.local_parent(param_id))
                         && let Some(param) = generics.params.iter().find(|p| p.def_id == param_id)
@@ -1811,7 +1811,7 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
                         self.tcx,
                         type_def_id,
                         constraint.ident,
-                        ty::AssocKind::Fn,
+                        ty::AssocTag::Fn,
                     ) {
                     bound_vars.extend(
                         self.tcx
@@ -1843,7 +1843,7 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
                     self.tcx,
                     type_def_id,
                     constraint.ident,
-                    ty::AssocKind::Type,
+                    ty::AssocTag::Type,
                 )
                 .map(|(bound_vars, _)| bound_vars);
                 self.with(scope, |this| {
@@ -1875,13 +1875,13 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
         tcx: TyCtxt<'tcx>,
         def_id: DefId,
         assoc_ident: Ident,
-        assoc_kind: ty::AssocKind,
+        assoc_tag: ty::AssocTag,
     ) -> Option<(Vec<ty::BoundVariableKind>, &'tcx ty::AssocItem)> {
         let trait_defines_associated_item_named = |trait_def_id: DefId| {
             tcx.associated_items(trait_def_id).find_by_ident_and_kind(
                 tcx,
                 assoc_ident,
-                assoc_kind,
+                assoc_tag,
                 trait_def_id,
             )
         };
@@ -1894,8 +1894,8 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
             let Some((def_id, bound_vars)) = stack.pop() else {
                 break None;
             };
-            // See issue #83753. If someone writes an associated type on a non-trait, just treat it as
-            // there being no supertrait HRTBs.
+            // See issue #83753. If someone writes an associated type on a non-trait, just treat it
+            // as there being no supertrait HRTBs.
             match tcx.def_kind(def_id) {
                 DefKind::Trait | DefKind::TraitAlias | DefKind::Impl { .. } => {}
                 _ => break None,
@@ -2067,7 +2067,7 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
                                     self.tcx,
                                     trait_def_id,
                                     item_segment.ident,
-                                    ty::AssocKind::Fn,
+                                    ty::AssocTag::Fn,
                                 )
                             });
 
@@ -2112,7 +2112,7 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
                             self.tcx,
                             trait_def_id,
                             item_segment.ident,
-                            ty::AssocKind::Fn,
+                            ty::AssocTag::Fn,
                         ) else {
                             return;
                         };
@@ -2440,7 +2440,7 @@ fn is_late_bound_map(
         }
 
         fn visit_lifetime(&mut self, lifetime_ref: &'v hir::Lifetime) {
-            if let hir::LifetimeName::Param(def_id) = lifetime_ref.res {
+            if let hir::LifetimeKind::Param(def_id) = lifetime_ref.kind {
                 self.regions.insert(def_id);
             }
         }
@@ -2453,7 +2453,7 @@ fn is_late_bound_map(
 
     impl<'tcx> Visitor<'tcx> for AllCollector {
         fn visit_lifetime(&mut self, lifetime_ref: &'tcx hir::Lifetime) {
-            if let hir::LifetimeName::Param(def_id) = lifetime_ref.res {
+            if let hir::LifetimeKind::Param(def_id) = lifetime_ref.kind {
                 self.regions.insert(def_id);
             }
         }
diff --git a/compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs b/compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs
index 772197a53ac..50e20a19eda 100644
--- a/compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs
+++ b/compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs
@@ -32,9 +32,11 @@ pub(super) fn find_opaque_ty_constraints_for_impl_trait_in_assoc_type(
     for &assoc_id in tcx.associated_item_def_ids(impl_def_id) {
         let assoc = tcx.associated_item(assoc_id);
         match assoc.kind {
-            ty::AssocKind::Const | ty::AssocKind::Fn => locator.check(assoc_id.expect_local()),
+            ty::AssocKind::Const { .. } | ty::AssocKind::Fn { .. } => {
+                locator.check(assoc_id.expect_local())
+            }
             // Associated types don't have bodies, so they can't constrain hidden types
-            ty::AssocKind::Type => {}
+            ty::AssocKind::Type { .. } => {}
         }
     }
 
diff --git a/compiler/rustc_hir_analysis/src/errors/wrong_number_of_generic_args.rs b/compiler/rustc_hir_analysis/src/errors/wrong_number_of_generic_args.rs
index 610b293a114..526ee30209c 100644
--- a/compiler/rustc_hir_analysis/src/errors/wrong_number_of_generic_args.rs
+++ b/compiler/rustc_hir_analysis/src/errors/wrong_number_of_generic_args.rs
@@ -4,7 +4,7 @@ use GenericArgsInfo::*;
 use rustc_errors::codes::*;
 use rustc_errors::{Applicability, Diag, Diagnostic, EmissionGuarantee, MultiSpan, pluralize};
 use rustc_hir as hir;
-use rustc_middle::ty::{self as ty, AssocItems, AssocKind, TyCtxt};
+use rustc_middle::ty::{self as ty, AssocItems, TyCtxt};
 use rustc_span::def_id::DefId;
 use tracing::debug;
 
@@ -486,13 +486,13 @@ impl<'a, 'tcx> WrongNumberOfGenericArgs<'a, 'tcx> {
             let items: &AssocItems = self.tcx.associated_items(self.def_id);
             items
                 .in_definition_order()
-                .filter(|item| item.kind == AssocKind::Type)
+                .filter(|item| item.is_type())
                 .filter(|item| {
                     !self
                         .gen_args
                         .constraints
                         .iter()
-                        .any(|constraint| constraint.ident.name == item.name)
+                        .any(|constraint| constraint.ident.name == item.name())
                 })
                 .filter(|item| !item.is_impl_trait_in_trait())
                 .map(|item| self.tcx.item_ident(item.def_id).to_string())
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs
index 24d05b49861..bf91eb1b8fd 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs
@@ -431,16 +431,16 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     ) -> Result<(), ErrorGuaranteed> {
         let tcx = self.tcx();
 
-        let assoc_kind = if constraint.gen_args.parenthesized
+        let assoc_tag = if constraint.gen_args.parenthesized
             == hir::GenericArgsParentheses::ReturnTypeNotation
         {
-            ty::AssocKind::Fn
+            ty::AssocTag::Fn
         } else if let hir::AssocItemConstraintKind::Equality { term: hir::Term::Const(_) } =
             constraint.kind
         {
-            ty::AssocKind::Const
+            ty::AssocTag::Const
         } else {
-            ty::AssocKind::Type
+            ty::AssocTag::Type
         };
 
         // Given something like `U: Trait<T = X>`, we want to produce a predicate like
@@ -453,7 +453,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         //     trait SuperTrait<A> { type T; }
         let candidate = if self.probe_trait_that_defines_assoc_item(
             trait_ref.def_id(),
-            assoc_kind,
+            assoc_tag,
             constraint.ident,
         ) {
             // Simple case: The assoc item is defined in the current trait.
@@ -464,7 +464,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             self.probe_single_bound_for_assoc_item(
                 || traits::supertraits(tcx, trait_ref),
                 AssocItemQSelf::Trait(trait_ref.def_id()),
-                assoc_kind,
+                assoc_tag,
                 constraint.ident,
                 path_span,
                 Some(constraint),
@@ -474,7 +474,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         let assoc_item = self
             .probe_assoc_item(
                 constraint.ident,
-                assoc_kind,
+                assoc_tag,
                 hir_ref_id,
                 constraint.span,
                 candidate.def_id(),
@@ -493,7 +493,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             })
             .or_insert(constraint.span);
 
-        let projection_term = if let ty::AssocKind::Fn = assoc_kind {
+        let projection_term = if let ty::AssocTag::Fn = assoc_tag {
             let bound_vars = tcx.late_bound_vars(constraint.hir_id);
             ty::Binder::bind_with_vars(
                 self.lower_return_type_notation_ty(candidate, assoc_item.def_id, path_span)?.into(),
@@ -542,7 +542,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         };
 
         match constraint.kind {
-            hir::AssocItemConstraintKind::Equality { .. } if let ty::AssocKind::Fn = assoc_kind => {
+            hir::AssocItemConstraintKind::Equality { .. } if let ty::AssocTag::Fn = assoc_tag => {
                 return Err(self.dcx().emit_err(crate::errors::ReturnTypeNotationEqualityBound {
                     span: constraint.span,
                 }));
@@ -679,7 +679,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         trait_def_id,
                         hir_ty.span,
                         item_segment,
-                        ty::AssocKind::Type,
+                        ty::AssocTag::Type,
                     );
                     return Ty::new_error(tcx, guar);
                 };
@@ -771,7 +771,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         )
                     },
                     AssocItemQSelf::SelfTyAlias,
-                    ty::AssocKind::Fn,
+                    ty::AssocTag::Fn,
                     assoc_ident,
                     span,
                     None,
@@ -783,7 +783,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             ) => self.probe_single_ty_param_bound_for_assoc_item(
                 param_did.expect_local(),
                 qself.span,
-                ty::AssocKind::Fn,
+                ty::AssocTag::Fn,
                 assoc_ident,
                 span,
             )?,
@@ -823,7 +823,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
         let trait_def_id = bound.def_id();
         let assoc_ty = self
-            .probe_assoc_item(assoc_ident, ty::AssocKind::Fn, qpath_hir_id, span, trait_def_id)
+            .probe_assoc_item(assoc_ident, ty::AssocTag::Fn, qpath_hir_id, span, trait_def_id)
             .expect("failed to find associated type");
 
         Ok((bound, assoc_ty.def_id))
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs
index e64cd8ec302..88f74589204 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs
@@ -201,7 +201,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                             tcx.associated_items(pred.trait_ref.def_id)
                                 .in_definition_order()
                                 // We only care about associated types.
-                                .filter(|item| item.kind == ty::AssocKind::Type)
+                                .filter(|item| item.is_type())
                                 // No RPITITs -- they're not dyn-compatible for now.
                                 .filter(|item| !item.is_impl_trait_in_trait())
                                 // If the associated type has a `where Self: Sized` bound,
@@ -415,7 +415,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     self.lower_lifetime(lifetime, RegionInferReason::ExplicitObjectLifetime)
                 } else {
                     let reason =
-                        if let hir::LifetimeName::ImplicitObjectLifetimeDefault = lifetime.res {
+                        if let hir::LifetimeKind::ImplicitObjectLifetimeDefault = lifetime.kind {
                             if let hir::Node::Ty(hir::Ty {
                                 kind: hir::TyKind::Ref(parent_lifetime, _),
                                 ..
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs
index 5a0524d33fd..3759a224ff7 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs
@@ -116,7 +116,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         &self,
         all_candidates: impl Fn() -> I,
         qself: AssocItemQSelf,
-        assoc_kind: ty::AssocKind,
+        assoc_tag: ty::AssocTag,
         assoc_ident: Ident,
         span: Span,
         constraint: Option<&hir::AssocItemConstraint<'tcx>>,
@@ -134,14 +134,14 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         }) {
             return self.complain_about_assoc_kind_mismatch(
                 assoc_item,
-                assoc_kind,
+                assoc_tag,
                 assoc_ident,
                 span,
                 constraint,
             );
         }
 
-        let assoc_kind_str = assoc_kind_str(assoc_kind);
+        let assoc_kind_str = assoc_tag_str(assoc_tag);
         let qself_str = qself.to_string(tcx);
 
         // The fallback span is needed because `assoc_name` might be an `Fn()`'s `Output` without a
@@ -168,7 +168,11 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         let all_candidate_names: Vec<_> = all_candidates()
             .flat_map(|r| tcx.associated_items(r.def_id()).in_definition_order())
             .filter_map(|item| {
-                (!item.is_impl_trait_in_trait() && item.kind == assoc_kind).then_some(item.name)
+                if !item.is_impl_trait_in_trait() && item.as_tag() == assoc_tag {
+                    item.opt_name()
+                } else {
+                    None
+                }
             })
             .collect();
 
@@ -200,7 +204,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             .iter()
             .flat_map(|trait_def_id| tcx.associated_items(*trait_def_id).in_definition_order())
             .filter_map(|item| {
-                (!item.is_impl_trait_in_trait() && item.kind == assoc_kind).then_some(item.name)
+                (!item.is_impl_trait_in_trait() && item.as_tag() == assoc_tag).then(|| item.name())
             })
             .collect();
 
@@ -213,7 +217,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 .filter(|trait_def_id| {
                     tcx.associated_items(trait_def_id)
                         .filter_by_name_unhygienic(suggested_name)
-                        .any(|item| item.kind == assoc_kind)
+                        .any(|item| item.as_tag() == assoc_tag)
                 })
                 .collect::<Vec<_>>()[..]
             {
@@ -330,14 +334,14 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     fn complain_about_assoc_kind_mismatch(
         &self,
         assoc_item: &ty::AssocItem,
-        assoc_kind: ty::AssocKind,
+        assoc_tag: ty::AssocTag,
         ident: Ident,
         span: Span,
         constraint: Option<&hir::AssocItemConstraint<'tcx>>,
     ) -> ErrorGuaranteed {
         let tcx = self.tcx();
 
-        let bound_on_assoc_const_label = if let ty::AssocKind::Const = assoc_item.kind
+        let bound_on_assoc_const_label = if let ty::AssocKind::Const { .. } = assoc_item.kind
             && let Some(constraint) = constraint
             && let hir::AssocItemConstraintKind::Bound { .. } = constraint.kind
         {
@@ -375,17 +379,17 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 hir::Term::Ty(ty) => ty.span,
                 hir::Term::Const(ct) => ct.span(),
             };
-            (span, Some(ident.span), assoc_item.kind, assoc_kind)
+            (span, Some(ident.span), assoc_item.as_tag(), assoc_tag)
         } else {
-            (ident.span, None, assoc_kind, assoc_item.kind)
+            (ident.span, None, assoc_tag, assoc_item.as_tag())
         };
 
         self.dcx().emit_err(errors::AssocKindMismatch {
             span,
-            expected: assoc_kind_str(expected),
-            got: assoc_kind_str(got),
+            expected: assoc_tag_str(expected),
+            got: assoc_tag_str(got),
             expected_because_label,
-            assoc_kind: assoc_kind_str(assoc_item.kind),
+            assoc_kind: assoc_tag_str(assoc_item.as_tag()),
             def_span: tcx.def_span(assoc_item.def_id),
             bound_on_assoc_const_label,
             wrap_in_braces_sugg,
@@ -398,9 +402,9 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         types: &[String],
         traits: &[String],
         name: Symbol,
-        kind: ty::AssocKind,
+        assoc_tag: ty::AssocTag,
     ) -> ErrorGuaranteed {
-        let kind_str = assoc_kind_str(kind);
+        let kind_str = assoc_tag_str(assoc_tag);
         let mut err =
             struct_span_code_err!(self.dcx(), span, E0223, "ambiguous associated {kind_str}");
         if self
@@ -569,7 +573,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         candidates: Vec<(DefId, (DefId, DefId))>,
         fulfillment_errors: Vec<FulfillmentError<'tcx>>,
         span: Span,
-        kind: ty::AssocKind,
+        assoc_tag: ty::AssocTag,
     ) -> ErrorGuaranteed {
         // FIXME(fmease): This was copied in parts from an old version of `rustc_hir_typeck::method::suggest`.
         // Either
@@ -579,14 +583,14 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
         let tcx = self.tcx();
 
-        let kind_str = assoc_kind_str(kind);
+        let assoc_tag_str = assoc_tag_str(assoc_tag);
         let adt_did = self_ty.ty_adt_def().map(|def| def.did());
         let add_def_label = |err: &mut Diag<'_>| {
             if let Some(did) = adt_did {
                 err.span_label(
                     tcx.def_span(did),
                     format!(
-                        "associated {kind_str} `{name}` not found for this {}",
+                        "associated {assoc_tag_str} `{name}` not found for this {}",
                         tcx.def_descr(did)
                     ),
                 );
@@ -615,11 +619,11 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 self.dcx(),
                 name.span,
                 E0220,
-                "associated {kind_str} `{name}` not found for `{self_ty}` in the current scope"
+                "associated {assoc_tag_str} `{name}` not found for `{self_ty}` in the current scope"
             );
             err.span_label(name.span, format!("associated item not found in `{self_ty}`"));
             err.note(format!(
-                "the associated {kind_str} was found for\n{type_candidates}{additional_types}",
+                "the associated {assoc_tag_str} was found for\n{type_candidates}{additional_types}",
             ));
             add_def_label(&mut err);
             return err.emit();
@@ -700,7 +704,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
         let mut err = self.dcx().struct_span_err(
             name.span,
-            format!("the associated {kind_str} `{name}` exists for `{self_ty}`, but its trait bounds were not satisfied")
+            format!("the associated {assoc_tag_str} `{name}` exists for `{self_ty}`, but its trait bounds were not satisfied")
         );
         if !bounds.is_empty() {
             err.note(format!(
@@ -710,7 +714,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         }
         err.span_label(
             name.span,
-            format!("associated {kind_str} cannot be referenced on `{self_ty}` due to unsatisfied trait bounds")
+            format!("associated {assoc_tag_str} cannot be referenced on `{self_ty}` due to unsatisfied trait bounds")
         );
 
         for (span, mut bounds) in bound_spans {
@@ -761,7 +765,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         // `issue-22560.rs`.
         let mut dyn_compatibility_violations = Ok(());
         for (assoc_item, trait_ref) in &missing_assoc_types {
-            names.entry(trait_ref).or_default().push(assoc_item.name);
+            names.entry(trait_ref).or_default().push(assoc_item.name());
             names_len += 1;
 
             let violations =
@@ -812,7 +816,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     let assoc_item = tcx.associated_items(trait_def).find_by_ident_and_kind(
                         tcx,
                         ident,
-                        ty::AssocKind::Type,
+                        ty::AssocTag::Type,
                         trait_def,
                     );
 
@@ -852,16 +856,17 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         let mut names: UnordMap<_, usize> = Default::default();
         for (item, _) in &missing_assoc_types {
             types_count += 1;
-            *names.entry(item.name).or_insert(0) += 1;
+            *names.entry(item.name()).or_insert(0) += 1;
         }
         let mut dupes = false;
         let mut shadows = false;
         for (item, trait_ref) in &missing_assoc_types {
-            let prefix = if names[&item.name] > 1 {
+            let name = item.name();
+            let prefix = if names[&name] > 1 {
                 let trait_def_id = trait_ref.def_id();
                 dupes = true;
                 format!("{}::", tcx.def_path_str(trait_def_id))
-            } else if bound_names.get(&item.name).is_some_and(|x| *x != item) {
+            } else if bound_names.get(&name).is_some_and(|x| *x != item) {
                 let trait_def_id = trait_ref.def_id();
                 shadows = true;
                 format!("{}::", tcx.def_path_str(trait_def_id))
@@ -871,7 +876,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
             let mut is_shadowed = false;
 
-            if let Some(assoc_item) = bound_names.get(&item.name)
+            if let Some(assoc_item) = bound_names.get(&name)
                 && *assoc_item != item
             {
                 is_shadowed = true;
@@ -880,17 +885,14 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     if assoc_item.def_id.is_local() { ", consider renaming it" } else { "" };
                 err.span_label(
                     tcx.def_span(assoc_item.def_id),
-                    format!("`{}{}` shadowed here{}", prefix, item.name, rename_message),
+                    format!("`{}{}` shadowed here{}", prefix, name, rename_message),
                 );
             }
 
             let rename_message = if is_shadowed { ", consider renaming it" } else { "" };
 
             if let Some(sp) = tcx.hir_span_if_local(item.def_id) {
-                err.span_label(
-                    sp,
-                    format!("`{}{}` defined here{}", prefix, item.name, rename_message),
-                );
+                err.span_label(sp, format!("`{}{}` defined here{}", prefix, name, rename_message));
             }
         }
         if potential_assoc_types.len() == missing_assoc_types.len() {
@@ -903,7 +905,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         {
             let types: Vec<_> = missing_assoc_types
                 .iter()
-                .map(|(item, _)| format!("{} = Type", item.name))
+                .map(|(item, _)| format!("{} = Type", item.name()))
                 .collect();
             let code = if let Some(snippet) = snippet.strip_suffix('>') {
                 // The user wrote `Trait<'a>` or similar and we don't have a type we can
@@ -938,16 +940,17 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             let mut names: FxIndexMap<_, usize> = FxIndexMap::default();
             for (item, _) in &missing_assoc_types {
                 types_count += 1;
-                *names.entry(item.name).or_insert(0) += 1;
+                *names.entry(item.name()).or_insert(0) += 1;
             }
             let mut label = vec![];
             for (item, trait_ref) in &missing_assoc_types {
-                let postfix = if names[&item.name] > 1 {
+                let name = item.name();
+                let postfix = if names[&name] > 1 {
                     format!(" (from trait `{}`)", trait_ref.print_trait_sugared())
                 } else {
                     String::new()
                 };
-                label.push(format!("`{}`{}", item.name, postfix));
+                label.push(format!("`{}`{}", name, postfix));
             }
             if !label.is_empty() {
                 err.span_label(
@@ -1022,12 +1025,13 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         .map(|simple_ty| tcx.incoherent_impls(simple_ty))
                 })
             && let name = Symbol::intern(&format!("{ident2}_{ident3}"))
-            && let Some(ty::AssocItem { kind: ty::AssocKind::Fn, .. }) = inherent_impls
+            && let Some(item) = inherent_impls
                 .iter()
                 .flat_map(|inherent_impl| {
                     tcx.associated_items(inherent_impl).filter_by_name_unhygienic(name)
                 })
                 .next()
+            && item.is_fn()
         {
             Err(struct_span_code_err!(self.dcx(), span, E0223, "ambiguous associated type")
                 .with_span_suggestion_verbose(
@@ -1629,10 +1633,10 @@ fn generics_args_err_extend<'a>(
     }
 }
 
-pub(crate) fn assoc_kind_str(kind: ty::AssocKind) -> &'static str {
-    match kind {
-        ty::AssocKind::Fn => "function",
-        ty::AssocKind::Const => "constant",
-        ty::AssocKind::Type => "type",
+pub(crate) fn assoc_tag_str(assoc_tag: ty::AssocTag) -> &'static str {
+    match assoc_tag {
+        ty::AssocTag::Fn => "function",
+        ty::AssocTag::Const => "constant",
+        ty::AssocTag::Type => "type",
     }
 }
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs
index 8e62dce2191..483b61add33 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs
@@ -501,8 +501,8 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         let names: Vec<_> = tcx
             .associated_items(trait_def_id)
             .in_definition_order()
-            .filter(|assoc| assoc.kind.namespace() == Namespace::ValueNS)
-            .map(|cand| cand.name)
+            .filter(|assoc| assoc.namespace() == Namespace::ValueNS)
+            .map(|cand| cand.name())
             .collect();
         if let Some(typo) = find_best_match_for_name(&names, segment.ident.name, None) {
             diag.span_suggestion_verbose(
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
index 83aa0d95620..7605c6c6a42 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
@@ -38,8 +38,8 @@ use rustc_middle::middle::stability::AllowUnstable;
 use rustc_middle::mir::interpret::LitToConstInput;
 use rustc_middle::ty::print::PrintPolyTraitRefExt as _;
 use rustc_middle::ty::{
-    self, Const, GenericArgKind, GenericArgsRef, GenericParamDefKind, ParamEnv, Ty, TyCtxt,
-    TypeVisitableExt, TypingMode, Upcast, fold_regions,
+    self, AssocTag, Const, GenericArgKind, GenericArgsRef, GenericParamDefKind, ParamEnv, Ty,
+    TyCtxt, TypeVisitableExt, TypingMode, Upcast, fold_regions,
 };
 use rustc_middle::{bug, span_bug};
 use rustc_session::lint::builtin::AMBIGUOUS_ASSOCIATED_ITEMS;
@@ -51,7 +51,7 @@ use rustc_trait_selection::traits::wf::object_region_bounds;
 use rustc_trait_selection::traits::{self, ObligationCtxt};
 use tracing::{debug, instrument};
 
-use self::errors::assoc_kind_str;
+use self::errors::assoc_tag_str;
 use crate::check::check_abi_fn_ptr;
 use crate::errors::{AmbiguousLifetimeBound, BadReturnTypeNotation, NoVariantNamed};
 use crate::hir_ty_lowering::errors::{GenericsArgsErrExtend, prohibit_assoc_item_constraint};
@@ -168,7 +168,7 @@ pub trait HirTyLowerer<'tcx> {
         item_def_id: DefId,
         item_segment: &hir::PathSegment<'tcx>,
         poly_trait_ref: ty::PolyTraitRef<'tcx>,
-        kind: ty::AssocKind,
+        assoc_tag: ty::AssocTag,
     ) -> Result<(DefId, GenericArgsRef<'tcx>), ErrorGuaranteed>;
 
     fn lower_fn_sig(
@@ -251,10 +251,10 @@ enum LowerAssocMode {
 }
 
 impl LowerAssocMode {
-    fn kind(self) -> ty::AssocKind {
+    fn assoc_tag(self) -> ty::AssocTag {
         match self {
-            LowerAssocMode::Type { .. } => ty::AssocKind::Type,
-            LowerAssocMode::Const => ty::AssocKind::Const,
+            LowerAssocMode::Type { .. } => ty::AssocTag::Type,
+            LowerAssocMode::Const => ty::AssocTag::Const,
         }
     }
 
@@ -268,7 +268,8 @@ impl LowerAssocMode {
     fn permit_variants(self) -> bool {
         match self {
             LowerAssocMode::Type { permit_variants } => permit_variants,
-            // FIXME(mgca): Support paths like `Option::<T>::None` or `Option::<T>::Some` which resolve to const ctors/fn items respectively
+            // FIXME(mgca): Support paths like `Option::<T>::None` or `Option::<T>::Some` which
+            // resolve to const ctors/fn items respectively.
             LowerAssocMode::Const => false,
         }
     }
@@ -932,12 +933,12 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     fn probe_trait_that_defines_assoc_item(
         &self,
         trait_def_id: DefId,
-        assoc_kind: ty::AssocKind,
+        assoc_tag: AssocTag,
         assoc_ident: Ident,
     ) -> bool {
         self.tcx()
             .associated_items(trait_def_id)
-            .find_by_ident_and_kind(self.tcx(), assoc_ident, assoc_kind, trait_def_id)
+            .find_by_ident_and_kind(self.tcx(), assoc_ident, assoc_tag, trait_def_id)
             .is_some()
     }
 
@@ -975,7 +976,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         &self,
         ty_param_def_id: LocalDefId,
         ty_param_span: Span,
-        kind: ty::AssocKind,
+        assoc_tag: AssocTag,
         assoc_ident: Ident,
         span: Span,
     ) -> Result<ty::PolyTraitRef<'tcx>, ErrorGuaranteed> {
@@ -993,7 +994,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 traits::transitive_bounds_that_define_assoc_item(tcx, trait_refs, assoc_ident)
             },
             AssocItemQSelf::TyParam(ty_param_def_id, ty_param_span),
-            kind,
+            assoc_tag,
             assoc_ident,
             span,
             None,
@@ -1010,7 +1011,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         &self,
         all_candidates: impl Fn() -> I,
         qself: AssocItemQSelf,
-        assoc_kind: ty::AssocKind,
+        assoc_tag: AssocTag,
         assoc_ident: Ident,
         span: Span,
         constraint: Option<&hir::AssocItemConstraint<'tcx>>,
@@ -1021,14 +1022,14 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         let tcx = self.tcx();
 
         let mut matching_candidates = all_candidates().filter(|r| {
-            self.probe_trait_that_defines_assoc_item(r.def_id(), assoc_kind, assoc_ident)
+            self.probe_trait_that_defines_assoc_item(r.def_id(), assoc_tag, assoc_ident)
         });
 
         let Some(bound) = matching_candidates.next() else {
             let reported = self.complain_about_assoc_item_not_found(
                 all_candidates,
                 qself,
-                assoc_kind,
+                assoc_tag,
                 assoc_ident,
                 span,
                 constraint,
@@ -1040,7 +1041,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         if let Some(bound2) = matching_candidates.next() {
             debug!(?bound2);
 
-            let assoc_kind_str = errors::assoc_kind_str(assoc_kind);
+            let assoc_kind_str = errors::assoc_tag_str(assoc_tag);
             let qself_str = qself.to_string(tcx);
             let mut err = self.dcx().create_err(crate::errors::AmbiguousAssocItem {
                 span,
@@ -1059,14 +1060,15 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 },
             );
 
-            // FIXME(#97583): Print associated item bindings properly (i.e., not as equality predicates!).
+            // FIXME(#97583): Print associated item bindings properly (i.e., not as equality
+            // predicates!).
             // FIXME: Turn this into a structured, translateable & more actionable suggestion.
             let mut where_bounds = vec![];
             for bound in [bound, bound2].into_iter().chain(matching_candidates) {
                 let bound_id = bound.def_id();
                 let bound_span = tcx
                     .associated_items(bound_id)
-                    .find_by_ident_and_kind(tcx, assoc_ident, assoc_kind, bound_id)
+                    .find_by_ident_and_kind(tcx, assoc_ident, assoc_tag, bound_id)
                     .and_then(|item| tcx.hir_span_if_local(item.def_id));
 
                 if let Some(bound_span) = bound_span {
@@ -1265,7 +1267,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 qself_ty,
                 hir_ref_id,
                 span,
-                mode.kind(),
+                mode.assoc_tag(),
             )? {
                 return Ok(LoweredAssoc::Term(did, args));
             }
@@ -1296,7 +1298,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         )
                     },
                     AssocItemQSelf::SelfTyAlias,
-                    mode.kind(),
+                    mode.assoc_tag(),
                     assoc_ident,
                     span,
                     None,
@@ -1308,12 +1310,12 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             ) => self.probe_single_ty_param_bound_for_assoc_item(
                 param_did.expect_local(),
                 qself.span,
-                mode.kind(),
+                mode.assoc_tag(),
                 assoc_ident,
                 span,
             )?,
             _ => {
-                let kind_str = assoc_kind_str(mode.kind());
+                let kind_str = assoc_tag_str(mode.assoc_tag());
                 let reported = if variant_resolution.is_some() {
                     // Variant in type position
                     let msg = format!("expected {kind_str}, found variant `{assoc_ident}`");
@@ -1420,7 +1422,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         &[qself_ty.to_string()],
                         &traits,
                         assoc_ident.name,
-                        mode.kind(),
+                        mode.assoc_tag(),
                     )
                 };
                 return Err(reported);
@@ -1429,10 +1431,15 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
         let trait_did = bound.def_id();
         let assoc_item = self
-            .probe_assoc_item(assoc_ident, mode.kind(), hir_ref_id, span, trait_did)
+            .probe_assoc_item(assoc_ident, mode.assoc_tag(), hir_ref_id, span, trait_did)
             .expect("failed to find associated item");
-        let (def_id, args) =
-            self.lower_assoc_shared(span, assoc_item.def_id, assoc_segment, bound, mode.kind())?;
+        let (def_id, args) = self.lower_assoc_shared(
+            span,
+            assoc_item.def_id,
+            assoc_segment,
+            bound,
+            mode.assoc_tag(),
+        )?;
         let result = LoweredAssoc::Term(def_id, args);
 
         if let Some(variant_def_id) = variant_resolution {
@@ -1469,20 +1476,21 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         self_ty: Ty<'tcx>,
         block: HirId,
         span: Span,
-        kind: ty::AssocKind,
+        assoc_tag: ty::AssocTag,
     ) -> Result<Option<(DefId, GenericArgsRef<'tcx>)>, ErrorGuaranteed> {
         let tcx = self.tcx();
 
         if !tcx.features().inherent_associated_types() {
-            match kind {
-                // Don't attempt to look up inherent associated types when the feature is not enabled.
-                // Theoretically it'd be fine to do so since we feature-gate their definition site.
-                // However, due to current limitations of the implementation (caused by us performing
-                // selection during HIR ty lowering instead of in the trait solver), IATs can lead to cycle
-                // errors (#108491) which mask the feature-gate error, needlessly confusing users
-                // who use IATs by accident (#113265).
-                ty::AssocKind::Type => return Ok(None),
-                ty::AssocKind::Const => {
+            match assoc_tag {
+                // Don't attempt to look up inherent associated types when the feature is not
+                // enabled. Theoretically it'd be fine to do so since we feature-gate their
+                // definition site. However, due to current limitations of the implementation
+                // (caused by us performing selection during HIR ty lowering instead of in the
+                // trait solver), IATs can lead to cycle errors (#108491) which mask the
+                // feature-gate error, needlessly confusing users who use IATs by accident
+                // (#113265).
+                ty::AssocTag::Type => return Ok(None),
+                ty::AssocTag::Const => {
                     // We also gate the mgca codepath for type-level uses of inherent consts
                     // with the inherent_associated_types feature gate since it relies on the
                     // same machinery and has similar rough edges.
@@ -1494,7 +1502,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     )
                     .emit());
                 }
-                ty::AssocKind::Fn => unreachable!(),
+                ty::AssocTag::Fn => unreachable!(),
             }
         }
 
@@ -1503,7 +1511,8 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             .inherent_impls(adt_did)
             .iter()
             .filter_map(|&impl_| {
-                let (item, scope) = self.probe_assoc_item_unchecked(name, kind, block, impl_)?;
+                let (item, scope) =
+                    self.probe_assoc_item_unchecked(name, assoc_tag, block, impl_)?;
                 Some((impl_, (item.def_id, scope)))
             })
             .collect();
@@ -1542,7 +1551,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             self_ty,
             |self_ty| {
                 self.select_inherent_assoc_candidates(
-                    infcx, name, span, self_ty, param_env, candidates, kind,
+                    infcx, name, span, self_ty, param_env, candidates, assoc_tag,
                 )
             },
         )?;
@@ -1570,7 +1579,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         self_ty: Ty<'tcx>,
         param_env: ParamEnv<'tcx>,
         candidates: Vec<(DefId, (DefId, DefId))>,
-        kind: ty::AssocKind,
+        assoc_tag: ty::AssocTag,
     ) -> Result<(DefId, (DefId, DefId)), ErrorGuaranteed> {
         let tcx = self.tcx();
         let mut fulfillment_errors = Vec::new();
@@ -1621,7 +1630,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 candidates,
                 fulfillment_errors,
                 span,
-                kind,
+                assoc_tag,
             )),
 
             &[applicable_candidate] => Ok(applicable_candidate),
@@ -1640,12 +1649,12 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     fn probe_assoc_item(
         &self,
         ident: Ident,
-        kind: ty::AssocKind,
+        assoc_tag: ty::AssocTag,
         block: HirId,
         span: Span,
         scope: DefId,
     ) -> Option<ty::AssocItem> {
-        let (item, scope) = self.probe_assoc_item_unchecked(ident, kind, block, scope)?;
+        let (item, scope) = self.probe_assoc_item_unchecked(ident, assoc_tag, block, scope)?;
         self.check_assoc_item(item.def_id, ident, scope, block, span);
         Some(item)
     }
@@ -1657,7 +1666,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     fn probe_assoc_item_unchecked(
         &self,
         ident: Ident,
-        kind: ty::AssocKind,
+        assoc_tag: ty::AssocTag,
         block: HirId,
         scope: DefId,
     ) -> Option<(ty::AssocItem, /*scope*/ DefId)> {
@@ -1670,7 +1679,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         let item = tcx
             .associated_items(scope)
             .filter_by_name_unhygienic(ident.name)
-            .find(|i| i.kind == kind && i.ident(tcx).normalize_to_macros_2_0() == ident)?;
+            .find(|i| i.as_tag() == assoc_tag && i.ident(tcx).normalize_to_macros_2_0() == ident)?;
 
         Some((*item, def_scope))
     }
@@ -1722,9 +1731,9 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 tcx.associated_items(*trait_def_id)
                         .in_definition_order()
                         .any(|i| {
-                            i.kind.namespace() == Namespace::TypeNS
+                            i.namespace() == Namespace::TypeNS
                                 && i.ident(tcx).normalize_to_macros_2_0() == assoc_ident
-                                && matches!(i.kind, ty::AssocKind::Type)
+                                && i.is_type()
                         })
                     // Consider only accessible traits
                     && tcx.visibility(*trait_def_id)
@@ -1770,7 +1779,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             item_def_id,
             trait_segment,
             item_segment,
-            ty::AssocKind::Type,
+            ty::AssocTag::Type,
         ) {
             Ok((item_def_id, item_args)) => {
                 Ty::new_projection_from_args(self.tcx(), item_def_id, item_args)
@@ -1795,7 +1804,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             item_def_id,
             trait_segment,
             item_segment,
-            ty::AssocKind::Const,
+            ty::AssocTag::Const,
         ) {
             Ok((item_def_id, item_args)) => {
                 let uv = ty::UnevaluatedConst::new(item_def_id, item_args);
@@ -1813,7 +1822,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         item_def_id: DefId,
         trait_segment: &hir::PathSegment<'tcx>,
         item_segment: &hir::PathSegment<'tcx>,
-        kind: ty::AssocKind,
+        assoc_tag: ty::AssocTag,
     ) -> Result<(DefId, GenericArgsRef<'tcx>), ErrorGuaranteed> {
         let tcx = self.tcx();
 
@@ -1821,7 +1830,12 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         debug!(?trait_def_id);
 
         let Some(self_ty) = opt_self_ty else {
-            return Err(self.error_missing_qpath_self_ty(trait_def_id, span, item_segment, kind));
+            return Err(self.error_missing_qpath_self_ty(
+                trait_def_id,
+                span,
+                item_segment,
+                assoc_tag,
+            ));
         };
         debug!(?self_ty);
 
@@ -1840,7 +1854,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         trait_def_id: DefId,
         span: Span,
         item_segment: &hir::PathSegment<'tcx>,
-        kind: ty::AssocKind,
+        assoc_tag: ty::AssocTag,
     ) -> ErrorGuaranteed {
         let tcx = self.tcx();
         let path_str = tcx.def_path_str(trait_def_id);
@@ -1877,7 +1891,13 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         // FIXME: also look at `tcx.generics_of(self.item_def_id()).params` any that
         // references the trait. Relevant for the first case in
         // `src/test/ui/associated-types/associated-types-in-ambiguous-context.rs`
-        self.report_ambiguous_assoc(span, &type_names, &[path_str], item_segment.ident.name, kind)
+        self.report_ambiguous_assoc(
+            span,
+            &type_names,
+            &[path_str],
+            item_segment.ident.name,
+            assoc_tag,
+        )
     }
 
     pub fn prohibit_generic_args<'a>(
@@ -2862,7 +2882,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         let assoc = tcx.associated_items(trait_ref.def_id).find_by_ident_and_kind(
             tcx,
             *ident,
-            ty::AssocKind::Fn,
+            ty::AssocTag::Fn,
             trait_ref.def_id,
         )?;
 
diff --git a/compiler/rustc_hir_analysis/src/impl_wf_check.rs b/compiler/rustc_hir_analysis/src/impl_wf_check.rs
index c30b39dfe76..cbdc501291b 100644
--- a/compiler/rustc_hir_analysis/src/impl_wf_check.rs
+++ b/compiler/rustc_hir_analysis/src/impl_wf_check.rs
@@ -112,14 +112,14 @@ pub(crate) fn enforce_impl_lifetime_params_are_constrained(
         .flat_map(|def_id| {
             let item = tcx.associated_item(def_id);
             match item.kind {
-                ty::AssocKind::Type => {
+                ty::AssocKind::Type { .. } => {
                     if item.defaultness(tcx).has_value() {
                         cgp::parameters_for(tcx, tcx.type_of(def_id).instantiate_identity(), true)
                     } else {
                         vec![]
                     }
                 }
-                ty::AssocKind::Fn | ty::AssocKind::Const => vec![],
+                ty::AssocKind::Fn { .. } | ty::AssocKind::Const { .. } => vec![],
             }
         })
         .collect();
diff --git a/compiler/rustc_hir_analysis/src/lib.rs b/compiler/rustc_hir_analysis/src/lib.rs
index e7ecd727a85..e1ad8124aea 100644
--- a/compiler/rustc_hir_analysis/src/lib.rs
+++ b/compiler/rustc_hir_analysis/src/lib.rs
@@ -59,7 +59,6 @@ This API is completely unstable and subject to change.
 #![allow(internal_features)]
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(assert_matches)]
diff --git a/compiler/rustc_hir_pretty/src/lib.rs b/compiler/rustc_hir_pretty/src/lib.rs
index e5ab317685f..ff4385c3bcc 100644
--- a/compiler/rustc_hir_pretty/src/lib.rs
+++ b/compiler/rustc_hir_pretty/src/lib.rs
@@ -1871,10 +1871,11 @@ impl<'a> State<'a> {
     fn print_pat(&mut self, pat: &hir::Pat<'_>) {
         self.maybe_print_comment(pat.span.lo());
         self.ann.pre(self, AnnNode::Pat(pat));
-        // Pat isn't normalized, but the beauty of it
-        // is that it doesn't matter
+        // Pat isn't normalized, but the beauty of it is that it doesn't matter.
         match pat.kind {
-            PatKind::Missing => unreachable!(),
+            // Printing `_` isn't ideal for a missing pattern, but it's easy and good enough.
+            // E.g. `fn(u32)` gets printed as `fn(_: u32)`.
+            PatKind::Missing => self.word("_"),
             PatKind::Wild => self.word("_"),
             PatKind::Never => self.word("!"),
             PatKind::Binding(BindingMode(by_ref, mutbl), _, ident, sub) => {
@@ -2164,7 +2165,9 @@ impl<'a> State<'a> {
             s.end();
         });
         if decl.c_variadic {
-            self.word(", ");
+            if !decl.inputs.is_empty() {
+                self.word(", ");
+            }
             print_arg(self, None);
             self.word("...");
         }
diff --git a/compiler/rustc_hir_typeck/src/cast.rs b/compiler/rustc_hir_typeck/src/cast.rs
index 8f5fddd19d7..b19d9efe2c6 100644
--- a/compiler/rustc_hir_typeck/src/cast.rs
+++ b/compiler/rustc_hir_typeck/src/cast.rs
@@ -1042,30 +1042,31 @@ impl<'a, 'tcx> CastCheck<'tcx> {
         m_cast: ty::TypeAndMut<'tcx>,
     ) -> Result<CastKind, CastError<'tcx>> {
         // array-ptr-cast: allow mut-to-mut, mut-to-const, const-to-const
-        if m_expr.mutbl >= m_cast.mutbl {
-            if let ty::Array(ety, _) = m_expr.ty.kind() {
-                // Due to the limitations of LLVM global constants,
-                // region pointers end up pointing at copies of
-                // vector elements instead of the original values.
-                // To allow raw pointers to work correctly, we
-                // need to special-case obtaining a raw pointer
-                // from a region pointer to a vector.
-
-                // Coerce to a raw pointer so that we generate RawPtr in MIR.
-                let array_ptr_type = Ty::new_ptr(fcx.tcx, m_expr.ty, m_expr.mutbl);
-                fcx.coerce(self.expr, self.expr_ty, array_ptr_type, AllowTwoPhase::No, None)
-                    .unwrap_or_else(|_| {
-                        bug!(
+        if m_expr.mutbl >= m_cast.mutbl
+            && let ty::Array(ety, _) = m_expr.ty.kind()
+            && fcx.can_eq(fcx.param_env, *ety, m_cast.ty)
+        {
+            // Due to the limitations of LLVM global constants,
+            // region pointers end up pointing at copies of
+            // vector elements instead of the original values.
+            // To allow raw pointers to work correctly, we
+            // need to special-case obtaining a raw pointer
+            // from a region pointer to a vector.
+
+            // Coerce to a raw pointer so that we generate RawPtr in MIR.
+            let array_ptr_type = Ty::new_ptr(fcx.tcx, m_expr.ty, m_expr.mutbl);
+            fcx.coerce(self.expr, self.expr_ty, array_ptr_type, AllowTwoPhase::No, None)
+                .unwrap_or_else(|_| {
+                    bug!(
                         "could not cast from reference to array to pointer to array ({:?} to {:?})",
                         self.expr_ty,
                         array_ptr_type,
                     )
-                    });
+                });
 
-                // this will report a type mismatch if needed
-                fcx.demand_eqtype(self.span, *ety, m_cast.ty);
-                return Ok(CastKind::ArrayPtrCast);
-            }
+            // this will report a type mismatch if needed
+            fcx.demand_eqtype(self.span, *ety, m_cast.ty);
+            return Ok(CastKind::ArrayPtrCast);
         }
 
         Err(CastError::IllegalCast)
diff --git a/compiler/rustc_hir_typeck/src/coercion.rs b/compiler/rustc_hir_typeck/src/coercion.rs
index f1571cf4c83..fd899425f62 100644
--- a/compiler/rustc_hir_typeck/src/coercion.rs
+++ b/compiler/rustc_hir_typeck/src/coercion.rs
@@ -103,15 +103,6 @@ fn coerce_mutbls<'tcx>(
     if from_mutbl >= to_mutbl { Ok(()) } else { Err(TypeError::Mutability) }
 }
 
-/// Do not require any adjustments, i.e. coerce `x -> x`.
-fn identity(_: Ty<'_>) -> Vec<Adjustment<'_>> {
-    vec![]
-}
-
-fn simple<'tcx>(kind: Adjust) -> impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>> {
-    move |target| vec![Adjustment { kind, target }]
-}
-
 /// This always returns `Ok(...)`.
 fn success<'tcx>(
     adj: Vec<Adjustment<'tcx>>,
@@ -131,7 +122,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         Coerce { fcx, cause, allow_two_phase, use_lub: false, coerce_never }
     }
 
-    fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> {
+    fn unify_raw(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> {
         debug!("unify(a: {:?}, b: {:?}, use_lub: {})", a, b, self.use_lub);
         self.commit_if_ok(|_| {
             let at = self.at(&self.cause, self.fcx.param_env);
@@ -161,13 +152,30 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         })
     }
 
+    /// Unify two types (using sub or lub).
+    fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
+        self.unify_raw(a, b)
+            .and_then(|InferOk { value: ty, obligations }| success(vec![], ty, obligations))
+    }
+
     /// Unify two types (using sub or lub) and produce a specific coercion.
-    fn unify_and<F>(&self, a: Ty<'tcx>, b: Ty<'tcx>, f: F) -> CoerceResult<'tcx>
-    where
-        F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
-    {
-        self.unify(a, b)
-            .and_then(|InferOk { value: ty, obligations }| success(f(ty), ty, obligations))
+    fn unify_and(
+        &self,
+        a: Ty<'tcx>,
+        b: Ty<'tcx>,
+        adjustments: impl IntoIterator<Item = Adjustment<'tcx>>,
+        final_adjustment: Adjust,
+    ) -> CoerceResult<'tcx> {
+        self.unify_raw(a, b).and_then(|InferOk { value: ty, obligations }| {
+            success(
+                adjustments
+                    .into_iter()
+                    .chain(std::iter::once(Adjustment { target: ty, kind: final_adjustment }))
+                    .collect(),
+                ty,
+                obligations,
+            )
+        })
     }
 
     #[instrument(skip(self))]
@@ -180,10 +188,14 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         // Coercing from `!` to any type is allowed:
         if a.is_never() {
             if self.coerce_never {
-                return success(simple(Adjust::NeverToAny)(b), b, PredicateObligations::new());
+                return success(
+                    vec![Adjustment { kind: Adjust::NeverToAny, target: b }],
+                    b,
+                    PredicateObligations::new(),
+                );
             } else {
                 // Otherwise the only coercion we can do is unification.
-                return self.unify_and(a, b, identity);
+                return self.unify(a, b);
             }
         }
 
@@ -191,7 +203,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         // we have no information about the source type. This will always
         // ultimately fall back to some form of subtyping.
         if a.is_ty_var() {
-            return self.coerce_from_inference_variable(a, b, identity);
+            return self.coerce_from_inference_variable(a, b);
         }
 
         // Consider coercing the subtype to a DST
@@ -247,7 +259,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             ty::FnPtr(a_sig_tys, a_hdr) => {
                 // We permit coercion of fn pointers to drop the
                 // unsafe qualifier.
-                self.coerce_from_fn_pointer(a, a_sig_tys.with(a_hdr), b)
+                self.coerce_from_fn_pointer(a_sig_tys.with(a_hdr), b)
             }
             ty::Closure(closure_def_id_a, args_a) => {
                 // Non-capturing closures are coercible to
@@ -257,7 +269,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             }
             _ => {
                 // Otherwise, just use unification rules.
-                self.unify_and(a, b, identity)
+                self.unify(a, b)
             }
         }
     }
@@ -265,12 +277,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
     /// Coercing *from* an inference variable. In this case, we have no information
     /// about the source type, so we can't really do a true coercion and we always
     /// fall back to subtyping (`unify_and`).
-    fn coerce_from_inference_variable(
-        &self,
-        a: Ty<'tcx>,
-        b: Ty<'tcx>,
-        make_adjustments: impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
-    ) -> CoerceResult<'tcx> {
+    fn coerce_from_inference_variable(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
         debug!("coerce_from_inference_variable(a={:?}, b={:?})", a, b);
         assert!(a.is_ty_var() && self.shallow_resolve(a) == a);
         assert!(self.shallow_resolve(b) == b);
@@ -298,12 +305,11 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 "coerce_from_inference_variable: two inference variables, target_ty={:?}, obligations={:?}",
                 target_ty, obligations
             );
-            let adjustments = make_adjustments(target_ty);
-            InferResult::Ok(InferOk { value: (adjustments, target_ty), obligations })
+            success(vec![], target_ty, obligations)
         } else {
             // One unresolved type variable: just apply subtyping, we may be able
             // to do something useful.
-            self.unify_and(a, b, make_adjustments)
+            self.unify(a, b)
         }
     }
 
@@ -331,7 +337,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 coerce_mutbls(mt_a.mutbl, mutbl_b)?;
                 (r_a, mt_a)
             }
-            _ => return self.unify_and(a, b, identity),
+            _ => return self.unify(a, b),
         };
 
         let span = self.cause.span;
@@ -437,7 +443,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 referent_ty,
                 mutbl_b, // [1] above
             );
-            match self.unify(derefd_ty_a, b) {
+            match self.unify_raw(derefd_ty_a, b) {
                 Ok(ok) => {
                     found = Some(ok);
                     break;
@@ -579,13 +585,13 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         // We only have the latter, so we use an inference variable
         // for the former and let type inference do the rest.
         let coerce_target = self.next_ty_var(self.cause.span);
-        let mut coercion = self.unify_and(coerce_target, target, |target| {
-            let unsize = Adjustment { kind: Adjust::Pointer(PointerCoercion::Unsize), target };
-            match reborrow {
-                None => vec![unsize],
-                Some((ref deref, ref autoref)) => vec![deref.clone(), autoref.clone(), unsize],
-            }
-        })?;
+
+        let mut coercion = self.unify_and(
+            coerce_target,
+            target,
+            reborrow.into_iter().flat_map(|(deref, autoref)| [deref, autoref]),
+            Adjust::Pointer(PointerCoercion::Unsize),
+        )?;
 
         let mut selcx = traits::SelectionContext::new(self);
 
@@ -708,7 +714,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             && let ty::Dynamic(b_data, _, ty::DynStar) = b.kind()
             && a_data.principal_def_id() == b_data.principal_def_id()
         {
-            return self.unify_and(a, b, |_| vec![]);
+            return self.unify(a, b);
         }
 
         // Check the obligations of the cast -- for example, when casting
@@ -808,23 +814,15 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
 
         // To complete the reborrow, we need to make sure we can unify the inner types, and if so we
         // add the adjustments.
-        self.unify_and(a, b, |_inner_ty| {
-            vec![Adjustment { kind: Adjust::ReborrowPin(mut_b), target: b }]
-        })
+        self.unify_and(a, b, [], Adjust::ReborrowPin(mut_b))
     }
 
-    fn coerce_from_safe_fn<F, G>(
+    fn coerce_from_safe_fn(
         &self,
-        a: Ty<'tcx>,
         fn_ty_a: ty::PolyFnSig<'tcx>,
         b: Ty<'tcx>,
-        to_unsafe: F,
-        normal: G,
-    ) -> CoerceResult<'tcx>
-    where
-        F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
-        G: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
-    {
+        adjustment: Option<Adjust>,
+    ) -> CoerceResult<'tcx> {
         self.commit_if_ok(|snapshot| {
             let outer_universe = self.infcx.universe();
 
@@ -833,9 +831,19 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 && hdr_b.safety.is_unsafe()
             {
                 let unsafe_a = self.tcx.safe_to_unsafe_fn_ty(fn_ty_a);
-                self.unify_and(unsafe_a, b, to_unsafe)
+                self.unify_and(
+                    unsafe_a,
+                    b,
+                    adjustment
+                        .map(|kind| Adjustment { kind, target: Ty::new_fn_ptr(self.tcx, fn_ty_a) }),
+                    Adjust::Pointer(PointerCoercion::UnsafeFnPointer),
+                )
             } else {
-                self.unify_and(a, b, normal)
+                let a = Ty::new_fn_ptr(self.tcx, fn_ty_a);
+                match adjustment {
+                    Some(adjust) => self.unify_and(a, b, [], adjust),
+                    None => self.unify(a, b),
+                }
             };
 
             // FIXME(#73154): This is a hack. Currently LUB can generate
@@ -852,7 +860,6 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
 
     fn coerce_from_fn_pointer(
         &self,
-        a: Ty<'tcx>,
         fn_ty_a: ty::PolyFnSig<'tcx>,
         b: Ty<'tcx>,
     ) -> CoerceResult<'tcx> {
@@ -861,15 +868,9 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         //!
 
         let b = self.shallow_resolve(b);
-        debug!("coerce_from_fn_pointer(a={:?}, b={:?})", a, b);
-
-        self.coerce_from_safe_fn(
-            a,
-            fn_ty_a,
-            b,
-            simple(Adjust::Pointer(PointerCoercion::UnsafeFnPointer)),
-            identity,
-        )
+        debug!(?fn_ty_a, ?b, "coerce_from_fn_pointer");
+
+        self.coerce_from_safe_fn(fn_ty_a, b, None)
     }
 
     fn coerce_from_fn_item(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
@@ -916,30 +917,16 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                     self.at(&self.cause, self.param_env).normalize(a_sig);
                 obligations.extend(o1);
 
-                let a_fn_pointer = Ty::new_fn_ptr(self.tcx, a_sig);
                 let InferOk { value, obligations: o2 } = self.coerce_from_safe_fn(
-                    a_fn_pointer,
                     a_sig,
                     b,
-                    |unsafe_ty| {
-                        vec![
-                            Adjustment {
-                                kind: Adjust::Pointer(PointerCoercion::ReifyFnPointer),
-                                target: a_fn_pointer,
-                            },
-                            Adjustment {
-                                kind: Adjust::Pointer(PointerCoercion::UnsafeFnPointer),
-                                target: unsafe_ty,
-                            },
-                        ]
-                    },
-                    simple(Adjust::Pointer(PointerCoercion::ReifyFnPointer)),
+                    Some(Adjust::Pointer(PointerCoercion::ReifyFnPointer)),
                 )?;
 
                 obligations.extend(o2);
                 Ok(InferOk { value, obligations })
             }
-            _ => self.unify_and(a, b, identity),
+            _ => self.unify(a, b),
         }
     }
 
@@ -983,10 +970,11 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 self.unify_and(
                     pointer_ty,
                     b,
-                    simple(Adjust::Pointer(PointerCoercion::ClosureFnPointer(safety))),
+                    [],
+                    Adjust::Pointer(PointerCoercion::ClosureFnPointer(safety)),
                 )
             }
-            _ => self.unify_and(a, b, identity),
+            _ => self.unify(a, b),
         }
     }
 
@@ -1001,7 +989,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         let (is_ref, mt_a) = match *a.kind() {
             ty::Ref(_, ty, mutbl) => (true, ty::TypeAndMut { ty, mutbl }),
             ty::RawPtr(ty, mutbl) => (false, ty::TypeAndMut { ty, mutbl }),
-            _ => return self.unify_and(a, b, identity),
+            _ => return self.unify(a, b),
         };
         coerce_mutbls(mt_a.mutbl, mutbl_b)?;
 
@@ -1011,16 +999,16 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         // representation, we still register an Adjust::DerefRef so that
         // regionck knows that the region for `a` must be valid here.
         if is_ref {
-            self.unify_and(a_raw, b, |target| {
-                vec![
-                    Adjustment { kind: Adjust::Deref(None), target: mt_a.ty },
-                    Adjustment { kind: Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)), target },
-                ]
-            })
+            self.unify_and(
+                a_raw,
+                b,
+                [Adjustment { kind: Adjust::Deref(None), target: mt_a.ty }],
+                Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)),
+            )
         } else if mt_a.mutbl != mutbl_b {
-            self.unify_and(a_raw, b, simple(Adjust::Pointer(PointerCoercion::MutToConstPointer)))
+            self.unify_and(a_raw, b, [], Adjust::Pointer(PointerCoercion::MutToConstPointer))
         } else {
-            self.unify_and(a_raw, b, identity)
+            self.unify(a_raw, b)
         }
     }
 }
@@ -1118,9 +1106,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let cause = self.cause(DUMMY_SP, ObligationCauseCode::ExprAssignable);
         // We don't ever need two-phase here since we throw out the result of the coercion.
         let coerce = Coerce::new(self, cause, AllowTwoPhase::No, true);
-        coerce
-            .autoderef(DUMMY_SP, expr_ty)
-            .find_map(|(ty, steps)| self.probe(|_| coerce.unify(ty, target)).ok().map(|_| steps))
+        coerce.autoderef(DUMMY_SP, expr_ty).find_map(|(ty, steps)| {
+            self.probe(|_| coerce.unify_raw(ty, target)).ok().map(|_| steps)
+        })
     }
 
     /// Given a type, this function will calculate and return the type given
diff --git a/compiler/rustc_hir_typeck/src/demand.rs b/compiler/rustc_hir_typeck/src/demand.rs
index fec45995410..d1bc54ed73e 100644
--- a/compiler/rustc_hir_typeck/src/demand.rs
+++ b/compiler/rustc_hir_typeck/src/demand.rs
@@ -1089,14 +1089,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     /// This function checks whether the method is not static and does not accept other parameters than `self`.
     fn has_only_self_parameter(&self, method: &AssocItem) -> bool {
-        match method.kind {
-            ty::AssocKind::Fn => {
-                method.fn_has_self_parameter
-                    && self.tcx.fn_sig(method.def_id).skip_binder().inputs().skip_binder().len()
-                        == 1
-            }
-            _ => false,
-        }
+        method.is_method()
+            && self.tcx.fn_sig(method.def_id).skip_binder().inputs().skip_binder().len() == 1
     }
 
     /// If the given `HirId` corresponds to a block with a trailing expression, return that expression
diff --git a/compiler/rustc_hir_typeck/src/errors.rs b/compiler/rustc_hir_typeck/src/errors.rs
index dfaa374592b..9e7305430e5 100644
--- a/compiler/rustc_hir_typeck/src/errors.rs
+++ b/compiler/rustc_hir_typeck/src/errors.rs
@@ -5,7 +5,7 @@ use std::borrow::Cow;
 use rustc_errors::codes::*;
 use rustc_errors::{
     Applicability, Diag, DiagArgValue, DiagSymbolList, EmissionGuarantee, IntoDiagArg, MultiSpan,
-    SubdiagMessageOp, Subdiagnostic,
+    Subdiagnostic,
 };
 use rustc_macros::{Diagnostic, LintDiagnostic, Subdiagnostic};
 use rustc_middle::ty::{self, Ty};
@@ -270,11 +270,7 @@ pub(crate) struct SuggestAnnotations {
     pub suggestions: Vec<SuggestAnnotation>,
 }
 impl Subdiagnostic for SuggestAnnotations {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         if self.suggestions.is_empty() {
             return;
         }
@@ -337,11 +333,7 @@ pub(crate) struct TypeMismatchFruTypo {
 }
 
 impl Subdiagnostic for TypeMismatchFruTypo {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         diag.arg("expr", self.expr.as_deref().unwrap_or("NONE"));
 
         // Only explain that `a ..b` is a range if it's split up
@@ -599,11 +591,7 @@ pub(crate) struct RemoveSemiForCoerce {
 }
 
 impl Subdiagnostic for RemoveSemiForCoerce {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         let mut multispan: MultiSpan = self.semi.into();
         multispan.push_span_label(self.expr, fluent::hir_typeck_remove_semi_for_coerce_expr);
         multispan.push_span_label(self.ret, fluent::hir_typeck_remove_semi_for_coerce_ret);
@@ -778,20 +766,16 @@ pub(crate) enum CastUnknownPointerSub {
 }
 
 impl rustc_errors::Subdiagnostic for CastUnknownPointerSub {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         match self {
             CastUnknownPointerSub::To(span) => {
-                let msg = f(diag, crate::fluent_generated::hir_typeck_label_to);
+                let msg = diag.eagerly_translate(fluent::hir_typeck_label_to);
                 diag.span_label(span, msg);
-                let msg = f(diag, crate::fluent_generated::hir_typeck_note);
+                let msg = diag.eagerly_translate(fluent::hir_typeck_note);
                 diag.note(msg);
             }
             CastUnknownPointerSub::From(span) => {
-                let msg = f(diag, crate::fluent_generated::hir_typeck_label_from);
+                let msg = diag.eagerly_translate(fluent::hir_typeck_label_from);
                 diag.span_label(span, msg);
             }
         }
diff --git a/compiler/rustc_hir_typeck/src/expr.rs b/compiler/rustc_hir_typeck/src/expr.rs
index de2f039cb1c..d6a7bfc446a 100644
--- a/compiler/rustc_hir_typeck/src/expr.rs
+++ b/compiler/rustc_hir_typeck/src/expr.rs
@@ -1600,11 +1600,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 Ok(method)
             }
             Err(error) => {
-                if segment.ident.name == kw::Empty {
-                    span_bug!(rcvr.span, "empty method name")
-                } else {
-                    Err(self.report_method_error(expr.hir_id, rcvr_t, error, expected, false))
-                }
+                Err(self.report_method_error(expr.hir_id, rcvr_t, error, expected, false))
             }
         };
 
@@ -2588,9 +2584,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 .into_iter()
                 .flat_map(|i| self.tcx.associated_items(i).in_definition_order())
                 // Only assoc fn with no receivers.
-                .filter(|item| {
-                    matches!(item.kind, ty::AssocKind::Fn) && !item.fn_has_self_parameter
-                })
+                .filter(|item| item.is_fn() && !item.is_method())
                 .filter_map(|item| {
                     // Only assoc fns that return `Self`
                     let fn_sig = self.tcx.fn_sig(item.def_id).skip_binder();
@@ -2603,8 +2597,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         return None;
                     }
                     let input_len = fn_sig.inputs().skip_binder().len();
-                    let order = !item.name.as_str().starts_with("new");
-                    Some((order, item.name, input_len))
+                    let name = item.name();
+                    let order = !name.as_str().starts_with("new");
+                    Some((order, name, input_len))
                 })
                 .collect::<Vec<_>>();
             items.sort_by_key(|(order, _, _)| *order);
@@ -2942,9 +2937,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return Ty::new_error(self.tcx(), guar);
         }
 
-        let guar = if field.name == kw::Empty {
-            self.dcx().span_bug(field.span, "field name with no name")
-        } else if self.method_exists_for_diagnostic(
+        let guar = if self.method_exists_for_diagnostic(
             field,
             base_ty,
             expr.hir_id,
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
index 91190a32ff5..87d92b3fbde 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
@@ -27,9 +27,9 @@ use rustc_middle::ty::{
 };
 use rustc_middle::{bug, span_bug};
 use rustc_session::lint;
+use rustc_span::Span;
 use rustc_span::def_id::LocalDefId;
 use rustc_span::hygiene::DesugaringKind;
-use rustc_span::{Span, kw};
 use rustc_trait_selection::error_reporting::infer::need_type_info::TypeAnnotationNeeded;
 use rustc_trait_selection::traits::{
     self, NormalizeExt, ObligationCauseCode, StructurallyNormalizeExt,
@@ -833,7 +833,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
                 let trait_missing_method =
                     matches!(error, method::MethodError::NoMatch(_)) && ty.normalized.is_trait();
-                assert_ne!(item_name.name, kw::Empty);
                 self.report_method_error(
                     hir_id,
                     ty.normalized,
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
index 81eb8510785..da0e8e362d6 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
@@ -616,7 +616,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 if let Some((DefKind::AssocFn, def_id)) =
                     self.typeck_results.borrow().type_dependent_def(call_expr.hir_id)
                     && let Some(assoc) = tcx.opt_associated_item(def_id)
-                    && assoc.fn_has_self_parameter
+                    && assoc.is_method()
                 {
                     Some(*receiver)
                 } else {
@@ -642,8 +642,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     TraitsInScope,
                     |mut ctxt| ctxt.probe_for_similar_candidate(),
                 )
-                && let ty::AssocKind::Fn = assoc.kind
-                && assoc.fn_has_self_parameter
+                && assoc.is_method()
             {
                 let args = self.infcx.fresh_args_for_item(call_name.span, assoc.def_id);
                 let fn_sig = tcx.fn_sig(assoc.def_id).instantiate(tcx, args);
@@ -684,10 +683,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     .all(|(expected, found)| self.may_coerce(*expected, *found))
                 && fn_sig.inputs()[1..].len() == input_types.len()
             {
+                let assoc_name = assoc.name();
                 err.span_suggestion_verbose(
                     call_name.span,
-                    format!("you might have meant to use `{}`", assoc.name),
-                    assoc.name,
+                    format!("you might have meant to use `{}`", assoc_name),
+                    assoc_name,
                     Applicability::MaybeIncorrect,
                 );
                 return;
@@ -707,7 +707,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     tcx.def_span(assoc.def_id),
                     format!(
                         "there's is a method with similar name `{}`, but the arguments don't match",
-                        assoc.name,
+                        assoc.name(),
                     ),
                 );
                 return;
@@ -719,7 +719,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     format!(
                         "there's is a method with similar name `{}`, but their argument count \
                          doesn't match",
-                        assoc.name,
+                        assoc.name(),
                     ),
                 );
                 return;
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs
index e14f1528d2c..74cc8181418 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs
@@ -314,7 +314,7 @@ impl<'tcx> HirTyLowerer<'tcx> for FnCtxt<'_, 'tcx> {
         item_def_id: DefId,
         item_segment: &rustc_hir::PathSegment<'tcx>,
         poly_trait_ref: ty::PolyTraitRef<'tcx>,
-        _kind: ty::AssocKind,
+        _assoc_tag: ty::AssocTag,
     ) -> Result<(DefId, ty::GenericArgsRef<'tcx>), ErrorGuaranteed> {
         let trait_ref = self.instantiate_binder_with_fresh_vars(
             span,
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs
index 912098c4e2d..91eb1989864 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs
@@ -381,9 +381,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             let mut suggestions = methods
                 .iter()
                 .filter_map(|conversion_method| {
+                    let conversion_method_name = conversion_method.name();
                     let receiver_method_ident = expr.method_ident();
                     if let Some(method_ident) = receiver_method_ident
-                        && method_ident.name == conversion_method.name
+                        && method_ident.name == conversion_method_name
                     {
                         return None; // do not suggest code that is already there (#53348)
                     }
@@ -391,20 +392,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     let method_call_list = [sym::to_vec, sym::to_string];
                     let mut sugg = if let ExprKind::MethodCall(receiver_method, ..) = expr.kind
                         && receiver_method.ident.name == sym::clone
-                        && method_call_list.contains(&conversion_method.name)
+                        && method_call_list.contains(&conversion_method_name)
                     // If receiver is `.clone()` and found type has one of those methods,
                     // we guess that the user wants to convert from a slice type (`&[]` or `&str`)
                     // to an owned type (`Vec` or `String`). These conversions clone internally,
                     // so we remove the user's `clone` call.
                     {
-                        vec![(receiver_method.ident.span, conversion_method.name.to_string())]
+                        vec![(receiver_method.ident.span, conversion_method_name.to_string())]
                     } else if expr.precedence() < ExprPrecedence::Unambiguous {
                         vec![
                             (expr.span.shrink_to_lo(), "(".to_string()),
-                            (expr.span.shrink_to_hi(), format!(").{}()", conversion_method.name)),
+                            (expr.span.shrink_to_hi(), format!(").{}()", conversion_method_name)),
                         ]
                     } else {
-                        vec![(expr.span.shrink_to_hi(), format!(".{}()", conversion_method.name))]
+                        vec![(expr.span.shrink_to_hi(), format!(".{}()", conversion_method_name))]
                     };
                     let struct_pat_shorthand_field =
                         self.tcx.hir_maybe_get_struct_pattern_shorthand_field(expr);
diff --git a/compiler/rustc_hir_typeck/src/lib.rs b/compiler/rustc_hir_typeck/src/lib.rs
index 1d86ff14471..af8ec373934 100644
--- a/compiler/rustc_hir_typeck/src/lib.rs
+++ b/compiler/rustc_hir_typeck/src/lib.rs
@@ -1,7 +1,6 @@
 // tidy-alphabetical-start
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![feature(array_windows)]
 #![feature(box_patterns)]
 #![feature(if_let_guard)]
@@ -266,7 +265,7 @@ fn infer_type_if_missing<'tcx>(fcx: &FnCtxt<'_, 'tcx>, node: Node<'tcx>) -> Opti
     let expected_type = if let Some(&hir::Ty { kind: hir::TyKind::Infer(()), span, .. }) = node.ty()
     {
         if let Some(item) = tcx.opt_associated_item(def_id.into())
-            && let ty::AssocKind::Const = item.kind
+            && let ty::AssocKind::Const { .. } = item.kind
             && let ty::AssocItemContainer::Impl = item.container
             && let Some(trait_item_def_id) = item.trait_item_def_id
         {
diff --git a/compiler/rustc_hir_typeck/src/method/mod.rs b/compiler/rustc_hir_typeck/src/method/mod.rs
index ddfd27ccf6b..1b67e2306aa 100644
--- a/compiler/rustc_hir_typeck/src/method/mod.rs
+++ b/compiler/rustc_hir_typeck/src/method/mod.rs
@@ -379,7 +379,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         };
 
         let def_id = method_item.def_id;
-        if method_item.kind != ty::AssocKind::Fn {
+        if !method_item.is_fn() {
             span_bug!(tcx.def_span(def_id), "expected `{m_name}` to be an associated function");
         }
 
@@ -529,7 +529,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
         }
 
-        let def_kind = pick.item.kind.as_def_kind();
+        let def_kind = pick.item.as_def_kind();
         tcx.check_stability(pick.item.def_id, Some(expr_id), span, Some(method_name.span));
         Ok((def_kind, pick.item.def_id))
     }
diff --git a/compiler/rustc_hir_typeck/src/method/probe.rs b/compiler/rustc_hir_typeck/src/method/probe.rs
index 0a01ec89a32..ba4396a5ab3 100644
--- a/compiler/rustc_hir_typeck/src/method/probe.rs
+++ b/compiler/rustc_hir_typeck/src/method/probe.rs
@@ -992,7 +992,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
 
     fn matches_return_type(&self, method: ty::AssocItem, expected: Ty<'tcx>) -> bool {
         match method.kind {
-            ty::AssocKind::Fn => self.probe(|_| {
+            ty::AssocKind::Fn { .. } => self.probe(|_| {
                 let args = self.fresh_args_for_item(self.span, method.def_id);
                 let fty = self.tcx.fn_sig(method.def_id).instantiate(self.tcx, args);
                 let fty = self.instantiate_binder_with_fresh_vars(self.span, infer::FnCall, fty);
@@ -1583,7 +1583,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                 },
                 None,
             ) {
-                self.private_candidate.set(Some((pick.item.kind.as_def_kind(), pick.item.def_id)));
+                self.private_candidate.set(Some((pick.item.as_def_kind(), pick.item.def_id)));
             }
         }
         None
@@ -1671,16 +1671,7 @@ impl<'tcx> Pick<'tcx> {
     /// Do not use for type checking.
     pub(crate) fn differs_from(&self, other: &Self) -> bool {
         let Self {
-            item:
-                AssocItem {
-                    def_id,
-                    name: _,
-                    kind: _,
-                    container: _,
-                    trait_item_def_id: _,
-                    fn_has_self_parameter: _,
-                    opt_rpitit_info: _,
-                },
+            item: AssocItem { def_id, kind: _, container: _, trait_item_def_id: _ },
             kind: _,
             import_ids: _,
             autoderefs: _,
@@ -1703,7 +1694,7 @@ impl<'tcx> Pick<'tcx> {
         if self.unstable_candidates.is_empty() {
             return;
         }
-        let def_kind = self.item.kind.as_def_kind();
+        let def_kind = self.item.as_def_kind();
         tcx.node_span_lint(lint::builtin::UNSTABLE_NAME_COLLISIONS, scope_expr_id, span, |lint| {
             lint.primary_message(format!(
                 "{} {} with this name may be added to the standard library in the future",
@@ -1712,7 +1703,7 @@ impl<'tcx> Pick<'tcx> {
             ));
 
             match (self.item.kind, self.item.container) {
-                (ty::AssocKind::Fn, _) => {
+                (ty::AssocKind::Fn { .. }, _) => {
                     // FIXME: This should be a `span_suggestion` instead of `help`
                     // However `self.span` only
                     // highlights the method name, so we can't use it. Also consider reusing
@@ -1723,17 +1714,12 @@ impl<'tcx> Pick<'tcx> {
                         tcx.def_path_str(self.item.def_id),
                     ));
                 }
-                (ty::AssocKind::Const, ty::AssocItemContainer::Trait) => {
+                (ty::AssocKind::Const { name }, ty::AssocItemContainer::Trait) => {
                     let def_id = self.item.container_id(tcx);
                     lint.span_suggestion(
                         span,
                         "use the fully qualified path to the associated const",
-                        format!(
-                            "<{} as {}>::{}",
-                            self.self_ty,
-                            tcx.def_path_str(def_id),
-                            self.item.name
-                        ),
+                        format!("<{} as {}>::{}", self.self_ty, tcx.def_path_str(def_id), name),
                         Applicability::MachineApplicable,
                     );
                 }
@@ -2222,7 +2208,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                 let best_name = {
                     let names = applicable_close_candidates
                         .iter()
-                        .map(|cand| cand.name)
+                        .map(|cand| cand.name())
                         .collect::<Vec<Symbol>>();
                     find_best_match_for_name_with_substrings(
                         &names,
@@ -2234,10 +2220,12 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                     applicable_close_candidates
                         .iter()
                         .find(|cand| self.matches_by_doc_alias(cand.def_id))
-                        .map(|cand| cand.name)
+                        .map(|cand| cand.name())
                 });
                 Ok(best_name.and_then(|best_name| {
-                    applicable_close_candidates.into_iter().find(|method| method.name == best_name)
+                    applicable_close_candidates
+                        .into_iter()
+                        .find(|method| method.name() == best_name)
                 }))
             }
         })
@@ -2252,10 +2240,10 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         // In Path mode (i.e., resolving a value like `T::next`), consider any
         // associated value (i.e., methods, constants) but not types.
         match self.mode {
-            Mode::MethodCall => item.fn_has_self_parameter,
+            Mode::MethodCall => item.is_method(),
             Mode::Path => match item.kind {
-                ty::AssocKind::Type => false,
-                ty::AssocKind::Fn | ty::AssocKind::Const => true,
+                ty::AssocKind::Type { .. } => false,
+                ty::AssocKind::Fn { .. } | ty::AssocKind::Const { .. } => true,
             },
         }
         // FIXME -- check for types that deref to `Self`,
@@ -2277,7 +2265,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         impl_ty: Ty<'tcx>,
         args: GenericArgsRef<'tcx>,
     ) -> (Ty<'tcx>, Option<Ty<'tcx>>) {
-        if item.kind == ty::AssocKind::Fn && self.mode == Mode::MethodCall {
+        if item.is_fn() && self.mode == Mode::MethodCall {
             let sig = self.xform_method_sig(item.def_id, args);
             (sig.inputs()[0], Some(sig.output()))
         } else {
@@ -2328,8 +2316,8 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
     /// Determine if the given associated item type is relevant in the current context.
     fn is_relevant_kind_for_mode(&self, kind: ty::AssocKind) -> bool {
         match (self.mode, kind) {
-            (Mode::MethodCall, ty::AssocKind::Fn) => true,
-            (Mode::Path, ty::AssocKind::Const | ty::AssocKind::Fn) => true,
+            (Mode::MethodCall, ty::AssocKind::Fn { .. }) => true,
+            (Mode::Path, ty::AssocKind::Const { .. } | ty::AssocKind::Fn { .. }) => true,
             _ => false,
         }
     }
@@ -2411,7 +2399,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                         }
                         match edit_distance_with_substrings(
                             name.as_str(),
-                            x.name.as_str(),
+                            x.name().as_str(),
                             max_dist,
                         ) {
                             Some(d) => d > 0,
diff --git a/compiler/rustc_hir_typeck/src/method/suggest.rs b/compiler/rustc_hir_typeck/src/method/suggest.rs
index 68f13d654d6..6a9fd7cdd48 100644
--- a/compiler/rustc_hir_typeck/src/method/suggest.rs
+++ b/compiler/rustc_hir_typeck/src/method/suggest.rs
@@ -713,7 +713,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 && let Some(candidate) = tcx.associated_items(impl_def_id).find_by_ident_and_kind(
                     self.tcx,
                     item_ident,
-                    ty::AssocKind::Type,
+                    ty::AssocTag::Type,
                     impl_def_id,
                 )
                 && let Some(adt_def) = tcx.type_of(candidate.def_id).skip_binder().ty_adt_def()
@@ -1442,7 +1442,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             if let Some(assoc) = self.associated_value(*def_id, item_ident) {
                                 // Check for both mode is the same so we avoid suggesting
                                 // incorrect associated item.
-                                match (mode, assoc.fn_has_self_parameter, source) {
+                                match (mode, assoc.is_method(), source) {
                                     (Mode::MethodCall, true, SelfSource::MethodCall(_)) => {
                                         // We check that the suggest type is actually
                                         // different from the received one
@@ -1722,7 +1722,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // that had unsatisfied trait bounds
             if unsatisfied_predicates.is_empty()
                 // ...or if we already suggested that name because of `rustc_confusable` annotation.
-                && Some(similar_candidate.name) != confusable_suggested
+                && Some(similar_candidate.name()) != confusable_suggested
             {
                 self.find_likely_intended_associated_item(
                     &mut err,
@@ -1819,12 +1819,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         mode: Mode,
     ) {
         let tcx = self.tcx;
-        let def_kind = similar_candidate.kind.as_def_kind();
+        let def_kind = similar_candidate.as_def_kind();
         let an = self.tcx.def_kind_descr_article(def_kind, similar_candidate.def_id);
+        let similar_candidate_name = similar_candidate.name();
         let msg = format!(
             "there is {an} {} `{}` with a similar name",
             self.tcx.def_kind_descr(def_kind, similar_candidate.def_id),
-            similar_candidate.name,
+            similar_candidate_name,
         );
         // Methods are defined within the context of a struct and their first parameter
         // is always `self`, which represents the instance of the struct the method is
@@ -1834,7 +1835,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             let ty_args = self.infcx.fresh_args_for_item(span, similar_candidate.def_id);
             let fn_sig = tcx.fn_sig(similar_candidate.def_id).instantiate(tcx, ty_args);
             let fn_sig = self.instantiate_binder_with_fresh_vars(span, infer::FnCall, fn_sig);
-            if similar_candidate.fn_has_self_parameter {
+            if similar_candidate.is_method() {
                 if let Some(args) = args
                     && fn_sig.inputs()[1..].len() == args.len()
                 {
@@ -1843,7 +1844,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     err.span_suggestion_verbose(
                         span,
                         msg,
-                        similar_candidate.name,
+                        similar_candidate_name,
                         Applicability::MaybeIncorrect,
                     );
                 } else {
@@ -1865,7 +1866,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 err.span_suggestion_verbose(
                     span,
                     msg,
-                    similar_candidate.name,
+                    similar_candidate_name,
                     Applicability::MaybeIncorrect,
                 );
             } else {
@@ -1878,7 +1879,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             err.span_suggestion_verbose(
                 span,
                 msg,
-                similar_candidate.name,
+                similar_candidate_name,
                 Applicability::MaybeIncorrect,
             );
         } else {
@@ -1902,7 +1903,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 {
                     if let Some(candidates) = find_attr!(self.tcx.get_all_attrs(inherent_method.def_id), AttributeKind::Confusables{symbols, ..} => symbols)
                         && candidates.contains(&item_name.name)
-                        && let ty::AssocKind::Fn = inherent_method.kind
+                        && inherent_method.is_fn()
                     {
                         let args =
                             ty::GenericArgs::identity_for_item(self.tcx, inherent_method.def_id)
@@ -1918,6 +1919,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             infer::FnCall,
                             fn_sig,
                         );
+                        let name = inherent_method.name();
                         if let Some(ref args) = call_args
                             && fn_sig.inputs()[1..]
                                 .iter()
@@ -1927,20 +1929,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         {
                             err.span_suggestion_verbose(
                                 item_name.span,
-                                format!("you might have meant to use `{}`", inherent_method.name),
-                                inherent_method.name,
+                                format!("you might have meant to use `{}`", name),
+                                name,
                                 Applicability::MaybeIncorrect,
                             );
-                            return Some(inherent_method.name);
+                            return Some(name);
                         } else if let None = call_args {
                             err.span_note(
                                 self.tcx.def_span(inherent_method.def_id),
-                                format!(
-                                    "you might have meant to use method `{}`",
-                                    inherent_method.name,
-                                ),
+                                format!("you might have meant to use method `{}`", name),
                             );
-                            return Some(inherent_method.name);
+                            return Some(name);
                         }
                     }
                 }
@@ -2116,8 +2115,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // Only assoc fn with no receivers and only if
             // they are resolvable
             .filter(|item| {
-                matches!(item.kind, ty::AssocKind::Fn)
-                    && !item.fn_has_self_parameter
+                matches!(item.kind, ty::AssocKind::Fn { has_self: false, .. })
                     && self
                         .probe_for_name(
                             Mode::Path,
@@ -2261,7 +2259,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 };
 
                 let assoc = self.associated_value(assoc_did, item_name)?;
-                if assoc.kind != ty::AssocKind::Fn {
+                if !assoc.is_fn() {
                     return None;
                 }
 
@@ -3208,7 +3206,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     // If this method receives `&self`, then the provided
                     // argument _should_ coerce, so it's valid to suggest
                     // just changing the path.
-                    && pick.item.fn_has_self_parameter
+                    && pick.item.is_method()
                     && let Some(self_ty) =
                         self.tcx.fn_sig(pick.item.def_id).instantiate_identity().inputs().skip_binder().get(0)
                     && self_ty.is_ref()
@@ -3560,7 +3558,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             || (("Pin::new" == *pre)
                                 && ((sym::as_ref == item_name.name) || !unpin))
                             || inputs_len.is_some_and(|inputs_len| {
-                                pick.item.kind == ty::AssocKind::Fn
+                                pick.item.is_fn()
                                     && self
                                         .tcx
                                         .fn_sig(pick.item.def_id)
@@ -3618,7 +3616,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     && pick.autoderefs == 0
                     // Check that the method of the same name that was found on the new `Pin<T>`
                     // receiver has the same number of arguments that appear in the user's code.
-                    && inputs_len.is_some_and(|inputs_len| pick.item.kind == ty::AssocKind::Fn && self.tcx.fn_sig(pick.item.def_id).skip_binder().skip_binder().inputs().len() == inputs_len)
+                    && inputs_len.is_some_and(|inputs_len| pick.item.is_fn() && self.tcx.fn_sig(pick.item.def_id).skip_binder().skip_binder().inputs().len() == inputs_len)
                 {
                     let indent = self
                         .tcx
@@ -3756,7 +3754,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     && self
                         .associated_value(info.def_id, item_name)
                         .filter(|item| {
-                            if let ty::AssocKind::Fn = item.kind {
+                            if item.is_fn() {
                                 let id = item
                                     .def_id
                                     .as_local()
@@ -4279,17 +4277,17 @@ fn print_disambiguation_help<'tcx>(
     item: ty::AssocItem,
 ) -> Option<String> {
     let trait_impl_type = trait_ref.self_ty().peel_refs();
-    let trait_ref = if item.fn_has_self_parameter {
+    let trait_ref = if item.is_method() {
         trait_ref.print_only_trait_name().to_string()
     } else {
         format!("<{} as {}>", trait_ref.args[0], trait_ref.print_only_trait_name())
     };
     Some(
-        if matches!(item.kind, ty::AssocKind::Fn)
+        if item.is_fn()
             && let SelfSource::MethodCall(receiver) = source
             && let Some(args) = args
         {
-            let def_kind_descr = tcx.def_kind_descr(item.kind.as_def_kind(), item.def_id);
+            let def_kind_descr = tcx.def_kind_descr(item.as_def_kind(), item.def_id);
             let item_name = item.ident(tcx);
             let first_input =
                 tcx.fn_sig(item.def_id).instantiate_identity().skip_binder().inputs().get(0);
@@ -4304,7 +4302,7 @@ fn print_disambiguation_help<'tcx>(
             let args = if let Some(first_arg_type) = first_arg_type
                 && (first_arg_type == tcx.types.self_param
                     || first_arg_type == trait_impl_type
-                    || item.fn_has_self_parameter)
+                    || item.is_method())
             {
                 Some(receiver)
             } else {
diff --git a/compiler/rustc_hir_typeck/src/op.rs b/compiler/rustc_hir_typeck/src/op.rs
index 93f77b8409f..0e42a84ca32 100644
--- a/compiler/rustc_hir_typeck/src/op.rs
+++ b/compiler/rustc_hir_typeck/src/op.rs
@@ -372,7 +372,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                 .associated_item_def_ids(def_id)
                                 .iter()
                                 .find(|item_def_id| {
-                                    self.tcx.associated_item(*item_def_id).name == sym::Output
+                                    self.tcx.associated_item(*item_def_id).name() == sym::Output
                                 })
                                 .cloned()
                         });
diff --git a/compiler/rustc_incremental/src/lib.rs b/compiler/rustc_incremental/src/lib.rs
index dabfb6a90ca..299ee487638 100644
--- a/compiler/rustc_incremental/src/lib.rs
+++ b/compiler/rustc_incremental/src/lib.rs
@@ -2,7 +2,6 @@
 
 // tidy-alphabetical-start
 #![allow(internal_features)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![deny(missing_docs)]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
diff --git a/compiler/rustc_incremental/src/persist/fs.rs b/compiler/rustc_incremental/src/persist/fs.rs
index 76a1ff3cf38..f0d24d27e85 100644
--- a/compiler/rustc_incremental/src/persist/fs.rs
+++ b/compiler/rustc_incremental/src/persist/fs.rs
@@ -290,7 +290,7 @@ pub(crate) fn prepare_session_directory(sess: &Session, crate_name: Symbol) {
 
             // Try to remove the session directory we just allocated. We don't
             // know if there's any garbage in it from the failed copy action.
-            if let Err(err) = safe_remove_dir_all(&session_dir) {
+            if let Err(err) = std_fs::remove_dir_all(&session_dir) {
                 sess.dcx().emit_warn(errors::DeletePartial { path: &session_dir, err });
             }
 
@@ -324,7 +324,7 @@ pub fn finalize_session_directory(sess: &Session, svh: Option<Svh>) {
             incr_comp_session_dir.display()
         );
 
-        if let Err(err) = safe_remove_dir_all(&*incr_comp_session_dir) {
+        if let Err(err) = std_fs::remove_dir_all(&*incr_comp_session_dir) {
             sess.dcx().emit_warn(errors::DeleteFull { path: &incr_comp_session_dir, err });
         }
 
@@ -715,7 +715,7 @@ pub(crate) fn garbage_collect_session_directories(sess: &Session) -> io::Result<
     for directory_name in session_directories {
         if !lock_file_to_session_dir.items().any(|(_, dir)| *dir == directory_name) {
             let path = crate_directory.join(directory_name);
-            if let Err(err) = safe_remove_dir_all(&path) {
+            if let Err(err) = std_fs::remove_dir_all(&path) {
                 sess.dcx().emit_warn(errors::InvalidGcFailed { path: &path, err });
             }
         }
@@ -821,7 +821,7 @@ pub(crate) fn garbage_collect_session_directories(sess: &Session) -> io::Result<
     all_except_most_recent(deletion_candidates).into_items().all(|(path, lock)| {
         debug!("garbage_collect_session_directories() - deleting `{}`", path.display());
 
-        if let Err(err) = safe_remove_dir_all(&path) {
+        if let Err(err) = std_fs::remove_dir_all(&path) {
             sess.dcx().emit_warn(errors::FinalizedGcFailed { path: &path, err });
         } else {
             delete_session_dir_lock_file(sess, &lock_file_path(&path));
@@ -839,7 +839,7 @@ pub(crate) fn garbage_collect_session_directories(sess: &Session) -> io::Result<
 fn delete_old(sess: &Session, path: &Path) {
     debug!("garbage_collect_session_directories() - deleting `{}`", path.display());
 
-    if let Err(err) = safe_remove_dir_all(path) {
+    if let Err(err) = std_fs::remove_dir_all(path) {
         sess.dcx().emit_warn(errors::SessionGcFailed { path, err });
     } else {
         delete_session_dir_lock_file(sess, &lock_file_path(path));
@@ -862,30 +862,8 @@ fn all_except_most_recent(
     }
 }
 
-/// Since paths of artifacts within session directories can get quite long, we
-/// need to support deleting files with very long paths. The regular
-/// WinApi functions only support paths up to 260 characters, however. In order
-/// to circumvent this limitation, we canonicalize the path of the directory
-/// before passing it to std::fs::remove_dir_all(). This will convert the path
-/// into the '\\?\' format, which supports much longer paths.
-fn safe_remove_dir_all(p: &Path) -> io::Result<()> {
-    let canonicalized = match try_canonicalize(p) {
-        Ok(canonicalized) => canonicalized,
-        Err(err) if err.kind() == io::ErrorKind::NotFound => return Ok(()),
-        Err(err) => return Err(err),
-    };
-
-    std_fs::remove_dir_all(canonicalized)
-}
-
 fn safe_remove_file(p: &Path) -> io::Result<()> {
-    let canonicalized = match try_canonicalize(p) {
-        Ok(canonicalized) => canonicalized,
-        Err(err) if err.kind() == io::ErrorKind::NotFound => return Ok(()),
-        Err(err) => return Err(err),
-    };
-
-    match std_fs::remove_file(canonicalized) {
+    match std_fs::remove_file(p) {
         Err(err) if err.kind() == io::ErrorKind::NotFound => Ok(()),
         result => result,
     }
diff --git a/compiler/rustc_index_macros/src/newtype.rs b/compiler/rustc_index_macros/src/newtype.rs
index f0b58eabbff..eedbe630cf2 100644
--- a/compiler/rustc_index_macros/src/newtype.rs
+++ b/compiler/rustc_index_macros/src/newtype.rs
@@ -257,6 +257,13 @@ impl Parse for Newtype {
                 }
             }
 
+            impl std::ops::AddAssign<usize> for #name {
+                #[inline]
+                fn add_assign(&mut self, other: usize) {
+                    *self = *self + other;
+                }
+            }
+
             impl rustc_index::Idx for #name {
                 #[inline]
                 fn new(value: usize) -> Self {
diff --git a/compiler/rustc_infer/src/infer/region_constraints/mod.rs b/compiler/rustc_infer/src/infer/region_constraints/mod.rs
index 8366aa6ec42..40e2e654b2e 100644
--- a/compiler/rustc_infer/src/infer/region_constraints/mod.rs
+++ b/compiler/rustc_infer/src/infer/region_constraints/mod.rs
@@ -618,9 +618,7 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
             RegionVid::from(value_count)..RegionVid::from(self.storage.unification_table.len());
         (
             range.clone(),
-            (range.start.index()..range.end.index())
-                .map(|index| self.storage.var_infos[ty::RegionVid::from(index)].origin)
-                .collect(),
+            (range.start..range.end).map(|index| self.storage.var_infos[index].origin).collect(),
         )
     }
 
diff --git a/compiler/rustc_infer/src/infer/snapshot/fudge.rs b/compiler/rustc_infer/src/infer/snapshot/fudge.rs
index b5d3c26b05e..39c8c40ea7d 100644
--- a/compiler/rustc_infer/src/infer/snapshot/fudge.rs
+++ b/compiler/rustc_infer/src/infer/snapshot/fudge.rs
@@ -30,11 +30,12 @@ fn const_vars_since_snapshot<'tcx>(
     snapshot_var_len: usize,
 ) -> (Range<ConstVid>, Vec<ConstVariableOrigin>) {
     let range = vars_since_snapshot(table, snapshot_var_len);
+    let range = range.start.vid..range.end.vid;
 
     (
-        range.start.vid..range.end.vid,
-        (range.start.index()..range.end.index())
-            .map(|index| match table.probe_value(ConstVid::from_u32(index)) {
+        range.clone(),
+        range
+            .map(|index| match table.probe_value(index) {
                 ConstVariableValue::Known { value: _ } => {
                     ConstVariableOrigin { param_def_id: None, span: rustc_span::DUMMY_SP }
                 }
diff --git a/compiler/rustc_infer/src/traits/mod.rs b/compiler/rustc_infer/src/traits/mod.rs
index b537750f1b5..6d5ad96e31c 100644
--- a/compiler/rustc_infer/src/traits/mod.rs
+++ b/compiler/rustc_infer/src/traits/mod.rs
@@ -12,6 +12,7 @@ use std::hash::{Hash, Hasher};
 
 use hir::def_id::LocalDefId;
 use rustc_hir as hir;
+use rustc_macros::{TypeFoldable, TypeVisitable};
 use rustc_middle::traits::query::NoSolution;
 use rustc_middle::traits::solve::Certainty;
 pub use rustc_middle::traits::*;
@@ -35,9 +36,11 @@ use crate::infer::InferCtxt;
 /// either identifying an `impl` (e.g., `impl Eq for i32`) that
 /// satisfies the obligation, or else finding a bound that is in
 /// scope. The eventual result is usually a `Selection` (defined below).
-#[derive(Clone)]
+#[derive(Clone, TypeFoldable, TypeVisitable)]
 pub struct Obligation<'tcx, T> {
     /// The reason we have to prove this thing.
+    #[type_foldable(identity)]
+    #[type_visitable(ignore)]
     pub cause: ObligationCause<'tcx>,
 
     /// The environment in which we should prove this thing.
@@ -51,6 +54,8 @@ pub struct Obligation<'tcx, T> {
     /// If this goes over a certain threshold, we abort compilation --
     /// in such cases, we can not say whether or not the predicate
     /// holds for certain. Stupid halting problem; such a drag.
+    #[type_foldable(identity)]
+    #[type_visitable(ignore)]
     pub recursion_depth: usize,
 }
 
diff --git a/compiler/rustc_infer/src/traits/structural_impls.rs b/compiler/rustc_infer/src/traits/structural_impls.rs
index 4335073d9bc..03661ebf7ec 100644
--- a/compiler/rustc_infer/src/traits/structural_impls.rs
+++ b/compiler/rustc_infer/src/traits/structural_impls.rs
@@ -1,8 +1,6 @@
 use std::fmt;
 
-use rustc_middle::ty::{
-    self, FallibleTypeFolder, TyCtxt, TypeFoldable, TypeVisitable, TypeVisitor, try_visit,
-};
+use rustc_middle::ty;
 
 use crate::traits;
 use crate::traits::project::Normalized;
@@ -34,31 +32,3 @@ impl<'tcx> fmt::Debug for traits::MismatchedProjectionTypes<'tcx> {
         write!(f, "MismatchedProjectionTypes({:?})", self.err)
     }
 }
-
-///////////////////////////////////////////////////////////////////////////
-// TypeFoldable implementations.
-
-impl<'tcx, O: TypeFoldable<TyCtxt<'tcx>>> TypeFoldable<TyCtxt<'tcx>>
-    for traits::Obligation<'tcx, O>
-{
-    fn try_fold_with<F: FallibleTypeFolder<TyCtxt<'tcx>>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
-        Ok(traits::Obligation {
-            cause: self.cause,
-            recursion_depth: self.recursion_depth,
-            predicate: self.predicate.try_fold_with(folder)?,
-            param_env: self.param_env.try_fold_with(folder)?,
-        })
-    }
-}
-
-impl<'tcx, O: TypeVisitable<TyCtxt<'tcx>>> TypeVisitable<TyCtxt<'tcx>>
-    for traits::Obligation<'tcx, O>
-{
-    fn visit_with<V: TypeVisitor<TyCtxt<'tcx>>>(&self, visitor: &mut V) -> V::Result {
-        try_visit!(self.predicate.visit_with(visitor));
-        self.param_env.visit_with(visitor)
-    }
-}
diff --git a/compiler/rustc_interface/src/tests.rs b/compiler/rustc_interface/src/tests.rs
index 75a1b61b416..d405d044cae 100644
--- a/compiler/rustc_interface/src/tests.rs
+++ b/compiler/rustc_interface/src/tests.rs
@@ -614,6 +614,7 @@ fn test_codegen_options_tracking_hash() {
     tracked!(control_flow_guard, CFGuard::Checks);
     tracked!(debug_assertions, Some(true));
     tracked!(debuginfo, DebugInfo::Limited);
+    tracked!(dwarf_version, Some(5));
     tracked!(embed_bitcode, false);
     tracked!(force_frame_pointers, FramePointer::Always);
     tracked!(force_unwind_tables, Some(true));
diff --git a/compiler/rustc_lint/messages.ftl b/compiler/rustc_lint/messages.ftl
index 782d328a951..60c183bd56b 100644
--- a/compiler/rustc_lint/messages.ftl
+++ b/compiler/rustc_lint/messages.ftl
@@ -271,7 +271,7 @@ lint_expectation = this lint expectation is unfulfilled
 lint_extern_crate_not_idiomatic = `extern crate` is not idiomatic in the new edition
     .suggestion = convert it to a `use`
 
-lint_extern_without_abi = extern declarations without an explicit ABI are deprecated
+lint_extern_without_abi = `extern` declarations without an explicit ABI are deprecated
     .label = ABI should be specified here
     .suggestion = explicitly specify the {$default_abi} ABI
 
diff --git a/compiler/rustc_lint/src/context.rs b/compiler/rustc_lint/src/context.rs
index 16c9e08c78d..3660bb3f780 100644
--- a/compiler/rustc_lint/src/context.rs
+++ b/compiler/rustc_lint/src/context.rs
@@ -83,11 +83,6 @@ enum TargetLint {
     Ignored,
 }
 
-pub enum FindLintError {
-    NotFound,
-    Removed,
-}
-
 struct LintAlias {
     name: &'static str,
     /// Whether deprecation warnings should be suppressed for this alias.
@@ -231,13 +226,24 @@ impl LintStore {
         }
     }
 
-    pub fn register_group_alias(&mut self, lint_name: &'static str, alias: &'static str) {
-        self.lint_groups.insert(
+    fn insert_group(&mut self, name: &'static str, group: LintGroup) {
+        let previous = self.lint_groups.insert(name, group);
+        if previous.is_some() {
+            bug!("group {name:?} already exists");
+        }
+    }
+
+    pub fn register_group_alias(&mut self, group_name: &'static str, alias: &'static str) {
+        let Some(LintGroup { lint_ids, .. }) = self.lint_groups.get(group_name) else {
+            bug!("group alias {alias:?} points to unregistered group {group_name:?}")
+        };
+
+        self.insert_group(
             alias,
             LintGroup {
-                lint_ids: vec![],
+                lint_ids: lint_ids.clone(),
                 is_externally_loaded: false,
-                depr: Some(LintAlias { name: lint_name, silent: true }),
+                depr: Some(LintAlias { name: group_name, silent: true }),
             },
         );
     }
@@ -249,24 +255,17 @@ impl LintStore {
         deprecated_name: Option<&'static str>,
         to: Vec<LintId>,
     ) {
-        let new = self
-            .lint_groups
-            .insert(name, LintGroup { lint_ids: to, is_externally_loaded, depr: None })
-            .is_none();
         if let Some(deprecated) = deprecated_name {
-            self.lint_groups.insert(
+            self.insert_group(
                 deprecated,
                 LintGroup {
-                    lint_ids: vec![],
+                    lint_ids: to.clone(),
                     is_externally_loaded,
                     depr: Some(LintAlias { name, silent: false }),
                 },
             );
         }
-
-        if !new {
-            bug!("duplicate specification of lint group {}", name);
-        }
+        self.insert_group(name, LintGroup { lint_ids: to, is_externally_loaded, depr: None });
     }
 
     /// This lint should give no warning and have no effect.
@@ -292,23 +291,15 @@ impl LintStore {
         self.by_name.insert(name.into(), Removed(reason.into()));
     }
 
-    pub fn find_lints(&self, mut lint_name: &str) -> Result<Vec<LintId>, FindLintError> {
+    pub fn find_lints(&self, lint_name: &str) -> Option<&[LintId]> {
         match self.by_name.get(lint_name) {
-            Some(&Id(lint_id)) => Ok(vec![lint_id]),
-            Some(&Renamed(_, lint_id)) => Ok(vec![lint_id]),
-            Some(&Removed(_)) => Err(FindLintError::Removed),
-            Some(&Ignored) => Ok(vec![]),
-            None => loop {
-                return match self.lint_groups.get(lint_name) {
-                    Some(LintGroup { lint_ids, depr, .. }) => {
-                        if let Some(LintAlias { name, .. }) = depr {
-                            lint_name = name;
-                            continue;
-                        }
-                        Ok(lint_ids.clone())
-                    }
-                    None => Err(FindLintError::Removed),
-                };
+            Some(Id(lint_id)) => Some(slice::from_ref(lint_id)),
+            Some(Renamed(_, lint_id)) => Some(slice::from_ref(lint_id)),
+            Some(Removed(_)) => None,
+            Some(Ignored) => Some(&[]),
+            None => match self.lint_groups.get(lint_name) {
+                Some(LintGroup { lint_ids, .. }) => Some(lint_ids),
+                None => None,
             },
         }
     }
@@ -374,8 +365,12 @@ impl LintStore {
                             CheckLintNameResult::MissingTool
                         };
                     }
-                    Some(LintGroup { lint_ids, .. }) => {
-                        return CheckLintNameResult::Tool(lint_ids, None);
+                    Some(LintGroup { lint_ids, depr, .. }) => {
+                        return if let &Some(LintAlias { name, silent: false }) = depr {
+                            CheckLintNameResult::Tool(lint_ids, Some(name.to_string()))
+                        } else {
+                            CheckLintNameResult::Tool(lint_ids, None)
+                        };
                     }
                 },
                 Some(Id(id)) => return CheckLintNameResult::Tool(slice::from_ref(id), None),
@@ -393,15 +388,11 @@ impl LintStore {
                 None => self.check_tool_name_for_backwards_compat(&complete_name, "clippy"),
                 Some(LintGroup { lint_ids, depr, .. }) => {
                     // Check if the lint group name is deprecated
-                    if let Some(LintAlias { name, silent }) = depr {
-                        let LintGroup { lint_ids, .. } = self.lint_groups.get(name).unwrap();
-                        return if *silent {
-                            CheckLintNameResult::Ok(lint_ids)
-                        } else {
-                            CheckLintNameResult::Tool(lint_ids, Some((*name).to_string()))
-                        };
+                    if let &Some(LintAlias { name, silent: false }) = depr {
+                        CheckLintNameResult::Tool(lint_ids, Some(name.to_string()))
+                    } else {
+                        CheckLintNameResult::Ok(lint_ids)
                     }
-                    CheckLintNameResult::Ok(lint_ids)
                 }
             },
             Some(Id(id)) => CheckLintNameResult::Ok(slice::from_ref(id)),
@@ -412,7 +403,7 @@ impl LintStore {
     fn no_lint_suggestion(&self, lint_name: &str, tool_name: &str) -> CheckLintNameResult<'_> {
         let name_lower = lint_name.to_lowercase();
 
-        if lint_name.chars().any(char::is_uppercase) && self.find_lints(&name_lower).is_ok() {
+        if lint_name.chars().any(char::is_uppercase) && self.find_lints(&name_lower).is_some() {
             // First check if the lint name is (partly) in upper case instead of lower case...
             return CheckLintNameResult::NoLint(Some((Symbol::intern(&name_lower), false)));
         }
@@ -455,18 +446,8 @@ impl LintStore {
             None => match self.lint_groups.get(&*complete_name) {
                 // Now we are sure, that this lint exists nowhere
                 None => self.no_lint_suggestion(lint_name, tool_name),
-                Some(LintGroup { lint_ids, depr, .. }) => {
-                    // Reaching this would be weird, but let's cover this case anyway
-                    if let Some(LintAlias { name, silent }) = depr {
-                        let LintGroup { lint_ids, .. } = self.lint_groups.get(name).unwrap();
-                        if *silent {
-                            CheckLintNameResult::Tool(lint_ids, Some(complete_name))
-                        } else {
-                            CheckLintNameResult::Tool(lint_ids, Some((*name).to_string()))
-                        }
-                    } else {
-                        CheckLintNameResult::Tool(lint_ids, Some(complete_name))
-                    }
+                Some(LintGroup { lint_ids, .. }) => {
+                    CheckLintNameResult::Tool(lint_ids, Some(complete_name))
                 }
             },
             Some(Id(id)) => CheckLintNameResult::Tool(slice::from_ref(id), Some(complete_name)),
@@ -855,11 +836,11 @@ impl<'tcx> LateContext<'tcx> {
         &self,
         self_ty: Ty<'tcx>,
         trait_id: DefId,
-        name: &str,
+        name: Symbol,
     ) -> Option<Ty<'tcx>> {
         let tcx = self.tcx;
         tcx.associated_items(trait_id)
-            .find_by_ident_and_kind(tcx, Ident::from_str(name), ty::AssocKind::Type, trait_id)
+            .find_by_ident_and_kind(tcx, Ident::with_dummy_span(name), ty::AssocTag::Type, trait_id)
             .and_then(|assoc| {
                 let proj = Ty::new_projection(tcx, assoc.def_id, [self_ty]);
                 tcx.try_normalize_erasing_regions(self.typing_env(), proj).ok()
diff --git a/compiler/rustc_lint/src/deref_into_dyn_supertrait.rs b/compiler/rustc_lint/src/deref_into_dyn_supertrait.rs
index ec8f8441575..5989ef9519c 100644
--- a/compiler/rustc_lint/src/deref_into_dyn_supertrait.rs
+++ b/compiler/rustc_lint/src/deref_into_dyn_supertrait.rs
@@ -69,7 +69,7 @@ impl<'tcx> LateLintPass<'tcx> for DerefIntoDynSupertrait {
             && let ty::Dynamic(data, _, ty::Dyn) = self_ty.kind()
             && let Some(self_principal) = data.principal()
             // `<T as Deref>::Target` is `dyn target_principal`
-            && let Some(target) = cx.get_associated_type(self_ty, did, "Target")
+            && let Some(target) = cx.get_associated_type(self_ty, did, sym::Target)
             && let ty::Dynamic(data, _, ty::Dyn) = target.kind()
             && let Some(target_principal) = data.principal()
             // `target_principal` is a supertrait of `t_principal`
diff --git a/compiler/rustc_lint/src/errors.rs b/compiler/rustc_lint/src/errors.rs
index d109a5c9030..586e55c8055 100644
--- a/compiler/rustc_lint/src/errors.rs
+++ b/compiler/rustc_lint/src/errors.rs
@@ -1,5 +1,5 @@
 use rustc_errors::codes::*;
-use rustc_errors::{Diag, EmissionGuarantee, SubdiagMessageOp, Subdiagnostic};
+use rustc_errors::{Diag, EmissionGuarantee, Subdiagnostic};
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_session::lint::Level;
 use rustc_span::{Span, Symbol};
@@ -26,11 +26,7 @@ pub(crate) enum OverruledAttributeSub {
 }
 
 impl Subdiagnostic for OverruledAttributeSub {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         match self {
             OverruledAttributeSub::DefaultSource { id } => {
                 diag.note(fluent::lint_default_source);
diff --git a/compiler/rustc_lint/src/if_let_rescope.rs b/compiler/rustc_lint/src/if_let_rescope.rs
index 39ea8d8e324..a9b04511c6b 100644
--- a/compiler/rustc_lint/src/if_let_rescope.rs
+++ b/compiler/rustc_lint/src/if_let_rescope.rs
@@ -3,9 +3,7 @@ use std::ops::ControlFlow;
 
 use hir::intravisit::{self, Visitor};
 use rustc_ast::Recovered;
-use rustc_errors::{
-    Applicability, Diag, EmissionGuarantee, SubdiagMessageOp, Subdiagnostic, SuggestionStyle,
-};
+use rustc_errors::{Applicability, Diag, EmissionGuarantee, Subdiagnostic, SuggestionStyle};
 use rustc_hir::{self as hir, HirIdSet};
 use rustc_macros::{LintDiagnostic, Subdiagnostic};
 use rustc_middle::ty::adjustment::Adjust;
@@ -327,11 +325,7 @@ struct IfLetRescopeRewrite {
 }
 
 impl Subdiagnostic for IfLetRescopeRewrite {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         let mut suggestions = vec![];
         for match_head in self.match_heads {
             match match_head {
@@ -360,7 +354,7 @@ impl Subdiagnostic for IfLetRescopeRewrite {
                 .chain(repeat('}').take(closing_brackets.count))
                 .collect(),
         ));
-        let msg = f(diag, crate::fluent_generated::lint_suggestion);
+        let msg = diag.eagerly_translate(crate::fluent_generated::lint_suggestion);
         diag.multipart_suggestion_with_style(
             msg,
             suggestions,
diff --git a/compiler/rustc_lint/src/levels.rs b/compiler/rustc_lint/src/levels.rs
index d0b1e7bf255..00775647ac6 100644
--- a/compiler/rustc_lint/src/levels.rs
+++ b/compiler/rustc_lint/src/levels.rs
@@ -517,11 +517,11 @@ impl<'s, P: LintLevelsProvider> LintLevelsBuilder<'s, P> {
 
             let lint_flag_val = Symbol::intern(lint_name);
 
-            let Ok(ids) = self.store.find_lints(lint_name) else {
+            let Some(ids) = self.store.find_lints(lint_name) else {
                 // errors already handled above
                 continue;
             };
-            for id in ids {
+            for &id in ids {
                 // ForceWarn and Forbid cannot be overridden
                 if let Some(LevelAndSource { level: Level::ForceWarn | Level::Forbid, .. }) =
                     self.current_specs().get(&id)
diff --git a/compiler/rustc_lint/src/lib.rs b/compiler/rustc_lint/src/lib.rs
index 9b5c564d332..212368bea82 100644
--- a/compiler/rustc_lint/src/lib.rs
+++ b/compiler/rustc_lint/src/lib.rs
@@ -21,7 +21,6 @@
 
 // tidy-alphabetical-start
 #![allow(internal_features)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(array_windows)]
@@ -125,9 +124,7 @@ use unused::*;
 
 #[rustfmt::skip]
 pub use builtin::{MissingDoc, SoftLints};
-pub use context::{
-    CheckLintNameResult, EarlyContext, FindLintError, LateContext, LintContext, LintStore,
-};
+pub use context::{CheckLintNameResult, EarlyContext, LateContext, LintContext, LintStore};
 pub use early::{EarlyCheckNode, check_ast_node};
 pub use late::{check_crate, late_lint_mod, unerased_lint_store};
 pub use levels::LintLevelsBuilder;
@@ -606,6 +603,11 @@ fn register_builtins(store: &mut LintStore) {
         "converted into hard error, see issue #127323 \
          <https://github.com/rust-lang/rust/issues/127323> for more information",
     );
+    store.register_removed(
+        "undefined_naked_function_abi",
+        "converted into hard error, see PR #139001 \
+         <https://github.com/rust-lang/rust/issues/139001> for more information",
+    );
 }
 
 fn register_internals(store: &mut LintStore) {
diff --git a/compiler/rustc_lint/src/lints.rs b/compiler/rustc_lint/src/lints.rs
index 51214c8e8a4..8ab64fbd127 100644
--- a/compiler/rustc_lint/src/lints.rs
+++ b/compiler/rustc_lint/src/lints.rs
@@ -6,7 +6,7 @@ use rustc_abi::ExternAbi;
 use rustc_errors::codes::*;
 use rustc_errors::{
     Applicability, Diag, DiagArgValue, DiagMessage, DiagStyledString, ElidedLifetimeInPathSubdiag,
-    EmissionGuarantee, LintDiagnostic, MultiSpan, SubdiagMessageOp, Subdiagnostic, SuggestionStyle,
+    EmissionGuarantee, LintDiagnostic, MultiSpan, Subdiagnostic, SuggestionStyle,
 };
 use rustc_hir::def::Namespace;
 use rustc_hir::def_id::DefId;
@@ -449,11 +449,7 @@ pub(crate) struct BuiltinUnpermittedTypeInitSub {
 }
 
 impl Subdiagnostic for BuiltinUnpermittedTypeInitSub {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         let mut err = self.err;
         loop {
             if let Some(span) = err.span {
@@ -504,11 +500,7 @@ pub(crate) struct BuiltinClashingExternSub<'a> {
 }
 
 impl Subdiagnostic for BuiltinClashingExternSub<'_> {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         let mut expected_str = DiagStyledString::new();
         expected_str.push(self.expected.fn_sig(self.tcx).to_string(), false);
         let mut found_str = DiagStyledString::new();
@@ -824,11 +816,7 @@ pub(crate) struct HiddenUnicodeCodepointsDiagLabels {
 }
 
 impl Subdiagnostic for HiddenUnicodeCodepointsDiagLabels {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         for (c, span) in self.spans {
             diag.span_label(span, format!("{c:?}"));
         }
@@ -842,11 +830,7 @@ pub(crate) enum HiddenUnicodeCodepointsDiagSub {
 
 // Used because of multiple multipart_suggestion and note
 impl Subdiagnostic for HiddenUnicodeCodepointsDiagSub {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         match self {
             HiddenUnicodeCodepointsDiagSub::Escape { spans } => {
                 diag.multipart_suggestion_with_style(
@@ -1015,11 +999,7 @@ pub(crate) struct NonBindingLetSub {
 }
 
 impl Subdiagnostic for NonBindingLetSub {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         let can_suggest_binding = self.drop_fn_start_end.is_some() || !self.is_assign_desugar;
 
         if can_suggest_binding {
@@ -1303,11 +1283,7 @@ pub(crate) enum NonSnakeCaseDiagSub {
 }
 
 impl Subdiagnostic for NonSnakeCaseDiagSub {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         match self {
             NonSnakeCaseDiagSub::Label { span } => {
                 diag.span_label(span, fluent::lint_label);
@@ -1629,11 +1605,7 @@ pub(crate) enum OverflowingBinHexSign {
 }
 
 impl Subdiagnostic for OverflowingBinHexSign {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         match self {
             OverflowingBinHexSign::Positive => {
                 diag.note(fluent::lint_positive_note);
diff --git a/compiler/rustc_lint/src/types.rs b/compiler/rustc_lint/src/types.rs
index a6c82f574a1..a084dacfb5b 100644
--- a/compiler/rustc_lint/src/types.rs
+++ b/compiler/rustc_lint/src/types.rs
@@ -864,8 +864,8 @@ fn ty_is_known_nonnull<'tcx>(
                 return true;
             }
 
-            // `UnsafeCell` has its niche hidden.
-            if def.is_unsafe_cell() {
+            // `UnsafeCell` and `UnsafePinned` have their niche hidden.
+            if def.is_unsafe_cell() || def.is_unsafe_pinned() {
                 return false;
             }
 
diff --git a/compiler/rustc_lint_defs/src/builtin.rs b/compiler/rustc_lint_defs/src/builtin.rs
index 8a761a0a096..b25d2a30681 100644
--- a/compiler/rustc_lint_defs/src/builtin.rs
+++ b/compiler/rustc_lint_defs/src/builtin.rs
@@ -110,7 +110,6 @@ declare_lint_pass! {
         UNCONDITIONAL_PANIC,
         UNCONDITIONAL_RECURSION,
         UNCOVERED_PARAM_IN_PROJECTION,
-        UNDEFINED_NAKED_FUNCTION_ABI,
         UNEXPECTED_CFGS,
         UNFULFILLED_LINT_EXPECTATIONS,
         UNINHABITED_STATIC,
@@ -2831,39 +2830,6 @@ declare_lint! {
 }
 
 declare_lint! {
-    /// The `undefined_naked_function_abi` lint detects naked function definitions that
-    /// either do not specify an ABI or specify the Rust ABI.
-    ///
-    /// ### Example
-    ///
-    /// ```rust
-    /// #![feature(asm_experimental_arch, naked_functions)]
-    ///
-    /// use std::arch::naked_asm;
-    ///
-    /// #[naked]
-    /// pub fn default_abi() -> u32 {
-    ///     unsafe { naked_asm!(""); }
-    /// }
-    ///
-    /// #[naked]
-    /// pub extern "Rust" fn rust_abi() -> u32 {
-    ///     unsafe { naked_asm!(""); }
-    /// }
-    /// ```
-    ///
-    /// {{produces}}
-    ///
-    /// ### Explanation
-    ///
-    /// The Rust ABI is currently undefined. Therefore, naked functions should
-    /// specify a non-Rust ABI.
-    pub UNDEFINED_NAKED_FUNCTION_ABI,
-    Warn,
-    "undefined naked function ABI"
-}
-
-declare_lint! {
     /// The `ineffective_unstable_trait_impl` lint detects `#[unstable]` attributes which are not used.
     ///
     /// ### Example
diff --git a/compiler/rustc_macros/src/diagnostics/subdiagnostic.rs b/compiler/rustc_macros/src/diagnostics/subdiagnostic.rs
index 909083d5e86..bc9516b2e0c 100644
--- a/compiler/rustc_macros/src/diagnostics/subdiagnostic.rs
+++ b/compiler/rustc_macros/src/diagnostics/subdiagnostic.rs
@@ -20,14 +20,12 @@ use crate::diagnostics::utils::{
 /// The central struct for constructing the `add_to_diag` method from an annotated struct.
 pub(crate) struct SubdiagnosticDerive {
     diag: syn::Ident,
-    f: syn::Ident,
 }
 
 impl SubdiagnosticDerive {
     pub(crate) fn new() -> Self {
         let diag = format_ident!("diag");
-        let f = format_ident!("f");
-        Self { diag, f }
+        Self { diag }
     }
 
     pub(crate) fn into_tokens(self, mut structure: Structure<'_>) -> TokenStream {
@@ -86,19 +84,16 @@ impl SubdiagnosticDerive {
         };
 
         let diag = &self.diag;
-        let f = &self.f;
 
         // FIXME(edition_2024): Fix the `keyword_idents_2024` lint to not trigger here?
         #[allow(keyword_idents_2024)]
         let ret = structure.gen_impl(quote! {
             gen impl rustc_errors::Subdiagnostic for @Self {
-                fn add_to_diag_with<__G, __F>(
+                fn add_to_diag<__G>(
                     self,
                     #diag: &mut rustc_errors::Diag<'_, __G>,
-                    #f: &__F
                 ) where
                     __G: rustc_errors::EmissionGuarantee,
-                    __F: rustc_errors::SubdiagMessageOp<__G>,
                 {
                     #implementation
                 }
@@ -384,11 +379,10 @@ impl<'parent, 'a> SubdiagnosticDeriveVariantBuilder<'parent, 'a> {
                 Ok(quote! {})
             }
             "subdiagnostic" => {
-                let f = &self.parent.f;
                 let diag = &self.parent.diag;
                 let binding = &info.binding;
                 self.has_subdiagnostic = true;
-                Ok(quote! { #binding.add_to_diag_with(#diag, #f); })
+                Ok(quote! { #binding.add_to_diag(#diag); })
             }
             _ => {
                 let mut span_attrs = vec![];
@@ -531,12 +525,11 @@ impl<'parent, 'a> SubdiagnosticDeriveVariantBuilder<'parent, 'a> {
         let span_field = self.span_field.value_ref();
 
         let diag = &self.parent.diag;
-        let f = &self.parent.f;
         let mut calls = TokenStream::new();
         for (kind, slug, no_span) in kind_slugs {
             let message = format_ident!("__message");
             calls.extend(
-                quote! { let #message = #f(#diag, crate::fluent_generated::#slug.into()); },
+                quote! { let #message = #diag.eagerly_translate(crate::fluent_generated::#slug); },
             );
 
             let name = format_ident!(
diff --git a/compiler/rustc_macros/src/type_foldable.rs b/compiler/rustc_macros/src/type_foldable.rs
index c4f584dca43..85051311bee 100644
--- a/compiler/rustc_macros/src/type_foldable.rs
+++ b/compiler/rustc_macros/src/type_foldable.rs
@@ -14,27 +14,13 @@ pub(super) fn type_foldable_derive(mut s: synstructure::Structure<'_>) -> proc_m
 
     s.add_bounds(synstructure::AddBounds::Generics);
     s.bind_with(|_| synstructure::BindStyle::Move);
-    let body_fold = s.each_variant(|vi| {
+    let try_body_fold = s.each_variant(|vi| {
         let bindings = vi.bindings();
         vi.construct(|_, index| {
             let bind = &bindings[index];
 
-            let mut fixed = false;
-
             // retain value of fields with #[type_foldable(identity)]
-            bind.ast().attrs.iter().for_each(|x| {
-                if !x.path().is_ident("type_foldable") {
-                    return;
-                }
-                let _ = x.parse_nested_meta(|nested| {
-                    if nested.path.is_ident("identity") {
-                        fixed = true;
-                    }
-                    Ok(())
-                });
-            });
-
-            if fixed {
+            if has_ignore_attr(&bind.ast().attrs, "type_foldable", "identity") {
                 bind.to_token_stream()
             } else {
                 quote! {
@@ -44,6 +30,22 @@ pub(super) fn type_foldable_derive(mut s: synstructure::Structure<'_>) -> proc_m
         })
     });
 
+    let body_fold = s.each_variant(|vi| {
+        let bindings = vi.bindings();
+        vi.construct(|_, index| {
+            let bind = &bindings[index];
+
+            // retain value of fields with #[type_foldable(identity)]
+            if has_ignore_attr(&bind.ast().attrs, "type_foldable", "identity") {
+                bind.to_token_stream()
+            } else {
+                quote! {
+                    ::rustc_middle::ty::TypeFoldable::fold_with(#bind, __folder)
+                }
+            }
+        })
+    });
+
     s.bound_impl(
         quote!(::rustc_middle::ty::TypeFoldable<::rustc_middle::ty::TyCtxt<'tcx>>),
         quote! {
@@ -51,8 +53,32 @@ pub(super) fn type_foldable_derive(mut s: synstructure::Structure<'_>) -> proc_m
                 self,
                 __folder: &mut __F
             ) -> Result<Self, __F::Error> {
-                Ok(match self { #body_fold })
+                Ok(match self { #try_body_fold })
+            }
+
+            fn fold_with<__F: ::rustc_middle::ty::TypeFolder<::rustc_middle::ty::TyCtxt<'tcx>>>(
+                self,
+                __folder: &mut __F
+            ) -> Self {
+                match self { #body_fold }
             }
         },
     )
 }
+
+fn has_ignore_attr(attrs: &[syn::Attribute], name: &'static str, meta: &'static str) -> bool {
+    let mut ignored = false;
+    attrs.iter().for_each(|attr| {
+        if !attr.path().is_ident(name) {
+            return;
+        }
+        let _ = attr.parse_nested_meta(|nested| {
+            if nested.path.is_ident(meta) {
+                ignored = true;
+            }
+            Ok(())
+        });
+    });
+
+    ignored
+}
diff --git a/compiler/rustc_metadata/src/lib.rs b/compiler/rustc_metadata/src/lib.rs
index 028d5c8b609..3b44c44fcb9 100644
--- a/compiler/rustc_metadata/src/lib.rs
+++ b/compiler/rustc_metadata/src/lib.rs
@@ -1,6 +1,5 @@
 // tidy-alphabetical-start
 #![allow(internal_features)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(coroutines)]
diff --git a/compiler/rustc_metadata/src/rmeta/decoder.rs b/compiler/rustc_metadata/src/rmeta/decoder.rs
index 4cc12ca2e0b..3c2245347f9 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder.rs
@@ -1332,29 +1332,30 @@ impl<'a> CrateMetadataRef<'a> {
     }
 
     fn get_associated_item(self, id: DefIndex, sess: &'a Session) -> ty::AssocItem {
-        let name = if self.root.tables.opt_rpitit_info.get(self, id).is_some() {
-            kw::Empty
-        } else {
-            self.item_name(id)
-        };
-        let (kind, has_self) = match self.def_kind(id) {
-            DefKind::AssocConst => (ty::AssocKind::Const, false),
-            DefKind::AssocFn => (ty::AssocKind::Fn, self.get_fn_has_self_parameter(id, sess)),
-            DefKind::AssocTy => (ty::AssocKind::Type, false),
+        let kind = match self.def_kind(id) {
+            DefKind::AssocConst => ty::AssocKind::Const { name: self.item_name(id) },
+            DefKind::AssocFn => ty::AssocKind::Fn {
+                name: self.item_name(id),
+                has_self: self.get_fn_has_self_parameter(id, sess),
+            },
+            DefKind::AssocTy => {
+                let data = if let Some(rpitit_info) = self.root.tables.opt_rpitit_info.get(self, id)
+                {
+                    ty::AssocTypeData::Rpitit(rpitit_info.decode(self))
+                } else {
+                    ty::AssocTypeData::Normal(self.item_name(id))
+                };
+                ty::AssocKind::Type { data }
+            }
             _ => bug!("cannot get associated-item of `{:?}`", self.def_key(id)),
         };
         let container = self.root.tables.assoc_container.get(self, id).unwrap();
-        let opt_rpitit_info =
-            self.root.tables.opt_rpitit_info.get(self, id).map(|d| d.decode(self));
 
         ty::AssocItem {
-            name,
             kind,
             def_id: self.local_def_id(id),
             trait_item_def_id: self.get_trait_item_def_id(id),
             container,
-            fn_has_self_parameter: has_self,
-            opt_rpitit_info,
         }
     }
 
diff --git a/compiler/rustc_metadata/src/rmeta/encoder.rs b/compiler/rustc_metadata/src/rmeta/encoder.rs
index 5c8e2888ec9..177318bfe15 100644
--- a/compiler/rustc_metadata/src/rmeta/encoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/encoder.rs
@@ -1338,7 +1338,7 @@ fn should_encode_const(def_kind: DefKind) -> bool {
 fn should_encode_fn_impl_trait_in_trait<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> bool {
     if let Some(assoc_item) = tcx.opt_associated_item(def_id)
         && assoc_item.container == ty::AssocItemContainer::Trait
-        && assoc_item.kind == ty::AssocKind::Fn
+        && assoc_item.is_fn()
     {
         true
     } else {
@@ -1691,7 +1691,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
 
         match item.container {
             AssocItemContainer::Trait => {
-                if let ty::AssocKind::Type = item.kind {
+                if item.is_type() {
                     self.encode_explicit_item_bounds(def_id);
                     self.encode_explicit_item_self_bounds(def_id);
                     if tcx.is_conditionally_const(def_id) {
@@ -1706,7 +1706,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                 }
             }
         }
-        if let Some(rpitit_info) = item.opt_rpitit_info {
+        if let ty::AssocKind::Type { data: ty::AssocTypeData::Rpitit(rpitit_info) } = item.kind {
             record!(self.tables.opt_rpitit_info[def_id] <- rpitit_info);
             if matches!(rpitit_info, ty::ImplTraitInTraitData::Trait { .. }) {
                 record_array!(
diff --git a/compiler/rustc_metadata/src/rmeta/mod.rs b/compiler/rustc_metadata/src/rmeta/mod.rs
index 2f27e5f6c5d..5aa81f41b7b 100644
--- a/compiler/rustc_metadata/src/rmeta/mod.rs
+++ b/compiler/rustc_metadata/src/rmeta/mod.rs
@@ -36,9 +36,7 @@ use rustc_serialize::opaque::FileEncoder;
 use rustc_session::config::{SymbolManglingVersion, TargetModifier};
 use rustc_session::cstore::{CrateDepKind, ForeignModule, LinkagePreference, NativeLib};
 use rustc_span::edition::Edition;
-use rustc_span::hygiene::{
-    ExpnIndex, MacroKind, SyntaxContextDataNonRecursive as SyntaxContextData,
-};
+use rustc_span::hygiene::{ExpnIndex, MacroKind, SyntaxContextKey};
 use rustc_span::{self, ExpnData, ExpnHash, ExpnId, Ident, Span, Symbol};
 use rustc_target::spec::{PanicStrategy, TargetTuple};
 use table::TableBuilder;
@@ -195,7 +193,7 @@ enum LazyState {
     Previous(NonZero<usize>),
 }
 
-type SyntaxContextTable = LazyTable<u32, Option<LazyValue<SyntaxContextData>>>;
+type SyntaxContextTable = LazyTable<u32, Option<LazyValue<SyntaxContextKey>>>;
 type ExpnDataTable = LazyTable<ExpnIndex, Option<LazyValue<ExpnData>>>;
 type ExpnHashTable = LazyTable<ExpnIndex, Option<LazyValue<ExpnHash>>>;
 
diff --git a/compiler/rustc_middle/src/arena.rs b/compiler/rustc_middle/src/arena.rs
index 98273a05446..d1bbb0598fe 100644
--- a/compiler/rustc_middle/src/arena.rs
+++ b/compiler/rustc_middle/src/arena.rs
@@ -89,7 +89,6 @@ macro_rules! arena_types {
             [] name_set: rustc_data_structures::unord::UnordSet<rustc_span::Symbol>,
             [] autodiff_item: rustc_ast::expand::autodiff_attrs::AutoDiffItem,
             [] ordered_name_set: rustc_data_structures::fx::FxIndexSet<rustc_span::Symbol>,
-            [] pats: rustc_middle::ty::PatternKind<'tcx>,
             [] valtree: rustc_middle::ty::ValTreeKind<'tcx>,
 
             // Note that this deliberately duplicates items in the `rustc_hir::arena`,
diff --git a/compiler/rustc_middle/src/hir/map.rs b/compiler/rustc_middle/src/hir/map.rs
index daf95420c43..fee707f7b4c 100644
--- a/compiler/rustc_middle/src/hir/map.rs
+++ b/compiler/rustc_middle/src/hir/map.rs
@@ -1,3 +1,7 @@
+//! This module used to contain a type called `Map`. That type has since been
+//! eliminated, and all its methods are now on `TyCtxt`. But the module name
+//! stays as `map` because there isn't an obviously better name for it.
+
 use rustc_abi::ExternAbi;
 use rustc_ast::visit::{VisitorResult, walk_list};
 use rustc_data_structures::fingerprint::Fingerprint;
@@ -18,16 +22,6 @@ use crate::middle::debugger_visualizer::DebuggerVisualizerFile;
 use crate::query::LocalCrate;
 use crate::ty::TyCtxt;
 
-// FIXME: the structure was necessary in the past but now it
-// only serves as "namespace" for HIR-related methods, and can be
-// removed if all the methods are reasonably renamed and moved to tcx
-// (https://github.com/rust-lang/rust/pull/118256#issuecomment-1826442834).
-#[allow(unused)] // FIXME: temporary
-#[derive(Copy, Clone)]
-pub struct Map<'hir> {
-    pub(super) tcx: TyCtxt<'hir>,
-}
-
 /// An iterator that walks up the ancestor tree of a given `HirId`.
 /// Constructed using `tcx.hir_parent_iter(hir_id)`.
 struct ParentHirIterator<'tcx> {
@@ -335,7 +329,7 @@ impl<'tcx> TyCtxt<'tcx> {
 
     /// Returns an iterator of the `DefId`s for all body-owners in this
     /// crate. If you would prefer to iterate over the bodies
-    /// themselves, you can do `self.hir().krate().body_ids.iter()`.
+    /// themselves, you can do `self.hir_crate(()).body_ids.iter()`.
     #[inline]
     pub fn hir_body_owners(self) -> impl Iterator<Item = LocalDefId> {
         self.hir_crate_items(()).body_owners.iter().copied()
diff --git a/compiler/rustc_middle/src/hir/mod.rs b/compiler/rustc_middle/src/hir/mod.rs
index 640ac70d3a7..a28dcb0cb8e 100644
--- a/compiler/rustc_middle/src/hir/mod.rs
+++ b/compiler/rustc_middle/src/hir/mod.rs
@@ -116,11 +116,6 @@ impl ModuleItems {
 }
 
 impl<'tcx> TyCtxt<'tcx> {
-    #[inline(always)]
-    pub fn hir(self) -> map::Map<'tcx> {
-        map::Map { tcx: self }
-    }
-
     pub fn parent_module(self, id: HirId) -> LocalModDefId {
         if !id.is_owner() && self.def_kind(id.owner) == DefKind::Mod {
             LocalModDefId::new_unchecked(id.owner.def_id)
diff --git a/compiler/rustc_middle/src/infer/canonical.rs b/compiler/rustc_middle/src/infer/canonical.rs
index 3cd148cd442..5b860374496 100644
--- a/compiler/rustc_middle/src/infer/canonical.rs
+++ b/compiler/rustc_middle/src/infer/canonical.rs
@@ -39,15 +39,6 @@ pub type CanonicalVarInfo<'tcx> = ir::CanonicalVarInfo<TyCtxt<'tcx>>;
 pub type CanonicalVarValues<'tcx> = ir::CanonicalVarValues<TyCtxt<'tcx>>;
 pub type CanonicalVarInfos<'tcx> = &'tcx List<CanonicalVarInfo<'tcx>>;
 
-impl<'tcx> ty::TypeFoldable<TyCtxt<'tcx>> for CanonicalVarInfos<'tcx> {
-    fn try_fold_with<F: ty::FallibleTypeFolder<TyCtxt<'tcx>>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
-        ty::util::fold_list(self, folder, |tcx, v| tcx.mk_canonical_var_infos(v))
-    }
-}
-
 /// When we canonicalize a value to form a query, we wind up replacing
 /// various parts of it with canonical variables. This struct stores
 /// those replaced bits to remember for when we process the query
diff --git a/compiler/rustc_middle/src/lib.rs b/compiler/rustc_middle/src/lib.rs
index 1e6178144c9..8fe2cc7101b 100644
--- a/compiler/rustc_middle/src/lib.rs
+++ b/compiler/rustc_middle/src/lib.rs
@@ -29,7 +29,6 @@
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::potential_query_instability)]
 #![allow(rustc::untranslatable_diagnostic)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(allocator_api)]
diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index 4dfb362f3a2..db19c858e7c 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -1636,8 +1636,8 @@ pub fn find_self_call<'tcx>(
         &body[block].terminator
         && let Operand::Constant(box ConstOperand { const_, .. }) = func
         && let ty::FnDef(def_id, fn_args) = *const_.ty().kind()
-        && let Some(ty::AssocItem { fn_has_self_parameter: true, .. }) =
-            tcx.opt_associated_item(def_id)
+        && let Some(item) = tcx.opt_associated_item(def_id)
+        && item.is_method()
         && let [Spanned { node: Operand::Move(self_place) | Operand::Copy(self_place), .. }, ..] =
             **args
     {
diff --git a/compiler/rustc_middle/src/mir/pretty.rs b/compiler/rustc_middle/src/mir/pretty.rs
index 5a038b27337..d57019fee0f 100644
--- a/compiler/rustc_middle/src/mir/pretty.rs
+++ b/compiler/rustc_middle/src/mir/pretty.rs
@@ -531,12 +531,12 @@ fn write_mir_intro<'tcx>(
 
     // construct a scope tree and write it out
     let mut scope_tree: FxHashMap<SourceScope, Vec<SourceScope>> = Default::default();
-    for (index, scope_data) in body.source_scopes.iter().enumerate() {
+    for (index, scope_data) in body.source_scopes.iter_enumerated() {
         if let Some(parent) = scope_data.parent_scope {
-            scope_tree.entry(parent).or_default().push(SourceScope::new(index));
+            scope_tree.entry(parent).or_default().push(index);
         } else {
             // Only the argument scope has no parent, because it's the root.
-            assert_eq!(index, OUTERMOST_SOURCE_SCOPE.index());
+            assert_eq!(index, OUTERMOST_SOURCE_SCOPE);
         }
     }
 
@@ -859,7 +859,7 @@ impl Debug for Statement<'_> {
             BackwardIncompatibleDropHint { ref place, reason: _ } => {
                 // For now, we don't record the reason because there is only one use case,
                 // which is to report breaking change in drop order by Edition 2024
-                write!(fmt, "backward incompatible drop({place:?})")
+                write!(fmt, "BackwardIncompatibleDropHint({place:?})")
             }
         }
     }
diff --git a/compiler/rustc_middle/src/mir/syntax.rs b/compiler/rustc_middle/src/mir/syntax.rs
index ff9d32ebb71..c7561f8afef 100644
--- a/compiler/rustc_middle/src/mir/syntax.rs
+++ b/compiler/rustc_middle/src/mir/syntax.rs
@@ -931,6 +931,8 @@ pub enum TerminatorKind<'tcx> {
         asm_macro: InlineAsmMacro,
 
         /// The template for the inline assembly, with placeholders.
+        #[type_foldable(identity)]
+        #[type_visitable(ignore)]
         template: &'tcx [InlineAsmTemplatePiece],
 
         /// The operands for the inline assembly, as `Operand`s or `Place`s.
@@ -941,6 +943,8 @@ pub enum TerminatorKind<'tcx> {
 
         /// Source spans for each line of the inline assembly code. These are
         /// used to map assembler errors back to the line in the source code.
+        #[type_foldable(identity)]
+        #[type_visitable(ignore)]
         line_spans: &'tcx [Span],
 
         /// Valid targets for the inline assembly.
diff --git a/compiler/rustc_middle/src/mir/visit.rs b/compiler/rustc_middle/src/mir/visit.rs
index 3c83d962900..de4d5140e85 100644
--- a/compiler/rustc_middle/src/mir/visit.rs
+++ b/compiler/rustc_middle/src/mir/visit.rs
@@ -457,9 +457,15 @@ macro_rules! make_mir_visitor {
                             }
                         }
                     }
+                    StatementKind::BackwardIncompatibleDropHint { place, .. } => {
+                        self.visit_place(
+                            place,
+                            PlaceContext::NonUse(NonUseContext::BackwardIncompatibleDropHint),
+                            location
+                        );
+                    }
                     StatementKind::ConstEvalCounter => {}
                     StatementKind::Nop => {}
-                    StatementKind::BackwardIncompatibleDropHint { .. } => {}
                 }
             }
 
@@ -1348,6 +1354,8 @@ pub enum NonUseContext {
     AscribeUserTy(ty::Variance),
     /// The data of a user variable, for debug info.
     VarDebugInfo,
+    /// A `BackwardIncompatibleDropHint` statement, meant for edition 2024 lints.
+    BackwardIncompatibleDropHint,
 }
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
@@ -1422,7 +1430,9 @@ impl PlaceContext {
         use NonUseContext::*;
         match self {
             PlaceContext::MutatingUse(_) => ty::Invariant,
-            PlaceContext::NonUse(StorageDead | StorageLive | VarDebugInfo) => ty::Invariant,
+            PlaceContext::NonUse(
+                StorageDead | StorageLive | VarDebugInfo | BackwardIncompatibleDropHint,
+            ) => ty::Invariant,
             PlaceContext::NonMutatingUse(
                 Inspect | Copy | Move | PlaceMention | SharedBorrow | FakeBorrow | RawBorrow
                 | Projection,
diff --git a/compiler/rustc_middle/src/query/mod.rs b/compiler/rustc_middle/src/query/mod.rs
index 28a59d3e73e..e94f088304b 100644
--- a/compiler/rustc_middle/src/query/mod.rs
+++ b/compiler/rustc_middle/src/query/mod.rs
@@ -161,11 +161,11 @@ rustc_queries! {
 
     /// Represents crate as a whole (as distinct from the top-level crate module).
     ///
-    /// If you call `hir_crate` (e.g., indirectly by calling `tcx.hir_crate()`),
-    /// we will have to assume that any change means that you need to be recompiled.
-    /// This is because the `hir_crate` query gives you access to all other items.
-    /// To avoid this fate, do not call `tcx.hir_crate()`; instead,
-    /// prefer wrappers like [`TyCtxt::hir_visit_all_item_likes_in_crate`].
+    /// If you call `tcx.hir_crate(())` we will have to assume that any change
+    /// means that you need to be recompiled. This is because the `hir_crate`
+    /// query gives you access to all other items. To avoid this fate, do not
+    /// call `tcx.hir_crate(())`; instead, prefer wrappers like
+    /// [`TyCtxt::hir_visit_all_item_likes_in_crate`].
     query hir_crate(key: ()) -> &'tcx Crate<'tcx> {
         arena_cache
         eval_always
@@ -197,7 +197,7 @@ rustc_queries! {
 
     /// Gives access to the HIR node's parent for the HIR owner `key`.
     ///
-    /// This can be conveniently accessed by methods on `tcx.hir()`.
+    /// This can be conveniently accessed by `tcx.hir_*` methods.
     /// Avoid calling this query directly.
     query hir_owner_parent(key: hir::OwnerId) -> hir::HirId {
         desc { |tcx| "getting HIR parent of `{}`", tcx.def_path_str(key) }
@@ -205,7 +205,7 @@ rustc_queries! {
 
     /// Gives access to the HIR nodes and bodies inside `key` if it's a HIR owner.
     ///
-    /// This can be conveniently accessed by methods on `tcx.hir()`.
+    /// This can be conveniently accessed by `tcx.hir_*` methods.
     /// Avoid calling this query directly.
     query opt_hir_owner_nodes(key: LocalDefId) -> Option<&'tcx hir::OwnerNodes<'tcx>> {
         desc { |tcx| "getting HIR owner items in `{}`", tcx.def_path_str(key) }
@@ -214,7 +214,7 @@ rustc_queries! {
 
     /// Gives access to the HIR attributes inside the HIR owner `key`.
     ///
-    /// This can be conveniently accessed by methods on `tcx.hir()`.
+    /// This can be conveniently accessed by `tcx.hir_*` methods.
     /// Avoid calling this query directly.
     query hir_attr_map(key: hir::OwnerId) -> &'tcx hir::AttributeMap<'tcx> {
         desc { |tcx| "getting HIR owner attributes in `{}`", tcx.def_path_str(key) }
diff --git a/compiler/rustc_middle/src/query/on_disk_cache.rs b/compiler/rustc_middle/src/query/on_disk_cache.rs
index f78398c7c37..5431097cb1d 100644
--- a/compiler/rustc_middle/src/query/on_disk_cache.rs
+++ b/compiler/rustc_middle/src/query/on_disk_cache.rs
@@ -16,8 +16,7 @@ use rustc_serialize::opaque::{FileEncodeResult, FileEncoder, IntEncodedWithFixed
 use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
 use rustc_session::Session;
 use rustc_span::hygiene::{
-    ExpnId, HygieneDecodeContext, HygieneEncodeContext, SyntaxContext,
-    SyntaxContextDataNonRecursive as SyntaxContextData,
+    ExpnId, HygieneDecodeContext, HygieneEncodeContext, SyntaxContext, SyntaxContextKey,
 };
 use rustc_span::source_map::Spanned;
 use rustc_span::{
@@ -567,7 +566,7 @@ impl<'a, 'tcx> SpanDecoder for CacheDecoder<'a, 'tcx> {
             // We look up the position of the associated `SyntaxData` and decode it.
             let pos = syntax_contexts.get(&id).unwrap();
             this.with_position(pos.to_usize(), |decoder| {
-                let data: SyntaxContextData = decode_tagged(decoder, TAG_SYNTAX_CONTEXT);
+                let data: SyntaxContextKey = decode_tagged(decoder, TAG_SYNTAX_CONTEXT);
                 data
             })
         })
diff --git a/compiler/rustc_middle/src/ty/adjustment.rs b/compiler/rustc_middle/src/ty/adjustment.rs
index f8ab555305f..3425da48559 100644
--- a/compiler/rustc_middle/src/ty/adjustment.rs
+++ b/compiler/rustc_middle/src/ty/adjustment.rs
@@ -5,7 +5,7 @@ use rustc_hir::lang_items::LangItem;
 use rustc_macros::{HashStable, TyDecodable, TyEncodable, TypeFoldable, TypeVisitable};
 use rustc_span::Span;
 
-use crate::ty::{self, Ty, TyCtxt};
+use crate::ty::{Ty, TyCtxt};
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable)]
 pub enum PointerCoercion {
@@ -133,7 +133,7 @@ impl OverloadedDeref {
         };
         tcx.associated_items(trait_def_id)
             .in_definition_order()
-            .find(|m| m.kind == ty::AssocKind::Fn)
+            .find(|item| item.is_fn())
             .unwrap()
             .def_id
     }
diff --git a/compiler/rustc_middle/src/ty/adt.rs b/compiler/rustc_middle/src/ty/adt.rs
index 00fe5cb0c5d..66517c97a68 100644
--- a/compiler/rustc_middle/src/ty/adt.rs
+++ b/compiler/rustc_middle/src/ty/adt.rs
@@ -53,6 +53,10 @@ bitflags::bitflags! {
         const IS_VARIANT_LIST_NON_EXHAUSTIVE = 1 << 8;
         /// Indicates whether the type is `UnsafeCell`.
         const IS_UNSAFE_CELL              = 1 << 9;
+        /// Indicates whether the type is `UnsafePinned`.
+        const IS_UNSAFE_PINNED              = 1 << 10;
+        /// Indicates whether the type is anonymous.
+        const IS_ANONYMOUS                = 1 << 11;
     }
 }
 rustc_data_structures::external_bitflags_debug! { AdtFlags }
@@ -302,6 +306,9 @@ impl AdtDefData {
         if tcx.is_lang_item(did, LangItem::UnsafeCell) {
             flags |= AdtFlags::IS_UNSAFE_CELL;
         }
+        if tcx.is_lang_item(did, LangItem::UnsafePinned) {
+            flags |= AdtFlags::IS_UNSAFE_PINNED;
+        }
 
         AdtDefData { did, variants, flags, repr }
     }
@@ -405,6 +412,12 @@ impl<'tcx> AdtDef<'tcx> {
         self.flags().contains(AdtFlags::IS_UNSAFE_CELL)
     }
 
+    /// Returns `true` if this is `UnsafePinned<T>`.
+    #[inline]
+    pub fn is_unsafe_pinned(self) -> bool {
+        self.flags().contains(AdtFlags::IS_UNSAFE_PINNED)
+    }
+
     /// Returns `true` if this is `ManuallyDrop<T>`.
     #[inline]
     pub fn is_manually_drop(self) -> bool {
diff --git a/compiler/rustc_middle/src/ty/assoc.rs b/compiler/rustc_middle/src/ty/assoc.rs
index bbaf735fbdb..0c44fd2758d 100644
--- a/compiler/rustc_middle/src/ty/assoc.rs
+++ b/compiler/rustc_middle/src/ty/assoc.rs
@@ -18,27 +18,33 @@ pub enum AssocItemContainer {
 #[derive(Copy, Clone, Debug, PartialEq, HashStable, Eq, Hash, Encodable, Decodable)]
 pub struct AssocItem {
     pub def_id: DefId,
-    pub name: Symbol,
     pub kind: AssocKind,
     pub container: AssocItemContainer,
 
     /// If this is an item in an impl of a trait then this is the `DefId` of
     /// the associated item on the trait that this implements.
     pub trait_item_def_id: Option<DefId>,
-
-    /// Whether this is a method with an explicit self
-    /// as its first parameter, allowing method calls.
-    pub fn_has_self_parameter: bool,
-
-    /// `Some` if the associated item (an associated type) comes from the
-    /// return-position `impl Trait` in trait desugaring. The `ImplTraitInTraitData`
-    /// provides additional information about its source.
-    pub opt_rpitit_info: Option<ty::ImplTraitInTraitData>,
 }
 
 impl AssocItem {
+    // Gets the identifier, if it has one.
+    pub fn opt_name(&self) -> Option<Symbol> {
+        match self.kind {
+            ty::AssocKind::Type { data: AssocTypeData::Normal(name) } => Some(name),
+            ty::AssocKind::Type { data: AssocTypeData::Rpitit(_) } => None,
+            ty::AssocKind::Const { name } => Some(name),
+            ty::AssocKind::Fn { name, .. } => Some(name),
+        }
+    }
+
+    // Gets the identifier name. Aborts if it lacks one, i.e. is an RPITIT
+    // associated type.
+    pub fn name(&self) -> Symbol {
+        self.opt_name().expect("name of non-Rpitit assoc item")
+    }
+
     pub fn ident(&self, tcx: TyCtxt<'_>) -> Ident {
-        Ident::new(self.name, tcx.def_ident_span(self.def_id).unwrap())
+        Ident::new(self.name(), tcx.def_ident_span(self.def_id).unwrap())
     }
 
     /// Gets the defaultness of the associated item.
@@ -78,35 +84,65 @@ impl AssocItem {
 
     pub fn signature(&self, tcx: TyCtxt<'_>) -> String {
         match self.kind {
-            ty::AssocKind::Fn => {
+            ty::AssocKind::Fn { .. } => {
                 // We skip the binder here because the binder would deanonymize all
                 // late-bound regions, and we don't want method signatures to show up
                 // `as for<'r> fn(&'r MyType)`. Pretty-printing handles late-bound
                 // regions just fine, showing `fn(&MyType)`.
                 tcx.fn_sig(self.def_id).instantiate_identity().skip_binder().to_string()
             }
-            ty::AssocKind::Type => format!("type {};", self.name),
-            ty::AssocKind::Const => {
-                format!(
-                    "const {}: {:?};",
-                    self.name,
-                    tcx.type_of(self.def_id).instantiate_identity()
-                )
+            ty::AssocKind::Type { .. } => format!("type {};", self.name()),
+            ty::AssocKind::Const { name } => {
+                format!("const {}: {:?};", name, tcx.type_of(self.def_id).instantiate_identity())
             }
         }
     }
 
     pub fn descr(&self) -> &'static str {
         match self.kind {
-            ty::AssocKind::Const => "associated const",
-            ty::AssocKind::Fn if self.fn_has_self_parameter => "method",
-            ty::AssocKind::Fn => "associated function",
-            ty::AssocKind::Type => "associated type",
+            ty::AssocKind::Const { .. } => "associated const",
+            ty::AssocKind::Fn { has_self: true, .. } => "method",
+            ty::AssocKind::Fn { has_self: false, .. } => "associated function",
+            ty::AssocKind::Type { .. } => "associated type",
+        }
+    }
+
+    pub fn namespace(&self) -> Namespace {
+        match self.kind {
+            ty::AssocKind::Type { .. } => Namespace::TypeNS,
+            ty::AssocKind::Const { .. } | ty::AssocKind::Fn { .. } => Namespace::ValueNS,
+        }
+    }
+
+    pub fn as_def_kind(&self) -> DefKind {
+        match self.kind {
+            AssocKind::Const { .. } => DefKind::AssocConst,
+            AssocKind::Fn { .. } => DefKind::AssocFn,
+            AssocKind::Type { .. } => DefKind::AssocTy,
+        }
+    }
+    pub fn is_type(&self) -> bool {
+        matches!(self.kind, ty::AssocKind::Type { .. })
+    }
+
+    pub fn is_fn(&self) -> bool {
+        matches!(self.kind, ty::AssocKind::Fn { .. })
+    }
+
+    pub fn is_method(&self) -> bool {
+        matches!(self.kind, ty::AssocKind::Fn { has_self: true, .. })
+    }
+
+    pub fn as_tag(&self) -> AssocTag {
+        match self.kind {
+            AssocKind::Const { .. } => AssocTag::Const,
+            AssocKind::Fn { .. } => AssocTag::Fn,
+            AssocKind::Type { .. } => AssocTag::Type,
         }
     }
 
     pub fn is_impl_trait_in_trait(&self) -> bool {
-        self.opt_rpitit_info.is_some()
+        matches!(self.kind, AssocKind::Type { data: AssocTypeData::Rpitit(_) })
     }
 
     /// Returns true if:
@@ -114,7 +150,7 @@ impl AssocItem {
     /// - If it is in a trait impl, the item from the original trait has this attribute, or
     /// - It is an inherent assoc const.
     pub fn is_type_const_capable(&self, tcx: TyCtxt<'_>) -> bool {
-        if self.kind != ty::AssocKind::Const {
+        if !matches!(self.kind, ty::AssocKind::Const { .. }) {
             return false;
         }
 
@@ -129,25 +165,34 @@ impl AssocItem {
 }
 
 #[derive(Copy, Clone, PartialEq, Debug, HashStable, Eq, Hash, Encodable, Decodable)]
+pub enum AssocTypeData {
+    Normal(Symbol),
+    /// The associated type comes from an RPITIT. It has no name, and the
+    /// `ImplTraitInTraitData` provides additional information about its
+    /// source.
+    Rpitit(ty::ImplTraitInTraitData),
+}
+
+#[derive(Copy, Clone, PartialEq, Debug, HashStable, Eq, Hash, Encodable, Decodable)]
 pub enum AssocKind {
-    Const,
-    Fn,
-    Type,
+    Const { name: Symbol },
+    Fn { name: Symbol, has_self: bool },
+    Type { data: AssocTypeData },
 }
 
 impl AssocKind {
     pub fn namespace(&self) -> Namespace {
         match *self {
-            ty::AssocKind::Type => Namespace::TypeNS,
-            ty::AssocKind::Const | ty::AssocKind::Fn => Namespace::ValueNS,
+            ty::AssocKind::Type { .. } => Namespace::TypeNS,
+            ty::AssocKind::Const { .. } | ty::AssocKind::Fn { .. } => Namespace::ValueNS,
         }
     }
 
     pub fn as_def_kind(&self) -> DefKind {
         match self {
-            AssocKind::Const => DefKind::AssocConst,
-            AssocKind::Fn => DefKind::AssocFn,
-            AssocKind::Type => DefKind::AssocTy,
+            AssocKind::Const { .. } => DefKind::AssocConst,
+            AssocKind::Fn { .. } => DefKind::AssocFn,
+            AssocKind::Type { .. } => DefKind::AssocTy,
         }
     }
 }
@@ -155,15 +200,22 @@ impl AssocKind {
 impl std::fmt::Display for AssocKind {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
-            // FIXME: fails to distinguish between "associated function" and
-            // "method" because `has_self` isn't known here.
-            AssocKind::Fn => write!(f, "method"),
-            AssocKind::Const => write!(f, "associated const"),
-            AssocKind::Type => write!(f, "associated type"),
+            AssocKind::Fn { has_self: true, .. } => write!(f, "method"),
+            AssocKind::Fn { has_self: false, .. } => write!(f, "associated function"),
+            AssocKind::Const { .. } => write!(f, "associated const"),
+            AssocKind::Type { .. } => write!(f, "associated type"),
         }
     }
 }
 
+// Like `AssocKind`, but just the tag, no fields. Used in various kinds of matching.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum AssocTag {
+    Const,
+    Fn,
+    Type,
+}
+
 /// A list of `ty::AssocItem`s in definition order that allows for efficient lookup by name.
 ///
 /// When doing lookup by name, we try to postpone hygienic comparison for as long as possible since
@@ -171,17 +223,17 @@ impl std::fmt::Display for AssocKind {
 /// done only on items with the same name.
 #[derive(Debug, Clone, PartialEq, HashStable)]
 pub struct AssocItems {
-    items: SortedIndexMultiMap<u32, Symbol, ty::AssocItem>,
+    items: SortedIndexMultiMap<u32, Option<Symbol>, ty::AssocItem>,
 }
 
 impl AssocItems {
     /// Constructs an `AssociatedItems` map from a series of `ty::AssocItem`s in definition order.
     pub fn new(items_in_def_order: impl IntoIterator<Item = ty::AssocItem>) -> Self {
-        let items = items_in_def_order.into_iter().map(|item| (item.name, item)).collect();
+        let items = items_in_def_order.into_iter().map(|item| (item.opt_name(), item)).collect();
         AssocItems { items }
     }
 
-    /// Returns a slice of associated items in the order they were defined.
+    /// Returns an iterator over associated items in the order they were defined.
     ///
     /// New code should avoid relying on definition order. If you need a particular associated item
     /// for a known trait, make that trait a lang item instead of indexing this array.
@@ -198,7 +250,8 @@ impl AssocItems {
         &self,
         name: Symbol,
     ) -> impl '_ + Iterator<Item = &ty::AssocItem> {
-        self.items.get_by_key(name)
+        assert!(!name.is_empty());
+        self.items.get_by_key(Some(name))
     }
 
     /// Returns the associated item with the given identifier and `AssocKind`, if one exists.
@@ -207,27 +260,14 @@ impl AssocItems {
         &self,
         tcx: TyCtxt<'_>,
         ident: Ident,
-        kind: AssocKind,
+        assoc_tag: AssocTag,
         parent_def_id: DefId,
     ) -> Option<&ty::AssocItem> {
         self.filter_by_name_unhygienic(ident.name)
-            .filter(|item| item.kind == kind)
+            .filter(|item| item.as_tag() == assoc_tag)
             .find(|item| tcx.hygienic_eq(ident, item.ident(tcx), parent_def_id))
     }
 
-    /// Returns the associated item with the given identifier and any of `AssocKind`, if one
-    /// exists. The identifier is matched hygienically.
-    pub fn find_by_ident_and_kinds(
-        &self,
-        tcx: TyCtxt<'_>,
-        ident: Ident,
-        // Sorted in order of what kinds to look at
-        kinds: &[AssocKind],
-        parent_def_id: DefId,
-    ) -> Option<&ty::AssocItem> {
-        kinds.iter().find_map(|kind| self.find_by_ident_and_kind(tcx, ident, *kind, parent_def_id))
-    }
-
     /// Returns the associated item with the given identifier in the given `Namespace`, if one
     /// exists. The identifier is matched hygienically.
     pub fn find_by_ident_and_namespace(
@@ -238,7 +278,7 @@ impl AssocItems {
         parent_def_id: DefId,
     ) -> Option<&ty::AssocItem> {
         self.filter_by_name_unhygienic(ident.name)
-            .filter(|item| item.kind.namespace() == ns)
+            .filter(|item| item.namespace() == ns)
             .find(|item| tcx.hygienic_eq(ident, item.ident(tcx), parent_def_id))
     }
 }
diff --git a/compiler/rustc_middle/src/ty/consts.rs b/compiler/rustc_middle/src/ty/consts.rs
index ae1c6c670cb..dc5fe2d8f8b 100644
--- a/compiler/rustc_middle/src/ty/consts.rs
+++ b/compiler/rustc_middle/src/ty/consts.rs
@@ -3,6 +3,7 @@ use std::borrow::Cow;
 use rustc_data_structures::intern::Interned;
 use rustc_error_messages::MultiSpan;
 use rustc_macros::HashStable;
+use rustc_type_ir::walk::TypeWalker;
 use rustc_type_ir::{self as ir, TypeFlags, WithCachedTypeInfo};
 
 use crate::ty::{self, Ty, TyCtxt};
@@ -243,4 +244,18 @@ impl<'tcx> Const<'tcx> {
     pub fn is_ct_infer(self) -> bool {
         matches!(self.kind(), ty::ConstKind::Infer(_))
     }
+
+    /// Iterator that walks `self` and any types reachable from
+    /// `self`, in depth-first order. Note that just walks the types
+    /// that appear in `self`, it does not descend into the fields of
+    /// structs or variants. For example:
+    ///
+    /// ```text
+    /// isize => { isize }
+    /// Foo<Bar<isize>> => { Foo<Bar<isize>>, Bar<isize>, isize }
+    /// [isize] => { [isize], isize }
+    /// ```
+    pub fn walk(self) -> TypeWalker<TyCtxt<'tcx>> {
+        TypeWalker::new(self.into())
+    }
 }
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index abf6cbbcd87..fa2f1cf1a1c 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -464,7 +464,7 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
     fn associated_type_def_ids(self, def_id: DefId) -> impl IntoIterator<Item = DefId> {
         self.associated_items(def_id)
             .in_definition_order()
-            .filter(|assoc_item| matches!(assoc_item.kind, ty::AssocKind::Type))
+            .filter(|assoc_item| assoc_item.is_type())
             .map(|assoc_item| assoc_item.def_id)
     }
 
@@ -870,7 +870,7 @@ impl<'tcx> CtxtInterners<'tcx> {
         Ty(Interned::new_unchecked(
             self.type_
                 .intern(kind, |kind| {
-                    let flags = super::flags::FlagComputation::for_kind(&kind);
+                    let flags = ty::FlagComputation::<TyCtxt<'tcx>>::for_kind(&kind);
                     let stable_hash = self.stable_hash(&flags, sess, untracked, &kind);
 
                     InternedInSet(self.arena.alloc(WithCachedTypeInfo {
@@ -896,7 +896,7 @@ impl<'tcx> CtxtInterners<'tcx> {
         Const(Interned::new_unchecked(
             self.const_
                 .intern(kind, |kind: ty::ConstKind<'_>| {
-                    let flags = super::flags::FlagComputation::for_const_kind(&kind);
+                    let flags = ty::FlagComputation::<TyCtxt<'tcx>>::for_const_kind(&kind);
                     let stable_hash = self.stable_hash(&flags, sess, untracked, &kind);
 
                     InternedInSet(self.arena.alloc(WithCachedTypeInfo {
@@ -912,7 +912,7 @@ impl<'tcx> CtxtInterners<'tcx> {
 
     fn stable_hash<'a, T: HashStable<StableHashingContext<'a>>>(
         &self,
-        flags: &ty::flags::FlagComputation,
+        flags: &ty::FlagComputation<TyCtxt<'tcx>>,
         sess: &'a Session,
         untracked: &'a Untracked,
         val: &T,
@@ -940,7 +940,7 @@ impl<'tcx> CtxtInterners<'tcx> {
         Predicate(Interned::new_unchecked(
             self.predicate
                 .intern(kind, |kind| {
-                    let flags = super::flags::FlagComputation::for_predicate(kind);
+                    let flags = ty::FlagComputation::<TyCtxt<'tcx>>::for_predicate(kind);
 
                     let stable_hash = self.stable_hash(&flags, sess, untracked, &kind);
 
@@ -961,7 +961,7 @@ impl<'tcx> CtxtInterners<'tcx> {
         } else {
             self.clauses
                 .intern_ref(clauses, || {
-                    let flags = super::flags::FlagComputation::for_clauses(clauses);
+                    let flags = ty::FlagComputation::<TyCtxt<'tcx>>::for_clauses(clauses);
 
                     InternedInSet(ListWithCachedTypeInfo::from_arena(
                         &*self.arena,
@@ -2147,7 +2147,7 @@ impl<'tcx> TyCtxt<'tcx> {
             return vec![];
         };
 
-        let mut v = TraitObjectVisitor(vec![], self.hir());
+        let mut v = TraitObjectVisitor(vec![]);
         v.visit_ty_unambig(hir_output);
         v.0
     }
@@ -2160,7 +2160,7 @@ impl<'tcx> TyCtxt<'tcx> {
         scope_def_id: LocalDefId,
     ) -> Option<(Vec<&'tcx hir::Ty<'tcx>>, Span, Option<Span>)> {
         let hir_id = self.local_def_id_to_hir_id(scope_def_id);
-        let mut v = TraitObjectVisitor(vec![], self.hir());
+        let mut v = TraitObjectVisitor(vec![]);
         // when the return type is a type alias
         if let Some(hir::FnDecl { output: hir::FnRetTy::Return(hir_output), .. }) = self.hir_fn_decl_by_hir_id(hir_id)
             && let hir::TyKind::Path(hir::QPath::Resolved(
diff --git a/compiler/rustc_middle/src/ty/diagnostics.rs b/compiler/rustc_middle/src/ty/diagnostics.rs
index d3abb3d64b8..fbb57b8df6b 100644
--- a/compiler/rustc_middle/src/ty/diagnostics.rs
+++ b/compiler/rustc_middle/src/ty/diagnostics.rs
@@ -571,15 +571,15 @@ pub fn suggest_constraining_type_params<'a>(
 }
 
 /// Collect al types that have an implicit `'static` obligation that we could suggest `'_` for.
-pub struct TraitObjectVisitor<'tcx>(pub Vec<&'tcx hir::Ty<'tcx>>, pub crate::hir::map::Map<'tcx>);
+pub(crate) struct TraitObjectVisitor<'tcx>(pub(crate) Vec<&'tcx hir::Ty<'tcx>>);
 
 impl<'v> hir::intravisit::Visitor<'v> for TraitObjectVisitor<'v> {
     fn visit_ty(&mut self, ty: &'v hir::Ty<'v, AmbigArg>) {
         match ty.kind {
             hir::TyKind::TraitObject(_, tagged_ptr)
                 if let hir::Lifetime {
-                    res:
-                        hir::LifetimeName::ImplicitObjectLifetimeDefault | hir::LifetimeName::Static,
+                    kind:
+                        hir::LifetimeKind::ImplicitObjectLifetimeDefault | hir::LifetimeKind::Static,
                     ..
                 } = tagged_ptr.pointer() =>
             {
@@ -592,18 +592,6 @@ impl<'v> hir::intravisit::Visitor<'v> for TraitObjectVisitor<'v> {
     }
 }
 
-/// Collect al types that have an implicit `'static` obligation that we could suggest `'_` for.
-pub struct StaticLifetimeVisitor<'tcx>(pub Vec<Span>, pub crate::hir::map::Map<'tcx>);
-
-impl<'v> hir::intravisit::Visitor<'v> for StaticLifetimeVisitor<'v> {
-    fn visit_lifetime(&mut self, lt: &'v hir::Lifetime) {
-        if let hir::LifetimeName::ImplicitObjectLifetimeDefault | hir::LifetimeName::Static = lt.res
-        {
-            self.0.push(lt.ident.span);
-        }
-    }
-}
-
 pub struct IsSuggestableVisitor<'tcx> {
     tcx: TyCtxt<'tcx>,
     infer_suggestable: bool,
diff --git a/compiler/rustc_middle/src/ty/flags.rs b/compiler/rustc_middle/src/ty/flags.rs
deleted file mode 100644
index 2424923fb78..00000000000
--- a/compiler/rustc_middle/src/ty/flags.rs
+++ /dev/null
@@ -1,359 +0,0 @@
-use std::slice;
-
-use crate::ty::{self, GenericArg, GenericArgKind, InferConst, Ty, TypeFlags};
-
-#[derive(Debug)]
-pub struct FlagComputation {
-    pub flags: TypeFlags,
-
-    /// see `Ty::outer_exclusive_binder` for details
-    pub outer_exclusive_binder: ty::DebruijnIndex,
-}
-
-impl FlagComputation {
-    fn new() -> FlagComputation {
-        FlagComputation { flags: TypeFlags::empty(), outer_exclusive_binder: ty::INNERMOST }
-    }
-
-    #[allow(rustc::usage_of_ty_tykind)]
-    pub fn for_kind(kind: &ty::TyKind<'_>) -> FlagComputation {
-        let mut result = FlagComputation::new();
-        result.add_kind(kind);
-        result
-    }
-
-    pub fn for_predicate(binder: ty::Binder<'_, ty::PredicateKind<'_>>) -> FlagComputation {
-        let mut result = FlagComputation::new();
-        result.add_predicate(binder);
-        result
-    }
-
-    pub fn for_const_kind(kind: &ty::ConstKind<'_>) -> FlagComputation {
-        let mut result = FlagComputation::new();
-        result.add_const_kind(kind);
-        result
-    }
-
-    pub fn for_clauses(clauses: &[ty::Clause<'_>]) -> FlagComputation {
-        let mut result = FlagComputation::new();
-        for c in clauses {
-            result.add_flags(c.as_predicate().flags());
-            result.add_exclusive_binder(c.as_predicate().outer_exclusive_binder());
-        }
-        result
-    }
-
-    fn add_flags(&mut self, flags: TypeFlags) {
-        self.flags = self.flags | flags;
-    }
-
-    /// indicates that `self` refers to something at binding level `binder`
-    fn add_bound_var(&mut self, binder: ty::DebruijnIndex) {
-        let exclusive_binder = binder.shifted_in(1);
-        self.add_exclusive_binder(exclusive_binder);
-    }
-
-    /// indicates that `self` refers to something *inside* binding
-    /// level `binder` -- not bound by `binder`, but bound by the next
-    /// binder internal to it
-    fn add_exclusive_binder(&mut self, exclusive_binder: ty::DebruijnIndex) {
-        self.outer_exclusive_binder = self.outer_exclusive_binder.max(exclusive_binder);
-    }
-
-    /// Adds the flags/depth from a set of types that appear within the current type, but within a
-    /// region binder.
-    fn bound_computation<T, F>(&mut self, value: ty::Binder<'_, T>, f: F)
-    where
-        F: FnOnce(&mut Self, T),
-    {
-        let mut computation = FlagComputation::new();
-
-        if !value.bound_vars().is_empty() {
-            computation.add_flags(TypeFlags::HAS_BINDER_VARS);
-        }
-
-        f(&mut computation, value.skip_binder());
-
-        self.add_flags(computation.flags);
-
-        // The types that contributed to `computation` occurred within
-        // a region binder, so subtract one from the region depth
-        // within when adding the depth to `self`.
-        let outer_exclusive_binder = computation.outer_exclusive_binder;
-        if outer_exclusive_binder > ty::INNERMOST {
-            self.add_exclusive_binder(outer_exclusive_binder.shifted_out(1));
-        } // otherwise, this binder captures nothing
-    }
-
-    #[allow(rustc::usage_of_ty_tykind)]
-    fn add_kind(&mut self, kind: &ty::TyKind<'_>) {
-        match kind {
-            &ty::Bool
-            | &ty::Char
-            | &ty::Int(_)
-            | &ty::Float(_)
-            | &ty::Uint(_)
-            | &ty::Never
-            | &ty::Str
-            | &ty::Foreign(..) => {}
-
-            &ty::Error(_) => self.add_flags(TypeFlags::HAS_ERROR),
-
-            &ty::Param(_) => {
-                self.add_flags(TypeFlags::HAS_TY_PARAM);
-            }
-
-            &ty::Closure(_, args)
-            | &ty::Coroutine(_, args)
-            | &ty::CoroutineClosure(_, args)
-            | &ty::CoroutineWitness(_, args) => {
-                self.add_args(args);
-            }
-
-            &ty::Bound(debruijn, _) => {
-                self.add_bound_var(debruijn);
-                self.add_flags(TypeFlags::HAS_TY_BOUND);
-            }
-
-            &ty::Placeholder(..) => {
-                self.add_flags(TypeFlags::HAS_TY_PLACEHOLDER);
-            }
-
-            &ty::Infer(infer) => match infer {
-                ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_) => {
-                    self.add_flags(TypeFlags::HAS_TY_FRESH)
-                }
-
-                ty::TyVar(_) | ty::IntVar(_) | ty::FloatVar(_) => {
-                    self.add_flags(TypeFlags::HAS_TY_INFER)
-                }
-            },
-
-            &ty::Adt(_, args) => {
-                self.add_args(args);
-            }
-
-            &ty::Alias(kind, data) => {
-                self.add_flags(match kind {
-                    ty::Projection => TypeFlags::HAS_TY_PROJECTION,
-                    ty::Weak => TypeFlags::HAS_TY_WEAK,
-                    ty::Opaque => TypeFlags::HAS_TY_OPAQUE,
-                    ty::Inherent => TypeFlags::HAS_TY_INHERENT,
-                });
-
-                self.add_alias_ty(data);
-            }
-
-            &ty::Dynamic(obj, r, _) => {
-                for predicate in obj.iter() {
-                    self.bound_computation(predicate, |computation, predicate| match predicate {
-                        ty::ExistentialPredicate::Trait(tr) => computation.add_args(tr.args),
-                        ty::ExistentialPredicate::Projection(p) => {
-                            computation.add_existential_projection(&p);
-                        }
-                        ty::ExistentialPredicate::AutoTrait(_) => {}
-                    });
-                }
-
-                self.add_region(r);
-            }
-
-            &ty::Array(tt, len) => {
-                self.add_ty(tt);
-                self.add_const(len);
-            }
-
-            &ty::Pat(ty, pat) => {
-                self.add_ty(ty);
-                match *pat {
-                    ty::PatternKind::Range { start, end } => {
-                        self.add_const(start);
-                        self.add_const(end);
-                    }
-                }
-            }
-
-            &ty::Slice(tt) => self.add_ty(tt),
-
-            &ty::RawPtr(ty, _) => {
-                self.add_ty(ty);
-            }
-
-            &ty::Ref(r, ty, _) => {
-                self.add_region(r);
-                self.add_ty(ty);
-            }
-
-            &ty::Tuple(types) => {
-                self.add_tys(types);
-            }
-
-            &ty::FnDef(_, args) => {
-                self.add_args(args);
-            }
-
-            &ty::FnPtr(sig_tys, _) => self.bound_computation(sig_tys, |computation, sig_tys| {
-                computation.add_tys(sig_tys.inputs_and_output);
-            }),
-
-            &ty::UnsafeBinder(bound_ty) => {
-                self.bound_computation(bound_ty.into(), |computation, ty| {
-                    computation.add_ty(ty);
-                })
-            }
-        }
-    }
-
-    fn add_predicate(&mut self, binder: ty::Binder<'_, ty::PredicateKind<'_>>) {
-        self.bound_computation(binder, |computation, atom| computation.add_predicate_atom(atom));
-    }
-
-    fn add_predicate_atom(&mut self, atom: ty::PredicateKind<'_>) {
-        match atom {
-            ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_pred)) => {
-                self.add_args(trait_pred.trait_ref.args);
-            }
-            ty::PredicateKind::Clause(ty::ClauseKind::HostEffect(ty::HostEffectPredicate {
-                trait_ref,
-                constness: _,
-            })) => {
-                self.add_args(trait_ref.args);
-            }
-            ty::PredicateKind::Clause(ty::ClauseKind::RegionOutlives(ty::OutlivesPredicate(
-                a,
-                b,
-            ))) => {
-                self.add_region(a);
-                self.add_region(b);
-            }
-            ty::PredicateKind::Clause(ty::ClauseKind::TypeOutlives(ty::OutlivesPredicate(
-                ty,
-                region,
-            ))) => {
-                self.add_ty(ty);
-                self.add_region(region);
-            }
-            ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(ct, ty)) => {
-                self.add_const(ct);
-                self.add_ty(ty);
-            }
-            ty::PredicateKind::Subtype(ty::SubtypePredicate { a_is_expected: _, a, b }) => {
-                self.add_ty(a);
-                self.add_ty(b);
-            }
-            ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) => {
-                self.add_ty(a);
-                self.add_ty(b);
-            }
-            ty::PredicateKind::Clause(ty::ClauseKind::Projection(ty::ProjectionPredicate {
-                projection_term,
-                term,
-            })) => {
-                self.add_alias_term(projection_term);
-                self.add_term(term);
-            }
-            ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(arg)) => {
-                self.add_args(slice::from_ref(&arg));
-            }
-            ty::PredicateKind::DynCompatible(_def_id) => {}
-            ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(uv)) => {
-                self.add_const(uv);
-            }
-            ty::PredicateKind::ConstEquate(expected, found) => {
-                self.add_const(expected);
-                self.add_const(found);
-            }
-            ty::PredicateKind::Ambiguous => {}
-            ty::PredicateKind::NormalizesTo(ty::NormalizesTo { alias, term }) => {
-                self.add_alias_term(alias);
-                self.add_term(term);
-            }
-            ty::PredicateKind::AliasRelate(t1, t2, _) => {
-                self.add_term(t1);
-                self.add_term(t2);
-            }
-        }
-    }
-
-    fn add_ty(&mut self, ty: Ty<'_>) {
-        self.add_flags(ty.flags());
-        self.add_exclusive_binder(ty.outer_exclusive_binder());
-    }
-
-    fn add_tys(&mut self, tys: &[Ty<'_>]) {
-        for &ty in tys {
-            self.add_ty(ty);
-        }
-    }
-
-    fn add_region(&mut self, r: ty::Region<'_>) {
-        self.add_flags(r.type_flags());
-        if let ty::ReBound(debruijn, _) = r.kind() {
-            self.add_bound_var(debruijn);
-        }
-    }
-
-    fn add_const(&mut self, c: ty::Const<'_>) {
-        self.add_flags(c.flags());
-        self.add_exclusive_binder(c.outer_exclusive_binder());
-    }
-
-    fn add_const_kind(&mut self, c: &ty::ConstKind<'_>) {
-        match *c {
-            ty::ConstKind::Unevaluated(uv) => {
-                self.add_args(uv.args);
-                self.add_flags(TypeFlags::HAS_CT_PROJECTION);
-            }
-            ty::ConstKind::Infer(infer) => match infer {
-                InferConst::Fresh(_) => self.add_flags(TypeFlags::HAS_CT_FRESH),
-                InferConst::Var(_) => self.add_flags(TypeFlags::HAS_CT_INFER),
-            },
-            ty::ConstKind::Bound(debruijn, _) => {
-                self.add_bound_var(debruijn);
-                self.add_flags(TypeFlags::HAS_CT_BOUND);
-            }
-            ty::ConstKind::Param(_) => {
-                self.add_flags(TypeFlags::HAS_CT_PARAM);
-            }
-            ty::ConstKind::Placeholder(_) => {
-                self.add_flags(TypeFlags::HAS_CT_PLACEHOLDER);
-            }
-            ty::ConstKind::Value(cv) => self.add_ty(cv.ty),
-            ty::ConstKind::Expr(e) => self.add_args(e.args()),
-            ty::ConstKind::Error(_) => self.add_flags(TypeFlags::HAS_ERROR),
-        }
-    }
-
-    fn add_existential_projection(&mut self, projection: &ty::ExistentialProjection<'_>) {
-        self.add_args(projection.args);
-        match projection.term.unpack() {
-            ty::TermKind::Ty(ty) => self.add_ty(ty),
-            ty::TermKind::Const(ct) => self.add_const(ct),
-        }
-    }
-
-    fn add_alias_ty(&mut self, alias_ty: ty::AliasTy<'_>) {
-        self.add_args(alias_ty.args);
-    }
-
-    fn add_alias_term(&mut self, alias_term: ty::AliasTerm<'_>) {
-        self.add_args(alias_term.args);
-    }
-
-    fn add_args(&mut self, args: &[GenericArg<'_>]) {
-        for kind in args {
-            match kind.unpack() {
-                GenericArgKind::Type(ty) => self.add_ty(ty),
-                GenericArgKind::Lifetime(lt) => self.add_region(lt),
-                GenericArgKind::Const(ct) => self.add_const(ct),
-            }
-        }
-    }
-
-    fn add_term(&mut self, term: ty::Term<'_>) {
-        match term.unpack() {
-            ty::TermKind::Ty(ty) => self.add_ty(ty),
-            ty::TermKind::Const(ct) => self.add_const(ct),
-        }
-    }
-}
diff --git a/compiler/rustc_middle/src/ty/fold.rs b/compiler/rustc_middle/src/ty/fold.rs
index 8dc73e4ce85..8d6871d2f1f 100644
--- a/compiler/rustc_middle/src/ty/fold.rs
+++ b/compiler/rustc_middle/src/ty/fold.rs
@@ -278,7 +278,7 @@ impl<'tcx> TyCtxt<'tcx> {
     where
         T: TypeFoldable<TyCtxt<'tcx>>,
     {
-        let shift_bv = |bv: ty::BoundVar| ty::BoundVar::from_usize(bv.as_usize() + bound_vars);
+        let shift_bv = |bv: ty::BoundVar| bv + bound_vars;
         self.replace_escaping_bound_vars_uncached(
             value,
             FnMutDelegate {
diff --git a/compiler/rustc_middle/src/ty/generic_args.rs b/compiler/rustc_middle/src/ty/generic_args.rs
index 9c1ff134f0f..8de64b3bfac 100644
--- a/compiler/rustc_middle/src/ty/generic_args.rs
+++ b/compiler/rustc_middle/src/ty/generic_args.rs
@@ -11,12 +11,13 @@ use rustc_hir::def_id::DefId;
 use rustc_macros::{HashStable, TyDecodable, TyEncodable, extension};
 use rustc_serialize::{Decodable, Encodable};
 use rustc_type_ir::WithCachedTypeInfo;
+use rustc_type_ir::walk::TypeWalker;
 use smallvec::SmallVec;
 
 use crate::ty::codec::{TyDecoder, TyEncoder};
 use crate::ty::{
     self, ClosureArgs, CoroutineArgs, CoroutineClosureArgs, FallibleTypeFolder, InlineConstArgs,
-    Lift, List, Ty, TyCtxt, TypeFoldable, TypeVisitable, TypeVisitor, VisitorResult,
+    Lift, List, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeVisitable, TypeVisitor, VisitorResult,
     walk_visitable_list,
 };
 
@@ -297,6 +298,20 @@ impl<'tcx> GenericArg<'tcx> {
             GenericArgKind::Const(ct) => ct.is_ct_infer(),
         }
     }
+
+    /// Iterator that walks `self` and any types reachable from
+    /// `self`, in depth-first order. Note that just walks the types
+    /// that appear in `self`, it does not descend into the fields of
+    /// structs or variants. For example:
+    ///
+    /// ```text
+    /// isize => { isize }
+    /// Foo<Bar<isize>> => { Foo<Bar<isize>>, Bar<isize>, isize }
+    /// [isize] => { [isize], isize }
+    /// ```
+    pub fn walk(self) -> TypeWalker<TyCtxt<'tcx>> {
+        TypeWalker::new(self)
+    }
 }
 
 impl<'a, 'tcx> Lift<TyCtxt<'tcx>> for GenericArg<'a> {
@@ -322,6 +337,14 @@ impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for GenericArg<'tcx> {
             GenericArgKind::Const(ct) => ct.try_fold_with(folder).map(Into::into),
         }
     }
+
+    fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, folder: &mut F) -> Self {
+        match self.unpack() {
+            GenericArgKind::Lifetime(lt) => lt.fold_with(folder).into(),
+            GenericArgKind::Type(ty) => ty.fold_with(folder).into(),
+            GenericArgKind::Const(ct) => ct.fold_with(folder).into(),
+        }
+    }
 }
 
 impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for GenericArg<'tcx> {
@@ -591,6 +614,27 @@ impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for GenericArgsRef<'tcx> {
                 }
             }
             0 => Ok(self),
+            _ => ty::util::try_fold_list(self, folder, |tcx, v| tcx.mk_args(v)),
+        }
+    }
+
+    fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, folder: &mut F) -> Self {
+        // See justification for this behavior in `try_fold_with`.
+        match self.len() {
+            1 => {
+                let param0 = self[0].fold_with(folder);
+                if param0 == self[0] { self } else { folder.cx().mk_args(&[param0]) }
+            }
+            2 => {
+                let param0 = self[0].fold_with(folder);
+                let param1 = self[1].fold_with(folder);
+                if param0 == self[0] && param1 == self[1] {
+                    self
+                } else {
+                    folder.cx().mk_args(&[param0, param1])
+                }
+            }
+            0 => self,
             _ => ty::util::fold_list(self, folder, |tcx, v| tcx.mk_args(v)),
         }
     }
@@ -626,6 +670,22 @@ impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for &'tcx ty::List<Ty<'tcx>> {
                     Ok(folder.cx().mk_type_list(&[param0, param1]))
                 }
             }
+            _ => ty::util::try_fold_list(self, folder, |tcx, v| tcx.mk_type_list(v)),
+        }
+    }
+
+    fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, folder: &mut F) -> Self {
+        // See comment justifying behavior in `try_fold_with`.
+        match self.len() {
+            2 => {
+                let param0 = self[0].fold_with(folder);
+                let param1 = self[1].fold_with(folder);
+                if param0 == self[0] && param1 == self[1] {
+                    self
+                } else {
+                    folder.cx().mk_type_list(&[param0, param1])
+                }
+            }
             _ => ty::util::fold_list(self, folder, |tcx, v| tcx.mk_type_list(v)),
         }
     }
diff --git a/compiler/rustc_middle/src/ty/instance.rs b/compiler/rustc_middle/src/ty/instance.rs
index 07f2a602f2b..faad0a82acb 100644
--- a/compiler/rustc_middle/src/ty/instance.rs
+++ b/compiler/rustc_middle/src/ty/instance.rs
@@ -746,7 +746,7 @@ impl<'tcx> Instance<'tcx> {
         let call_once = tcx
             .associated_items(fn_once)
             .in_definition_order()
-            .find(|it| it.kind == ty::AssocKind::Fn)
+            .find(|it| it.is_fn())
             .unwrap()
             .def_id;
         let track_caller =
diff --git a/compiler/rustc_middle/src/ty/list.rs b/compiler/rustc_middle/src/ty/list.rs
index 0fd370a5619..0cf5820959e 100644
--- a/compiler/rustc_middle/src/ty/list.rs
+++ b/compiler/rustc_middle/src/ty/list.rs
@@ -7,9 +7,9 @@ use std::{fmt, iter, mem, ptr, slice};
 use rustc_data_structures::aligned::{Aligned, align_of};
 use rustc_data_structures::sync::DynSync;
 use rustc_serialize::{Encodable, Encoder};
+use rustc_type_ir::FlagComputation;
 
-use super::flags::FlagComputation;
-use super::{DebruijnIndex, TypeFlags};
+use super::{DebruijnIndex, TyCtxt, TypeFlags};
 use crate::arena::Arena;
 
 /// `List<T>` is a bit like `&[T]`, but with some critical differences.
@@ -299,8 +299,8 @@ impl TypeInfo {
     }
 }
 
-impl From<FlagComputation> for TypeInfo {
-    fn from(computation: FlagComputation) -> TypeInfo {
+impl<'tcx> From<FlagComputation<TyCtxt<'tcx>>> for TypeInfo {
+    fn from(computation: FlagComputation<TyCtxt<'tcx>>) -> TypeInfo {
         TypeInfo {
             flags: computation.flags,
             outer_exclusive_binder: computation.outer_exclusive_binder,
diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
index a2b3acac3f2..3698cfb9870 100644
--- a/compiler/rustc_middle/src/ty/mod.rs
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -117,7 +117,6 @@ pub mod cast;
 pub mod codec;
 pub mod error;
 pub mod fast_reject;
-pub mod flags;
 pub mod inhabitedness;
 pub mod layout;
 pub mod normalize_erasing_regions;
@@ -128,7 +127,6 @@ pub mod significant_drop_order;
 pub mod trait_def;
 pub mod util;
 pub mod vtable;
-pub mod walk;
 
 mod adt;
 mod assoc;
@@ -539,6 +537,13 @@ impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for Term<'tcx> {
             ty::TermKind::Const(ct) => ct.try_fold_with(folder).map(Into::into),
         }
     }
+
+    fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, folder: &mut F) -> Self {
+        match self.unpack() {
+            ty::TermKind::Ty(ty) => ty.fold_with(folder).into(),
+            ty::TermKind::Const(ct) => ct.fold_with(folder).into(),
+        }
+    }
 }
 
 impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for Term<'tcx> {
@@ -1464,7 +1469,7 @@ impl<'tcx> TyCtxt<'tcx> {
     pub fn provided_trait_methods(self, id: DefId) -> impl 'tcx + Iterator<Item = &'tcx AssocItem> {
         self.associated_items(id)
             .in_definition_order()
-            .filter(move |item| item.kind == AssocKind::Fn && item.defaultness(self).has_value())
+            .filter(move |item| item.is_fn() && item.defaultness(self).has_value())
     }
 
     pub fn repr_options_of_def(self, did: LocalDefId) -> ReprOptions {
@@ -1610,8 +1615,11 @@ impl<'tcx> TyCtxt<'tcx> {
     /// return-position `impl Trait` from a trait, then provide the source info
     /// about where that RPITIT came from.
     pub fn opt_rpitit_info(self, def_id: DefId) -> Option<ImplTraitInTraitData> {
-        if let DefKind::AssocTy = self.def_kind(def_id) {
-            self.associated_item(def_id).opt_rpitit_info
+        if let DefKind::AssocTy = self.def_kind(def_id)
+            && let AssocKind::Type { data: AssocTypeData::Rpitit(rpitit_info) } =
+                self.associated_item(def_id).kind
+        {
+            Some(rpitit_info)
         } else {
             None
         }
diff --git a/compiler/rustc_middle/src/ty/parameterized.rs b/compiler/rustc_middle/src/ty/parameterized.rs
index b56e0862692..ecd6132b3ef 100644
--- a/compiler/rustc_middle/src/ty/parameterized.rs
+++ b/compiler/rustc_middle/src/ty/parameterized.rs
@@ -113,7 +113,7 @@ trivially_parameterized_over_tcx! {
     rustc_span::Span,
     rustc_span::Symbol,
     rustc_span::def_id::DefPathHash,
-    rustc_span::hygiene::SyntaxContextDataNonRecursive,
+    rustc_span::hygiene::SyntaxContextKey,
     rustc_span::Ident,
     rustc_type_ir::Variance,
     rustc_hir::Attribute,
diff --git a/compiler/rustc_middle/src/ty/pattern.rs b/compiler/rustc_middle/src/ty/pattern.rs
index 4cad1ab2099..758adc42e3e 100644
--- a/compiler/rustc_middle/src/ty/pattern.rs
+++ b/compiler/rustc_middle/src/ty/pattern.rs
@@ -1,14 +1,40 @@
 use std::fmt;
 
 use rustc_data_structures::intern::Interned;
-use rustc_macros::{HashStable, TyDecodable, TyEncodable, TypeFoldable, TypeVisitable};
+use rustc_macros::HashStable;
+use rustc_type_ir::ir_print::IrPrint;
+use rustc_type_ir::{
+    FlagComputation, Flags, {self as ir},
+};
 
+use super::TyCtxt;
 use crate::ty;
 
+pub type PatternKind<'tcx> = ir::PatternKind<TyCtxt<'tcx>>;
+
 #[derive(Copy, Clone, PartialEq, Eq, Hash, HashStable)]
 #[rustc_pass_by_value]
 pub struct Pattern<'tcx>(pub Interned<'tcx, PatternKind<'tcx>>);
 
+impl<'tcx> Flags for Pattern<'tcx> {
+    fn flags(&self) -> rustc_type_ir::TypeFlags {
+        match &**self {
+            ty::PatternKind::Range { start, end } => {
+                FlagComputation::for_const_kind(&start.kind()).flags
+                    | FlagComputation::for_const_kind(&end.kind()).flags
+            }
+        }
+    }
+
+    fn outer_exclusive_binder(&self) -> rustc_type_ir::DebruijnIndex {
+        match &**self {
+            ty::PatternKind::Range { start, end } => {
+                start.outer_exclusive_binder().max(end.outer_exclusive_binder())
+            }
+        }
+    }
+}
+
 impl<'tcx> std::ops::Deref for Pattern<'tcx> {
     type Target = PatternKind<'tcx>;
 
@@ -23,9 +49,9 @@ impl<'tcx> fmt::Debug for Pattern<'tcx> {
     }
 }
 
-impl<'tcx> fmt::Debug for PatternKind<'tcx> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        match *self {
+impl<'tcx> IrPrint<PatternKind<'tcx>> for TyCtxt<'tcx> {
+    fn print(t: &PatternKind<'tcx>, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *t {
             PatternKind::Range { start, end } => {
                 write!(f, "{start}")?;
 
@@ -53,10 +79,15 @@ impl<'tcx> fmt::Debug for PatternKind<'tcx> {
             }
         }
     }
+
+    fn print_debug(t: &PatternKind<'tcx>, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        Self::print(t, fmt)
+    }
 }
 
-#[derive(Clone, PartialEq, Eq, Hash)]
-#[derive(HashStable, TyEncodable, TyDecodable, TypeVisitable, TypeFoldable)]
-pub enum PatternKind<'tcx> {
-    Range { start: ty::Const<'tcx>, end: ty::Const<'tcx> },
+impl<'tcx> rustc_type_ir::inherent::IntoKind for Pattern<'tcx> {
+    type Kind = PatternKind<'tcx>;
+    fn kind(self) -> Self::Kind {
+        *self
+    }
 }
diff --git a/compiler/rustc_middle/src/ty/print/pretty.rs b/compiler/rustc_middle/src/ty/print/pretty.rs
index 2f93197bcef..d739218af5e 100644
--- a/compiler/rustc_middle/src/ty/print/pretty.rs
+++ b/compiler/rustc_middle/src/ty/print/pretty.rs
@@ -1214,7 +1214,7 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
                             && assoc
                                 .trait_container(tcx)
                                 .is_some_and(|def_id| tcx.is_lang_item(def_id, LangItem::Coroutine))
-                            && assoc.name == rustc_span::sym::Return
+                            && assoc.opt_name() == Some(rustc_span::sym::Return)
                         {
                             if let ty::Coroutine(_, args) = args.type_at(0).kind() {
                                 let return_ty = args.as_coroutine().return_ty();
@@ -1237,7 +1237,7 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
                             p!(", ");
                         }
 
-                        p!(write("{} = ", tcx.associated_item(assoc_item_def_id).name));
+                        p!(write("{} = ", tcx.associated_item(assoc_item_def_id).name()));
 
                         match term.unpack() {
                             TermKind::Ty(ty) => p!(print(ty)),
@@ -3291,7 +3291,7 @@ define_print! {
     }
 
     ty::ExistentialProjection<'tcx> {
-        let name = cx.tcx().associated_item(self.def_id).name;
+        let name = cx.tcx().associated_item(self.def_id).name();
         // The args don't contain the self ty (as it has been erased) but the corresp.
         // generics do as the trait always has a self ty param. We need to offset.
         let args = &self.args[cx.tcx().generics_of(self.def_id).parent_count - 1..];
diff --git a/compiler/rustc_middle/src/ty/structural_impls.rs b/compiler/rustc_middle/src/ty/structural_impls.rs
index 798ef352c40..40eef541423 100644
--- a/compiler/rustc_middle/src/ty/structural_impls.rs
+++ b/compiler/rustc_middle/src/ty/structural_impls.rs
@@ -6,20 +6,19 @@
 use std::fmt::{self, Debug};
 
 use rustc_abi::TyAndLayout;
-use rustc_ast::InlineAsmTemplatePiece;
 use rustc_hir::def::Namespace;
 use rustc_hir::def_id::LocalDefId;
-use rustc_span::Span;
 use rustc_span::source_map::Spanned;
-use rustc_type_ir::{ConstKind, VisitorResult, try_visit};
+use rustc_type_ir::{ConstKind, TypeFolder, VisitorResult, try_visit};
 
 use super::print::PrettyPrinter;
 use super::{GenericArg, GenericArgKind, Pattern, Region};
+use crate::infer::canonical::CanonicalVarInfos;
 use crate::mir::PlaceElem;
 use crate::ty::print::{FmtPrinter, Printer, with_no_trimmed_paths};
 use crate::ty::{
-    self, FallibleTypeFolder, InferConst, Lift, Term, TermKind, Ty, TyCtxt, TypeFoldable,
-    TypeSuperFoldable, TypeSuperVisitable, TypeVisitable, TypeVisitor,
+    self, FallibleTypeFolder, Lift, Term, TermKind, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable,
+    TypeSuperVisitable, TypeVisitable, TypeVisitor,
 };
 
 impl fmt::Debug for ty::TraitDef {
@@ -271,6 +270,7 @@ TrivialTypeTraversalImpls! {
     crate::ty::AssocKind,
     crate::ty::BoundRegion,
     crate::ty::BoundVar,
+    crate::ty::InferConst,
     crate::ty::Placeholder<crate::ty::BoundRegion>,
     crate::ty::Placeholder<crate::ty::BoundTy>,
     crate::ty::Placeholder<ty::BoundVar>,
@@ -337,24 +337,6 @@ impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for ty::AdtDef<'tcx> {
     }
 }
 
-impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>> {
-    fn try_fold_with<F: FallibleTypeFolder<TyCtxt<'tcx>>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
-        ty::util::fold_list(self, folder, |tcx, v| tcx.mk_poly_existential_predicates(v))
-    }
-}
-
-impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for &'tcx ty::List<ty::Const<'tcx>> {
-    fn try_fold_with<F: FallibleTypeFolder<TyCtxt<'tcx>>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
-        ty::util::fold_list(self, folder, |tcx, v| tcx.mk_const_list(v))
-    }
-}
-
 impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for Pattern<'tcx> {
     fn try_fold_with<F: FallibleTypeFolder<TyCtxt<'tcx>>>(
         self,
@@ -363,6 +345,11 @@ impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for Pattern<'tcx> {
         let pat = (*self).clone().try_fold_with(folder)?;
         Ok(if pat == *self { self } else { folder.cx().mk_pat(pat) })
     }
+
+    fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, folder: &mut F) -> Self {
+        let pat = (*self).clone().fold_with(folder);
+        if pat == *self { self } else { folder.cx().mk_pat(pat) }
+    }
 }
 
 impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for Pattern<'tcx> {
@@ -378,6 +365,10 @@ impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for Ty<'tcx> {
     ) -> Result<Self, F::Error> {
         folder.try_fold_ty(self)
     }
+
+    fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, folder: &mut F) -> Self {
+        folder.fold_ty(self)
+    }
 }
 
 impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for Ty<'tcx> {
@@ -436,6 +427,45 @@ impl<'tcx> TypeSuperFoldable<TyCtxt<'tcx>> for Ty<'tcx> {
 
         Ok(if *self.kind() == kind { self } else { folder.cx().mk_ty_from_kind(kind) })
     }
+
+    fn super_fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, folder: &mut F) -> Self {
+        let kind = match *self.kind() {
+            ty::RawPtr(ty, mutbl) => ty::RawPtr(ty.fold_with(folder), mutbl),
+            ty::Array(typ, sz) => ty::Array(typ.fold_with(folder), sz.fold_with(folder)),
+            ty::Slice(typ) => ty::Slice(typ.fold_with(folder)),
+            ty::Adt(tid, args) => ty::Adt(tid, args.fold_with(folder)),
+            ty::Dynamic(trait_ty, region, representation) => {
+                ty::Dynamic(trait_ty.fold_with(folder), region.fold_with(folder), representation)
+            }
+            ty::Tuple(ts) => ty::Tuple(ts.fold_with(folder)),
+            ty::FnDef(def_id, args) => ty::FnDef(def_id, args.fold_with(folder)),
+            ty::FnPtr(sig_tys, hdr) => ty::FnPtr(sig_tys.fold_with(folder), hdr),
+            ty::UnsafeBinder(f) => ty::UnsafeBinder(f.fold_with(folder)),
+            ty::Ref(r, ty, mutbl) => ty::Ref(r.fold_with(folder), ty.fold_with(folder), mutbl),
+            ty::Coroutine(did, args) => ty::Coroutine(did, args.fold_with(folder)),
+            ty::CoroutineWitness(did, args) => ty::CoroutineWitness(did, args.fold_with(folder)),
+            ty::Closure(did, args) => ty::Closure(did, args.fold_with(folder)),
+            ty::CoroutineClosure(did, args) => ty::CoroutineClosure(did, args.fold_with(folder)),
+            ty::Alias(kind, data) => ty::Alias(kind, data.fold_with(folder)),
+            ty::Pat(ty, pat) => ty::Pat(ty.fold_with(folder), pat.fold_with(folder)),
+
+            ty::Bool
+            | ty::Char
+            | ty::Str
+            | ty::Int(_)
+            | ty::Uint(_)
+            | ty::Float(_)
+            | ty::Error(_)
+            | ty::Infer(_)
+            | ty::Param(..)
+            | ty::Bound(..)
+            | ty::Placeholder(..)
+            | ty::Never
+            | ty::Foreign(..) => return self,
+        };
+
+        if *self.kind() == kind { self } else { folder.cx().mk_ty_from_kind(kind) }
+    }
 }
 
 impl<'tcx> TypeSuperVisitable<TyCtxt<'tcx>> for Ty<'tcx> {
@@ -496,6 +526,10 @@ impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for ty::Region<'tcx> {
     ) -> Result<Self, F::Error> {
         folder.try_fold_region(self)
     }
+
+    fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, folder: &mut F) -> Self {
+        folder.fold_region(self)
+    }
 }
 
 impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for ty::Region<'tcx> {
@@ -511,6 +545,10 @@ impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for ty::Predicate<'tcx> {
     ) -> Result<Self, F::Error> {
         folder.try_fold_predicate(self)
     }
+
+    fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, folder: &mut F) -> Self {
+        folder.fold_predicate(self)
+    }
 }
 
 // FIXME(clause): This is wonky
@@ -521,6 +559,10 @@ impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for ty::Clause<'tcx> {
     ) -> Result<Self, F::Error> {
         Ok(folder.try_fold_predicate(self.as_predicate())?.expect_clause())
     }
+
+    fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, folder: &mut F) -> Self {
+        folder.fold_predicate(self.as_predicate()).expect_clause()
+    }
 }
 
 impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for ty::Predicate<'tcx> {
@@ -543,6 +585,11 @@ impl<'tcx> TypeSuperFoldable<TyCtxt<'tcx>> for ty::Predicate<'tcx> {
         let new = self.kind().try_fold_with(folder)?;
         Ok(folder.cx().reuse_or_mk_predicate(self, new))
     }
+
+    fn super_fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, folder: &mut F) -> Self {
+        let new = self.kind().fold_with(folder);
+        folder.cx().reuse_or_mk_predicate(self, new)
+    }
 }
 
 impl<'tcx> TypeSuperVisitable<TyCtxt<'tcx>> for ty::Predicate<'tcx> {
@@ -563,15 +610,6 @@ impl<'tcx> TypeSuperVisitable<TyCtxt<'tcx>> for ty::Clauses<'tcx> {
     }
 }
 
-impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for ty::Clauses<'tcx> {
-    fn try_fold_with<F: FallibleTypeFolder<TyCtxt<'tcx>>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
-        ty::util::fold_list(self, folder, |tcx, v| tcx.mk_clauses(v))
-    }
-}
-
 impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for ty::Const<'tcx> {
     fn try_fold_with<F: FallibleTypeFolder<TyCtxt<'tcx>>>(
         self,
@@ -579,6 +617,10 @@ impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for ty::Const<'tcx> {
     ) -> Result<Self, F::Error> {
         folder.try_fold_const(self)
     }
+
+    fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, folder: &mut F) -> Self {
+        folder.fold_const(self)
+    }
 }
 
 impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for ty::Const<'tcx> {
@@ -606,6 +648,20 @@ impl<'tcx> TypeSuperFoldable<TyCtxt<'tcx>> for ty::Const<'tcx> {
         };
         if kind != self.kind() { Ok(folder.cx().mk_ct_from_kind(kind)) } else { Ok(self) }
     }
+
+    fn super_fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, folder: &mut F) -> Self {
+        let kind = match self.kind() {
+            ConstKind::Param(p) => ConstKind::Param(p.fold_with(folder)),
+            ConstKind::Infer(i) => ConstKind::Infer(i.fold_with(folder)),
+            ConstKind::Bound(d, b) => ConstKind::Bound(d.fold_with(folder), b.fold_with(folder)),
+            ConstKind::Placeholder(p) => ConstKind::Placeholder(p.fold_with(folder)),
+            ConstKind::Unevaluated(uv) => ConstKind::Unevaluated(uv.fold_with(folder)),
+            ConstKind::Value(v) => ConstKind::Value(v.fold_with(folder)),
+            ConstKind::Error(e) => ConstKind::Error(e.fold_with(folder)),
+            ConstKind::Expr(e) => ConstKind::Expr(e.fold_with(folder)),
+        };
+        if kind != self.kind() { folder.cx().mk_ct_from_kind(kind) } else { self }
+    }
 }
 
 impl<'tcx> TypeSuperVisitable<TyCtxt<'tcx>> for ty::Const<'tcx> {
@@ -639,20 +695,9 @@ impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for rustc_span::ErrorGuaranteed {
     ) -> Result<Self, F::Error> {
         Ok(self)
     }
-}
 
-impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for InferConst {
-    fn try_fold_with<F: FallibleTypeFolder<TyCtxt<'tcx>>>(
-        self,
-        _folder: &mut F,
-    ) -> Result<Self, F::Error> {
-        Ok(self)
-    }
-}
-
-impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for InferConst {
-    fn visit_with<V: TypeVisitor<TyCtxt<'tcx>>>(&self, _visitor: &mut V) -> V::Result {
-        V::Result::output()
+    fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, _folder: &mut F) -> Self {
+        self
     }
 }
 
@@ -683,40 +728,50 @@ impl<'tcx, T: TypeFoldable<TyCtxt<'tcx>> + Debug + Clone> TypeFoldable<TyCtxt<'t
             span: self.span.try_fold_with(folder)?,
         })
     }
-}
 
-impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for &'tcx [InlineAsmTemplatePiece] {
-    fn try_fold_with<F: FallibleTypeFolder<TyCtxt<'tcx>>>(
-        self,
-        _folder: &mut F,
-    ) -> Result<Self, F::Error> {
-        Ok(self)
+    fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, folder: &mut F) -> Self {
+        Spanned { node: self.node.fold_with(folder), span: self.span.fold_with(folder) }
     }
 }
 
-impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for &'tcx [Span] {
+impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for &'tcx ty::List<LocalDefId> {
     fn try_fold_with<F: FallibleTypeFolder<TyCtxt<'tcx>>>(
         self,
         _folder: &mut F,
     ) -> Result<Self, F::Error> {
         Ok(self)
     }
-}
 
-impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for &'tcx ty::List<LocalDefId> {
-    fn try_fold_with<F: FallibleTypeFolder<TyCtxt<'tcx>>>(
-        self,
-        _folder: &mut F,
-    ) -> Result<Self, F::Error> {
-        Ok(self)
+    fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, _folder: &mut F) -> Self {
+        self
     }
 }
 
-impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for &'tcx ty::List<PlaceElem<'tcx>> {
-    fn try_fold_with<F: FallibleTypeFolder<TyCtxt<'tcx>>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
-        ty::util::fold_list(self, folder, |tcx, v| tcx.mk_place_elems(v))
+macro_rules! list_fold {
+    ($($ty:ty : $mk:ident),+ $(,)?) => {
+        $(
+            impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for $ty {
+                fn try_fold_with<F: FallibleTypeFolder<TyCtxt<'tcx>>>(
+                    self,
+                    folder: &mut F,
+                ) -> Result<Self, F::Error> {
+                    ty::util::try_fold_list(self, folder, |tcx, v| tcx.$mk(v))
+                }
+
+                fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(
+                    self,
+                    folder: &mut F,
+                ) -> Self {
+                    ty::util::fold_list(self, folder, |tcx, v| tcx.$mk(v))
+                }
+            }
+        )*
     }
 }
+
+list_fold! {
+    ty::Clauses<'tcx> : mk_clauses,
+    &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>> : mk_poly_existential_predicates,
+    &'tcx ty::List<PlaceElem<'tcx>> : mk_place_elems,
+    CanonicalVarInfos<'tcx> : mk_canonical_var_infos,
+}
diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
index 27ee363f1c1..301ae604574 100644
--- a/compiler/rustc_middle/src/ty/sty.rs
+++ b/compiler/rustc_middle/src/ty/sty.rs
@@ -16,6 +16,7 @@ use rustc_hir::def_id::DefId;
 use rustc_macros::{HashStable, TyDecodable, TyEncodable, TypeFoldable, extension};
 use rustc_span::{DUMMY_SP, Span, Symbol, sym};
 use rustc_type_ir::TyKind::*;
+use rustc_type_ir::walk::TypeWalker;
 use rustc_type_ir::{self as ir, BoundVar, CollectAndApply, DynKind, TypeVisitableExt, elaborate};
 use tracing::instrument;
 use ty::util::{AsyncDropGlueMorphology, IntTypeExt};
@@ -734,7 +735,7 @@ impl<'tcx> Ty<'tcx> {
                     .map(|principal| {
                         tcx.associated_items(principal.def_id())
                             .in_definition_order()
-                            .filter(|item| item.kind == ty::AssocKind::Type)
+                            .filter(|item| item.is_type())
                             .filter(|item| !item.is_impl_trait_in_trait())
                             .filter(|item| !tcx.generics_require_sized_self(item.def_id))
                             .count()
@@ -1872,14 +1873,14 @@ impl<'tcx> Ty<'tcx> {
 
     /// Fast path helper for testing if a type is `Sized`.
     ///
-    /// Returning true means the type is known to be sized. Returning
-    /// `false` means nothing -- could be sized, might not be.
+    /// Returning true means the type is known to implement `Sized`. Returning `false` means
+    /// nothing -- could be sized, might not be.
     ///
-    /// Note that we could never rely on the fact that a type such as `[_]` is
-    /// trivially `!Sized` because we could be in a type environment with a
-    /// bound such as `[_]: Copy`. A function with such a bound obviously never
-    /// can be called, but that doesn't mean it shouldn't typecheck. This is why
-    /// this method doesn't return `Option<bool>`.
+    /// Note that we could never rely on the fact that a type such as `[_]` is trivially `!Sized`
+    /// because we could be in a type environment with a bound such as `[_]: Copy`. A function with
+    /// such a bound obviously never can be called, but that doesn't mean it shouldn't typecheck.
+    /// This is why this method doesn't return `Option<bool>`.
+    #[instrument(skip(tcx), level = "debug")]
     pub fn is_trivially_sized(self, tcx: TyCtxt<'tcx>) -> bool {
         match self.kind() {
             ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
@@ -2029,6 +2030,20 @@ impl<'tcx> Ty<'tcx> {
     pub fn is_known_rigid(self) -> bool {
         self.kind().is_known_rigid()
     }
+
+    /// Iterator that walks `self` and any types reachable from
+    /// `self`, in depth-first order. Note that just walks the types
+    /// that appear in `self`, it does not descend into the fields of
+    /// structs or variants. For example:
+    ///
+    /// ```text
+    /// isize => { isize }
+    /// Foo<Bar<isize>> => { Foo<Bar<isize>>, Bar<isize>, isize }
+    /// [isize] => { [isize], isize }
+    /// ```
+    pub fn walk(self) -> TypeWalker<TyCtxt<'tcx>> {
+        TypeWalker::new(self.into())
+    }
 }
 
 impl<'tcx> rustc_type_ir::inherent::Tys<TyCtxt<'tcx>> for &'tcx ty::List<Ty<'tcx>> {
diff --git a/compiler/rustc_middle/src/ty/util.rs b/compiler/rustc_middle/src/ty/util.rs
index 857b462b9eb..08cee1101d0 100644
--- a/compiler/rustc_middle/src/ty/util.rs
+++ b/compiler/rustc_middle/src/ty/util.rs
@@ -819,7 +819,7 @@ impl<'tcx> TyCtxt<'tcx> {
     /// Get an English description for the item's kind.
     pub fn def_kind_descr(self, def_kind: DefKind, def_id: DefId) -> &'static str {
         match def_kind {
-            DefKind::AssocFn if self.associated_item(def_id).fn_has_self_parameter => "method",
+            DefKind::AssocFn if self.associated_item(def_id).is_method() => "method",
             DefKind::Closure if let Some(coroutine_kind) = self.coroutine_kind(def_id) => {
                 match coroutine_kind {
                     hir::CoroutineKind::Desugared(
@@ -873,7 +873,7 @@ impl<'tcx> TyCtxt<'tcx> {
     /// Gets an English article for the [`TyCtxt::def_kind_descr`].
     pub fn def_kind_descr_article(self, def_kind: DefKind, def_id: DefId) -> &'static str {
         match def_kind {
-            DefKind::AssocFn if self.associated_item(def_id).fn_has_self_parameter => "a",
+            DefKind::AssocFn if self.associated_item(def_id).is_method() => "a",
             DefKind::Closure if let Some(coroutine_kind) = self.coroutine_kind(def_id) => {
                 match coroutine_kind {
                     hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::Async, ..) => "an",
@@ -1647,6 +1647,42 @@ pub fn fold_list<'tcx, F, L, T>(
     list: L,
     folder: &mut F,
     intern: impl FnOnce(TyCtxt<'tcx>, &[T]) -> L,
+) -> L
+where
+    F: TypeFolder<TyCtxt<'tcx>>,
+    L: AsRef<[T]>,
+    T: TypeFoldable<TyCtxt<'tcx>> + PartialEq + Copy,
+{
+    let slice = list.as_ref();
+    let mut iter = slice.iter().copied();
+    // Look for the first element that changed
+    match iter.by_ref().enumerate().find_map(|(i, t)| {
+        let new_t = t.fold_with(folder);
+        if new_t != t { Some((i, new_t)) } else { None }
+    }) {
+        Some((i, new_t)) => {
+            // An element changed, prepare to intern the resulting list
+            let mut new_list = SmallVec::<[_; 8]>::with_capacity(slice.len());
+            new_list.extend_from_slice(&slice[..i]);
+            new_list.push(new_t);
+            for t in iter {
+                new_list.push(t.fold_with(folder))
+            }
+            intern(folder.cx(), &new_list)
+        }
+        None => list,
+    }
+}
+
+/// Does the equivalent of
+/// ```ignore (illustrative)
+/// let v = self.iter().map(|p| p.try_fold_with(folder)).collect::<SmallVec<[_; 8]>>();
+/// folder.tcx().intern_*(&v)
+/// ```
+pub fn try_fold_list<'tcx, F, L, T>(
+    list: L,
+    folder: &mut F,
+    intern: impl FnOnce(TyCtxt<'tcx>, &[T]) -> L,
 ) -> Result<L, F::Error>
 where
     F: FallibleTypeFolder<TyCtxt<'tcx>>,
diff --git a/compiler/rustc_middle/src/ty/visit.rs b/compiler/rustc_middle/src/ty/visit.rs
index b341b30af6a..44c7b6a7c9e 100644
--- a/compiler/rustc_middle/src/ty/visit.rs
+++ b/compiler/rustc_middle/src/ty/visit.rs
@@ -231,9 +231,7 @@ impl MaxUniverse {
 impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for MaxUniverse {
     fn visit_ty(&mut self, t: Ty<'tcx>) {
         if let ty::Placeholder(placeholder) = t.kind() {
-            self.max_universe = ty::UniverseIndex::from_u32(
-                self.max_universe.as_u32().max(placeholder.universe.as_u32()),
-            );
+            self.max_universe = self.max_universe.max(placeholder.universe);
         }
 
         t.super_visit_with(self)
@@ -241,9 +239,7 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for MaxUniverse {
 
     fn visit_const(&mut self, c: ty::consts::Const<'tcx>) {
         if let ty::ConstKind::Placeholder(placeholder) = c.kind() {
-            self.max_universe = ty::UniverseIndex::from_u32(
-                self.max_universe.as_u32().max(placeholder.universe.as_u32()),
-            );
+            self.max_universe = self.max_universe.max(placeholder.universe);
         }
 
         c.super_visit_with(self)
@@ -251,9 +247,7 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for MaxUniverse {
 
     fn visit_region(&mut self, r: ty::Region<'tcx>) {
         if let ty::RePlaceholder(placeholder) = r.kind() {
-            self.max_universe = ty::UniverseIndex::from_u32(
-                self.max_universe.as_u32().max(placeholder.universe.as_u32()),
-            );
+            self.max_universe = self.max_universe.max(placeholder.universe);
         }
     }
 }
diff --git a/compiler/rustc_mir_build/src/builder/matches/test.rs b/compiler/rustc_mir_build/src/builder/matches/test.rs
index d1f9d4c34fe..73cc3c86e22 100644
--- a/compiler/rustc_mir_build/src/builder/matches/test.rs
+++ b/compiler/rustc_mir_build/src/builder/matches/test.rs
@@ -140,8 +140,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 let success_block = target_block(TestBranch::Success);
                 let fail_block = target_block(TestBranch::Failure);
 
-                let expect_ty = value.ty();
-                let expect = self.literal_operand(test.span, value);
+                let mut expect_ty = value.ty();
+                let mut expect = self.literal_operand(test.span, value);
 
                 let mut place = place;
                 let mut block = block;
@@ -174,6 +174,31 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                         place = ref_str;
                         ty = ref_str_ty;
                     }
+                    &ty::Pat(base, _) => {
+                        assert_eq!(ty, value.ty());
+                        assert!(base.is_trivially_pure_clone_copy());
+
+                        let transmuted_place = self.temp(base, test.span);
+                        self.cfg.push_assign(
+                            block,
+                            self.source_info(scrutinee_span),
+                            transmuted_place,
+                            Rvalue::Cast(CastKind::Transmute, Operand::Copy(place), base),
+                        );
+
+                        let transmuted_expect = self.temp(base, test.span);
+                        self.cfg.push_assign(
+                            block,
+                            self.source_info(test.span),
+                            transmuted_expect,
+                            Rvalue::Cast(CastKind::Transmute, expect, base),
+                        );
+
+                        place = transmuted_place;
+                        expect = Operand::Copy(transmuted_expect);
+                        ty = base;
+                        expect_ty = base;
+                    }
                     _ => {}
                 }
 
@@ -715,7 +740,7 @@ fn trait_method<'tcx>(
     let item = tcx
         .associated_items(trait_def_id)
         .filter_by_name_unhygienic(method_name)
-        .find(|item| item.kind == ty::AssocKind::Fn)
+        .find(|item| item.is_fn())
         .expect("trait method not found");
 
     let method_ty = Ty::new_fn_def(tcx, item.def_id, args);
diff --git a/compiler/rustc_mir_build/src/errors.rs b/compiler/rustc_mir_build/src/errors.rs
index 0e16f871b16..ae09db50235 100644
--- a/compiler/rustc_mir_build/src/errors.rs
+++ b/compiler/rustc_mir_build/src/errors.rs
@@ -2,7 +2,7 @@ use rustc_data_structures::fx::FxIndexMap;
 use rustc_errors::codes::*;
 use rustc_errors::{
     Applicability, Diag, DiagArgValue, DiagCtxtHandle, Diagnostic, EmissionGuarantee, Level,
-    MultiSpan, SubdiagMessageOp, Subdiagnostic, pluralize,
+    MultiSpan, Subdiagnostic, pluralize,
 };
 use rustc_macros::{Diagnostic, LintDiagnostic, Subdiagnostic};
 use rustc_middle::ty::{self, Ty};
@@ -546,11 +546,7 @@ pub(crate) struct UnsafeNotInheritedLintNote {
 }
 
 impl Subdiagnostic for UnsafeNotInheritedLintNote {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         diag.span_note(self.signature_span, fluent::mir_build_unsafe_fn_safe_body);
         let body_start = self.body_span.shrink_to_lo();
         let body_end = self.body_span.shrink_to_hi();
@@ -1031,11 +1027,7 @@ pub(crate) struct Variant {
 }
 
 impl<'tcx> Subdiagnostic for AdtDefinedHere<'tcx> {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         diag.arg("ty", self.ty);
         let mut spans = MultiSpan::from(self.adt_def_span);
 
@@ -1117,11 +1109,7 @@ pub(crate) struct Rust2024IncompatiblePatSugg {
 }
 
 impl Subdiagnostic for Rust2024IncompatiblePatSugg {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         // Format and emit explanatory notes about default binding modes. Reversing the spans' order
         // means if we have nested spans, the innermost ones will be visited first.
         for (span, def_br_mutbl) in self.default_mode_labels.into_iter().rev() {
diff --git a/compiler/rustc_mir_build/src/lib.rs b/compiler/rustc_mir_build/src/lib.rs
index a25697ba086..8e96d46dac2 100644
--- a/compiler/rustc_mir_build/src/lib.rs
+++ b/compiler/rustc_mir_build/src/lib.rs
@@ -3,7 +3,6 @@
 // tidy-alphabetical-start
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![feature(assert_matches)]
 #![feature(box_patterns)]
 #![feature(if_let_guard)]
diff --git a/compiler/rustc_mir_build/src/thir/cx/expr.rs b/compiler/rustc_mir_build/src/thir/cx/expr.rs
index 31e22e69111..fde23413972 100644
--- a/compiler/rustc_mir_build/src/thir/cx/expr.rs
+++ b/compiler/rustc_mir_build/src/thir/cx/expr.rs
@@ -191,7 +191,7 @@ impl<'tcx> ThirBuildCx<'tcx> {
                 let pointer_target = ExprKind::Field {
                     lhs: self.thir.exprs.push(expr),
                     variant_index: FIRST_VARIANT,
-                    name: FieldIdx::from(0u32),
+                    name: FieldIdx::ZERO,
                 };
                 let arg = Expr { temp_lifetime, ty: pin_ty, span, kind: pointer_target };
                 let arg = self.thir.exprs.push(arg);
@@ -226,7 +226,7 @@ impl<'tcx> ThirBuildCx<'tcx> {
                     adt_def: self.tcx.adt_def(pin_did),
                     variant_index: FIRST_VARIANT,
                     args,
-                    fields: Box::new([FieldExpr { name: FieldIdx::from(0u32), expr }]),
+                    fields: Box::new([FieldExpr { name: FieldIdx::ZERO, expr }]),
                     user_ty: None,
                     base: AdtExprBase::None,
                 }));
diff --git a/compiler/rustc_mir_dataflow/src/framework/graphviz.rs b/compiler/rustc_mir_dataflow/src/framework/graphviz.rs
index 448fad2dc3e..c436b8c0fb0 100644
--- a/compiler/rustc_mir_dataflow/src/framework/graphviz.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/graphviz.rs
@@ -122,7 +122,7 @@ impl RustcMirAttrs {
                 })
             } else if attr.has_name(sym::borrowck_graphviz_format) {
                 Self::set_field(&mut ret.formatter, tcx, &attr, |s| match s {
-                    sym::gen_kill | sym::two_phase => Ok(s),
+                    sym::two_phase => Ok(s),
                     _ => {
                         tcx.dcx().emit_err(UnknownFormatter { span: attr.span() });
                         Err(())
diff --git a/compiler/rustc_mir_dataflow/src/lib.rs b/compiler/rustc_mir_dataflow/src/lib.rs
index 82c57ef5678..a0efc623b8e 100644
--- a/compiler/rustc_mir_dataflow/src/lib.rs
+++ b/compiler/rustc_mir_dataflow/src/lib.rs
@@ -1,5 +1,4 @@
 // tidy-alphabetical-start
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![feature(assert_matches)]
 #![feature(associated_type_defaults)]
 #![feature(box_patterns)]
diff --git a/compiler/rustc_mir_transform/src/check_undefined_transmutes.rs b/compiler/rustc_mir_transform/src/check_undefined_transmutes.rs
index ed3b1ae4f42..daddb5dedbc 100644
--- a/compiler/rustc_mir_transform/src/check_undefined_transmutes.rs
+++ b/compiler/rustc_mir_transform/src/check_undefined_transmutes.rs
@@ -42,7 +42,7 @@ impl<'a, 'tcx> UndefinedTransmutesChecker<'a, 'tcx> {
         if self.tcx.is_const_fn(def_id)
             || matches!(
                 self.tcx.opt_associated_item(def_id),
-                Some(AssocItem { kind: AssocKind::Const, .. })
+                Some(AssocItem { kind: AssocKind::Const { .. }, .. })
             )
         {
             let fn_sig = function.ty(self.body, self.tcx).fn_sig(self.tcx).skip_binder();
diff --git a/compiler/rustc_mir_transform/src/cleanup_post_borrowck.rs b/compiler/rustc_mir_transform/src/cleanup_post_borrowck.rs
index cb844019857..4be67b873f7 100644
--- a/compiler/rustc_mir_transform/src/cleanup_post_borrowck.rs
+++ b/compiler/rustc_mir_transform/src/cleanup_post_borrowck.rs
@@ -35,7 +35,8 @@ impl<'tcx> crate::MirPass<'tcx> for CleanupPostBorrowck {
                         // MIR building, and are not needed after InstrumentCoverage.
                         CoverageKind::BlockMarker { .. } | CoverageKind::SpanMarker { .. },
                     )
-                    | StatementKind::FakeRead(..) => statement.make_nop(),
+                    | StatementKind::FakeRead(..)
+                    | StatementKind::BackwardIncompatibleDropHint { .. } => statement.make_nop(),
                     StatementKind::Assign(box (
                         _,
                         Rvalue::Cast(
diff --git a/compiler/rustc_mir_transform/src/coroutine.rs b/compiler/rustc_mir_transform/src/coroutine.rs
index 04d96f11707..92d5e152f58 100644
--- a/compiler/rustc_mir_transform/src/coroutine.rs
+++ b/compiler/rustc_mir_transform/src/coroutine.rs
@@ -203,7 +203,7 @@ struct TransformVisitor<'tcx> {
 
 impl<'tcx> TransformVisitor<'tcx> {
     fn insert_none_ret_block(&self, body: &mut Body<'tcx>) -> BasicBlock {
-        let block = BasicBlock::new(body.basic_blocks.len());
+        let block = body.basic_blocks.next_index();
         let source_info = SourceInfo::outermost(body.span);
 
         let none_value = match self.coroutine_kind {
@@ -547,7 +547,7 @@ fn transform_async_context<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
 
     let get_context_def_id = tcx.require_lang_item(LangItem::GetContext, None);
 
-    for bb in START_BLOCK..body.basic_blocks.next_index() {
+    for bb in body.basic_blocks.indices() {
         let bb_data = &body[bb];
         if bb_data.is_cleanup {
             continue;
@@ -556,11 +556,11 @@ fn transform_async_context<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         match &bb_data.terminator().kind {
             TerminatorKind::Call { func, .. } => {
                 let func_ty = func.ty(body, tcx);
-                if let ty::FnDef(def_id, _) = *func_ty.kind() {
-                    if def_id == get_context_def_id {
-                        let local = eliminate_get_context_call(&mut body[bb]);
-                        replace_resume_ty_local(tcx, body, local, context_mut_ref);
-                    }
+                if let ty::FnDef(def_id, _) = *func_ty.kind()
+                    && def_id == get_context_def_id
+                {
+                    let local = eliminate_get_context_call(&mut body[bb]);
+                    replace_resume_ty_local(tcx, body, local, context_mut_ref);
                 }
             }
             TerminatorKind::Yield { resume_arg, .. } => {
@@ -1057,7 +1057,7 @@ fn insert_switch<'tcx>(
     let blocks = body.basic_blocks_mut().iter_mut();
 
     for target in blocks.flat_map(|b| b.terminator_mut().successors_mut()) {
-        *target = BasicBlock::new(target.index() + 1);
+        *target += 1;
     }
 }
 
@@ -1169,6 +1169,13 @@ fn create_coroutine_drop_shim<'tcx>(
     dump_mir(tcx, false, "coroutine_drop", &0, &body, |_, _| Ok(()));
     body.source.instance = drop_instance;
 
+    // Creating a coroutine drop shim happens on `Analysis(PostCleanup) -> Runtime(Initial)`
+    // but the pass manager doesn't update the phase of the coroutine drop shim. Update the
+    // phase of the drop shim so that later on when we run the pass manager on the shim, in
+    // the `mir_shims` query, we don't ICE on the intra-pass validation before we've updated
+    // the phase of the body from analysis.
+    body.phase = MirPhase::Runtime(RuntimePhase::Initial);
+
     body
 }
 
@@ -1186,7 +1193,7 @@ fn insert_panic_block<'tcx>(
     body: &mut Body<'tcx>,
     message: AssertMessage<'tcx>,
 ) -> BasicBlock {
-    let assert_block = BasicBlock::new(body.basic_blocks.len());
+    let assert_block = body.basic_blocks.next_index();
     let kind = TerminatorKind::Assert {
         cond: Operand::Constant(Box::new(ConstOperand {
             span: body.span,
@@ -1209,14 +1216,8 @@ fn can_return<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, typing_env: ty::Typing
     }
 
     // If there's a return terminator the function may return.
-    for block in body.basic_blocks.iter() {
-        if let TerminatorKind::Return = block.terminator().kind {
-            return true;
-        }
-    }
-
+    body.basic_blocks.iter().any(|block| matches!(block.terminator().kind, TerminatorKind::Return))
     // Otherwise the function can't return.
-    false
 }
 
 fn can_unwind<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>) -> bool {
@@ -1293,12 +1294,12 @@ fn create_coroutine_resume_function<'tcx>(
                         kind: TerminatorKind::Goto { target: poison_block },
                     };
                 }
-            } else if !block.is_cleanup {
+            } else if !block.is_cleanup
                 // Any terminators that *can* unwind but don't have an unwind target set are also
                 // pointed at our poisoning block (unless they're part of the cleanup path).
-                if let Some(unwind @ UnwindAction::Continue) = block.terminator_mut().unwind_mut() {
-                    *unwind = UnwindAction::Cleanup(poison_block);
-                }
+                && let Some(unwind @ UnwindAction::Continue) = block.terminator_mut().unwind_mut()
+            {
+                *unwind = UnwindAction::Cleanup(poison_block);
             }
         }
     }
@@ -1340,12 +1341,14 @@ fn create_coroutine_resume_function<'tcx>(
     make_coroutine_state_argument_indirect(tcx, body);
 
     match transform.coroutine_kind {
+        CoroutineKind::Coroutine(_)
+        | CoroutineKind::Desugared(CoroutineDesugaring::Async | CoroutineDesugaring::AsyncGen, _) =>
+        {
+            make_coroutine_state_argument_pinned(tcx, body);
+        }
         // Iterator::next doesn't accept a pinned argument,
         // unlike for all other coroutine kinds.
         CoroutineKind::Desugared(CoroutineDesugaring::Gen, _) => {}
-        _ => {
-            make_coroutine_state_argument_pinned(tcx, body);
-        }
     }
 
     // Make sure we remove dead blocks to remove
@@ -1408,8 +1411,7 @@ fn create_cases<'tcx>(
                 let mut statements = Vec::new();
 
                 // Create StorageLive instructions for locals with live storage
-                for i in 0..(body.local_decls.len()) {
-                    let l = Local::new(i);
+                for l in body.local_decls.indices() {
                     let needs_storage_live = point.storage_liveness.contains(l)
                         && !transform.remap.contains(l)
                         && !transform.always_live_locals.contains(l);
@@ -1535,15 +1537,10 @@ impl<'tcx> crate::MirPass<'tcx> for StateTransform {
         let coroutine_kind = body.coroutine_kind().unwrap();
 
         // Get the discriminant type and args which typeck computed
-        let (discr_ty, movable) = match *coroutine_ty.kind() {
-            ty::Coroutine(_, args) => {
-                let args = args.as_coroutine();
-                (args.discr_ty(tcx), coroutine_kind.movability() == hir::Movability::Movable)
-            }
-            _ => {
-                tcx.dcx().span_bug(body.span, format!("unexpected coroutine type {coroutine_ty}"));
-            }
+        let ty::Coroutine(_, args) = coroutine_ty.kind() else {
+            tcx.dcx().span_bug(body.span, format!("unexpected coroutine type {coroutine_ty}"));
         };
+        let discr_ty = args.as_coroutine().discr_ty(tcx);
 
         let new_ret_ty = match coroutine_kind {
             CoroutineKind::Desugared(CoroutineDesugaring::Async, _) => {
@@ -1610,6 +1607,7 @@ impl<'tcx> crate::MirPass<'tcx> for StateTransform {
 
         let always_live_locals = always_storage_live_locals(body);
 
+        let movable = coroutine_kind.movability() == hir::Movability::Movable;
         let liveness_info =
             locals_live_across_suspend_points(tcx, body, &always_live_locals, movable);
 
diff --git a/compiler/rustc_mir_transform/src/early_otherwise_branch.rs b/compiler/rustc_mir_transform/src/early_otherwise_branch.rs
index 57f7893be1b..d49f5d9f9c3 100644
--- a/compiler/rustc_mir_transform/src/early_otherwise_branch.rs
+++ b/compiler/rustc_mir_transform/src/early_otherwise_branch.rs
@@ -103,9 +103,8 @@ impl<'tcx> crate::MirPass<'tcx> for EarlyOtherwiseBranch {
         let mut should_cleanup = false;
 
         // Also consider newly generated bbs in the same pass
-        for i in 0..body.basic_blocks.len() {
+        for parent in body.basic_blocks.indices() {
             let bbs = &*body.basic_blocks;
-            let parent = BasicBlock::from_usize(i);
             let Some(opt_data) = evaluate_candidate(tcx, body, parent) else { continue };
 
             trace!("SUCCESS: found optimization possibility to apply: {opt_data:?}");
diff --git a/compiler/rustc_mir_transform/src/elaborate_drop.rs b/compiler/rustc_mir_transform/src/elaborate_drop.rs
index 0d8cf524661..e3057a2f648 100644
--- a/compiler/rustc_mir_transform/src/elaborate_drop.rs
+++ b/compiler/rustc_mir_transform/src/elaborate_drop.rs
@@ -258,31 +258,27 @@ where
     ) -> Vec<(Place<'tcx>, Option<D::Path>)> {
         variant
             .fields
-            .iter()
-            .enumerate()
-            .map(|(i, f)| {
-                let field = FieldIdx::new(i);
-                let subpath = self.elaborator.field_subpath(variant_path, field);
+            .iter_enumerated()
+            .map(|(field_idx, field)| {
+                let subpath = self.elaborator.field_subpath(variant_path, field_idx);
                 let tcx = self.tcx();
 
                 assert_eq!(self.elaborator.typing_env().typing_mode, ty::TypingMode::PostAnalysis);
-                // The type error for normalization may have been in dropck: see
-                // `compute_drop_data` in rustc_borrowck, in which case we wouldn't have
-                // deleted the MIR body and could have an error here as well.
-                let field_ty = match tcx
-                    .try_normalize_erasing_regions(self.elaborator.typing_env(), f.ty(tcx, args))
-                {
+                let field_ty = match tcx.try_normalize_erasing_regions(
+                    self.elaborator.typing_env(),
+                    field.ty(tcx, args),
+                ) {
                     Ok(t) => t,
                     Err(_) => Ty::new_error(
                         self.tcx(),
-                        self.elaborator
-                            .body()
-                            .tainted_by_errors
-                            .expect("Error in drop elaboration not found by dropck."),
+                        self.tcx().dcx().span_delayed_bug(
+                            self.elaborator.body().span,
+                            "Error normalizing in drop elaboration.",
+                        ),
                     ),
                 };
 
-                (tcx.mk_place_field(base_place, field, field_ty), subpath)
+                (tcx.mk_place_field(base_place, field_idx, field_ty), subpath)
             })
             .collect()
     }
diff --git a/compiler/rustc_mir_transform/src/inline.rs b/compiler/rustc_mir_transform/src/inline.rs
index 0ab24e48d44..69e80ed54ea 100644
--- a/compiler/rustc_mir_transform/src/inline.rs
+++ b/compiler/rustc_mir_transform/src/inline.rs
@@ -903,9 +903,9 @@ fn inline_call<'tcx, I: Inliner<'tcx>>(
 
     let mut integrator = Integrator {
         args: &args,
-        new_locals: Local::new(caller_body.local_decls.len())..,
-        new_scopes: SourceScope::new(caller_body.source_scopes.len())..,
-        new_blocks: BasicBlock::new(caller_body.basic_blocks.len())..,
+        new_locals: caller_body.local_decls.next_index()..,
+        new_scopes: caller_body.source_scopes.next_index()..,
+        new_blocks: caller_body.basic_blocks.next_index()..,
         destination: destination_local,
         callsite_scope: caller_body.source_scopes[callsite.source_info.scope].clone(),
         callsite,
@@ -1169,7 +1169,7 @@ impl Integrator<'_, '_> {
             if idx < self.args.len() {
                 self.args[idx]
             } else {
-                Local::new(self.new_locals.start.index() + (idx - self.args.len()))
+                self.new_locals.start + (idx - self.args.len())
             }
         };
         trace!("mapping local `{:?}` to `{:?}`", local, new);
@@ -1177,13 +1177,13 @@ impl Integrator<'_, '_> {
     }
 
     fn map_scope(&self, scope: SourceScope) -> SourceScope {
-        let new = SourceScope::new(self.new_scopes.start.index() + scope.index());
+        let new = self.new_scopes.start + scope.index();
         trace!("mapping scope `{:?}` to `{:?}`", scope, new);
         new
     }
 
     fn map_block(&self, block: BasicBlock) -> BasicBlock {
-        let new = BasicBlock::new(self.new_blocks.start.index() + block.index());
+        let new = self.new_blocks.start + block.index();
         trace!("mapping block `{:?}` to `{:?}`", block, new);
         new
     }
diff --git a/compiler/rustc_mir_transform/src/instsimplify.rs b/compiler/rustc_mir_transform/src/instsimplify.rs
index a8d6aaa50a2..5f0c55ddc09 100644
--- a/compiler/rustc_mir_transform/src/instsimplify.rs
+++ b/compiler/rustc_mir_transform/src/instsimplify.rs
@@ -10,7 +10,6 @@ use rustc_middle::ty::{self, GenericArgsRef, Ty, TyCtxt, layout};
 use rustc_span::{DUMMY_SP, Symbol, sym};
 
 use crate::simplify::simplify_duplicate_switch_targets;
-use crate::take_array;
 
 pub(super) enum InstSimplify {
     BeforeInline,
@@ -214,7 +213,9 @@ impl<'tcx> InstSimplifyContext<'_, 'tcx> {
         terminator: &mut Terminator<'tcx>,
         statements: &mut Vec<Statement<'tcx>>,
     ) {
-        let TerminatorKind::Call { func, args, destination, target, .. } = &mut terminator.kind
+        let TerminatorKind::Call {
+            func, args, destination, target: Some(destination_block), ..
+        } = &terminator.kind
         else {
             return;
         };
@@ -222,15 +223,8 @@ impl<'tcx> InstSimplifyContext<'_, 'tcx> {
         // It's definitely not a clone if there are multiple arguments
         let [arg] = &args[..] else { return };
 
-        let Some(destination_block) = *target else { return };
-
         // Only bother looking more if it's easy to know what we're calling
-        let Some((fn_def_id, fn_args)) = func.const_fn_def() else { return };
-
-        // Clone needs one arg, so we can cheaply rule out other stuff
-        if fn_args.len() != 1 {
-            return;
-        }
+        let Some((fn_def_id, ..)) = func.const_fn_def() else { return };
 
         // These types are easily available from locals, so check that before
         // doing DefId lookups to figure out what we're actually calling.
@@ -238,15 +232,12 @@ impl<'tcx> InstSimplifyContext<'_, 'tcx> {
 
         let ty::Ref(_region, inner_ty, Mutability::Not) = *arg_ty.kind() else { return };
 
-        if !inner_ty.is_trivially_pure_clone_copy() {
-            return;
-        }
-
-        if !self.tcx.is_lang_item(fn_def_id, LangItem::CloneFn) {
+        if !self.tcx.is_lang_item(fn_def_id, LangItem::CloneFn)
+            || !inner_ty.is_trivially_pure_clone_copy()
+        {
             return;
         }
 
-        let Ok([arg]) = take_array(args) else { return };
         let Some(arg_place) = arg.node.place() else { return };
 
         statements.push(Statement {
@@ -258,7 +249,7 @@ impl<'tcx> InstSimplifyContext<'_, 'tcx> {
                 )),
             ))),
         });
-        terminator.kind = TerminatorKind::Goto { target: destination_block };
+        terminator.kind = TerminatorKind::Goto { target: *destination_block };
     }
 
     fn simplify_nounwind_call(&self, terminator: &mut Terminator<'tcx>) {
diff --git a/compiler/rustc_mir_transform/src/jump_threading.rs b/compiler/rustc_mir_transform/src/jump_threading.rs
index 0a72a9d669f..8b4b214a3d4 100644
--- a/compiler/rustc_mir_transform/src/jump_threading.rs
+++ b/compiler/rustc_mir_transform/src/jump_threading.rs
@@ -90,7 +90,11 @@ impl<'tcx> crate::MirPass<'tcx> for JumpThreading {
         };
 
         for bb in body.basic_blocks.indices() {
-            finder.start_from_switch(bb);
+            let old_len = finder.opportunities.len();
+            // If we have any const-eval errors discard any opportunities found
+            if finder.start_from_switch(bb).is_none() {
+                finder.opportunities.truncate(old_len);
+            }
         }
 
         let opportunities = finder.opportunities;
@@ -150,14 +154,6 @@ impl Condition {
     fn matches(&self, value: ScalarInt) -> bool {
         (self.value == value) == (self.polarity == Polarity::Eq)
     }
-
-    fn inv(mut self) -> Self {
-        self.polarity = match self.polarity {
-            Polarity::Eq => Polarity::Ne,
-            Polarity::Ne => Polarity::Eq,
-        };
-        self
-    }
 }
 
 #[derive(Copy, Clone, Debug)]
@@ -180,8 +176,21 @@ impl<'a> ConditionSet<'a> {
         self.iter().filter(move |c| c.matches(value))
     }
 
-    fn map(self, arena: &'a DroplessArena, f: impl Fn(Condition) -> Condition) -> ConditionSet<'a> {
-        ConditionSet(arena.alloc_from_iter(self.iter().map(f)))
+    fn map(
+        self,
+        arena: &'a DroplessArena,
+        f: impl Fn(Condition) -> Option<Condition>,
+    ) -> Option<ConditionSet<'a>> {
+        let mut all_ok = true;
+        let set = arena.alloc_from_iter(self.iter().map_while(|c| {
+            if let Some(c) = f(c) {
+                Some(c)
+            } else {
+                all_ok = false;
+                None
+            }
+        }));
+        all_ok.then_some(ConditionSet(set))
     }
 }
 
@@ -192,28 +201,28 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
 
     /// Recursion entry point to find threading opportunities.
     #[instrument(level = "trace", skip(self))]
-    fn start_from_switch(&mut self, bb: BasicBlock) {
+    fn start_from_switch(&mut self, bb: BasicBlock) -> Option<()> {
         let bbdata = &self.body[bb];
         if bbdata.is_cleanup || self.loop_headers.contains(bb) {
-            return;
+            return Some(());
         }
-        let Some((discr, targets)) = bbdata.terminator().kind.as_switch() else { return };
-        let Some(discr) = discr.place() else { return };
+        let Some((discr, targets)) = bbdata.terminator().kind.as_switch() else { return Some(()) };
+        let Some(discr) = discr.place() else { return Some(()) };
         debug!(?discr, ?bb);
 
         let discr_ty = discr.ty(self.body, self.tcx).ty;
         let Ok(discr_layout) = self.ecx.layout_of(discr_ty) else {
-            return;
+            return Some(());
         };
 
-        let Some(discr) = self.map.find(discr.as_ref()) else { return };
+        let Some(discr) = self.map.find(discr.as_ref()) else { return Some(()) };
         debug!(?discr);
 
         let cost = CostChecker::new(self.tcx, self.typing_env, None, self.body);
         let mut state = State::new_reachable();
 
         let conds = if let Some((value, then, else_)) = targets.as_static_if() {
-            let Some(value) = ScalarInt::try_from_uint(value, discr_layout.size) else { return };
+            let value = ScalarInt::try_from_uint(value, discr_layout.size)?;
             self.arena.alloc_from_iter([
                 Condition { value, polarity: Polarity::Eq, target: then },
                 Condition { value, polarity: Polarity::Ne, target: else_ },
@@ -227,7 +236,7 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
         let conds = ConditionSet(conds);
         state.insert_value_idx(discr, conds, &self.map);
 
-        self.find_opportunity(bb, state, cost, 0);
+        self.find_opportunity(bb, state, cost, 0)
     }
 
     /// Recursively walk statements backwards from this bb's terminator to find threading
@@ -239,10 +248,10 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
         mut state: State<ConditionSet<'a>>,
         mut cost: CostChecker<'_, 'tcx>,
         depth: usize,
-    ) {
+    ) -> Option<()> {
         // Do not thread through loop headers.
         if self.loop_headers.contains(bb) {
-            return;
+            return Some(());
         }
 
         debug!(cost = ?cost.cost());
@@ -250,16 +259,16 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
             self.body.basic_blocks[bb].statements.iter().enumerate().rev()
         {
             if self.is_empty(&state) {
-                return;
+                return Some(());
             }
 
             cost.visit_statement(stmt, Location { block: bb, statement_index });
             if cost.cost() > MAX_COST {
-                return;
+                return Some(());
             }
 
             // Attempt to turn the `current_condition` on `lhs` into a condition on another place.
-            self.process_statement(bb, stmt, &mut state);
+            self.process_statement(bb, stmt, &mut state)?;
 
             // When a statement mutates a place, assignments to that place that happen
             // above the mutation cannot fulfill a condition.
@@ -271,7 +280,7 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
         }
 
         if self.is_empty(&state) || depth >= MAX_BACKTRACK {
-            return;
+            return Some(());
         }
 
         let last_non_rec = self.opportunities.len();
@@ -284,9 +293,9 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
             match term.kind {
                 TerminatorKind::SwitchInt { ref discr, ref targets } => {
                     self.process_switch_int(discr, targets, bb, &mut state);
-                    self.find_opportunity(pred, state, cost, depth + 1);
+                    self.find_opportunity(pred, state, cost, depth + 1)?;
                 }
-                _ => self.recurse_through_terminator(pred, || state, &cost, depth),
+                _ => self.recurse_through_terminator(pred, || state, &cost, depth)?,
             }
         } else if let &[ref predecessors @ .., last_pred] = &predecessors[..] {
             for &pred in predecessors {
@@ -311,12 +320,13 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
             let first = &mut new_tos[0];
             *first = ThreadingOpportunity { chain: vec![bb], target: first.target };
             self.opportunities.truncate(last_non_rec + 1);
-            return;
+            return Some(());
         }
 
         for op in self.opportunities[last_non_rec..].iter_mut() {
             op.chain.push(bb);
         }
+        Some(())
     }
 
     /// Extract the mutated place from a statement.
@@ -430,23 +440,23 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
         lhs: PlaceIndex,
         rhs: &Operand<'tcx>,
         state: &mut State<ConditionSet<'a>>,
-    ) {
+    ) -> Option<()> {
         match rhs {
             // If we expect `lhs ?= A`, we have an opportunity if we assume `constant == A`.
             Operand::Constant(constant) => {
-                let Some(constant) =
-                    self.ecx.eval_mir_constant(&constant.const_, constant.span, None).discard_err()
-                else {
-                    return;
-                };
+                let constant = self
+                    .ecx
+                    .eval_mir_constant(&constant.const_, constant.span, None)
+                    .discard_err()?;
                 self.process_constant(bb, lhs, constant, state);
             }
             // Transfer the conditions on the copied rhs.
             Operand::Move(rhs) | Operand::Copy(rhs) => {
-                let Some(rhs) = self.map.find(rhs.as_ref()) else { return };
+                let Some(rhs) = self.map.find(rhs.as_ref()) else { return Some(()) };
                 state.insert_place_idx(rhs, lhs, &self.map);
             }
         }
+        Some(())
     }
 
     #[instrument(level = "trace", skip(self))]
@@ -456,14 +466,18 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
         lhs_place: &Place<'tcx>,
         rhs: &Rvalue<'tcx>,
         state: &mut State<ConditionSet<'a>>,
-    ) {
-        let Some(lhs) = self.map.find(lhs_place.as_ref()) else { return };
+    ) -> Option<()> {
+        let Some(lhs) = self.map.find(lhs_place.as_ref()) else {
+            return Some(());
+        };
         match rhs {
-            Rvalue::Use(operand) => self.process_operand(bb, lhs, operand, state),
+            Rvalue::Use(operand) => self.process_operand(bb, lhs, operand, state)?,
             // Transfer the conditions on the copy rhs.
-            Rvalue::CopyForDeref(rhs) => self.process_operand(bb, lhs, &Operand::Copy(*rhs), state),
+            Rvalue::CopyForDeref(rhs) => {
+                self.process_operand(bb, lhs, &Operand::Copy(*rhs), state)?
+            }
             Rvalue::Discriminant(rhs) => {
-                let Some(rhs) = self.map.find_discr(rhs.as_ref()) else { return };
+                let Some(rhs) = self.map.find_discr(rhs.as_ref()) else { return Some(()) };
                 state.insert_place_idx(rhs, lhs, &self.map);
             }
             // If we expect `lhs ?= A`, we have an opportunity if we assume `constant == A`.
@@ -471,7 +485,7 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
                 let agg_ty = lhs_place.ty(self.body, self.tcx).ty;
                 let lhs = match kind {
                     // Do not support unions.
-                    AggregateKind::Adt(.., Some(_)) => return,
+                    AggregateKind::Adt(.., Some(_)) => return Some(()),
                     AggregateKind::Adt(_, variant_index, ..) if agg_ty.is_enum() => {
                         if let Some(discr_target) = self.map.apply(lhs, TrackElem::Discriminant)
                             && let Some(discr_value) = self
@@ -484,30 +498,31 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
                         if let Some(idx) = self.map.apply(lhs, TrackElem::Variant(*variant_index)) {
                             idx
                         } else {
-                            return;
+                            return Some(());
                         }
                     }
                     _ => lhs,
                 };
                 for (field_index, operand) in operands.iter_enumerated() {
                     if let Some(field) = self.map.apply(lhs, TrackElem::Field(field_index)) {
-                        self.process_operand(bb, field, operand, state);
+                        self.process_operand(bb, field, operand, state)?;
                     }
                 }
             }
-            // Transfer the conditions on the copy rhs, after inversing polarity.
+            // Transfer the conditions on the copy rhs, after inverting the value of the condition.
             Rvalue::UnaryOp(UnOp::Not, Operand::Move(place) | Operand::Copy(place)) => {
-                if !place.ty(self.body, self.tcx).ty.is_bool() {
-                    // Constructing the conditions by inverting the polarity
-                    // of equality is only correct for bools. That is to say,
-                    // `!a == b` is not `a != b` for integers greater than 1 bit.
-                    return;
-                }
-                let Some(conditions) = state.try_get_idx(lhs, &self.map) else { return };
-                let Some(place) = self.map.find(place.as_ref()) else { return };
-                // FIXME: I think This could be generalized to not bool if we
-                // actually perform a logical not on the condition's value.
-                let conds = conditions.map(self.arena, Condition::inv);
+                let layout = self.ecx.layout_of(place.ty(self.body, self.tcx).ty).unwrap();
+                let Some(conditions) = state.try_get_idx(lhs, &self.map) else { return Some(()) };
+                let Some(place) = self.map.find(place.as_ref()) else { return Some(()) };
+                let conds = conditions.map(self.arena, |mut cond| {
+                    cond.value = self
+                        .ecx
+                        .unary_op(UnOp::Not, &ImmTy::from_scalar_int(cond.value, layout))
+                        .discard_err()?
+                        .to_scalar_int()
+                        .discard_err()?;
+                    Some(cond)
+                })?;
                 state.insert_value_idx(place, conds, &self.map);
             }
             // We expect `lhs ?= A`. We found `lhs = Eq(rhs, B)`.
@@ -517,34 +532,34 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
                 box (Operand::Move(place) | Operand::Copy(place), Operand::Constant(value))
                 | box (Operand::Constant(value), Operand::Move(place) | Operand::Copy(place)),
             ) => {
-                let Some(conditions) = state.try_get_idx(lhs, &self.map) else { return };
-                let Some(place) = self.map.find(place.as_ref()) else { return };
+                let Some(conditions) = state.try_get_idx(lhs, &self.map) else { return Some(()) };
+                let Some(place) = self.map.find(place.as_ref()) else { return Some(()) };
                 let equals = match op {
                     BinOp::Eq => ScalarInt::TRUE,
                     BinOp::Ne => ScalarInt::FALSE,
-                    _ => return,
+                    _ => return Some(()),
                 };
                 if value.const_.ty().is_floating_point() {
                     // Floating point equality does not follow bit-patterns.
                     // -0.0 and NaN both have special rules for equality,
                     // and therefore we cannot use integer comparisons for them.
                     // Avoid handling them, though this could be extended in the future.
-                    return;
+                    return Some(());
                 }
-                let Some(value) = value.const_.try_eval_scalar_int(self.tcx, self.typing_env)
-                else {
-                    return;
-                };
-                let conds = conditions.map(self.arena, |c| Condition {
-                    value,
-                    polarity: if c.matches(equals) { Polarity::Eq } else { Polarity::Ne },
-                    ..c
-                });
+                let value = value.const_.try_eval_scalar_int(self.tcx, self.typing_env)?;
+                let conds = conditions.map(self.arena, |c| {
+                    Some(Condition {
+                        value,
+                        polarity: if c.matches(equals) { Polarity::Eq } else { Polarity::Ne },
+                        ..c
+                    })
+                })?;
                 state.insert_value_idx(place, conds, &self.map);
             }
 
             _ => {}
         }
+        Some(())
     }
 
     #[instrument(level = "trace", skip(self))]
@@ -553,7 +568,7 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
         bb: BasicBlock,
         stmt: &Statement<'tcx>,
         state: &mut State<ConditionSet<'a>>,
-    ) {
+    ) -> Option<()> {
         let register_opportunity = |c: Condition| {
             debug!(?bb, ?c.target, "register");
             self.opportunities.push(ThreadingOpportunity { chain: vec![bb], target: c.target })
@@ -566,30 +581,32 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
             // If we expect `discriminant(place) ?= A`,
             // we have an opportunity if `variant_index ?= A`.
             StatementKind::SetDiscriminant { box place, variant_index } => {
-                let Some(discr_target) = self.map.find_discr(place.as_ref()) else { return };
+                let Some(discr_target) = self.map.find_discr(place.as_ref()) else {
+                    return Some(());
+                };
                 let enum_ty = place.ty(self.body, self.tcx).ty;
                 // `SetDiscriminant` guarantees that the discriminant is now `variant_index`.
                 // Even if the discriminant write does nothing due to niches, it is UB to set the
                 // discriminant when the data does not encode the desired discriminant.
-                let Some(discr) =
-                    self.ecx.discriminant_for_variant(enum_ty, *variant_index).discard_err()
-                else {
-                    return;
-                };
+                let discr =
+                    self.ecx.discriminant_for_variant(enum_ty, *variant_index).discard_err()?;
                 self.process_immediate(bb, discr_target, discr, state);
             }
             // If we expect `lhs ?= true`, we have an opportunity if we assume `lhs == true`.
             StatementKind::Intrinsic(box NonDivergingIntrinsic::Assume(
                 Operand::Copy(place) | Operand::Move(place),
             )) => {
-                let Some(conditions) = state.try_get(place.as_ref(), &self.map) else { return };
+                let Some(conditions) = state.try_get(place.as_ref(), &self.map) else {
+                    return Some(());
+                };
                 conditions.iter_matches(ScalarInt::TRUE).for_each(register_opportunity);
             }
             StatementKind::Assign(box (lhs_place, rhs)) => {
-                self.process_assign(bb, lhs_place, rhs, state);
+                self.process_assign(bb, lhs_place, rhs, state)?;
             }
             _ => {}
         }
+        Some(())
     }
 
     #[instrument(level = "trace", skip(self, state, cost))]
@@ -600,7 +617,7 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
         state: impl FnOnce() -> State<ConditionSet<'a>>,
         cost: &CostChecker<'_, 'tcx>,
         depth: usize,
-    ) {
+    ) -> Option<()> {
         let term = self.body.basic_blocks[bb].terminator();
         let place_to_flood = match term.kind {
             // We come from a target, so those are not possible.
@@ -615,9 +632,9 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
             | TerminatorKind::FalseUnwind { .. }
             | TerminatorKind::Yield { .. } => bug!("{term:?} invalid"),
             // Cannot reason about inline asm.
-            TerminatorKind::InlineAsm { .. } => return,
+            TerminatorKind::InlineAsm { .. } => return Some(()),
             // `SwitchInt` is handled specially.
-            TerminatorKind::SwitchInt { .. } => return,
+            TerminatorKind::SwitchInt { .. } => return Some(()),
             // We can recurse, no thing particular to do.
             TerminatorKind::Goto { .. } => None,
             // Flood the overwritten place, and progress through.
@@ -632,7 +649,7 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
         if let Some(place_to_flood) = place_to_flood {
             state.flood_with(place_to_flood.as_ref(), &self.map, ConditionSet::BOTTOM);
         }
-        self.find_opportunity(bb, state, cost.clone(), depth + 1);
+        self.find_opportunity(bb, state, cost.clone(), depth + 1)
     }
 
     #[instrument(level = "trace", skip(self))]
diff --git a/compiler/rustc_mir_transform/src/lib.rs b/compiler/rustc_mir_transform/src/lib.rs
index dfd07f0fb16..4d74ecddfb0 100644
--- a/compiler/rustc_mir_transform/src/lib.rs
+++ b/compiler/rustc_mir_transform/src/lib.rs
@@ -1,5 +1,4 @@
 // tidy-alphabetical-start
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![feature(array_windows)]
 #![feature(assert_matches)]
 #![feature(box_patterns)]
diff --git a/compiler/rustc_mir_transform/src/lint_tail_expr_drop_order.rs b/compiler/rustc_mir_transform/src/lint_tail_expr_drop_order.rs
index 29a9133abe9..537f152938e 100644
--- a/compiler/rustc_mir_transform/src/lint_tail_expr_drop_order.rs
+++ b/compiler/rustc_mir_transform/src/lint_tail_expr_drop_order.rs
@@ -512,23 +512,17 @@ struct LocalLabel<'a> {
 
 /// A custom `Subdiagnostic` implementation so that the notes are delivered in a specific order
 impl Subdiagnostic for LocalLabel<'_> {
-    fn add_to_diag_with<
-        G: rustc_errors::EmissionGuarantee,
-        F: rustc_errors::SubdiagMessageOp<G>,
-    >(
-        self,
-        diag: &mut rustc_errors::Diag<'_, G>,
-        f: &F,
-    ) {
+    fn add_to_diag<G: rustc_errors::EmissionGuarantee>(self, diag: &mut rustc_errors::Diag<'_, G>) {
         diag.arg("name", self.name);
         diag.arg("is_generated_name", self.is_generated_name);
         diag.arg("is_dropped_first_edition_2024", self.is_dropped_first_edition_2024);
-        let msg = f(diag, crate::fluent_generated::mir_transform_tail_expr_local.into());
+        let msg = diag.eagerly_translate(crate::fluent_generated::mir_transform_tail_expr_local);
         diag.span_label(self.span, msg);
         for dtor in self.destructors {
-            dtor.add_to_diag_with(diag, f);
+            dtor.add_to_diag(diag);
         }
-        let msg = f(diag, crate::fluent_generated::mir_transform_label_local_epilogue);
+        let msg =
+            diag.eagerly_translate(crate::fluent_generated::mir_transform_label_local_epilogue);
         diag.span_label(self.span, msg);
     }
 }
diff --git a/compiler/rustc_mir_transform/src/match_branches.rs b/compiler/rustc_mir_transform/src/match_branches.rs
index 0d9d0368d37..5059837328e 100644
--- a/compiler/rustc_mir_transform/src/match_branches.rs
+++ b/compiler/rustc_mir_transform/src/match_branches.rs
@@ -20,13 +20,11 @@ impl<'tcx> crate::MirPass<'tcx> for MatchBranchSimplification {
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         let typing_env = body.typing_env(tcx);
         let mut should_cleanup = false;
-        for i in 0..body.basic_blocks.len() {
-            let bbs = &*body.basic_blocks;
-            let bb_idx = BasicBlock::from_usize(i);
-            match bbs[bb_idx].terminator().kind {
+        for bb_idx in body.basic_blocks.indices() {
+            match &body.basic_blocks[bb_idx].terminator().kind {
                 TerminatorKind::SwitchInt {
-                    discr: ref _discr @ (Operand::Copy(_) | Operand::Move(_)),
-                    ref targets,
+                    discr: Operand::Copy(_) | Operand::Move(_),
+                    targets,
                     ..
                     // We require that the possible target blocks don't contain this block.
                 } if !targets.all_targets().contains(&bb_idx) => {}
@@ -66,9 +64,10 @@ trait SimplifyMatch<'tcx> {
         typing_env: ty::TypingEnv<'tcx>,
     ) -> Option<()> {
         let bbs = &body.basic_blocks;
-        let (discr, targets) = match bbs[switch_bb_idx].terminator().kind {
-            TerminatorKind::SwitchInt { ref discr, ref targets, .. } => (discr, targets),
-            _ => unreachable!(),
+        let TerminatorKind::SwitchInt { discr, targets, .. } =
+            &bbs[switch_bb_idx].terminator().kind
+        else {
+            unreachable!();
         };
 
         let discr_ty = discr.ty(body.local_decls(), tcx);
diff --git a/compiler/rustc_mir_transform/src/multiple_return_terminators.rs b/compiler/rustc_mir_transform/src/multiple_return_terminators.rs
index c63bfdcee85..f59b849e85c 100644
--- a/compiler/rustc_mir_transform/src/multiple_return_terminators.rs
+++ b/compiler/rustc_mir_transform/src/multiple_return_terminators.rs
@@ -18,19 +18,17 @@ impl<'tcx> crate::MirPass<'tcx> for MultipleReturnTerminators {
         // find basic blocks with no statement and a return terminator
         let mut bbs_simple_returns = DenseBitSet::new_empty(body.basic_blocks.len());
         let bbs = body.basic_blocks_mut();
-        for idx in bbs.indices() {
-            if bbs[idx].statements.is_empty()
-                && bbs[idx].terminator().kind == TerminatorKind::Return
-            {
+        for (idx, bb) in bbs.iter_enumerated() {
+            if bb.statements.is_empty() && bb.terminator().kind == TerminatorKind::Return {
                 bbs_simple_returns.insert(idx);
             }
         }
 
         for bb in bbs {
-            if let TerminatorKind::Goto { target } = bb.terminator().kind {
-                if bbs_simple_returns.contains(target) {
-                    bb.terminator_mut().kind = TerminatorKind::Return;
-                }
+            if let TerminatorKind::Goto { target } = bb.terminator().kind
+                && bbs_simple_returns.contains(target)
+            {
+                bb.terminator_mut().kind = TerminatorKind::Return;
             }
         }
 
diff --git a/compiler/rustc_mir_transform/src/patch.rs b/compiler/rustc_mir_transform/src/patch.rs
index 6a177faeac8..12ace04c5e2 100644
--- a/compiler/rustc_mir_transform/src/patch.rs
+++ b/compiler/rustc_mir_transform/src/patch.rs
@@ -181,7 +181,7 @@ impl<'tcx> MirPatch<'tcx> {
 
     /// Queues the addition of a new basic block.
     pub(crate) fn new_block(&mut self, data: BasicBlockData<'tcx>) -> BasicBlock {
-        let block = BasicBlock::new(self.term_patch_map.len());
+        let block = self.term_patch_map.next_index();
         debug!("MirPatch: new_block: {:?}: {:?}", block, data);
         self.new_blocks.push(data);
         self.term_patch_map.push(None);
diff --git a/compiler/rustc_mir_transform/src/promote_consts.rs b/compiler/rustc_mir_transform/src/promote_consts.rs
index c8d8dc147e9..47d43830970 100644
--- a/compiler/rustc_mir_transform/src/promote_consts.rs
+++ b/compiler/rustc_mir_transform/src/promote_consts.rs
@@ -18,7 +18,7 @@ use either::{Left, Right};
 use rustc_const_eval::check_consts::{ConstCx, qualifs};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir as hir;
-use rustc_index::{Idx, IndexSlice, IndexVec};
+use rustc_index::{IndexSlice, IndexVec};
 use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, GenericArgs, List, Ty, TyCtxt, TypeVisitableExt};
@@ -864,17 +864,21 @@ impl<'a, 'tcx> Promoter<'a, 'tcx> {
         new_temp
     }
 
-    fn promote_candidate(mut self, candidate: Candidate, next_promoted_id: usize) -> Body<'tcx> {
+    fn promote_candidate(
+        mut self,
+        candidate: Candidate,
+        next_promoted_index: Promoted,
+    ) -> Body<'tcx> {
         let def = self.source.source.def_id();
         let (mut rvalue, promoted_op) = {
             let promoted = &mut self.promoted;
-            let promoted_id = Promoted::new(next_promoted_id);
             let tcx = self.tcx;
             let mut promoted_operand = |ty, span| {
                 promoted.span = span;
                 promoted.local_decls[RETURN_PLACE] = LocalDecl::new(ty, span);
                 let args = tcx.erase_regions(GenericArgs::identity_for_item(tcx, def));
-                let uneval = mir::UnevaluatedConst { def, args, promoted: Some(promoted_id) };
+                let uneval =
+                    mir::UnevaluatedConst { def, args, promoted: Some(next_promoted_index) };
 
                 ConstOperand { span, user_ty: None, const_: Const::Unevaluated(uneval, ty) }
             };
@@ -1034,7 +1038,7 @@ fn promote_candidates<'tcx>(
             required_consts: Vec::new(),
         };
 
-        let mut promoted = promoter.promote_candidate(candidate, promotions.len());
+        let mut promoted = promoter.promote_candidate(candidate, promotions.next_index());
         promoted.source.promoted = Some(promotions.next_index());
         promotions.push(promoted);
     }
diff --git a/compiler/rustc_mir_transform/src/shim.rs b/compiler/rustc_mir_transform/src/shim.rs
index c9771467e49..c13ffae3649 100644
--- a/compiler/rustc_mir_transform/src/shim.rs
+++ b/compiler/rustc_mir_transform/src/shim.rs
@@ -66,7 +66,7 @@ fn make_shim<'tcx>(tcx: TyCtxt<'tcx>, instance: ty::InstanceKind<'tcx>) -> Body<
             let call_mut = tcx
                 .associated_items(fn_mut)
                 .in_definition_order()
-                .find(|it| it.kind == ty::AssocKind::Fn)
+                .find(|it| it.is_fn())
                 .unwrap()
                 .def_id;
 
diff --git a/compiler/rustc_mir_transform/src/simplify.rs b/compiler/rustc_mir_transform/src/simplify.rs
index 84905f4a400..5947637cded 100644
--- a/compiler/rustc_mir_transform/src/simplify.rs
+++ b/compiler/rustc_mir_transform/src/simplify.rs
@@ -597,20 +597,6 @@ impl<'tcx> MutVisitor<'tcx> for LocalUpdater<'tcx> {
         self.tcx
     }
 
-    fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
-        if let StatementKind::BackwardIncompatibleDropHint { place, reason: _ } =
-            &mut statement.kind
-        {
-            self.visit_local(
-                &mut place.local,
-                PlaceContext::MutatingUse(MutatingUseContext::Store),
-                location,
-            );
-        } else {
-            self.super_statement(statement, location);
-        }
-    }
-
     fn visit_local(&mut self, l: &mut Local, _: PlaceContext, _: Location) {
         *l = self.map[*l].unwrap();
     }
diff --git a/compiler/rustc_mir_transform/src/validate.rs b/compiler/rustc_mir_transform/src/validate.rs
index e7930f0a1e3..66fe3ef4141 100644
--- a/compiler/rustc_mir_transform/src/validate.rs
+++ b/compiler/rustc_mir_transform/src/validate.rs
@@ -221,12 +221,11 @@ impl<'a, 'tcx> CfgChecker<'a, 'tcx> {
 
         // Check for cycles
         let mut stack = FxHashSet::default();
-        for i in 0..parent.len() {
-            let mut bb = BasicBlock::from_usize(i);
+        for (mut bb, parent) in parent.iter_enumerated_mut() {
             stack.clear();
             stack.insert(bb);
             loop {
-                let Some(parent) = parent[bb].take() else { break };
+                let Some(parent) = parent.take() else { break };
                 let no_cycle = stack.insert(parent);
                 if !no_cycle {
                     self.fail(
diff --git a/compiler/rustc_monomorphize/src/collector.rs b/compiler/rustc_monomorphize/src/collector.rs
index 1dbb35f92c2..d1d0f7cacae 100644
--- a/compiler/rustc_monomorphize/src/collector.rs
+++ b/compiler/rustc_monomorphize/src/collector.rs
@@ -688,7 +688,7 @@ impl<'a, 'tcx> MirVisitor<'tcx> for MirUsedCollector<'a, 'tcx> {
                 let target_ty = self.monomorphize(target_ty);
                 let source_ty = self.monomorphize(source_ty);
                 let (source_ty, target_ty) =
-                    find_vtable_types_for_unsizing(self.tcx.at(span), source_ty, target_ty);
+                    find_tails_for_unsizing(self.tcx.at(span), source_ty, target_ty);
                 // This could also be a different Unsize instruction, like
                 // from a fixed sized array to a slice. But we are only
                 // interested in things that produce a vtable.
@@ -1037,36 +1037,35 @@ fn should_codegen_locally<'tcx>(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) ->
 ///
 /// Finally, there is also the case of custom unsizing coercions, e.g., for
 /// smart pointers such as `Rc` and `Arc`.
-fn find_vtable_types_for_unsizing<'tcx>(
+fn find_tails_for_unsizing<'tcx>(
     tcx: TyCtxtAt<'tcx>,
     source_ty: Ty<'tcx>,
     target_ty: Ty<'tcx>,
 ) -> (Ty<'tcx>, Ty<'tcx>) {
-    let ptr_vtable = |inner_source: Ty<'tcx>, inner_target: Ty<'tcx>| {
-        let typing_env = ty::TypingEnv::fully_monomorphized();
-        if tcx.type_has_metadata(inner_source, typing_env) {
-            (inner_source, inner_target)
-        } else {
-            tcx.struct_lockstep_tails_for_codegen(inner_source, inner_target, typing_env)
-        }
-    };
+    let typing_env = ty::TypingEnv::fully_monomorphized();
+    debug_assert!(!source_ty.has_param(), "{source_ty} should be fully monomorphic");
+    debug_assert!(!target_ty.has_param(), "{target_ty} should be fully monomorphic");
 
     match (source_ty.kind(), target_ty.kind()) {
-        (&ty::Ref(_, a, _), &ty::Ref(_, b, _) | &ty::RawPtr(b, _))
-        | (&ty::RawPtr(a, _), &ty::RawPtr(b, _)) => ptr_vtable(a, b),
+        (
+            &ty::Ref(_, source_pointee, _),
+            &ty::Ref(_, target_pointee, _) | &ty::RawPtr(target_pointee, _),
+        )
+        | (&ty::RawPtr(source_pointee, _), &ty::RawPtr(target_pointee, _)) => {
+            tcx.struct_lockstep_tails_for_codegen(source_pointee, target_pointee, typing_env)
+        }
+
+        // `Box<T>` could go through the ADT code below, b/c it'll unpeel to `Unique<T>`,
+        // and eventually bottom out in a raw ref, but we can micro-optimize it here.
         (_, _)
             if let Some(source_boxed) = source_ty.boxed_ty()
                 && let Some(target_boxed) = target_ty.boxed_ty() =>
         {
-            ptr_vtable(source_boxed, target_boxed)
+            tcx.struct_lockstep_tails_for_codegen(source_boxed, target_boxed, typing_env)
         }
 
-        // T as dyn* Trait
-        (_, &ty::Dynamic(_, _, ty::DynStar)) => ptr_vtable(source_ty, target_ty),
-
         (&ty::Adt(source_adt_def, source_args), &ty::Adt(target_adt_def, target_args)) => {
             assert_eq!(source_adt_def, target_adt_def);
-
             let CustomCoerceUnsized::Struct(coerce_index) =
                 match crate::custom_coerce_unsize_info(tcx, source_ty, target_ty) {
                     Ok(ccu) => ccu,
@@ -1075,21 +1074,23 @@ fn find_vtable_types_for_unsizing<'tcx>(
                         return (e, e);
                     }
                 };
+            let coerce_field = &source_adt_def.non_enum_variant().fields[coerce_index];
+            // We're getting a possibly unnormalized type, so normalize it.
+            let source_field =
+                tcx.normalize_erasing_regions(typing_env, coerce_field.ty(*tcx, source_args));
+            let target_field =
+                tcx.normalize_erasing_regions(typing_env, coerce_field.ty(*tcx, target_args));
+            find_tails_for_unsizing(tcx, source_field, target_field)
+        }
 
-            let source_fields = &source_adt_def.non_enum_variant().fields;
-            let target_fields = &target_adt_def.non_enum_variant().fields;
-
-            assert!(
-                coerce_index.index() < source_fields.len()
-                    && source_fields.len() == target_fields.len()
-            );
+        // `T` as `dyn* Trait` unsizes *directly*.
+        //
+        // FIXME(dyn_star): This case is a bit awkward, b/c we're not really computing
+        // a tail here. We probably should handle this separately in the *caller* of
+        // this function, rather than returning something that is semantically different
+        // than what we return above.
+        (_, &ty::Dynamic(_, _, ty::DynStar)) => (source_ty, target_ty),
 
-            find_vtable_types_for_unsizing(
-                tcx,
-                source_fields[coerce_index].ty(*tcx, source_args),
-                target_fields[coerce_index].ty(*tcx, target_args),
-            )
-        }
         _ => bug!(
             "find_vtable_types_for_unsizing: invalid coercion {:?} -> {:?}",
             source_ty,
@@ -1308,7 +1309,7 @@ fn visit_mentioned_item<'tcx>(
         }
         MentionedItem::UnsizeCast { source_ty, target_ty } => {
             let (source_ty, target_ty) =
-                find_vtable_types_for_unsizing(tcx.at(span), source_ty, target_ty);
+                find_tails_for_unsizing(tcx.at(span), source_ty, target_ty);
             // This could also be a different Unsize instruction, like
             // from a fixed sized array to a slice. But we are only
             // interested in things that produce a vtable.
diff --git a/compiler/rustc_monomorphize/src/lib.rs b/compiler/rustc_monomorphize/src/lib.rs
index 5dbae50c499..8f6914f3d72 100644
--- a/compiler/rustc_monomorphize/src/lib.rs
+++ b/compiler/rustc_monomorphize/src/lib.rs
@@ -1,5 +1,4 @@
 // tidy-alphabetical-start
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![feature(array_windows)]
 #![feature(file_buffered)]
 #![feature(if_let_guard)]
diff --git a/compiler/rustc_monomorphize/src/mono_checks/move_check.rs b/compiler/rustc_monomorphize/src/mono_checks/move_check.rs
index 55d52d5075d..7251ef478c6 100644
--- a/compiler/rustc_monomorphize/src/mono_checks/move_check.rs
+++ b/compiler/rustc_monomorphize/src/mono_checks/move_check.rs
@@ -3,7 +3,7 @@ use rustc_data_structures::fx::FxIndexSet;
 use rustc_hir::def_id::DefId;
 use rustc_middle::mir::visit::Visitor as MirVisitor;
 use rustc_middle::mir::{self, Location, traversal};
-use rustc_middle::ty::{self, AssocKind, Instance, Ty, TyCtxt, TypeFoldable};
+use rustc_middle::ty::{self, AssocTag, Instance, Ty, TyCtxt, TypeFoldable};
 use rustc_session::Limit;
 use rustc_session::lint::builtin::LARGE_ASSIGNMENTS;
 use rustc_span::source_map::Spanned;
@@ -194,7 +194,7 @@ fn assoc_fn_of_type<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId, fn_ident: Ident) ->
         if let Some(new) = tcx.associated_items(impl_def_id).find_by_ident_and_kind(
             tcx,
             fn_ident,
-            AssocKind::Fn,
+            AssocTag::Fn,
             def_id,
         ) {
             return Some(new.def_id);
diff --git a/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs b/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
index 384a304c4a9..ee000b11748 100644
--- a/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
@@ -792,37 +792,46 @@ where
         };
 
         match proven_via {
-            // Even when a trait bound has been proven using a where-bound, we
-            // still need to consider alias-bounds for normalization, see
-            // tests/ui/next-solver/alias-bound-shadowed-by-env.rs.
-            //
-            // FIXME(const_trait_impl): should this behavior also be used by
-            // constness checking. Doing so is *at least theoretically* breaking,
-            // see github.com/rust-lang/rust/issues/133044#issuecomment-2500709754
             TraitGoalProvenVia::ParamEnv | TraitGoalProvenVia::AliasBound => {
-                let mut candidates_from_env_and_bounds: Vec<_> = candidates
-                    .iter()
-                    .filter(|c| {
-                        matches!(
-                            c.source,
-                            CandidateSource::AliasBound | CandidateSource::ParamEnv(_)
-                        )
-                    })
-                    .map(|c| c.result)
-                    .collect();
+                let mut considered_candidates = Vec::new();
+                considered_candidates.extend(
+                    candidates
+                        .iter()
+                        .filter(|c| matches!(c.source, CandidateSource::ParamEnv(_)))
+                        .map(|c| c.result),
+                );
+
+                // Even when a trait bound has been proven using a where-bound, we
+                // still need to consider alias-bounds for normalization, see
+                // tests/ui/next-solver/alias-bound-shadowed-by-env.rs.
+                //
+                // We still need to prefer where-bounds over alias-bounds however.
+                // See tests/ui/winnowing/norm-where-bound-gt-alias-bound.rs.
+                //
+                // FIXME(const_trait_impl): should this behavior also be used by
+                // constness checking. Doing so is *at least theoretically* breaking,
+                // see github.com/rust-lang/rust/issues/133044#issuecomment-2500709754
+                if considered_candidates.is_empty() {
+                    considered_candidates.extend(
+                        candidates
+                            .iter()
+                            .filter(|c| matches!(c.source, CandidateSource::AliasBound))
+                            .map(|c| c.result),
+                    );
+                }
 
                 // If the trait goal has been proven by using the environment, we want to treat
                 // aliases as rigid if there are no applicable projection bounds in the environment.
-                if candidates_from_env_and_bounds.is_empty() {
+                if considered_candidates.is_empty() {
                     if let Ok(response) = inject_normalize_to_rigid_candidate(self) {
-                        candidates_from_env_and_bounds.push(response);
+                        considered_candidates.push(response);
                     }
                 }
 
-                if let Some(response) = self.try_merge_responses(&candidates_from_env_and_bounds) {
+                if let Some(response) = self.try_merge_responses(&considered_candidates) {
                     Ok(response)
                 } else {
-                    self.flounder(&candidates_from_env_and_bounds)
+                    self.flounder(&considered_candidates)
                 }
             }
             TraitGoalProvenVia::Misc => {
diff --git a/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/canonical.rs b/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/canonical.rs
index 4edc293ad80..d56b0e5847e 100644
--- a/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/canonical.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/canonical.rs
@@ -355,7 +355,7 @@ where
                     // exist at all (see the FIXME at the start of this method), we have to deal with
                     // them for now.
                     delegate.instantiate_canonical_var_with_infer(info, span, |idx| {
-                        ty::UniverseIndex::from(prev_universe.index() + idx.index())
+                        prev_universe + idx.index()
                     })
                 } else if info.is_existential() {
                     // As an optimization we sometimes avoid creating a new inference variable here.
diff --git a/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs b/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
index 148ba02252d..9994c85d0d0 100644
--- a/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
@@ -286,18 +286,23 @@ where
             // fixing it may cause inference breakage or introduce ambiguity.
             GoalSource::Misc => PathKind::Unknown,
             GoalSource::NormalizeGoal(path_kind) => path_kind,
-            GoalSource::ImplWhereBound => {
+            GoalSource::ImplWhereBound => match self.current_goal_kind {
                 // We currently only consider a cycle coinductive if it steps
                 // into a where-clause of a coinductive trait.
+                CurrentGoalKind::CoinductiveTrait => PathKind::Coinductive,
+                // While normalizing via an impl does step into a where-clause of
+                // an impl, accessing the associated item immediately steps out of
+                // it again. This means cycles/recursive calls are not guarded
+                // by impls used for normalization.
                 //
+                // See tests/ui/traits/next-solver/cycles/normalizes-to-is-not-productive.rs
+                // for how this can go wrong.
+                CurrentGoalKind::NormalizesTo => PathKind::Inductive,
                 // We probably want to make all traits coinductive in the future,
-                // so we treat cycles involving their where-clauses as ambiguous.
-                if let CurrentGoalKind::CoinductiveTrait = self.current_goal_kind {
-                    PathKind::Coinductive
-                } else {
-                    PathKind::Unknown
-                }
-            }
+                // so we treat cycles involving where-clauses of not-yet coinductive
+                // traits as ambiguous for now.
+                CurrentGoalKind::Misc => PathKind::Unknown,
+            },
             // Relating types is always unproductive. If we were to map proof trees to
             // corecursive functions as explained in #136824, relating types never
             // introduces a constructor which could cause the recursion to be guarded.
diff --git a/compiler/rustc_next_trait_solver/src/solve/normalizes_to/mod.rs b/compiler/rustc_next_trait_solver/src/solve/normalizes_to/mod.rs
index 2d027f16e5d..fdeb276a58e 100644
--- a/compiler/rustc_next_trait_solver/src/solve/normalizes_to/mod.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/normalizes_to/mod.rs
@@ -45,7 +45,6 @@ where
                             goal,
                             goal.predicate.alias,
                         );
-                        this.add_goal(GoalSource::AliasWellFormed, goal.with(cx, trait_ref));
                         this.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
                     })
                 })
diff --git a/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs b/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
index d42c9980f46..9262da2906d 100644
--- a/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
@@ -164,6 +164,7 @@ where
         ecx: &mut EvalCtxt<'_, D>,
         goal: Goal<I, Self>,
     ) -> Result<Candidate<I>, NoSolution> {
+        let cx = ecx.cx();
         if goal.predicate.polarity != ty::PredicatePolarity::Positive {
             return Err(NoSolution);
         }
@@ -174,20 +175,37 @@ where
 
         // Only consider auto impls of unsafe traits when there are no unsafe
         // fields.
-        if ecx.cx().trait_is_unsafe(goal.predicate.def_id())
+        if cx.trait_is_unsafe(goal.predicate.def_id())
             && goal.predicate.self_ty().has_unsafe_fields()
         {
             return Err(NoSolution);
         }
 
-        // We only look into opaque types during analysis for opaque types
-        // outside of their defining scope. Doing so for opaques in the
-        // defining scope may require calling `typeck` on the same item we're
-        // currently type checking, which will result in a fatal cycle that
-        // ideally we want to avoid, since we can make progress on this goal
-        // via an alias bound or a locally-inferred hidden type instead.
+        // We leak the implemented auto traits of opaques outside of their defining scope.
+        // This depends on `typeck` of the defining scope of that opaque, which may result in
+        // fatal query cycles.
+        //
+        // We only get to this point if we're outside of the defining scope as we'd otherwise
+        // be able to normalize the opaque type. We may also cycle in case `typeck` of a defining
+        // scope relies on the current context, e.g. either because it also leaks auto trait
+        // bounds of opaques defined in the current context or by evaluating the current item.
+        //
+        // To avoid this we don't try to leak auto trait bounds if they can also be proven via
+        // item bounds of the opaque. These bounds are always applicable as auto traits must not
+        // have any generic parameters. They would also get preferred over the impl candidate
+        // when merging candidates anyways.
+        //
+        // See tests/ui/impl-trait/auto-trait-leakage/avoid-query-cycle-via-item-bound.rs.
         if let ty::Alias(ty::Opaque, opaque_ty) = goal.predicate.self_ty().kind() {
             debug_assert!(ecx.opaque_type_is_rigid(opaque_ty.def_id));
+            for item_bound in cx.item_self_bounds(opaque_ty.def_id).skip_binder() {
+                if item_bound
+                    .as_trait_clause()
+                    .is_some_and(|b| b.def_id() == goal.predicate.def_id())
+                {
+                    return Err(NoSolution);
+                }
+            }
         }
 
         ecx.probe_and_evaluate_goal_for_constituent_tys(
@@ -1238,10 +1256,11 @@ where
     D: SolverDelegate<Interner = I>,
     I: Interner,
 {
+    #[instrument(level = "debug", skip(self, goal), ret)]
     pub(super) fn merge_trait_candidates(
         &mut self,
         goal: Goal<I, TraitPredicate<I>>,
-        candidates: Vec<Candidate<I>>,
+        mut candidates: Vec<Candidate<I>>,
     ) -> Result<(CanonicalResponse<I>, Option<TraitGoalProvenVia>), NoSolution> {
         if let TypingMode::Coherence = self.typing_mode() {
             let all_candidates: Vec<_> = candidates.into_iter().map(|c| c.result).collect();
@@ -1323,13 +1342,16 @@ where
 
         // If there are *only* global where bounds, then make sure to return that this
         // is still reported as being proven-via the param-env so that rigid projections
-        // operate correctly.
+        // operate correctly. Otherwise, drop all global where-bounds before merging the
+        // remaining candidates.
         let proven_via =
             if candidates.iter().all(|c| matches!(c.source, CandidateSource::ParamEnv(_))) {
                 TraitGoalProvenVia::ParamEnv
             } else {
+                candidates.retain(|c| !matches!(c.source, CandidateSource::ParamEnv(_)));
                 TraitGoalProvenVia::Misc
             };
+
         let all_candidates: Vec<_> = candidates.into_iter().map(|c| c.result).collect();
         if let Some(response) = self.try_merge_responses(&all_candidates) {
             Ok((response, Some(proven_via)))
diff --git a/compiler/rustc_parse/messages.ftl b/compiler/rustc_parse/messages.ftl
index 93fa89b68b9..e7f17bb6f99 100644
--- a/compiler/rustc_parse/messages.ftl
+++ b/compiler/rustc_parse/messages.ftl
@@ -543,7 +543,7 @@ parse_maybe_recover_from_bad_qpath_stage_2 =
     .suggestion = types that don't start with an identifier need to be surrounded with angle brackets in qualified paths
 
 parse_maybe_recover_from_bad_type_plus =
-    expected a path on the left-hand side of `+`, not `{$ty}`
+    expected a path on the left-hand side of `+`
 
 parse_maybe_report_ambiguous_plus =
     ambiguous `+` in a type
@@ -642,7 +642,9 @@ parse_mut_on_nested_ident_pattern = `mut` must be attached to each individual bi
     .suggestion = add `mut` to each binding
 parse_mut_on_non_ident_pattern = `mut` must be followed by a named binding
     .suggestion = remove the `mut` prefix
-parse_need_plus_after_trait_object_lifetime = lifetime in trait object type must be followed by `+`
+
+parse_need_plus_after_trait_object_lifetime = lifetimes must be followed by `+` to form a trait object type
+    .suggestion = consider adding a trait bound after the potential lifetime bound
 
 parse_nested_adt = `{$kw_str}` definition cannot be nested inside `{$keyword}`
     .suggestion = consider creating a new `{$kw_str}` definition instead of nesting
@@ -806,9 +808,6 @@ parse_trait_alias_cannot_be_unsafe = trait aliases cannot be `unsafe`
 parse_transpose_dyn_or_impl = `for<...>` expected after `{$kw}`, not before
     .suggestion = move `{$kw}` before the `for<...>`
 
-parse_type_ascription_removed =
-    if you meant to annotate an expression with a type, the type ascription syntax has been removed, see issue #101728 <https://github.com/rust-lang/rust/issues/101728>
-
 parse_unclosed_unicode_escape = unterminated unicode escape
     .label = missing a closing `{"}"}`
     .terminate = terminate the unicode escape
diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index dfdef018bc3..44b4e1a3e47 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -7,8 +7,7 @@ use rustc_ast::util::parser::ExprPrecedence;
 use rustc_ast::{Path, Visibility};
 use rustc_errors::codes::*;
 use rustc_errors::{
-    Applicability, Diag, DiagCtxtHandle, Diagnostic, EmissionGuarantee, Level, SubdiagMessageOp,
-    Subdiagnostic,
+    Applicability, Diag, DiagCtxtHandle, Diagnostic, EmissionGuarantee, Level, Subdiagnostic,
 };
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_session::errors::ExprParenthesesNeeded;
@@ -30,7 +29,6 @@ pub(crate) struct AmbiguousPlus {
 #[derive(Diagnostic)]
 #[diag(parse_maybe_recover_from_bad_type_plus, code = E0178)]
 pub(crate) struct BadTypePlus {
-    pub ty: String,
     #[primary_span]
     pub span: Span,
     #[subdiagnostic]
@@ -1551,11 +1549,7 @@ pub(crate) struct FnTraitMissingParen {
 }
 
 impl Subdiagnostic for FnTraitMissingParen {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         diag.span_label(self.span, crate::fluent_generated::parse_fn_trait_missing_paren);
         diag.span_suggestion_short(
             self.span.shrink_to_hi(),
@@ -1598,9 +1592,6 @@ pub(crate) struct PathSingleColon {
 
     #[suggestion(applicability = "machine-applicable", code = ":", style = "verbose")]
     pub suggestion: Span,
-
-    #[note(parse_type_ascription_removed)]
-    pub type_ascription: bool,
 }
 
 #[derive(Diagnostic)]
@@ -1617,9 +1608,6 @@ pub(crate) struct ColonAsSemi {
     #[primary_span]
     #[suggestion(applicability = "machine-applicable", code = ";", style = "verbose")]
     pub span: Span,
-
-    #[note(parse_type_ascription_removed)]
-    pub type_ascription: bool,
 }
 
 #[derive(Diagnostic)]
@@ -2806,6 +2794,8 @@ pub(crate) struct ReturnTypesUseThinArrow {
 pub(crate) struct NeedPlusAfterTraitObjectLifetime {
     #[primary_span]
     pub span: Span,
+    #[suggestion(code = " + /* Trait */", applicability = "has-placeholders")]
+    pub suggestion: Span,
 }
 
 #[derive(Diagnostic)]
diff --git a/compiler/rustc_parse/src/lexer/unicode_chars.rs b/compiler/rustc_parse/src/lexer/unicode_chars.rs
index ff03b42484b..2bfa1ea4e05 100644
--- a/compiler/rustc_parse/src/lexer/unicode_chars.rs
+++ b/compiler/rustc_parse/src/lexer/unicode_chars.rs
@@ -376,7 +376,7 @@ pub(super) fn check_for_substitution(
             ascii_name,
         })
     };
-    (token.clone(), sugg)
+    (*token, sugg)
 }
 
 /// Extract string if found at current position with given delimiters
diff --git a/compiler/rustc_parse/src/lib.rs b/compiler/rustc_parse/src/lib.rs
index 79939aab7fc..2edc8c83017 100644
--- a/compiler/rustc_parse/src/lib.rs
+++ b/compiler/rustc_parse/src/lib.rs
@@ -4,7 +4,6 @@
 #![allow(internal_features)]
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![feature(array_windows)]
 #![feature(assert_matches)]
 #![feature(box_patterns)]
diff --git a/compiler/rustc_parse/src/parser/attr_wrapper.rs b/compiler/rustc_parse/src/parser/attr_wrapper.rs
index cff998fa137..f1bd6a22730 100644
--- a/compiler/rustc_parse/src/parser/attr_wrapper.rs
+++ b/compiler/rustc_parse/src/parser/attr_wrapper.rs
@@ -120,7 +120,7 @@ impl ToAttrTokenStream for LazyAttrTokenStreamImpl {
         // produce an empty `TokenStream` if no calls were made, and omit the
         // final token otherwise.
         let mut cursor_snapshot = self.cursor_snapshot.clone();
-        let tokens = iter::once(FlatToken::Token(self.start_token.clone()))
+        let tokens = iter::once(FlatToken::Token(self.start_token))
             .chain(iter::repeat_with(|| FlatToken::Token(cursor_snapshot.next())))
             .take(self.num_calls as usize);
 
@@ -186,7 +186,7 @@ impl ToAttrTokenStream for LazyAttrTokenStreamImpl {
 impl<'a> Parser<'a> {
     pub(super) fn collect_pos(&self) -> CollectPos {
         CollectPos {
-            start_token: (self.token.clone(), self.token_spacing),
+            start_token: (self.token, self.token_spacing),
             cursor_snapshot: self.token_cursor.clone(),
             start_pos: self.num_bump_calls,
         }
diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index ef044fe9d63..7c8e0146c3d 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/compiler/rustc_parse/src/parser/diagnostics.rs
@@ -322,7 +322,7 @@ impl<'a> Parser<'a> {
         let mut recovered_ident = None;
         // we take this here so that the correct original token is retained in
         // the diagnostic, regardless of eager recovery.
-        let bad_token = self.token.clone();
+        let bad_token = self.token;
 
         // suggest prepending a keyword in identifier position with `r#`
         let suggest_raw = if let Some((ident, IdentIsRaw::No)) = self.token.ident()
@@ -382,7 +382,7 @@ impl<'a> Parser<'a> {
             // if the previous token is a valid keyword
             // that might use a generic, then suggest a correct
             // generic placement (later on)
-            let maybe_keyword = self.prev_token.clone();
+            let maybe_keyword = self.prev_token;
             if valid_prev_keywords.into_iter().any(|x| maybe_keyword.is_keyword(x)) {
                 // if we have a valid keyword, attempt to parse generics
                 // also obtain the keywords symbol
@@ -530,7 +530,7 @@ impl<'a> Parser<'a> {
                 //   let y = 42;
                 let guar = self.dcx().emit_err(ExpectedSemi {
                     span: self.token.span,
-                    token: self.token.clone(),
+                    token: self.token,
                     unexpected_token_label: None,
                     sugg: ExpectedSemiSugg::ChangeToSemi(self.token.span),
                 });
@@ -555,7 +555,7 @@ impl<'a> Parser<'a> {
                 let span = self.prev_token.span.shrink_to_hi();
                 let guar = self.dcx().emit_err(ExpectedSemi {
                     span,
-                    token: self.token.clone(),
+                    token: self.token,
                     unexpected_token_label: Some(self.token.span),
                     sugg: ExpectedSemiSugg::AddSemi(span),
                 });
@@ -609,6 +609,8 @@ impl<'a> Parser<'a> {
         // FIXME: translation requires list formatting (for `expect`)
         let mut err = self.dcx().struct_span_err(self.token.span, msg_exp);
 
+        self.label_expected_raw_ref(&mut err);
+
         // Look for usages of '=>' where '>=' was probably intended
         if self.token == token::FatArrow
             && expected.iter().any(|tok| matches!(tok, TokenType::Operator | TokenType::Le))
@@ -750,6 +752,25 @@ impl<'a> Parser<'a> {
         Err(err)
     }
 
+    /// Adds a label when `&raw EXPR` was written instead of `&raw const EXPR`/`&raw mut EXPR`.
+    ///
+    /// Given that not all parser diagnostics flow through `expected_one_of_not_found`, this
+    /// label may need added to other diagnostics emission paths as needed.
+    pub(super) fn label_expected_raw_ref(&mut self, err: &mut Diag<'_>) {
+        if self.prev_token.is_keyword(kw::Raw)
+            && self.expected_token_types.contains(TokenType::KwMut)
+            && self.expected_token_types.contains(TokenType::KwConst)
+            && self.token.can_begin_expr()
+        {
+            err.span_suggestions(
+                self.prev_token.span.shrink_to_hi(),
+                "`&raw` must be followed by `const` or `mut` to be a raw reference expression",
+                [" const".to_string(), " mut".to_string()],
+                Applicability::MaybeIncorrect,
+            );
+        }
+    }
+
     /// Checks if the current token or the previous token are misspelled keywords
     /// and adds a helpful suggestion.
     fn check_for_misspelled_kw(&self, err: &mut Diag<'_>, expected: &[TokenType]) {
@@ -801,7 +822,7 @@ impl<'a> Parser<'a> {
         let span = self.prev_token.span.shrink_to_hi();
         let mut err = self.dcx().create_err(ExpectedSemi {
             span,
-            token: self.token.clone(),
+            token: self.token,
             unexpected_token_label: Some(self.token.span),
             sugg: ExpectedSemiSugg::AddSemi(span),
         });
@@ -1636,19 +1657,19 @@ impl<'a> Parser<'a> {
 
         self.bump(); // `+`
         let _bounds = self.parse_generic_bounds()?;
-        let sum_span = ty.span.to(self.prev_token.span);
-
         let sub = match &ty.kind {
             TyKind::Ref(_lifetime, mut_ty) => {
                 let lo = mut_ty.ty.span.shrink_to_lo();
                 let hi = self.prev_token.span.shrink_to_hi();
                 BadTypePlusSub::AddParen { suggestion: AddParen { lo, hi } }
             }
-            TyKind::Ptr(..) | TyKind::BareFn(..) => BadTypePlusSub::ForgotParen { span: sum_span },
-            _ => BadTypePlusSub::ExpectPath { span: sum_span },
+            TyKind::Ptr(..) | TyKind::BareFn(..) => {
+                BadTypePlusSub::ForgotParen { span: ty.span.to(self.prev_token.span) }
+            }
+            _ => BadTypePlusSub::ExpectPath { span: ty.span },
         };
 
-        self.dcx().emit_err(BadTypePlus { ty: pprust::ty_to_string(ty), span: sum_span, sub });
+        self.dcx().emit_err(BadTypePlus { span: ty.span, sub });
 
         Ok(())
     }
@@ -1922,10 +1943,7 @@ impl<'a> Parser<'a> {
             && self.token == token::Colon
             && self.look_ahead(1, |next| line_idx(self.token.span) < line_idx(next.span))
         {
-            self.dcx().emit_err(ColonAsSemi {
-                span: self.token.span,
-                type_ascription: self.psess.unstable_features.is_nightly_build(),
-            });
+            self.dcx().emit_err(ColonAsSemi { span: self.token.span });
             self.bump();
             return true;
         }
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index d52e36fcfac..df44b3cc23c 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -344,7 +344,7 @@ impl<'a> Parser<'a> {
     fn error_found_expr_would_be_stmt(&self, lhs: &Expr) {
         self.dcx().emit_err(errors::FoundExprWouldBeStmt {
             span: self.token.span,
-            token: self.token.clone(),
+            token: self.token,
             suggestion: ExprParenthesesNeeded::surrounding(lhs.span),
         });
     }
@@ -417,7 +417,7 @@ impl<'a> Parser<'a> {
         cur_op_span: Span,
     ) -> PResult<'a, P<Expr>> {
         let rhs = if self.is_at_start_of_range_notation_rhs() {
-            let maybe_lt = self.token.clone();
+            let maybe_lt = self.token;
             let attrs = self.parse_outer_attributes()?;
             Some(
                 self.parse_expr_assoc_with(Bound::Excluded(prec), attrs)
@@ -611,7 +611,7 @@ impl<'a> Parser<'a> {
 
     /// Recover on `not expr` in favor of `!expr`.
     fn recover_not_expr(&mut self, lo: Span) -> PResult<'a, (Span, ExprKind)> {
-        let negated_token = self.look_ahead(1, |t| t.clone());
+        let negated_token = self.look_ahead(1, |t| *t);
 
         let sub_diag = if negated_token.is_numeric_lit() {
             errors::NotAsNegationOperatorSub::SuggestNotBitwise
@@ -637,9 +637,7 @@ impl<'a> Parser<'a> {
     /// Returns the span of expr if it was not interpolated, or the span of the interpolated token.
     fn interpolated_or_expr_span(&self, expr: &Expr) -> Span {
         match self.prev_token.kind {
-            TokenKind::NtIdent(..) | TokenKind::NtLifetime(..) | TokenKind::Interpolated(..) => {
-                self.prev_token.span
-            }
+            TokenKind::NtIdent(..) | TokenKind::NtLifetime(..) => self.prev_token.span,
             TokenKind::CloseDelim(Delimiter::Invisible(InvisibleOrigin::MetaVar(_))) => {
                 // `expr.span` is the interpolated span, because invisible open
                 // and close delims both get marked with the same span, one
@@ -829,6 +827,18 @@ impl<'a> Parser<'a> {
         if let Some(lt) = lifetime {
             self.error_remove_borrow_lifetime(span, lt.ident.span.until(expr.span));
         }
+
+        // Add expected tokens if we parsed `&raw` as an expression.
+        // This will make sure we see "expected `const`, `mut`", and
+        // guides recovery in case we write `&raw expr`.
+        if borrow_kind == ast::BorrowKind::Ref
+            && mutbl == ast::Mutability::Not
+            && matches!(&expr.kind, ExprKind::Path(None, p) if p.is_ident(kw::Raw))
+        {
+            self.expected_token_types.insert(TokenType::KwMut);
+            self.expected_token_types.insert(TokenType::KwConst);
+        }
+
         Ok((span, ExprKind::AddrOf(borrow_kind, mutbl, expr)))
     }
 
@@ -1386,15 +1396,7 @@ impl<'a> Parser<'a> {
         maybe_recover_from_interpolated_ty_qpath!(self, true);
 
         let span = self.token.span;
-        if let token::Interpolated(nt) = &self.token.kind {
-            match &**nt {
-                token::NtBlock(block) => {
-                    let block = block.clone();
-                    self.bump();
-                    return Ok(self.mk_expr(self.prev_token.span, ExprKind::Block(block, None)));
-                }
-            };
-        } else if let Some(expr) = self.eat_metavar_seq_with_matcher(
+        if let Some(expr) = self.eat_metavar_seq_with_matcher(
             |mv_kind| matches!(mv_kind, MetaVarKind::Expr { .. }),
             |this| {
                 // Force collection (as opposed to just `parse_expr`) is required to avoid the
@@ -1415,9 +1417,13 @@ impl<'a> Parser<'a> {
             self.eat_metavar_seq(MetaVarKind::Literal, |this| this.parse_literal_maybe_minus())
         {
             return Ok(lit);
-        } else if let Some(path) = self.eat_metavar_seq(MetaVarKind::Path, |this| {
-            this.collect_tokens_no_attrs(|this| this.parse_path(PathStyle::Type))
-        }) {
+        } else if let Some(block) =
+            self.eat_metavar_seq(MetaVarKind::Block, |this| this.parse_block())
+        {
+            return Ok(self.mk_expr(span, ExprKind::Block(block, None)));
+        } else if let Some(path) =
+            self.eat_metavar_seq(MetaVarKind::Path, |this| this.parse_path(PathStyle::Type))
+        {
             return Ok(self.mk_expr(span, ExprKind::Path(None, path)));
         }
 
@@ -1612,7 +1618,7 @@ impl<'a> Parser<'a> {
     }
 
     fn parse_expr_path_start(&mut self) -> PResult<'a, P<Expr>> {
-        let maybe_eq_tok = self.prev_token.clone();
+        let maybe_eq_tok = self.prev_token;
         let (qself, path) = if self.eat_lt() {
             let lt_span = self.prev_token.span;
             let (qself, path) = self.parse_qpath(PathStyle::Expr).map_err(|mut err| {
@@ -1671,7 +1677,7 @@ impl<'a> Parser<'a> {
         } else if self.eat_keyword(exp!(Loop)) {
             self.parse_expr_loop(label, lo)
         } else if self.check_noexpect(&token::OpenDelim(Delimiter::Brace))
-            || self.token.is_whole_block()
+            || self.token.is_metavar_block()
         {
             self.parse_expr_block(label, lo, BlockCheckMode::Default)
         } else if !ate_colon
@@ -2073,7 +2079,7 @@ impl<'a> Parser<'a> {
         &mut self,
         mk_lit_char: impl FnOnce(Symbol, Span) -> L,
     ) -> PResult<'a, L> {
-        let token = self.token.clone();
+        let token = self.token;
         let err = |self_: &Self| {
             let msg = format!("unexpected token: {}", super::token_descr(&token));
             self_.dcx().struct_span_err(token.span, msg)
@@ -2354,7 +2360,7 @@ impl<'a> Parser<'a> {
             }
         }
 
-        if self.token.is_whole_block() {
+        if self.token.is_metavar_block() {
             self.dcx().emit_err(errors::InvalidBlockMacroSegment {
                 span: self.token.span,
                 context: lo.to(self.token.span),
@@ -2379,7 +2385,7 @@ impl<'a> Parser<'a> {
     fn parse_expr_closure(&mut self) -> PResult<'a, P<Expr>> {
         let lo = self.token.span;
 
-        let before = self.prev_token.clone();
+        let before = self.prev_token;
         let binder = if self.check_keyword(exp!(For)) {
             let lo = self.token.span;
             let (lifetime_defs, _) = self.parse_late_bound_lifetime_defs()?;
@@ -2411,8 +2417,8 @@ impl<'a> Parser<'a> {
             FnRetTy::Default(_) => {
                 let restrictions =
                     self.restrictions - Restrictions::STMT_EXPR - Restrictions::ALLOW_LET;
-                let prev = self.prev_token.clone();
-                let token = self.token.clone();
+                let prev = self.prev_token;
+                let token = self.token;
                 let attrs = self.parse_outer_attributes()?;
                 match self.parse_expr_res(restrictions, attrs) {
                     Ok((expr, _)) => expr,
@@ -2477,7 +2483,7 @@ impl<'a> Parser<'a> {
         if self.may_recover()
             && self.token.can_begin_expr()
             && !matches!(self.token.kind, TokenKind::OpenDelim(Delimiter::Brace))
-            && !self.token.is_whole_block()
+            && !self.token.is_metavar_block()
         {
             let snapshot = self.create_snapshot_for_diagnostic();
             let restrictions =
@@ -2659,7 +2665,7 @@ impl<'a> Parser<'a> {
             }
         } else {
             let attrs = self.parse_outer_attributes()?; // For recovery.
-            let maybe_fatarrow = self.token.clone();
+            let maybe_fatarrow = self.token;
             let block = if self.check(exp!(OpenBrace)) {
                 self.parse_block()?
             } else if let Some(block) = recover_block_from_condition(self) {
@@ -3524,7 +3530,7 @@ impl<'a> Parser<'a> {
         self.token.is_keyword(kw::Do)
             && self.is_keyword_ahead(1, &[kw::Catch])
             && self
-                .look_ahead(2, |t| *t == token::OpenDelim(Delimiter::Brace) || t.is_whole_block())
+                .look_ahead(2, |t| *t == token::OpenDelim(Delimiter::Brace) || t.is_metavar_block())
             && !self.restrictions.contains(Restrictions::NO_STRUCT_LITERAL)
     }
 
@@ -3535,7 +3541,7 @@ impl<'a> Parser<'a> {
     fn is_try_block(&self) -> bool {
         self.token.is_keyword(kw::Try)
             && self
-                .look_ahead(1, |t| *t == token::OpenDelim(Delimiter::Brace) || t.is_whole_block())
+                .look_ahead(1, |t| *t == token::OpenDelim(Delimiter::Brace) || t.is_metavar_block())
             && self.token_uninterpolated_span().at_least_rust_2018()
     }
 
@@ -3569,12 +3575,12 @@ impl<'a> Parser<'a> {
                 // `async move {`
                 self.is_keyword_ahead(lookahead + 1, &[kw::Move, kw::Use])
                     && self.look_ahead(lookahead + 2, |t| {
-                        *t == token::OpenDelim(Delimiter::Brace) || t.is_whole_block()
+                        *t == token::OpenDelim(Delimiter::Brace) || t.is_metavar_block()
                     })
             ) || (
                 // `async {`
                 self.look_ahead(lookahead + 1, |t| {
-                    *t == token::OpenDelim(Delimiter::Brace) || t.is_whole_block()
+                    *t == token::OpenDelim(Delimiter::Brace) || t.is_metavar_block()
                 })
             ))
     }
@@ -3867,7 +3873,7 @@ impl<'a> Parser<'a> {
                 return Err(this.dcx().create_err(errors::ExpectedStructField {
                     span: this.look_ahead(1, |t| t.span),
                     ident_span: this.token.span,
-                    token: this.look_ahead(1, |t| t.clone()),
+                    token: this.look_ahead(1, |t| *t),
                 }));
             }
             let (ident, expr) = if is_shorthand {
diff --git a/compiler/rustc_parse/src/parser/item.rs b/compiler/rustc_parse/src/parser/item.rs
index 06501816340..39a0291cb1e 100644
--- a/compiler/rustc_parse/src/parser/item.rs
+++ b/compiler/rustc_parse/src/parser/item.rs
@@ -1732,8 +1732,7 @@ impl<'a> Parser<'a> {
             self.expect_semi()?;
             body
         } else {
-            let err =
-                errors::UnexpectedTokenAfterStructName::new(self.token.span, self.token.clone());
+            let err = errors::UnexpectedTokenAfterStructName::new(self.token.span, self.token);
             return Err(self.dcx().create_err(err));
         };
 
@@ -2303,7 +2302,7 @@ impl<'a> Parser<'a> {
             || self.token.is_keyword(kw::Union))
             && self.look_ahead(1, |t| t.is_ident())
         {
-            let kw_token = self.token.clone();
+            let kw_token = self.token;
             let kw_str = pprust::token_to_string(&kw_token);
             let item = self.parse_item(ForceCollect::No)?;
             let mut item = item.unwrap().span;
@@ -2536,7 +2535,7 @@ impl<'a> Parser<'a> {
             self.expect_semi()?;
             *sig_hi = self.prev_token.span;
             (AttrVec::new(), None)
-        } else if self.check(exp!(OpenBrace)) || self.token.is_whole_block() {
+        } else if self.check(exp!(OpenBrace)) || self.token.is_metavar_block() {
             self.parse_block_common(self.token.span, BlockCheckMode::Default, None)
                 .map(|(attrs, body)| (attrs, Some(body)))?
         } else if self.token == token::Eq {
diff --git a/compiler/rustc_parse/src/parser/mod.rs b/compiler/rustc_parse/src/parser/mod.rs
index fafd1b1ae00..2221a261b4c 100644
--- a/compiler/rustc_parse/src/parser/mod.rs
+++ b/compiler/rustc_parse/src/parser/mod.rs
@@ -13,7 +13,6 @@ mod ty;
 
 use std::assert_matches::debug_assert_matches;
 use std::ops::Range;
-use std::sync::Arc;
 use std::{fmt, mem, slice};
 
 use attr_wrapper::{AttrWrapper, UsePreAttrPos};
@@ -24,8 +23,8 @@ pub use pat::{CommaRecoveryMode, RecoverColon, RecoverComma};
 use path::PathStyle;
 use rustc_ast::ptr::P;
 use rustc_ast::token::{
-    self, Delimiter, IdentIsRaw, InvisibleOrigin, MetaVarKind, Nonterminal, NtExprKind, NtPatKind,
-    Token, TokenKind,
+    self, Delimiter, IdentIsRaw, InvisibleOrigin, MetaVarKind, NtExprKind, NtPatKind, Token,
+    TokenKind,
 };
 use rustc_ast::tokenstream::{AttrsTarget, Spacing, TokenStream, TokenTree};
 use rustc_ast::util::case::Case;
@@ -98,21 +97,6 @@ pub enum ForceCollect {
     No,
 }
 
-#[macro_export]
-macro_rules! maybe_whole {
-    ($p:expr, $constructor:ident, |$x:ident| $e:expr) => {
-        #[allow(irrefutable_let_patterns)] // FIXME: temporary
-        if let token::Interpolated(nt) = &$p.token.kind
-            && let token::$constructor(x) = &**nt
-        {
-            #[allow(unused_mut)]
-            let mut $x = x.clone();
-            $p.bump();
-            return Ok($e);
-        }
-    };
-}
-
 /// If the next tokens are ill-formed `$ty::` recover them as `<$ty>::`.
 #[macro_export]
 macro_rules! maybe_recover_from_interpolated_ty_qpath {
@@ -342,12 +326,12 @@ impl TokenCursor {
             // below can be removed.
             if let Some(tree) = self.curr.curr() {
                 match tree {
-                    &TokenTree::Token(ref token, spacing) => {
+                    &TokenTree::Token(token, spacing) => {
                         debug_assert!(!matches!(
                             token.kind,
                             token::OpenDelim(_) | token::CloseDelim(_)
                         ));
-                        let res = (token.clone(), spacing);
+                        let res = (token, spacing);
                         self.curr.bump();
                         return res;
                     }
@@ -459,7 +443,6 @@ pub fn token_descr(token: &Token) -> String {
         (Some(TokenDescription::MetaVar(kind)), _) => format!("`{kind}` metavariable"),
         (None, TokenKind::NtIdent(..)) => format!("identifier `{s}`"),
         (None, TokenKind::NtLifetime(..)) => format!("lifetime `{s}`"),
-        (None, TokenKind::Interpolated(node)) => format!("{} `{s}`", node.descr()),
         (None, _) => format!("`{s}`"),
     }
 }
@@ -855,8 +838,10 @@ impl<'a> Parser<'a> {
     fn check_inline_const(&self, dist: usize) -> bool {
         self.is_keyword_ahead(dist, &[kw::Const])
             && self.look_ahead(dist + 1, |t| match &t.kind {
-                token::Interpolated(nt) => matches!(&**nt, token::NtBlock(..)),
                 token::OpenDelim(Delimiter::Brace) => true,
+                token::OpenDelim(Delimiter::Invisible(InvisibleOrigin::MetaVar(
+                    MetaVarKind::Block,
+                ))) => true,
                 _ => false,
             })
     }
@@ -1402,7 +1387,7 @@ impl<'a> Parser<'a> {
         // Avoid const blocks and const closures to be parsed as const items
         if (self.check_const_closure() == is_closure)
             && !self
-                .look_ahead(1, |t| *t == token::OpenDelim(Delimiter::Brace) || t.is_whole_block())
+                .look_ahead(1, |t| *t == token::OpenDelim(Delimiter::Brace) || t.is_metavar_block())
             && self.eat_keyword_case(exp!(Const), case)
         {
             Const::Yes(self.prev_token_uninterpolated_span())
@@ -1532,7 +1517,7 @@ impl<'a> Parser<'a> {
             _ => {
                 let prev_spacing = self.token_spacing;
                 self.bump();
-                TokenTree::Token(self.prev_token.clone(), prev_spacing)
+                TokenTree::Token(self.prev_token, prev_spacing)
             }
         }
     }
@@ -1718,7 +1703,7 @@ impl<'a> Parser<'a> {
             dbg_fmt.field("prev_token", &self.prev_token);
             let mut tokens = vec![];
             for i in 0..lookahead {
-                let tok = self.look_ahead(i, |tok| tok.kind.clone());
+                let tok = self.look_ahead(i, |tok| tok.kind);
                 let is_eof = tok == TokenKind::Eof;
                 tokens.push(tok);
                 if is_eof {
@@ -1759,7 +1744,6 @@ impl<'a> Parser<'a> {
     pub fn token_uninterpolated_span(&self) -> Span {
         match &self.token.kind {
             token::NtIdent(ident, _) | token::NtLifetime(ident, _) => ident.span,
-            token::Interpolated(nt) => nt.use_span(),
             token::OpenDelim(Delimiter::Invisible(InvisibleOrigin::MetaVar(_))) => {
                 self.look_ahead(1, |t| t.span)
             }
@@ -1771,7 +1755,6 @@ impl<'a> Parser<'a> {
     pub fn prev_token_uninterpolated_span(&self) -> Span {
         match &self.prev_token.kind {
             token::NtIdent(ident, _) | token::NtLifetime(ident, _) => ident.span,
-            token::Interpolated(nt) => nt.use_span(),
             token::OpenDelim(Delimiter::Invisible(InvisibleOrigin::MetaVar(_))) => {
                 self.look_ahead(0, |t| t.span)
             }
@@ -1828,6 +1811,7 @@ pub enum ParseNtResult {
     Ident(Ident, IdentIsRaw),
     Lifetime(Ident, IdentIsRaw),
     Item(P<ast::Item>),
+    Block(P<ast::Block>),
     Stmt(P<ast::Stmt>),
     Pat(P<ast::Pat>, NtPatKind),
     Expr(P<ast::Expr>, NtExprKind),
@@ -1836,7 +1820,4 @@ pub enum ParseNtResult {
     Meta(P<ast::AttrItem>),
     Path(P<ast::Path>),
     Vis(P<ast::Visibility>),
-
-    /// This variant will eventually be removed, along with `Token::Interpolate`.
-    Nt(Arc<Nonterminal>),
 }
diff --git a/compiler/rustc_parse/src/parser/nonterminal.rs b/compiler/rustc_parse/src/parser/nonterminal.rs
index b4e540d670d..b6e89cd7fa4 100644
--- a/compiler/rustc_parse/src/parser/nonterminal.rs
+++ b/compiler/rustc_parse/src/parser/nonterminal.rs
@@ -1,14 +1,7 @@
-use std::sync::Arc;
-
-use rustc_ast::HasTokens;
 use rustc_ast::ptr::P;
-use rustc_ast::token::Nonterminal::*;
 use rustc_ast::token::NtExprKind::*;
 use rustc_ast::token::NtPatKind::*;
-use rustc_ast::token::{
-    self, Delimiter, InvisibleOrigin, MetaVarKind, Nonterminal, NonterminalKind, Token,
-};
-use rustc_ast_pretty::pprust;
+use rustc_ast::token::{self, Delimiter, InvisibleOrigin, MetaVarKind, NonterminalKind, Token};
 use rustc_errors::PResult;
 use rustc_span::{Ident, kw};
 
@@ -45,13 +38,6 @@ impl<'a> Parser<'a> {
             }
         }
 
-        /// Old variant of `may_be_ident`. Being phased out.
-        fn nt_may_be_ident(nt: &Nonterminal) -> bool {
-            match nt {
-                NtBlock(_) => false,
-            }
-        }
-
         match kind {
             // `expr_2021` and earlier
             NonterminalKind::Expr(Expr2021 { .. }) => {
@@ -83,16 +69,12 @@ impl<'a> Parser<'a> {
                 | token::Ident(..)
                 | token::NtIdent(..)
                 | token::NtLifetime(..)
-                | token::Interpolated(_)
                 | token::OpenDelim(Delimiter::Invisible(InvisibleOrigin::MetaVar(_))) => true,
                 _ => token.can_begin_type(),
             },
             NonterminalKind::Block => match &token.kind {
                 token::OpenDelim(Delimiter::Brace) => true,
                 token::NtLifetime(..) => true,
-                token::Interpolated(nt) => match &**nt {
-                    NtBlock(_) => true,
-                },
                 token::OpenDelim(Delimiter::Invisible(InvisibleOrigin::MetaVar(k))) => match k {
                     MetaVarKind::Block
                     | MetaVarKind::Stmt
@@ -112,7 +94,6 @@ impl<'a> Parser<'a> {
             },
             NonterminalKind::Path | NonterminalKind::Meta => match &token.kind {
                 token::PathSep | token::Ident(..) | token::NtIdent(..) => true,
-                token::Interpolated(nt) => nt_may_be_ident(nt),
                 token::OpenDelim(Delimiter::Invisible(InvisibleOrigin::MetaVar(kind))) => {
                     may_be_ident(*kind)
                 }
@@ -136,110 +117,85 @@ impl<'a> Parser<'a> {
         // A `macro_rules!` invocation may pass a captured item/expr to a proc-macro,
         // which requires having captured tokens available. Since we cannot determine
         // in advance whether or not a proc-macro will be (transitively) invoked,
-        // we always capture tokens for any `Nonterminal` which needs them.
-        let mut nt = match kind {
+        // we always capture tokens for any nonterminal that needs them.
+        match kind {
             // Note that TT is treated differently to all the others.
-            NonterminalKind::TT => return Ok(ParseNtResult::Tt(self.parse_token_tree())),
+            NonterminalKind::TT => Ok(ParseNtResult::Tt(self.parse_token_tree())),
             NonterminalKind::Item => match self.parse_item(ForceCollect::Yes)? {
-                Some(item) => return Ok(ParseNtResult::Item(item)),
-                None => {
-                    return Err(self
-                        .dcx()
-                        .create_err(UnexpectedNonterminal::Item(self.token.span)));
-                }
+                Some(item) => Ok(ParseNtResult::Item(item)),
+                None => Err(self.dcx().create_err(UnexpectedNonterminal::Item(self.token.span))),
             },
             NonterminalKind::Block => {
                 // While a block *expression* may have attributes (e.g. `#[my_attr] { ... }`),
                 // the ':block' matcher does not support them
-                NtBlock(self.collect_tokens_no_attrs(|this| this.parse_block())?)
+                Ok(ParseNtResult::Block(self.collect_tokens_no_attrs(|this| this.parse_block())?))
             }
             NonterminalKind::Stmt => match self.parse_stmt(ForceCollect::Yes)? {
-                Some(stmt) => return Ok(ParseNtResult::Stmt(P(stmt))),
+                Some(stmt) => Ok(ParseNtResult::Stmt(P(stmt))),
                 None => {
-                    return Err(self
-                        .dcx()
-                        .create_err(UnexpectedNonterminal::Statement(self.token.span)));
+                    Err(self.dcx().create_err(UnexpectedNonterminal::Statement(self.token.span)))
                 }
             },
-            NonterminalKind::Pat(pat_kind) => {
-                return Ok(ParseNtResult::Pat(
-                    self.collect_tokens_no_attrs(|this| match pat_kind {
-                        PatParam { .. } => this.parse_pat_no_top_alt(None, None),
-                        PatWithOr => this.parse_pat_no_top_guard(
-                            None,
-                            RecoverComma::No,
-                            RecoverColon::No,
-                            CommaRecoveryMode::EitherTupleOrPipe,
-                        ),
-                    })?,
-                    pat_kind,
-                ));
-            }
+            NonterminalKind::Pat(pat_kind) => Ok(ParseNtResult::Pat(
+                self.collect_tokens_no_attrs(|this| match pat_kind {
+                    PatParam { .. } => this.parse_pat_no_top_alt(None, None),
+                    PatWithOr => this.parse_pat_no_top_guard(
+                        None,
+                        RecoverComma::No,
+                        RecoverColon::No,
+                        CommaRecoveryMode::EitherTupleOrPipe,
+                    ),
+                })?,
+                pat_kind,
+            )),
             NonterminalKind::Expr(expr_kind) => {
-                return Ok(ParseNtResult::Expr(self.parse_expr_force_collect()?, expr_kind));
+                Ok(ParseNtResult::Expr(self.parse_expr_force_collect()?, expr_kind))
             }
             NonterminalKind::Literal => {
                 // The `:literal` matcher does not support attributes.
-                return Ok(ParseNtResult::Literal(
+                Ok(ParseNtResult::Literal(
                     self.collect_tokens_no_attrs(|this| this.parse_literal_maybe_minus())?,
-                ));
-            }
-            NonterminalKind::Ty => {
-                return Ok(ParseNtResult::Ty(
-                    self.collect_tokens_no_attrs(|this| this.parse_ty_no_question_mark_recover())?,
-                ));
+                ))
             }
-            // this could be handled like a token, since it is one
+            NonterminalKind::Ty => Ok(ParseNtResult::Ty(
+                self.collect_tokens_no_attrs(|this| this.parse_ty_no_question_mark_recover())?,
+            )),
+            // This could be handled like a token, since it is one.
             NonterminalKind::Ident => {
-                return if let Some((ident, is_raw)) = get_macro_ident(&self.token) {
+                if let Some((ident, is_raw)) = get_macro_ident(&self.token) {
                     self.bump();
                     Ok(ParseNtResult::Ident(ident, is_raw))
                 } else {
                     Err(self.dcx().create_err(UnexpectedNonterminal::Ident {
                         span: self.token.span,
-                        token: self.token.clone(),
+                        token: self.token,
                     }))
-                };
-            }
-            NonterminalKind::Path => {
-                return Ok(ParseNtResult::Path(P(
-                    self.collect_tokens_no_attrs(|this| this.parse_path(PathStyle::Type))?
-                )));
+                }
             }
+            NonterminalKind::Path => Ok(ParseNtResult::Path(P(
+                self.collect_tokens_no_attrs(|this| this.parse_path(PathStyle::Type))?
+            ))),
             NonterminalKind::Meta => {
-                return Ok(ParseNtResult::Meta(P(self.parse_attr_item(ForceCollect::Yes)?)));
+                Ok(ParseNtResult::Meta(P(self.parse_attr_item(ForceCollect::Yes)?)))
             }
             NonterminalKind::Vis => {
-                return Ok(ParseNtResult::Vis(P(self.collect_tokens_no_attrs(|this| {
-                    this.parse_visibility(FollowedByType::Yes)
-                })?)));
+                Ok(ParseNtResult::Vis(P(self
+                    .collect_tokens_no_attrs(|this| this.parse_visibility(FollowedByType::Yes))?)))
             }
             NonterminalKind::Lifetime => {
                 // We want to keep `'keyword` parsing, just like `keyword` is still
                 // an ident for nonterminal purposes.
-                return if let Some((ident, is_raw)) = self.token.lifetime() {
+                if let Some((ident, is_raw)) = self.token.lifetime() {
                     self.bump();
                     Ok(ParseNtResult::Lifetime(ident, is_raw))
                 } else {
                     Err(self.dcx().create_err(UnexpectedNonterminal::Lifetime {
                         span: self.token.span,
-                        token: self.token.clone(),
+                        token: self.token,
                     }))
-                };
+                }
             }
-        };
-
-        // If tokens are supported at all, they should be collected.
-        if matches!(nt.tokens_mut(), Some(None)) {
-            panic!(
-                "Missing tokens for nt {:?} at {:?}: {:?}",
-                nt,
-                nt.use_span(),
-                pprust::nonterminal_to_string(&nt)
-            );
         }
-
-        Ok(ParseNtResult::Nt(Arc::new(nt)))
     }
 }
 
diff --git a/compiler/rustc_parse/src/parser/pat.rs b/compiler/rustc_parse/src/parser/pat.rs
index 9612f71b2af..d5f469f9aa9 100644
--- a/compiler/rustc_parse/src/parser/pat.rs
+++ b/compiler/rustc_parse/src/parser/pat.rs
@@ -363,7 +363,7 @@ impl<'a> Parser<'a> {
                 self.dcx().emit_err(TrailingVertNotAllowed {
                     span: self.token.span,
                     start: lo,
-                    token: self.token.clone(),
+                    token: self.token,
                     note_double_vert: matches!(self.token.kind, token::OrOr),
                 });
                 self.bump();
@@ -1519,8 +1519,8 @@ impl<'a> Parser<'a> {
                 etc = PatFieldsRest::Rest;
                 let mut etc_sp = self.token.span;
                 if first_etc_and_maybe_comma_span.is_none() {
-                    if let Some(comma_tok) = self
-                        .look_ahead(1, |t| if *t == token::Comma { Some(t.clone()) } else { None })
+                    if let Some(comma_tok) =
+                        self.look_ahead(1, |&t| if t == token::Comma { Some(t) } else { None })
                     {
                         let nw_span = self
                             .psess
diff --git a/compiler/rustc_parse/src/parser/path.rs b/compiler/rustc_parse/src/parser/path.rs
index 9c6830c3672..02883655662 100644
--- a/compiler/rustc_parse/src/parser/path.rs
+++ b/compiler/rustc_parse/src/parser/path.rs
@@ -273,7 +273,6 @@ impl<'a> Parser<'a> {
                         self.dcx().emit_err(PathSingleColon {
                             span: self.prev_token.span,
                             suggestion: self.prev_token.span.shrink_to_hi(),
-                            type_ascription: self.psess.unstable_features.is_nightly_build(),
                         });
                     }
                     continue;
@@ -348,7 +347,6 @@ impl<'a> Parser<'a> {
                             err = self.dcx().create_err(PathSingleColon {
                                 span: self.token.span,
                                 suggestion: self.prev_token.span.shrink_to_hi(),
-                                type_ascription: self.psess.unstable_features.is_nightly_build(),
                             });
                         }
                         // Attempt to find places where a missing `>` might belong.
@@ -393,8 +391,8 @@ impl<'a> Parser<'a> {
                 } else {
                     // `(T, U) -> R`
 
-                    let prev_token_before_parsing = self.prev_token.clone();
-                    let token_before_parsing = self.token.clone();
+                    let prev_token_before_parsing = self.prev_token;
+                    let token_before_parsing = self.token;
                     let mut snapshot = None;
                     if self.may_recover()
                         && prev_token_before_parsing == token::PathSep
diff --git a/compiler/rustc_parse/src/parser/stmt.rs b/compiler/rustc_parse/src/parser/stmt.rs
index e00fd40ecee..0cc8b605018 100644
--- a/compiler/rustc_parse/src/parser/stmt.rs
+++ b/compiler/rustc_parse/src/parser/stmt.rs
@@ -23,8 +23,8 @@ use super::{
     AttrWrapper, BlockMode, FnParseMode, ForceCollect, Parser, Restrictions, SemiColonMode,
     Trailing, UsePreAttrPos,
 };
-use crate::errors::MalformedLoopLabel;
-use crate::{errors, exp, maybe_whole};
+use crate::errors::{self, MalformedLoopLabel};
+use crate::exp;
 
 impl<'a> Parser<'a> {
     /// Parses a statement. This stops just before trailing semicolons on everything but items.
@@ -518,7 +518,11 @@ impl<'a> Parser<'a> {
         let prev = self.prev_token.span;
         let sp = self.token.span;
         let mut e = self.dcx().struct_span_err(sp, msg);
-        let do_not_suggest_help = self.token.is_keyword(kw::In) || self.token == token::Colon;
+        self.label_expected_raw_ref(&mut e);
+
+        let do_not_suggest_help = self.token.is_keyword(kw::In)
+            || self.token == token::Colon
+            || self.prev_token.is_keyword(kw::Raw);
 
         // Check to see if the user has written something like
         //
@@ -696,9 +700,11 @@ impl<'a> Parser<'a> {
         blk_mode: BlockCheckMode,
         loop_header: Option<Span>,
     ) -> PResult<'a, (AttrVec, P<Block>)> {
-        maybe_whole!(self, NtBlock, |block| (AttrVec::new(), block));
+        if let Some(block) = self.eat_metavar_seq(MetaVarKind::Block, |this| this.parse_block()) {
+            return Ok((AttrVec::new(), block));
+        }
 
-        let maybe_ident = self.prev_token.clone();
+        let maybe_ident = self.prev_token;
         self.maybe_recover_unexpected_block_label(loop_header);
         if !self.eat(exp!(OpenBrace)) {
             return self.error_block_no_opening_brace();
@@ -765,10 +771,6 @@ impl<'a> Parser<'a> {
                                     Applicability::MaybeIncorrect,
                                 );
                             }
-                            if self.psess.unstable_features.is_nightly_build() {
-                                // FIXME(Nilstrieb): Remove this again after a few months.
-                                err.note("type ascription syntax has been removed, see issue #101728 <https://github.com/rust-lang/rust/issues/101728>");
-                            }
                         }
                     }
 
@@ -911,7 +913,7 @@ impl<'a> Parser<'a> {
                                 {
                                     if self.token == token::Colon
                                         && self.look_ahead(1, |token| {
-                                            token.is_whole_block()
+                                            token.is_metavar_block()
                                                 || matches!(
                                                     token.kind,
                                                     token::Ident(
diff --git a/compiler/rustc_parse/src/parser/tests.rs b/compiler/rustc_parse/src/parser/tests.rs
index 49ae6cb9b72..2f958f4d492 100644
--- a/compiler/rustc_parse/src/parser/tests.rs
+++ b/compiler/rustc_parse/src/parser/tests.rs
@@ -2554,7 +2554,7 @@ fn look(p: &Parser<'_>, dist: usize, kind: rustc_ast::token::TokenKind) {
     // Do the `assert_eq` outside the closure so that `track_caller` works.
     // (`#![feature(closure_track_caller)]` + `#[track_caller]` on the closure
     // doesn't give the line number in the test below if the assertion fails.)
-    let tok = p.look_ahead(dist, |tok| tok.clone());
+    let tok = p.look_ahead(dist, |tok| *tok);
     assert_eq!(kind, tok.kind);
 }
 
diff --git a/compiler/rustc_parse/src/parser/ty.rs b/compiler/rustc_parse/src/parser/ty.rs
index 93705da22c4..42ebf26784d 100644
--- a/compiler/rustc_parse/src/parser/ty.rs
+++ b/compiler/rustc_parse/src/parser/ty.rs
@@ -7,7 +7,7 @@ use rustc_ast::{
     Pinnedness, PolyTraitRef, PreciseCapturingArg, TraitBoundModifiers, TraitObjectSyntax, Ty,
     TyKind, UnsafeBinderTy,
 };
-use rustc_errors::{Applicability, PResult};
+use rustc_errors::{Applicability, Diag, PResult};
 use rustc_span::{ErrorGuaranteed, Ident, Span, kw, sym};
 use thin_vec::{ThinVec, thin_vec};
 
@@ -411,6 +411,9 @@ impl<'a> Parser<'a> {
                 TyKind::Path(None, path) if maybe_bounds => {
                     self.parse_remaining_bounds_path(ThinVec::new(), path, lo, true)
                 }
+                // For `('a) + …`, we know that `'a` in type position already lead to an error being
+                // emitted. To reduce output, let's indirectly suppress E0178 (bad `+` in type) and
+                // other irrelevant consequential errors.
                 TyKind::TraitObject(bounds, TraitObjectSyntax::None)
                     if maybe_bounds && bounds.len() == 1 && !trailing_plus =>
                 {
@@ -425,12 +428,60 @@ impl<'a> Parser<'a> {
     }
 
     fn parse_bare_trait_object(&mut self, lo: Span, allow_plus: AllowPlus) -> PResult<'a, TyKind> {
-        let lt_no_plus = self.check_lifetime() && !self.look_ahead(1, |t| t.is_like_plus());
-        let bounds = self.parse_generic_bounds_common(allow_plus)?;
-        if lt_no_plus {
-            self.dcx().emit_err(NeedPlusAfterTraitObjectLifetime { span: lo });
+        // A lifetime only begins a bare trait object type if it is followed by `+`!
+        if self.token.is_lifetime() && !self.look_ahead(1, |t| t.is_like_plus()) {
+            // In Rust 2021 and beyond, we assume that the user didn't intend to write a bare trait
+            // object type with a leading lifetime bound since that seems very unlikely given the
+            // fact that `dyn`-less trait objects are *semantically* invalid.
+            if self.psess.edition.at_least_rust_2021() {
+                let lt = self.expect_lifetime();
+                let mut err = self.dcx().struct_span_err(lo, "expected type, found lifetime");
+                err.span_label(lo, "expected type");
+                return Ok(match self.maybe_recover_ref_ty_no_leading_ampersand(lt, lo, err) {
+                    Ok(ref_ty) => ref_ty,
+                    Err(err) => TyKind::Err(err.emit()),
+                });
+            }
+
+            self.dcx().emit_err(NeedPlusAfterTraitObjectLifetime {
+                span: lo,
+                suggestion: lo.shrink_to_hi(),
+            });
+        }
+        Ok(TyKind::TraitObject(
+            self.parse_generic_bounds_common(allow_plus)?,
+            TraitObjectSyntax::None,
+        ))
+    }
+
+    fn maybe_recover_ref_ty_no_leading_ampersand<'cx>(
+        &mut self,
+        lt: Lifetime,
+        lo: Span,
+        mut err: Diag<'cx>,
+    ) -> Result<TyKind, Diag<'cx>> {
+        if !self.may_recover() {
+            return Err(err);
+        }
+        let snapshot = self.create_snapshot_for_diagnostic();
+        let mutbl = self.parse_mutability();
+        match self.parse_ty_no_plus() {
+            Ok(ty) => {
+                err.span_suggestion_verbose(
+                    lo.shrink_to_lo(),
+                    "you might have meant to write a reference type here",
+                    "&",
+                    Applicability::MaybeIncorrect,
+                );
+                err.emit();
+                Ok(TyKind::Ref(Some(lt), MutTy { ty, mutbl }))
+            }
+            Err(diag) => {
+                diag.cancel();
+                self.restore_snapshot(snapshot);
+                Err(err)
+            }
         }
-        Ok(TyKind::TraitObject(bounds, TraitObjectSyntax::None))
     }
 
     fn parse_remaining_bounds_path(
@@ -547,7 +598,7 @@ impl<'a> Parser<'a> {
 
             // Recovery
             mutbl = Mutability::Mut;
-            let (dyn_tok, dyn_tok_sp) = (self.token.clone(), self.token_spacing);
+            let (dyn_tok, dyn_tok_sp) = (self.token, self.token_spacing);
             self.bump();
             self.bump_with((dyn_tok, dyn_tok_sp));
         }
@@ -886,7 +937,7 @@ impl<'a> Parser<'a> {
     /// ```
     fn parse_generic_bound(&mut self) -> PResult<'a, GenericBound> {
         let lo = self.token.span;
-        let leading_token = self.prev_token.clone();
+        let leading_token = self.prev_token;
         let has_parens = self.eat(exp!(OpenParen));
 
         let bound = if self.token.is_lifetime() {
diff --git a/compiler/rustc_parse/src/validate_attr.rs b/compiler/rustc_parse/src/validate_attr.rs
index 6bbd650dcdf..b518fca7a65 100644
--- a/compiler/rustc_parse/src/validate_attr.rs
+++ b/compiler/rustc_parse/src/validate_attr.rs
@@ -157,7 +157,7 @@ fn is_attr_template_compatible(template: &AttributeTemplate, meta: &ast::MetaIte
 pub fn check_attribute_safety(psess: &ParseSess, safety: AttributeSafety, attr: &Attribute) {
     let attr_item = attr.get_normal_item();
 
-    if safety == AttributeSafety::Unsafe {
+    if let AttributeSafety::Unsafe { unsafe_since } = safety {
         if let ast::Safety::Default = attr_item.unsafety {
             let path_span = attr_item.path.span;
 
@@ -167,7 +167,13 @@ pub fn check_attribute_safety(psess: &ParseSess, safety: AttributeSafety, attr:
             // square bracket respectively.
             let diag_span = attr_item.span();
 
-            if attr.span.at_least_rust_2024() {
+            // Attributes can be safe in earlier editions, and become unsafe in later ones.
+            let emit_error = match unsafe_since {
+                None => true,
+                Some(unsafe_since) => attr.span.edition() >= unsafe_since,
+            };
+
+            if emit_error {
                 psess.dcx().emit_err(errors::UnsafeAttrOutsideUnsafe {
                     span: path_span,
                     suggestion: errors::UnsafeAttrOutsideUnsafeSuggestion {
@@ -188,6 +194,12 @@ pub fn check_attribute_safety(psess: &ParseSess, safety: AttributeSafety, attr:
             }
         }
     } else if let Safety::Unsafe(unsafe_span) = attr_item.unsafety {
+        // Allow (but don't require) `#[unsafe(naked)]` so that compiler-builtins can upgrade to it.
+        // FIXME(#139797): remove this special case when compiler-builtins has upgraded.
+        if attr.has_name(sym::naked) {
+            return;
+        }
+
         psess.dcx().emit_err(errors::InvalidAttrUnsafe {
             span: unsafe_span,
             name: attr_item.path.clone(),
diff --git a/compiler/rustc_passes/messages.ftl b/compiler/rustc_passes/messages.ftl
index bea86801ed7..6ee5e356435 100644
--- a/compiler/rustc_passes/messages.ftl
+++ b/compiler/rustc_passes/messages.ftl
@@ -742,9 +742,6 @@ passes_trait_impl_const_stable =
 passes_transparent_incompatible =
     transparent {$target} cannot have other repr hints
 
-passes_undefined_naked_function_abi =
-    Rust ABI is unsupported in naked functions
-
 passes_unknown_external_lang_item =
     unknown external lang item: `{$lang_item}`
 
diff --git a/compiler/rustc_passes/src/check_attr.rs b/compiler/rustc_passes/src/check_attr.rs
index 9161b23428a..42279258e87 100644
--- a/compiler/rustc_passes/src/check_attr.rs
+++ b/compiler/rustc_passes/src/check_attr.rs
@@ -624,6 +624,21 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
         match target {
             Target::Fn
             | Target::Method(MethodKind::Trait { body: true } | MethodKind::Inherent) => {
+                let fn_sig = self.tcx.hir_node(hir_id).fn_sig().unwrap();
+                let abi = fn_sig.header.abi;
+                if abi.is_rustic_abi() && !self.tcx.features().naked_functions_rustic_abi() {
+                    feature_err(
+                        &self.tcx.sess,
+                        sym::naked_functions_rustic_abi,
+                        fn_sig.span,
+                        format!(
+                            "`#[naked]` is currently unstable on `extern \"{}\"` functions",
+                            abi.as_str()
+                        ),
+                    )
+                    .emit();
+                }
+
                 for other_attr in attrs {
                     // this covers "sugared doc comments" of the form `/// ...`
                     // it does not cover `#[doc = "..."]`, which is handled below
@@ -1109,6 +1124,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
             ItemKind::Trait(_, _, _, generics, _, items)
                 if generics.params.len() != 0
                     || items.iter().any(|item| matches!(item.kind, AssocItemKind::Type)) => {}
+            ItemKind::TyAlias(_, _, generics) if generics.params.len() != 0 => {}
             _ => {
                 self.dcx().emit_err(errors::DocSearchUnboxInvalid { span: meta.span() });
             }
diff --git a/compiler/rustc_passes/src/errors.rs b/compiler/rustc_passes/src/errors.rs
index 4e3e0324205..995fc85676e 100644
--- a/compiler/rustc_passes/src/errors.rs
+++ b/compiler/rustc_passes/src/errors.rs
@@ -5,7 +5,7 @@ use rustc_ast::Label;
 use rustc_errors::codes::*;
 use rustc_errors::{
     Applicability, Diag, DiagCtxtHandle, DiagSymbolList, Diagnostic, EmissionGuarantee, Level,
-    MultiSpan, SubdiagMessageOp, Subdiagnostic,
+    MultiSpan, Subdiagnostic,
 };
 use rustc_hir::{self as hir, ExprKind, Target};
 use rustc_macros::{Diagnostic, LintDiagnostic, Subdiagnostic};
@@ -1197,10 +1197,6 @@ pub(crate) struct UnlabeledCfInWhileCondition<'a> {
     pub cf_type: &'a str,
 }
 
-#[derive(LintDiagnostic)]
-#[diag(passes_undefined_naked_function_abi)]
-pub(crate) struct UndefinedNakedFunctionAbi;
-
 #[derive(Diagnostic)]
 #[diag(passes_no_patterns)]
 pub(crate) struct NoPatterns {
@@ -1856,11 +1852,7 @@ pub(crate) struct UnusedVariableStringInterp {
 }
 
 impl Subdiagnostic for UnusedVariableStringInterp {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         diag.span_label(self.lit, crate::fluent_generated::passes_maybe_string_interpolation);
         diag.multipart_suggestion(
             crate::fluent_generated::passes_string_interpolation_only_works,
diff --git a/compiler/rustc_passes/src/lib.rs b/compiler/rustc_passes/src/lib.rs
index 6f6115af96c..93ff0f66d69 100644
--- a/compiler/rustc_passes/src/lib.rs
+++ b/compiler/rustc_passes/src/lib.rs
@@ -6,7 +6,6 @@
 
 // tidy-alphabetical-start
 #![allow(internal_features)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(let_chains)]
diff --git a/compiler/rustc_passes/src/liveness.rs b/compiler/rustc_passes/src/liveness.rs
index 06eb76c30c5..4e9b7fd44d4 100644
--- a/compiler/rustc_passes/src/liveness.rs
+++ b/compiler/rustc_passes/src/liveness.rs
@@ -578,8 +578,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
     where
         F: FnMut(Variable) -> bool,
     {
-        for var_idx in 0..self.ir.var_kinds.len() {
-            let var = Variable::from(var_idx);
+        for var in self.ir.var_kinds.indices() {
             if test(var) {
                 write!(wr, " {var:?}")?;
             }
@@ -609,8 +608,8 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
         debug!(
             "^^ liveness computation results for body {} (entry={:?})",
             {
-                for ln_idx in 0..self.ir.lnks.len() {
-                    debug!("{:?}", self.ln_str(LiveNode::from(ln_idx)));
+                for ln_idx in self.ir.lnks.indices() {
+                    debug!("{:?}", self.ln_str(ln_idx));
                 }
                 hir_id
             },
@@ -1021,7 +1020,10 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
             }
 
             hir::ExprKind::Call(ref f, args) => {
-                let succ = self.check_is_ty_uninhabited(expr, succ);
+                let is_ctor = |f: &Expr<'_>| matches!(f.kind, hir::ExprKind::Path(hir::QPath::Resolved(_, path)) if matches!(path.res, rustc_hir::def::Res::Def(rustc_hir::def::DefKind::Ctor(_, _), _)));
+                let succ =
+                    if !is_ctor(f) { self.check_is_ty_uninhabited(expr, succ) } else { succ };
+
                 let succ = self.propagate_through_exprs(args, succ);
                 self.propagate_through_expr(f, succ)
             }
diff --git a/compiler/rustc_passes/src/naked_functions.rs b/compiler/rustc_passes/src/naked_functions.rs
index d35aedf9a56..3c9f8b72c36 100644
--- a/compiler/rustc_passes/src/naked_functions.rs
+++ b/compiler/rustc_passes/src/naked_functions.rs
@@ -1,6 +1,5 @@
 //! Checks validity of naked functions.
 
-use rustc_abi::ExternAbi;
 use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{LocalDefId, LocalModDefId};
@@ -10,12 +9,11 @@ use rustc_middle::hir::nested_filter::OnlyBodies;
 use rustc_middle::query::Providers;
 use rustc_middle::span_bug;
 use rustc_middle::ty::TyCtxt;
-use rustc_session::lint::builtin::UNDEFINED_NAKED_FUNCTION_ABI;
 use rustc_span::{Span, sym};
 
 use crate::errors::{
     NakedAsmOutsideNakedFn, NakedFunctionsAsmBlock, NakedFunctionsMustNakedAsm, NoPatterns,
-    ParamsNotAllowed, UndefinedNakedFunctionAbi,
+    ParamsNotAllowed,
 };
 
 pub(crate) fn provide(providers: &mut Providers) {
@@ -29,26 +27,21 @@ fn check_mod_naked_functions(tcx: TyCtxt<'_>, module_def_id: LocalModDefId) {
             continue;
         }
 
-        let (fn_header, body_id) = match tcx.hir_node_by_def_id(def_id) {
+        let body = match tcx.hir_node_by_def_id(def_id) {
             hir::Node::Item(hir::Item {
-                kind: hir::ItemKind::Fn { sig, body: body_id, .. },
-                ..
+                kind: hir::ItemKind::Fn { body: body_id, .. }, ..
             })
             | hir::Node::TraitItem(hir::TraitItem {
-                kind: hir::TraitItemKind::Fn(sig, hir::TraitFn::Provided(body_id)),
+                kind: hir::TraitItemKind::Fn(_, hir::TraitFn::Provided(body_id)),
                 ..
             })
             | hir::Node::ImplItem(hir::ImplItem {
-                kind: hir::ImplItemKind::Fn(sig, body_id),
-                ..
-            }) => (sig.header, *body_id),
+                kind: hir::ImplItemKind::Fn(_, body_id), ..
+            }) => tcx.hir_body(*body_id),
             _ => continue,
         };
 
-        let body = tcx.hir_body(body_id);
-
         if tcx.has_attr(def_id, sym::naked) {
-            check_abi(tcx, def_id, fn_header.abi);
             check_no_patterns(tcx, body.params);
             check_no_parameters_use(tcx, body);
             check_asm(tcx, def_id, body);
@@ -60,20 +53,6 @@ fn check_mod_naked_functions(tcx: TyCtxt<'_>, module_def_id: LocalModDefId) {
     }
 }
 
-/// Checks that function uses non-Rust ABI.
-fn check_abi(tcx: TyCtxt<'_>, def_id: LocalDefId, abi: ExternAbi) {
-    if abi == ExternAbi::Rust {
-        let hir_id = tcx.local_def_id_to_hir_id(def_id);
-        let span = tcx.def_span(def_id);
-        tcx.emit_node_span_lint(
-            UNDEFINED_NAKED_FUNCTION_ABI,
-            hir_id,
-            span,
-            UndefinedNakedFunctionAbi,
-        );
-    }
-}
-
 /// Checks that parameters don't use patterns. Mirrors the checks for function declarations.
 fn check_no_patterns(tcx: TyCtxt<'_>, params: &[hir::Param<'_>]) {
     for param in params {
diff --git a/compiler/rustc_pattern_analysis/src/errors.rs b/compiler/rustc_pattern_analysis/src/errors.rs
index 1f7852e5190..e60930d6cd2 100644
--- a/compiler/rustc_pattern_analysis/src/errors.rs
+++ b/compiler/rustc_pattern_analysis/src/errors.rs
@@ -1,4 +1,4 @@
-use rustc_errors::{Diag, EmissionGuarantee, SubdiagMessageOp, Subdiagnostic};
+use rustc_errors::{Diag, EmissionGuarantee, Subdiagnostic};
 use rustc_macros::{LintDiagnostic, Subdiagnostic};
 use rustc_middle::ty::Ty;
 use rustc_span::Span;
@@ -55,11 +55,7 @@ pub struct Overlap {
 }
 
 impl Subdiagnostic for Overlap {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         let Overlap { span, range } = self;
 
         // FIXME(mejrs) unfortunately `#[derive(LintDiagnostic)]`
@@ -103,11 +99,7 @@ pub struct GappedRange {
 }
 
 impl Subdiagnostic for GappedRange {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         let GappedRange { span, gap, first_range } = self;
 
         // FIXME(mejrs) unfortunately `#[derive(LintDiagnostic)]`
diff --git a/compiler/rustc_privacy/src/lib.rs b/compiler/rustc_privacy/src/lib.rs
index 643b82f4753..c7bab828659 100644
--- a/compiler/rustc_privacy/src/lib.rs
+++ b/compiler/rustc_privacy/src/lib.rs
@@ -1,6 +1,5 @@
 // tidy-alphabetical-start
 #![allow(internal_features)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(associated_type_defaults)]
diff --git a/compiler/rustc_query_impl/src/lib.rs b/compiler/rustc_query_impl/src/lib.rs
index 30a9e718d23..3c329dd0a0e 100644
--- a/compiler/rustc_query_impl/src/lib.rs
+++ b/compiler/rustc_query_impl/src/lib.rs
@@ -3,7 +3,6 @@
 // tidy-alphabetical-start
 #![allow(internal_features)]
 #![allow(unused_parens)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(min_specialization)]
diff --git a/compiler/rustc_query_system/src/dep_graph/graph.rs b/compiler/rustc_query_system/src/dep_graph/graph.rs
index 56fd3a660d0..0d56db16099 100644
--- a/compiler/rustc_query_system/src/dep_graph/graph.rs
+++ b/compiler/rustc_query_system/src/dep_graph/graph.rs
@@ -1174,8 +1174,7 @@ pub(super) struct CurrentDepGraph<D: Deps> {
     /// ID from the previous session. In order to side-step this problem, we make
     /// sure that anonymous `NodeId`s allocated in different sessions don't overlap.
     /// This is implemented by mixing a session-key into the ID fingerprint of
-    /// each anon node. The session-key is just a random number generated when
-    /// the `DepGraph` is created.
+    /// each anon node. The session-key is a hash of the number of previous sessions.
     anon_id_seed: Fingerprint,
 
     /// These are simple counters that are for profiling and
@@ -1193,12 +1192,8 @@ impl<D: Deps> CurrentDepGraph<D> {
         record_stats: bool,
         previous: Arc<SerializedDepGraph>,
     ) -> Self {
-        use std::time::{SystemTime, UNIX_EPOCH};
-
-        let duration = SystemTime::now().duration_since(UNIX_EPOCH).unwrap();
-        let nanos = duration.as_nanos();
         let mut stable_hasher = StableHasher::new();
-        nanos.hash(&mut stable_hasher);
+        previous.session_count().hash(&mut stable_hasher);
         let anon_id_seed = stable_hasher.finish();
 
         #[cfg(debug_assertions)]
diff --git a/compiler/rustc_query_system/src/dep_graph/serialized.rs b/compiler/rustc_query_system/src/dep_graph/serialized.rs
index 7556a2456aa..ac24628447d 100644
--- a/compiler/rustc_query_system/src/dep_graph/serialized.rs
+++ b/compiler/rustc_query_system/src/dep_graph/serialized.rs
@@ -92,6 +92,9 @@ pub struct SerializedDepGraph {
     /// Stores a map from fingerprints to nodes per dep node kind.
     /// This is the reciprocal of `nodes`.
     index: Vec<UnhashMap<PackedFingerprint, SerializedDepNodeIndex>>,
+    /// The number of previous compilation sessions. This is used to generate
+    /// unique anon dep nodes per session.
+    session_count: u64,
 }
 
 impl SerializedDepGraph {
@@ -146,6 +149,11 @@ impl SerializedDepGraph {
     pub fn node_count(&self) -> usize {
         self.nodes.len()
     }
+
+    #[inline]
+    pub fn session_count(&self) -> u64 {
+        self.session_count
+    }
 }
 
 /// A packed representation of an edge's start index and byte width.
@@ -226,12 +234,12 @@ impl SerializedDepGraph {
 
             // If the length of this node's edge list is small, the length is stored in the header.
             // If it is not, we fall back to another decoder call.
-            let num_edges = node_header.len().unwrap_or_else(|| d.read_usize());
+            let num_edges = node_header.len().unwrap_or_else(|| d.read_u32());
 
             // The edges index list uses the same varint strategy as rmeta tables; we select the
             // number of byte elements per-array not per-element. This lets us read the whole edge
             // list for a node with one decoder call and also use the on-disk format in memory.
-            let edges_len_bytes = node_header.bytes_per_index() * num_edges;
+            let edges_len_bytes = node_header.bytes_per_index() * (num_edges as usize);
             // The in-memory structure for the edges list stores the byte width of the edges on
             // this node with the offset into the global edge data array.
             let edges_header = node_header.edges_header(&edge_list_data);
@@ -252,6 +260,8 @@ impl SerializedDepGraph {
             .map(|_| UnhashMap::with_capacity_and_hasher(d.read_u32() as usize, Default::default()))
             .collect();
 
+        let session_count = d.read_u64();
+
         for (idx, node) in nodes.iter_enumerated() {
             if index[node.kind.as_usize()].insert(node.hash, idx).is_some() {
                 // Side effect nodes can have duplicates
@@ -273,6 +283,7 @@ impl SerializedDepGraph {
             edge_list_indices,
             edge_list_data,
             index,
+            session_count,
         })
     }
 }
@@ -296,7 +307,7 @@ struct SerializedNodeHeader<D> {
 // The fields of a `SerializedNodeHeader`, this struct is an implementation detail and exists only
 // to make the implementation of `SerializedNodeHeader` simpler.
 struct Unpacked {
-    len: Option<usize>,
+    len: Option<u32>,
     bytes_per_index: usize,
     kind: DepKind,
     hash: PackedFingerprint,
@@ -352,7 +363,7 @@ impl<D: Deps> SerializedNodeHeader<D> {
             assert_eq!(fingerprint, res.fingerprint());
             assert_eq!(node, res.node());
             if let Some(len) = res.len() {
-                assert_eq!(edge_count, len);
+                assert_eq!(edge_count, len as usize);
             }
         }
         Self { bytes, _marker: PhantomData }
@@ -366,7 +377,7 @@ impl<D: Deps> SerializedNodeHeader<D> {
 
         let kind = head & mask(Self::KIND_BITS) as u16;
         let bytes_per_index = (head >> Self::KIND_BITS) & mask(Self::WIDTH_BITS) as u16;
-        let len = (head as usize) >> (Self::WIDTH_BITS + Self::KIND_BITS);
+        let len = (head as u32) >> (Self::WIDTH_BITS + Self::KIND_BITS);
 
         Unpacked {
             len: len.checked_sub(1),
@@ -378,7 +389,7 @@ impl<D: Deps> SerializedNodeHeader<D> {
     }
 
     #[inline]
-    fn len(&self) -> Option<usize> {
+    fn len(&self) -> Option<u32> {
         self.unpack().len
     }
 
@@ -421,7 +432,8 @@ impl NodeInfo {
         e.write_array(header.bytes);
 
         if header.len().is_none() {
-            e.emit_usize(edges.len());
+            // The edges are all unique and the number of unique indices is less than u32::MAX.
+            e.emit_u32(edges.len().try_into().unwrap());
         }
 
         let bytes_per_index = header.bytes_per_index();
@@ -456,7 +468,8 @@ impl NodeInfo {
         e.write_array(header.bytes);
 
         if header.len().is_none() {
-            e.emit_usize(edge_count);
+            // The edges are all unique and the number of unique indices is less than u32::MAX.
+            e.emit_u32(edge_count.try_into().unwrap());
         }
 
         let bytes_per_index = header.bytes_per_index();
@@ -601,7 +614,7 @@ impl<D: Deps> EncoderState<D> {
             stats: _,
             kind_stats,
             marker: _,
-            previous: _,
+            previous,
         } = self;
 
         let node_count = total_node_count.try_into().unwrap();
@@ -612,6 +625,8 @@ impl<D: Deps> EncoderState<D> {
             count.encode(&mut encoder);
         }
 
+        previous.session_count.checked_add(1).unwrap().encode(&mut encoder);
+
         debug!(?node_count, ?edge_count);
         debug!("position: {:?}", encoder.position());
         IntEncodedWithFixedSize(node_count).encode(&mut encoder);
diff --git a/compiler/rustc_resolve/src/imports.rs b/compiler/rustc_resolve/src/imports.rs
index 3f3b455f4db..762e08b2be5 100644
--- a/compiler/rustc_resolve/src/imports.rs
+++ b/compiler/rustc_resolve/src/imports.rs
@@ -1012,7 +1012,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                         // HACK(eddyb) `lint_if_path_starts_with_module` needs at least
                         // 2 segments, so the `resolve_path` above won't trigger it.
                         let mut full_path = import.module_path.clone();
-                        full_path.push(Segment::from_ident(Ident::empty()));
+                        full_path.push(Segment::from_ident(Ident::dummy()));
                         self.lint_if_path_starts_with_module(Some(finalize), &full_path, None);
                     }
 
diff --git a/compiler/rustc_resolve/src/late.rs b/compiler/rustc_resolve/src/late.rs
index a7170edb986..bae2fdeecaf 100644
--- a/compiler/rustc_resolve/src/late.rs
+++ b/compiler/rustc_resolve/src/late.rs
@@ -1948,7 +1948,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
 
         self.record_lifetime_res(
             anchor_id,
-            LifetimeRes::ElidedAnchor { start: id, end: NodeId::from_u32(id.as_u32() + 1) },
+            LifetimeRes::ElidedAnchor { start: id, end: id + 1 },
             LifetimeElisionCandidate::Ignore,
         );
         self.resolve_anonymous_lifetime(&lt, anchor_id, true);
@@ -4606,6 +4606,11 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
                     }
                 };
 
+                // Fix up partial res of segment from `resolve_path` call.
+                if let Some(id) = path[0].id {
+                    self.r.partial_res_map.insert(id, PartialRes::new(Res::PrimTy(prim)));
+                }
+
                 PartialRes::with_unresolved_segments(Res::PrimTy(prim), path.len() - 1)
             }
             PathResult::Module(ModuleOrUniformRoot::Module(module)) => {
diff --git a/compiler/rustc_resolve/src/late/diagnostics.rs b/compiler/rustc_resolve/src/late/diagnostics.rs
index c2761bd2717..d4fe446cc9f 100644
--- a/compiler/rustc_resolve/src/late/diagnostics.rs
+++ b/compiler/rustc_resolve/src/late/diagnostics.rs
@@ -1994,7 +1994,7 @@ impl<'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
             .iter()
             .flat_map(|i| self.r.tcx.associated_items(i).in_definition_order())
             // Only assoc fn with no receivers.
-            .filter(|item| matches!(item.kind, ty::AssocKind::Fn) && !item.fn_has_self_parameter)
+            .filter(|item| item.is_fn() && !item.is_method())
             .filter_map(|item| {
                 // Only assoc fns that return `Self`
                 let fn_sig = self.r.tcx.fn_sig(item.def_id).skip_binder();
@@ -2007,8 +2007,9 @@ impl<'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
                 if def.did() != def_id {
                     return None;
                 }
-                let order = !item.name.as_str().starts_with("new");
-                Some((order, item.name, input_len))
+                let name = item.name();
+                let order = !name.as_str().starts_with("new");
+                Some((order, name, input_len))
             })
             .collect::<Vec<_>>();
         items.sort_by_key(|(order, _, _)| *order);
diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
index a92912c9639..b121755acd9 100644
--- a/compiler/rustc_resolve/src/lib.rs
+++ b/compiler/rustc_resolve/src/lib.rs
@@ -2007,16 +2007,16 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                 result,
                 result.map(|r| r.expn_data())
             );
-            // Then find the last semi-transparent mark from the end if it exists.
+            // Then find the last semi-opaque mark from the end if it exists.
             for (mark, transparency) in iter {
-                if transparency == Transparency::SemiTransparent {
+                if transparency == Transparency::SemiOpaque {
                     result = Some(mark);
                 } else {
                     break;
                 }
             }
             debug!(
-                "resolve_crate_root: found semi-transparent mark {:?} {:?}",
+                "resolve_crate_root: found semi-opaque mark {:?} {:?}",
                 result,
                 result.map(|r| r.expn_data())
             );
diff --git a/compiler/rustc_sanitizers/src/cfi/typeid/itanium_cxx_abi/transform.rs b/compiler/rustc_sanitizers/src/cfi/typeid/itanium_cxx_abi/transform.rs
index 129a32c6edd..d2917478e4e 100644
--- a/compiler/rustc_sanitizers/src/cfi/typeid/itanium_cxx_abi/transform.rs
+++ b/compiler/rustc_sanitizers/src/cfi/typeid/itanium_cxx_abi/transform.rs
@@ -240,7 +240,7 @@ fn trait_object_ty<'tcx>(tcx: TyCtxt<'tcx>, poly_trait_ref: ty::PolyTraitRef<'tc
         .flat_map(|super_poly_trait_ref| {
             tcx.associated_items(super_poly_trait_ref.def_id())
                 .in_definition_order()
-                .filter(|item| item.kind == ty::AssocKind::Type)
+                .filter(|item| item.is_type())
                 .filter(|item| !tcx.generics_require_sized_self(item.def_id))
                 .map(move |assoc_ty| {
                     super_poly_trait_ref.map_bound(|super_trait_ref| {
@@ -446,7 +446,7 @@ pub(crate) fn transform_instance<'tcx>(
             let call = tcx
                 .associated_items(trait_id)
                 .in_definition_order()
-                .find(|it| it.kind == ty::AssocKind::Fn)
+                .find(|it| it.is_fn())
                 .expect("No call-family function on closure-like Fn trait?")
                 .def_id;
 
diff --git a/compiler/rustc_sanitizers/src/lib.rs b/compiler/rustc_sanitizers/src/lib.rs
index c4a9cab24d0..e4792563e71 100644
--- a/compiler/rustc_sanitizers/src/lib.rs
+++ b/compiler/rustc_sanitizers/src/lib.rs
@@ -4,7 +4,6 @@
 //! compiler.
 
 // tidy-alphabetical-start
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![feature(let_chains)]
 // tidy-alphabetical-end
 
diff --git a/compiler/rustc_serialize/src/opaque.rs b/compiler/rustc_serialize/src/opaque.rs
index 81f6266f8d1..0a55504a556 100644
--- a/compiler/rustc_serialize/src/opaque.rs
+++ b/compiler/rustc_serialize/src/opaque.rs
@@ -19,7 +19,7 @@ pub type FileEncodeResult = Result<usize, (PathBuf, io::Error)>;
 pub const MAGIC_END_BYTES: &[u8] = b"rust-end-file";
 
 /// The size of the buffer in `FileEncoder`.
-const BUF_SIZE: usize = 8192;
+const BUF_SIZE: usize = 64 * 1024;
 
 /// `FileEncoder` encodes data to file via fixed-size buffer.
 ///
diff --git a/compiler/rustc_session/src/code_stats.rs b/compiler/rustc_session/src/code_stats.rs
index b4597ae2515..6b18d450e9e 100644
--- a/compiler/rustc_session/src/code_stats.rs
+++ b/compiler/rustc_session/src/code_stats.rs
@@ -72,7 +72,7 @@ pub struct TypeSizeInfo {
 
 #[derive(Default)]
 pub struct CodeStats {
-    type_sizes: Lock<FxHashSet<TypeSizeInfo>>,
+    pub type_sizes: Lock<FxHashSet<TypeSizeInfo>>,
 }
 
 impl CodeStats {
diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index bdd54a15147..202378560ee 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -14,6 +14,7 @@ use std::str::{self, FromStr};
 use std::sync::LazyLock;
 use std::{cmp, fmt, fs, iter};
 
+use externs::{ExternOpt, split_extern_opt};
 use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
 use rustc_data_structures::stable_hasher::{StableOrd, ToStableHashKey};
 use rustc_errors::emitter::HumanReadableErrorType;
@@ -39,6 +40,7 @@ use crate::utils::CanonicalizedPath;
 use crate::{EarlyDiagCtxt, HashStableContext, Session, filesearch, lint};
 
 mod cfg;
+mod externs;
 mod native_libs;
 pub mod sigpipe;
 
@@ -1560,9 +1562,10 @@ The default is {DEFAULT_EDITION} and the latest stable edition is {LATEST_STABLE
     )
 });
 
-static PRINT_KINDS_STRING: LazyLock<String> = LazyLock::new(|| {
+static PRINT_HELP: LazyLock<String> = LazyLock::new(|| {
     format!(
-        "[{}]",
+        "Compiler information to print on stdout (or to a file)\n\
+        INFO may be one of ({}).",
         PRINT_KINDS.iter().map(|(name, _)| format!("{name}")).collect::<Vec<_>>().join("|")
     )
 });
@@ -1621,14 +1624,7 @@ pub fn rustc_optgroups() -> Vec<RustcOptGroup> {
             "Comma separated list of types of output for the compiler to emit",
             "[asm|llvm-bc|llvm-ir|obj|metadata|link|dep-info|mir]",
         ),
-        opt(
-            Stable,
-            Multi,
-            "",
-            "print",
-            "Compiler information to print on stdout",
-            &PRINT_KINDS_STRING,
-        ),
+        opt(Stable, Multi, "", "print", &PRINT_HELP, "INFO[=FILE]"),
         opt(Stable, FlagMulti, "g", "", "Equivalent to -C debuginfo=2", ""),
         opt(Stable, FlagMulti, "O", "", "Equivalent to -C opt-level=3", ""),
         opt(Stable, Opt, "o", "", "Write output to <filename>", "FILENAME"),
@@ -2211,44 +2207,13 @@ pub fn parse_externs(
     matches: &getopts::Matches,
     unstable_opts: &UnstableOptions,
 ) -> Externs {
-    fn is_ascii_ident(string: &str) -> bool {
-        let mut chars = string.chars();
-        if let Some(start) = chars.next()
-            && (start.is_ascii_alphabetic() || start == '_')
-        {
-            chars.all(|char| char.is_ascii_alphanumeric() || char == '_')
-        } else {
-            false
-        }
-    }
-
     let is_unstable_enabled = unstable_opts.unstable_options;
     let mut externs: BTreeMap<String, ExternEntry> = BTreeMap::new();
     for arg in matches.opt_strs("extern") {
-        let (name, path) = match arg.split_once('=') {
-            None => (arg, None),
-            Some((name, path)) => (name.to_string(), Some(Path::new(path))),
-        };
-        let (options, name) = match name.split_once(':') {
-            None => (None, name),
-            Some((opts, name)) => (Some(opts), name.to_string()),
-        };
-
-        if !is_ascii_ident(&name) {
-            let mut error = early_dcx.early_struct_fatal(format!(
-                "crate name `{name}` passed to `--extern` is not a valid ASCII identifier"
-            ));
-            let adjusted_name = name.replace('-', "_");
-            if is_ascii_ident(&adjusted_name) {
-                #[allow(rustc::diagnostic_outside_of_impl)] // FIXME
-                error.help(format!(
-                    "consider replacing the dashes with underscores: `{adjusted_name}`"
-                ));
-            }
-            error.emit();
-        }
+        let ExternOpt { crate_name: name, path, options } =
+            split_extern_opt(early_dcx, unstable_opts, &arg).unwrap_or_else(|e| e.emit());
 
-        let path = path.map(|p| CanonicalizedPath::new(p));
+        let path = path.map(|p| CanonicalizedPath::new(p.as_path()));
 
         let entry = externs.entry(name.to_owned());
 
diff --git a/compiler/rustc_session/src/config/externs.rs b/compiler/rustc_session/src/config/externs.rs
new file mode 100644
index 00000000000..1420ee38bf2
--- /dev/null
+++ b/compiler/rustc_session/src/config/externs.rs
@@ -0,0 +1,79 @@
+//! This module contains code to help parse and manipulate `--extern` arguments.
+
+use std::path::PathBuf;
+
+use rustc_errors::{Diag, FatalAbort};
+
+use super::UnstableOptions;
+use crate::EarlyDiagCtxt;
+
+#[cfg(test)]
+mod tests;
+
+/// Represents the pieces of an `--extern` argument.
+pub(crate) struct ExternOpt {
+    pub(crate) crate_name: String,
+    pub(crate) path: Option<PathBuf>,
+    pub(crate) options: Option<String>,
+}
+
+/// Breaks out the major components of an `--extern` argument.
+///
+/// The options field will be a string containing comma-separated options that will need further
+/// parsing and processing.
+pub(crate) fn split_extern_opt<'a>(
+    early_dcx: &'a EarlyDiagCtxt,
+    unstable_opts: &UnstableOptions,
+    extern_opt: &str,
+) -> Result<ExternOpt, Diag<'a, FatalAbort>> {
+    let (name, path) = match extern_opt.split_once('=') {
+        None => (extern_opt.to_string(), None),
+        Some((name, path)) => (name.to_string(), Some(PathBuf::from(path))),
+    };
+    let (options, crate_name) = match name.split_once(':') {
+        None => (None, name),
+        Some((opts, crate_name)) => {
+            if unstable_opts.namespaced_crates && crate_name.starts_with(':') {
+                // If the name starts with `:`, we know this was actually something like `foo::bar` and
+                // not a set of options. We can just use the original name as the crate name.
+                (None, name)
+            } else {
+                (Some(opts.to_string()), crate_name.to_string())
+            }
+        }
+    };
+
+    if !valid_crate_name(&crate_name, unstable_opts) {
+        let mut error = early_dcx.early_struct_fatal(format!(
+            "crate name `{crate_name}` passed to `--extern` is not a valid ASCII identifier"
+        ));
+        let adjusted_name = crate_name.replace('-', "_");
+        if is_ascii_ident(&adjusted_name) {
+            #[allow(rustc::diagnostic_outside_of_impl)] // FIXME
+            error
+                .help(format!("consider replacing the dashes with underscores: `{adjusted_name}`"));
+        }
+        return Err(error);
+    }
+
+    Ok(ExternOpt { crate_name, path, options })
+}
+
+fn valid_crate_name(name: &str, unstable_opts: &UnstableOptions) -> bool {
+    match name.split_once("::") {
+        Some((a, b)) if unstable_opts.namespaced_crates => is_ascii_ident(a) && is_ascii_ident(b),
+        Some(_) => false,
+        None => is_ascii_ident(name),
+    }
+}
+
+fn is_ascii_ident(string: &str) -> bool {
+    let mut chars = string.chars();
+    if let Some(start) = chars.next()
+        && (start.is_ascii_alphabetic() || start == '_')
+    {
+        chars.all(|char| char.is_ascii_alphanumeric() || char == '_')
+    } else {
+        false
+    }
+}
diff --git a/compiler/rustc_session/src/config/externs/tests.rs b/compiler/rustc_session/src/config/externs/tests.rs
new file mode 100644
index 00000000000..65448869515
--- /dev/null
+++ b/compiler/rustc_session/src/config/externs/tests.rs
@@ -0,0 +1,92 @@
+use std::path::PathBuf;
+
+use super::split_extern_opt;
+use crate::EarlyDiagCtxt;
+use crate::config::UnstableOptions;
+
+/// Verifies split_extern_opt handles the supported cases.
+#[test]
+fn test_split_extern_opt() {
+    let early_dcx = EarlyDiagCtxt::new(<_>::default());
+    let unstable_opts = &UnstableOptions::default();
+
+    let extern_opt =
+        split_extern_opt(&early_dcx, unstable_opts, "priv,noprelude:foo=libbar.rlib").unwrap();
+    assert_eq!(extern_opt.crate_name, "foo");
+    assert_eq!(extern_opt.path, Some(PathBuf::from("libbar.rlib")));
+    assert_eq!(extern_opt.options, Some("priv,noprelude".to_string()));
+
+    let extern_opt = split_extern_opt(&early_dcx, unstable_opts, "priv,noprelude:foo").unwrap();
+    assert_eq!(extern_opt.crate_name, "foo");
+    assert_eq!(extern_opt.path, None);
+    assert_eq!(extern_opt.options, Some("priv,noprelude".to_string()));
+
+    let extern_opt = split_extern_opt(&early_dcx, unstable_opts, "foo=libbar.rlib").unwrap();
+    assert_eq!(extern_opt.crate_name, "foo");
+    assert_eq!(extern_opt.path, Some(PathBuf::from("libbar.rlib")));
+    assert_eq!(extern_opt.options, None);
+
+    let extern_opt = split_extern_opt(&early_dcx, unstable_opts, "foo").unwrap();
+    assert_eq!(extern_opt.crate_name, "foo");
+    assert_eq!(extern_opt.path, None);
+    assert_eq!(extern_opt.options, None);
+}
+
+/// Tests some invalid cases for split_extern_opt.
+#[test]
+fn test_split_extern_opt_invalid() {
+    let early_dcx = EarlyDiagCtxt::new(<_>::default());
+    let unstable_opts = &UnstableOptions::default();
+
+    // too many `:`s
+    let result = split_extern_opt(&early_dcx, unstable_opts, "priv:noprelude:foo=libbar.rlib");
+    assert!(result.is_err());
+    let _ = result.map_err(|e| e.cancel());
+
+    // can't nest externs without the unstable flag
+    let result = split_extern_opt(&early_dcx, unstable_opts, "noprelude:foo::bar=libbar.rlib");
+    assert!(result.is_err());
+    let _ = result.map_err(|e| e.cancel());
+}
+
+/// Tests some cases for split_extern_opt with nested crates like `foo::bar`.
+#[test]
+fn test_split_extern_opt_nested() {
+    let early_dcx = EarlyDiagCtxt::new(<_>::default());
+    let unstable_opts = &UnstableOptions { namespaced_crates: true, ..Default::default() };
+
+    let extern_opt =
+        split_extern_opt(&early_dcx, unstable_opts, "priv,noprelude:foo::bar=libbar.rlib").unwrap();
+    assert_eq!(extern_opt.crate_name, "foo::bar");
+    assert_eq!(extern_opt.path, Some(PathBuf::from("libbar.rlib")));
+    assert_eq!(extern_opt.options, Some("priv,noprelude".to_string()));
+
+    let extern_opt =
+        split_extern_opt(&early_dcx, unstable_opts, "priv,noprelude:foo::bar").unwrap();
+    assert_eq!(extern_opt.crate_name, "foo::bar");
+    assert_eq!(extern_opt.path, None);
+    assert_eq!(extern_opt.options, Some("priv,noprelude".to_string()));
+
+    let extern_opt = split_extern_opt(&early_dcx, unstable_opts, "foo::bar=libbar.rlib").unwrap();
+    assert_eq!(extern_opt.crate_name, "foo::bar");
+    assert_eq!(extern_opt.path, Some(PathBuf::from("libbar.rlib")));
+    assert_eq!(extern_opt.options, None);
+
+    let extern_opt = split_extern_opt(&early_dcx, unstable_opts, "foo::bar").unwrap();
+    assert_eq!(extern_opt.crate_name, "foo::bar");
+    assert_eq!(extern_opt.path, None);
+    assert_eq!(extern_opt.options, None);
+}
+
+/// Tests some invalid cases for split_extern_opt with nested crates like `foo::bar`.
+#[test]
+fn test_split_extern_opt_nested_invalid() {
+    let early_dcx = EarlyDiagCtxt::new(<_>::default());
+    let unstable_opts = &UnstableOptions { namespaced_crates: true, ..Default::default() };
+
+    // crates can only be nested one deep.
+    let result =
+        split_extern_opt(&early_dcx, unstable_opts, "priv,noprelude:foo::bar::baz=libbar.rlib");
+    assert!(result.is_err());
+    let _ = result.map_err(|e| e.cancel());
+}
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index c70f1500d39..1b0794f79d3 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -1956,6 +1956,9 @@ options! {
         "allow the linker to link its default libraries (default: no)"),
     dlltool: Option<PathBuf> = (None, parse_opt_pathbuf, [UNTRACKED],
         "import library generation tool (ignored except when targeting windows-gnu)"),
+    #[rustc_lint_opt_deny_field_access("use `Session::dwarf_version` instead of this field")]
+    dwarf_version: Option<u32> = (None, parse_opt_number, [TRACKED],
+        "version of DWARF debug information to emit (default: 2 or 4, depending on platform)"),
     embed_bitcode: bool = (true, parse_bool, [TRACKED],
         "emit bitcode in rlibs (default: yes)"),
     extra_filename: String = (String::new(), parse_string, [UNTRACKED],
@@ -2331,6 +2334,8 @@ options! {
         "the size at which the `large_assignments` lint starts to be emitted"),
     mutable_noalias: bool = (true, parse_bool, [TRACKED],
         "emit noalias metadata for mutable references (default: yes)"),
+    namespaced_crates: bool = (false, parse_bool, [TRACKED],
+        "allow crates to be namespaced by other crates (default: no)"),
     next_solver: NextSolverConfig = (NextSolverConfig::default(), parse_next_solver_config, [TRACKED],
         "enable and configure the next generation trait solver used by rustc"),
     nll_facts: bool = (false, parse_bool, [UNTRACKED],
diff --git a/compiler/rustc_session/src/output.rs b/compiler/rustc_session/src/output.rs
index a24919e434c..46dae9144cd 100644
--- a/compiler/rustc_session/src/output.rs
+++ b/compiler/rustc_session/src/output.rs
@@ -177,6 +177,13 @@ pub fn collect_crate_types(session: &Session, attrs: &[ast::Attribute]) -> Vec<C
     // If we're generating a test executable, then ignore all other output
     // styles at all other locations
     if session.opts.test {
+        if !session.target.executables {
+            session.dcx().emit_warn(errors::UnsupportedCrateTypeForTarget {
+                crate_type: CrateType::Executable,
+                target_triple: &session.opts.target_triple,
+            });
+            return Vec::new();
+        }
         return vec![CrateType::Executable];
     }
 
diff --git a/compiler/rustc_session/src/session.rs b/compiler/rustc_session/src/session.rs
index 1359f7eb7bb..010ae42c280 100644
--- a/compiler/rustc_session/src/session.rs
+++ b/compiler/rustc_session/src/session.rs
@@ -764,7 +764,11 @@ impl Session {
 
     /// Returns the DWARF version passed on the CLI or the default for the target.
     pub fn dwarf_version(&self) -> u32 {
-        self.opts.unstable_opts.dwarf_version.unwrap_or(self.target.default_dwarf_version)
+        self.opts
+            .cg
+            .dwarf_version
+            .or(self.opts.unstable_opts.dwarf_version)
+            .unwrap_or(self.target.default_dwarf_version)
     }
 
     pub fn stack_protector(&self) -> StackProtector {
@@ -1327,7 +1331,9 @@ fn validate_commandline_args_with_session_available(sess: &Session) {
         sess.dcx().emit_err(errors::BranchProtectionRequiresAArch64);
     }
 
-    if let Some(dwarf_version) = sess.opts.unstable_opts.dwarf_version {
+    if let Some(dwarf_version) =
+        sess.opts.cg.dwarf_version.or(sess.opts.unstable_opts.dwarf_version)
+    {
         // DWARF 1 is not supported by LLVM and DWARF 6 is not yet finalized.
         if dwarf_version < 2 || dwarf_version > 5 {
             sess.dcx().emit_err(errors::UnsupportedDwarfVersion { dwarf_version });
diff --git a/compiler/rustc_smir/src/lib.rs b/compiler/rustc_smir/src/lib.rs
index 5d465bca4ee..77e5f3423d1 100644
--- a/compiler/rustc_smir/src/lib.rs
+++ b/compiler/rustc_smir/src/lib.rs
@@ -9,7 +9,6 @@
 // tidy-alphabetical-start
 #![allow(internal_features)]
 #![allow(rustc::usage_of_ty_tykind)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(
     html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/",
     test(attr(allow(unused_variables), deny(warnings)))
diff --git a/compiler/rustc_smir/src/rustc_internal/mod.rs b/compiler/rustc_smir/src/rustc_internal/mod.rs
index a546a44c870..146ba17d14f 100644
--- a/compiler/rustc_smir/src/rustc_internal/mod.rs
+++ b/compiler/rustc_smir/src/rustc_internal/mod.rs
@@ -244,6 +244,7 @@ where
 /// ```ignore(needs-extern-crate)
 /// # extern crate rustc_driver;
 /// # extern crate rustc_interface;
+/// # extern crate rustc_middle;
 /// # #[macro_use]
 /// # extern crate rustc_smir;
 /// # extern crate stable_mir;
@@ -264,6 +265,7 @@ where
 /// ```ignore(needs-extern-crate)
 /// # extern crate rustc_driver;
 /// # extern crate rustc_interface;
+/// # extern crate rustc_middle;
 /// # #[macro_use]
 /// # extern crate rustc_smir;
 /// # extern crate stable_mir;
@@ -328,6 +330,7 @@ macro_rules! run_driver {
         use rustc_driver::{Callbacks, Compilation, run_compiler};
         use rustc_middle::ty::TyCtxt;
         use rustc_interface::interface;
+        use rustc_smir::rustc_internal;
         use stable_mir::CompilerError;
         use std::ops::ControlFlow;
 
diff --git a/compiler/rustc_smir/src/rustc_smir/convert/ty.rs b/compiler/rustc_smir/src/rustc_smir/convert/ty.rs
index 28fc68d5e49..a757329bcf2 100644
--- a/compiler/rustc_smir/src/rustc_smir/convert/ty.rs
+++ b/compiler/rustc_smir/src/rustc_smir/convert/ty.rs
@@ -894,12 +894,21 @@ impl<'tcx> Stable<'tcx> for rustc_session::cstore::ForeignModule {
 impl<'tcx> Stable<'tcx> for ty::AssocKind {
     type T = stable_mir::ty::AssocKind;
 
-    fn stable(&self, _tables: &mut Tables<'_>) -> Self::T {
-        use stable_mir::ty::AssocKind;
-        match self {
-            ty::AssocKind::Const => AssocKind::Const,
-            ty::AssocKind::Fn => AssocKind::Fn,
-            ty::AssocKind::Type => AssocKind::Type,
+    fn stable(&self, tables: &mut Tables<'_>) -> Self::T {
+        use stable_mir::ty::{AssocKind, AssocTypeData};
+        match *self {
+            ty::AssocKind::Const { name } => AssocKind::Const { name: name.to_string() },
+            ty::AssocKind::Fn { name, has_self } => {
+                AssocKind::Fn { name: name.to_string(), has_self }
+            }
+            ty::AssocKind::Type { data } => AssocKind::Type {
+                data: match data {
+                    ty::AssocTypeData::Normal(name) => AssocTypeData::Normal(name.to_string()),
+                    ty::AssocTypeData::Rpitit(rpitit) => {
+                        AssocTypeData::Rpitit(rpitit.stable(tables))
+                    }
+                },
+            },
         }
     }
 }
@@ -922,12 +931,9 @@ impl<'tcx> Stable<'tcx> for ty::AssocItem {
     fn stable(&self, tables: &mut Tables<'_>) -> Self::T {
         stable_mir::ty::AssocItem {
             def_id: tables.assoc_def(self.def_id),
-            name: self.name.to_string(),
             kind: self.kind.stable(tables),
             container: self.container.stable(tables),
             trait_item_def_id: self.trait_item_def_id.map(|did| tables.assoc_def(did)),
-            fn_has_self_parameter: self.fn_has_self_parameter,
-            opt_rpitit_info: self.opt_rpitit_info.map(|rpitit| rpitit.stable(tables)),
         }
     }
 }
diff --git a/compiler/rustc_smir/src/stable_mir/mir/pretty.rs b/compiler/rustc_smir/src/stable_mir/mir/pretty.rs
index 439ebe978e5..8a6be0cd37a 100644
--- a/compiler/rustc_smir/src/stable_mir/mir/pretty.rs
+++ b/compiler/rustc_smir/src/stable_mir/mir/pretty.rs
@@ -22,9 +22,10 @@ impl Display for Ty {
 impl Display for AssocKind {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
         match self {
-            AssocKind::Fn => write!(f, "method"),
-            AssocKind::Const => write!(f, "associated const"),
-            AssocKind::Type => write!(f, "associated type"),
+            AssocKind::Fn { has_self: true, .. } => write!(f, "method"),
+            AssocKind::Fn { has_self: false, .. } => write!(f, "associated function"),
+            AssocKind::Const { .. } => write!(f, "associated const"),
+            AssocKind::Type { .. } => write!(f, "associated type"),
         }
     }
 }
diff --git a/compiler/rustc_smir/src/stable_mir/ty.rs b/compiler/rustc_smir/src/stable_mir/ty.rs
index 3fcbbb0e138..4b153007bd8 100644
--- a/compiler/rustc_smir/src/stable_mir/ty.rs
+++ b/compiler/rustc_smir/src/stable_mir/ty.rs
@@ -1578,29 +1578,28 @@ crate_def! {
 #[derive(Clone, Debug, Eq, PartialEq, Serialize)]
 pub struct AssocItem {
     pub def_id: AssocDef,
-    pub name: Symbol,
     pub kind: AssocKind,
     pub container: AssocItemContainer,
 
     /// If this is an item in an impl of a trait then this is the `DefId` of
     /// the associated item on the trait that this implements.
     pub trait_item_def_id: Option<AssocDef>,
+}
 
-    /// Whether this is a method with an explicit self
-    /// as its first parameter, allowing method calls.
-    pub fn_has_self_parameter: bool,
-
-    /// `Some` if the associated item (an associated type) comes from the
-    /// return-position `impl Trait` in trait desugaring. The `ImplTraitInTraitData`
-    /// provides additional information about its source.
-    pub opt_rpitit_info: Option<ImplTraitInTraitData>,
+#[derive(Clone, PartialEq, Debug, Eq, Serialize)]
+pub enum AssocTypeData {
+    Normal(Symbol),
+    /// The associated type comes from an RPITIT. It has no name, and the
+    /// `ImplTraitInTraitData` provides additional information about its
+    /// source.
+    Rpitit(ImplTraitInTraitData),
 }
 
 #[derive(Clone, Debug, Eq, PartialEq, Serialize)]
 pub enum AssocKind {
-    Const,
-    Fn,
-    Type,
+    Const { name: Symbol },
+    Fn { name: Symbol, has_self: bool },
+    Type { data: AssocTypeData },
 }
 
 #[derive(Clone, Debug, Eq, PartialEq, Serialize)]
@@ -1617,6 +1616,6 @@ pub enum ImplTraitInTraitData {
 
 impl AssocItem {
     pub fn is_impl_trait_in_trait(&self) -> bool {
-        self.opt_rpitit_info.is_some()
+        matches!(self.kind, AssocKind::Type { data: AssocTypeData::Rpitit(_) })
     }
 }
diff --git a/compiler/rustc_span/src/hygiene.rs b/compiler/rustc_span/src/hygiene.rs
index 9be16f8ce0c..d1da68ec236 100644
--- a/compiler/rustc_span/src/hygiene.rs
+++ b/compiler/rustc_span/src/hygiene.rs
@@ -56,44 +56,33 @@ impl !PartialOrd for SyntaxContext {}
 
 /// If this part of two syntax contexts is equal, then the whole syntax contexts should be equal.
 /// The other fields are only for caching.
-type SyntaxContextKey = (SyntaxContext, ExpnId, Transparency);
+pub type SyntaxContextKey = (SyntaxContext, ExpnId, Transparency);
 
 #[derive(Clone, Copy, Debug)]
 struct SyntaxContextData {
     outer_expn: ExpnId,
     outer_transparency: Transparency,
     parent: SyntaxContext,
-    /// This context, but with all transparent and semi-transparent expansions filtered away.
+    /// This context, but with all transparent and semi-opaque expansions filtered away.
     opaque: SyntaxContext,
     /// This context, but with all transparent expansions filtered away.
-    opaque_and_semitransparent: SyntaxContext,
+    opaque_and_semiopaque: SyntaxContext,
     /// Name of the crate to which `$crate` with this context would resolve.
     dollar_crate_name: Symbol,
 }
 
-/// Same as `SyntaxContextData`, but `opaque(_and_semitransparent)` cannot be recursive
-/// and use `None` if they need to refer to self. Used for encoding and decoding metadata.
-#[derive(Encodable, Decodable)]
-pub struct SyntaxContextDataNonRecursive {
-    outer_expn: ExpnId,
-    outer_transparency: Transparency,
-    parent: SyntaxContext,
-    opaque: Option<SyntaxContext>,
-    opaque_and_semitransparent: Option<SyntaxContext>,
-}
-
 impl SyntaxContextData {
     fn new(
         (parent, outer_expn, outer_transparency): SyntaxContextKey,
         opaque: SyntaxContext,
-        opaque_and_semitransparent: SyntaxContext,
+        opaque_and_semiopaque: SyntaxContext,
     ) -> SyntaxContextData {
         SyntaxContextData {
             outer_expn,
             outer_transparency,
             parent,
             opaque,
-            opaque_and_semitransparent,
+            opaque_and_semiopaque,
             dollar_crate_name: kw::DollarCrate,
         }
     }
@@ -104,7 +93,7 @@ impl SyntaxContextData {
             outer_transparency: Transparency::Opaque,
             parent: SyntaxContext::root(),
             opaque: SyntaxContext::root(),
-            opaque_and_semitransparent: SyntaxContext::root(),
+            opaque_and_semiopaque: SyntaxContext::root(),
             dollar_crate_name: kw::DollarCrate,
         }
     }
@@ -122,19 +111,6 @@ impl SyntaxContextData {
     }
 }
 
-impl SyntaxContextDataNonRecursive {
-    fn recursive(&self, ctxt: SyntaxContext) -> SyntaxContextData {
-        SyntaxContextData {
-            outer_expn: self.outer_expn,
-            outer_transparency: self.outer_transparency,
-            parent: self.parent,
-            opaque: self.opaque.unwrap_or(ctxt),
-            opaque_and_semitransparent: self.opaque_and_semitransparent.unwrap_or(ctxt),
-            dollar_crate_name: kw::DollarCrate,
-        }
-    }
-}
-
 rustc_index::newtype_index! {
     /// A unique ID associated with a macro invocation and expansion.
     #[orderable]
@@ -228,13 +204,13 @@ pub enum Transparency {
     /// Identifier produced by a transparent expansion is always resolved at call-site.
     /// Call-site spans in procedural macros, hygiene opt-out in `macro` should use this.
     Transparent,
-    /// Identifier produced by a semi-transparent expansion may be resolved
+    /// Identifier produced by a semi-opaque expansion may be resolved
     /// either at call-site or at definition-site.
     /// If it's a local variable, label or `$crate` then it's resolved at def-site.
     /// Otherwise it's resolved at call-site.
     /// `macro_rules` macros behave like this, built-in macros currently behave like this too,
     /// but that's an implementation detail.
-    SemiTransparent,
+    SemiOpaque,
     /// Identifier produced by an opaque expansion is always resolved at definition-site.
     /// Def-site spans in procedural macros, identifiers from `macro` by default use this.
     Opaque,
@@ -242,7 +218,7 @@ pub enum Transparency {
 
 impl Transparency {
     pub fn fallback(macro_rules: bool) -> Self {
-        if macro_rules { Transparency::SemiTransparent } else { Transparency::Opaque }
+        if macro_rules { Transparency::SemiOpaque } else { Transparency::Opaque }
     }
 }
 
@@ -490,7 +466,7 @@ impl HygieneData {
 
     fn normalize_to_macro_rules(&self, ctxt: SyntaxContext) -> SyntaxContext {
         debug_assert!(!self.syntax_context_data[ctxt.0 as usize].is_decode_placeholder());
-        self.syntax_context_data[ctxt.0 as usize].opaque_and_semitransparent
+        self.syntax_context_data[ctxt.0 as usize].opaque_and_semiopaque
     }
 
     fn outer_expn(&self, ctxt: SyntaxContext) -> ExpnId {
@@ -583,7 +559,7 @@ impl HygieneData {
         }
 
         let call_site_ctxt = self.expn_data(expn_id).call_site.ctxt();
-        let mut call_site_ctxt = if transparency == Transparency::SemiTransparent {
+        let mut call_site_ctxt = if transparency == Transparency::SemiOpaque {
             self.normalize_to_macros_2_0(call_site_ctxt)
         } else {
             self.normalize_to_macro_rules(call_site_ctxt)
@@ -629,48 +605,34 @@ impl HygieneData {
         self.syntax_context_data.push(SyntaxContextData::decode_placeholder());
         self.syntax_context_map.insert(key, ctxt);
 
-        // Opaque and semi-transparent versions of the parent. Note that they may be equal to the
+        // Opaque and semi-opaque versions of the parent. Note that they may be equal to the
         // parent itself. E.g. `parent_opaque` == `parent` if the expn chain contains only opaques,
-        // and `parent_opaque_and_semitransparent` == `parent` if the expn contains only opaques
-        // and semi-transparents.
+        // and `parent_opaque_and_semiopaque` == `parent` if the expn contains only (semi-)opaques.
         let parent_opaque = self.syntax_context_data[parent.0 as usize].opaque;
-        let parent_opaque_and_semitransparent =
-            self.syntax_context_data[parent.0 as usize].opaque_and_semitransparent;
+        let parent_opaque_and_semiopaque =
+            self.syntax_context_data[parent.0 as usize].opaque_and_semiopaque;
 
-        // Evaluate opaque and semi-transparent versions of the new syntax context.
-        let (opaque, opaque_and_semitransparent) = match transparency {
-            Transparency::Transparent => (parent_opaque, parent_opaque_and_semitransparent),
-            Transparency::SemiTransparent => (
+        // Evaluate opaque and semi-opaque versions of the new syntax context.
+        let (opaque, opaque_and_semiopaque) = match transparency {
+            Transparency::Transparent => (parent_opaque, parent_opaque_and_semiopaque),
+            Transparency::SemiOpaque => (
                 parent_opaque,
-                // Will be the same as `ctxt` if the expn chain contains only opaques and semi-transparents.
-                self.alloc_ctxt(parent_opaque_and_semitransparent, expn_id, transparency),
+                // Will be the same as `ctxt` if the expn chain contains only (semi-)opaques.
+                self.alloc_ctxt(parent_opaque_and_semiopaque, expn_id, transparency),
             ),
             Transparency::Opaque => (
                 // Will be the same as `ctxt` if the expn chain contains only opaques.
                 self.alloc_ctxt(parent_opaque, expn_id, transparency),
-                // Will be the same as `ctxt` if the expn chain contains only opaques and semi-transparents.
-                self.alloc_ctxt(parent_opaque_and_semitransparent, expn_id, transparency),
+                // Will be the same as `ctxt` if the expn chain contains only (semi-)opaques.
+                self.alloc_ctxt(parent_opaque_and_semiopaque, expn_id, transparency),
             ),
         };
 
         // Fill the full data, now that we have it.
         self.syntax_context_data[ctxt.as_u32() as usize] =
-            SyntaxContextData::new(key, opaque, opaque_and_semitransparent);
+            SyntaxContextData::new(key, opaque, opaque_and_semiopaque);
         ctxt
     }
-
-    fn non_recursive_ctxt(&self, ctxt: SyntaxContext) -> SyntaxContextDataNonRecursive {
-        debug_assert!(!self.syntax_context_data[ctxt.0 as usize].is_decode_placeholder());
-        let data = &self.syntax_context_data[ctxt.0 as usize];
-        SyntaxContextDataNonRecursive {
-            outer_expn: data.outer_expn,
-            outer_transparency: data.outer_transparency,
-            parent: data.parent,
-            opaque: (data.opaque != ctxt).then_some(data.opaque),
-            opaque_and_semitransparent: (data.opaque_and_semitransparent != ctxt)
-                .then_some(data.opaque_and_semitransparent),
-        }
-    }
 }
 
 pub fn walk_chain(span: Span, to: SyntaxContext) -> Span {
@@ -1300,7 +1262,7 @@ impl HygieneEncodeContext {
     pub fn encode<T>(
         &self,
         encoder: &mut T,
-        mut encode_ctxt: impl FnMut(&mut T, u32, &SyntaxContextDataNonRecursive),
+        mut encode_ctxt: impl FnMut(&mut T, u32, &SyntaxContextKey),
         mut encode_expn: impl FnMut(&mut T, ExpnId, &ExpnData, ExpnHash),
     ) {
         // When we serialize a `SyntaxContextData`, we may end up serializing
@@ -1425,10 +1387,7 @@ pub fn decode_expn_id(
 // to track which `SyntaxContext`s we have already decoded.
 // The provided closure will be invoked to deserialize a `SyntaxContextData`
 // if we haven't already seen the id of the `SyntaxContext` we are deserializing.
-pub fn decode_syntax_context<
-    D: Decoder,
-    F: FnOnce(&mut D, u32) -> SyntaxContextDataNonRecursive,
->(
+pub fn decode_syntax_context<D: Decoder, F: FnOnce(&mut D, u32) -> SyntaxContextKey>(
     d: &mut D,
     context: &HygieneDecodeContext,
     decode_data: F,
@@ -1450,16 +1409,9 @@ pub fn decode_syntax_context<
 
     // Don't try to decode data while holding the lock, since we need to
     // be able to recursively decode a SyntaxContext
-    let ctxt_data = decode_data(d, raw_id);
-
-    let ctxt = HygieneData::with(|hygiene_data| {
-        let ctxt_key = (ctxt_data.parent, ctxt_data.outer_expn, ctxt_data.outer_transparency);
-        *hygiene_data.syntax_context_map.entry(ctxt_key).or_insert_with(|| {
-            let ctxt = SyntaxContext::from_usize(hygiene_data.syntax_context_data.len());
-            hygiene_data.syntax_context_data.push(ctxt_data.recursive(ctxt));
-            ctxt
-        })
-    });
+    let (parent, expn_id, transparency) = decode_data(d, raw_id);
+    let ctxt =
+        HygieneData::with(|hygiene_data| hygiene_data.alloc_ctxt(parent, expn_id, transparency));
 
     let mut inner = context.inner.lock();
     let new_len = raw_id as usize + 1;
@@ -1471,12 +1423,21 @@ pub fn decode_syntax_context<
     ctxt
 }
 
-fn for_all_ctxts_in<F: FnMut(u32, SyntaxContext, &SyntaxContextDataNonRecursive)>(
+fn for_all_ctxts_in<F: FnMut(u32, SyntaxContext, &SyntaxContextKey)>(
     ctxts: impl Iterator<Item = SyntaxContext>,
     mut f: F,
 ) {
-    let all_data: Vec<_> =
-        HygieneData::with(|data| ctxts.map(|ctxt| (ctxt, data.non_recursive_ctxt(ctxt))).collect());
+    let all_data: Vec<_> = HygieneData::with(|data| {
+        ctxts
+            .map(|ctxt| {
+                (ctxt, {
+                    let item = data.syntax_context_data[ctxt.0 as usize];
+                    debug_assert!(!item.is_decode_placeholder());
+                    item.key()
+                })
+            })
+            .collect()
+    });
     for (ctxt, data) in all_data.into_iter() {
         f(ctxt.0, ctxt, &data);
     }
diff --git a/compiler/rustc_span/src/lib.rs b/compiler/rustc_span/src/lib.rs
index c95c03b4e4f..f788fd48037 100644
--- a/compiler/rustc_span/src/lib.rs
+++ b/compiler/rustc_span/src/lib.rs
@@ -1117,7 +1117,7 @@ impl Span {
     /// Equivalent of `Span::mixed_site` from the proc macro API,
     /// except that the location is taken from the `self` span.
     pub fn with_mixed_site_ctxt(self, expn_id: ExpnId) -> Span {
-        self.with_ctxt_from_mark(expn_id, Transparency::SemiTransparent)
+        self.with_ctxt_from_mark(expn_id, Transparency::SemiOpaque)
     }
 
     /// Produces a span with the same location as `self` and context produced by a macro with the
diff --git a/compiler/rustc_span/src/source_map.rs b/compiler/rustc_span/src/source_map.rs
index 6fdf8e46fec..0273bb040f4 100644
--- a/compiler/rustc_span/src/source_map.rs
+++ b/compiler/rustc_span/src/source_map.rs
@@ -633,6 +633,24 @@ impl SourceMap {
         sp
     }
 
+    /// Extends the given `Span` to just before the previous occurrence of `c`. Return the same span
+    /// if an error occurred while retrieving the code snippet.
+    pub fn span_extend_to_prev_char_before(
+        &self,
+        sp: Span,
+        c: char,
+        accept_newlines: bool,
+    ) -> Span {
+        if let Ok(prev_source) = self.span_to_prev_source(sp) {
+            let prev_source = prev_source.rsplit(c).next().unwrap_or("");
+            if accept_newlines || !prev_source.contains('\n') {
+                return sp.with_lo(BytePos(sp.lo().0 - prev_source.len() as u32 - 1_u32));
+            }
+        }
+
+        sp
+    }
+
     /// Extends the given `Span` to just after the previous occurrence of `pat` when surrounded by
     /// whitespace. Returns None if the pattern could not be found or if an error occurred while
     /// retrieving the code snippet.
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 986370f5019..8f75cc5e5e6 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -916,6 +916,7 @@ symbols! {
         expf16,
         expf32,
         expf64,
+        explicit_extern_abis,
         explicit_generic_args_with_impl_trait,
         explicit_tail_calls,
         export_name,
@@ -1068,7 +1069,6 @@ symbols! {
         ge,
         gen_blocks,
         gen_future,
-        gen_kill,
         generator_clone,
         generators,
         generic_arg_infer,
@@ -1400,6 +1400,7 @@ symbols! {
         naked,
         naked_asm,
         naked_functions,
+        naked_functions_rustic_abi,
         naked_functions_target_feature,
         name,
         names,
@@ -1882,6 +1883,7 @@ symbols! {
         select_unpredictable,
         self_in_typedefs,
         self_struct_ctor,
+        semiopaque,
         semitransparent,
         sha2,
         sha3,
@@ -2215,6 +2217,8 @@ symbols! {
         unsafe_fields,
         unsafe_no_drop_flag,
         unsafe_pin_internals,
+        unsafe_pinned,
+        unsafe_unpin,
         unsize,
         unsized_const_param_ty,
         unsized_const_params,
diff --git a/compiler/rustc_symbol_mangling/src/lib.rs b/compiler/rustc_symbol_mangling/src/lib.rs
index c9b15151a2c..cc33974cc62 100644
--- a/compiler/rustc_symbol_mangling/src/lib.rs
+++ b/compiler/rustc_symbol_mangling/src/lib.rs
@@ -89,7 +89,6 @@
 
 // tidy-alphabetical-start
 #![allow(internal_features)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(let_chains)]
diff --git a/compiler/rustc_symbol_mangling/src/v0.rs b/compiler/rustc_symbol_mangling/src/v0.rs
index d28f10ba9e3..f310aa65500 100644
--- a/compiler/rustc_symbol_mangling/src/v0.rs
+++ b/compiler/rustc_symbol_mangling/src/v0.rs
@@ -615,7 +615,7 @@ impl<'tcx> Printer<'tcx> for SymbolMangler<'tcx> {
                         cx.print_def_path(trait_ref.def_id, trait_ref.args)?;
                     }
                     ty::ExistentialPredicate::Projection(projection) => {
-                        let name = cx.tcx.associated_item(projection.def_id).name;
+                        let name = cx.tcx.associated_item(projection.def_id).name();
                         cx.push("p");
                         cx.push_ident(name.as_str());
                         match projection.term.unpack() {
@@ -776,7 +776,7 @@ impl<'tcx> Printer<'tcx> for SymbolMangler<'tcx> {
                                     self.push_disambiguator(
                                         disambiguated_field.disambiguator as u64,
                                     );
-                                    self.push_ident(field_name.unwrap_or(kw::Empty).as_str());
+                                    self.push_ident(field_name.unwrap().as_str());
 
                                     field.print(self)?;
                                 }
diff --git a/compiler/rustc_target/src/spec/base/lynxos178.rs b/compiler/rustc_target/src/spec/base/lynxos178.rs
new file mode 100644
index 00000000000..b9434ff5faa
--- /dev/null
+++ b/compiler/rustc_target/src/spec/base/lynxos178.rs
@@ -0,0 +1,31 @@
+use std::borrow::Cow;
+
+use crate::spec::{
+    PanicStrategy, RelocModel, RelroLevel, SplitDebuginfo, StackProbeType, TargetOptions, cvs,
+};
+
+pub(crate) fn opts() -> TargetOptions {
+    TargetOptions {
+        os: "lynxos178".into(),
+        dynamic_linking: false,
+        families: cvs!["unix"],
+        position_independent_executables: false,
+        static_position_independent_executables: false,
+        relro_level: RelroLevel::Full,
+        has_thread_local: false,
+        crt_static_respected: true,
+        panic_strategy: PanicStrategy::Abort,
+        linker: Some(Cow::Borrowed("x86_64-lynx-lynxos178-gcc")),
+        no_default_libraries: false,
+        eh_frame_header: false, // GNU ld (GNU Binutils) 2.37.50 does not support --eh-frame-hdr
+        max_atomic_width: Some(64),
+        supported_split_debuginfo: Cow::Borrowed(&[
+            SplitDebuginfo::Packed,
+            SplitDebuginfo::Unpacked,
+            SplitDebuginfo::Off,
+        ]),
+        relocation_model: RelocModel::Static,
+        stack_probes: StackProbeType::Inline,
+        ..Default::default()
+    }
+}
diff --git a/compiler/rustc_target/src/spec/base/mod.rs b/compiler/rustc_target/src/spec/base/mod.rs
index 71b6528c2dd..b368d93f007 100644
--- a/compiler/rustc_target/src/spec/base/mod.rs
+++ b/compiler/rustc_target/src/spec/base/mod.rs
@@ -19,6 +19,7 @@ pub(crate) mod linux_musl;
 pub(crate) mod linux_ohos;
 pub(crate) mod linux_uclibc;
 pub(crate) mod linux_wasm;
+pub(crate) mod lynxos178;
 pub(crate) mod msvc;
 pub(crate) mod netbsd;
 pub(crate) mod nto_qnx;
diff --git a/compiler/rustc_target/src/spec/mod.rs b/compiler/rustc_target/src/spec/mod.rs
index 64171fcc7ab..3c769dad630 100644
--- a/compiler/rustc_target/src/spec/mod.rs
+++ b/compiler/rustc_target/src/spec/mod.rs
@@ -2078,6 +2078,7 @@ supported_targets! {
     ("riscv32imafc-unknown-nuttx-elf", riscv32imafc_unknown_nuttx_elf),
     ("riscv64imac-unknown-nuttx-elf", riscv64imac_unknown_nuttx_elf),
     ("riscv64gc-unknown-nuttx-elf", riscv64gc_unknown_nuttx_elf),
+    ("x86_64-lynx-lynxos178", x86_64_lynx_lynxos178),
 
     ("x86_64-pc-cygwin", x86_64_pc_cygwin),
 }
diff --git a/compiler/rustc_target/src/spec/targets/riscv64_linux_android.rs b/compiler/rustc_target/src/spec/targets/riscv64_linux_android.rs
index 9f02ed4bcbe..b9176c939f8 100644
--- a/compiler/rustc_target/src/spec/targets/riscv64_linux_android.rs
+++ b/compiler/rustc_target/src/spec/targets/riscv64_linux_android.rs
@@ -19,7 +19,7 @@ pub(crate) fn target() -> Target {
         options: TargetOptions {
             code_model: Some(CodeModel::Medium),
             cpu: "generic-rv64".into(),
-            features: "+m,+a,+f,+d,+c,+zicsr,+zifencei,+zba,+zbb,+zbs,+v".into(),
+            features: "+m,+a,+f,+d,+c,+b,+v,+zicsr,+zifencei".into(),
             llvm_abiname: "lp64d".into(),
             supported_sanitizers: SanitizerSet::ADDRESS,
             max_atomic_width: Some(64),
diff --git a/compiler/rustc_target/src/spec/targets/x86_64_lynx_lynxos178.rs b/compiler/rustc_target/src/spec/targets/x86_64_lynx_lynxos178.rs
new file mode 100644
index 00000000000..654ae7c9c5b
--- /dev/null
+++ b/compiler/rustc_target/src/spec/targets/x86_64_lynx_lynxos178.rs
@@ -0,0 +1,34 @@
+use crate::spec::{SanitizerSet, StackProbeType, Target, base};
+
+pub(crate) fn target() -> Target {
+    let mut base = base::lynxos178::opts();
+    base.cpu = "x86-64".into();
+    base.plt_by_default = false;
+    base.max_atomic_width = Some(64);
+    base.stack_probes = StackProbeType::Inline;
+    base.static_position_independent_executables = false;
+    base.supported_sanitizers = SanitizerSet::ADDRESS
+        | SanitizerSet::CFI
+        | SanitizerSet::KCFI
+        | SanitizerSet::DATAFLOW
+        | SanitizerSet::LEAK
+        | SanitizerSet::MEMORY
+        | SanitizerSet::SAFESTACK
+        | SanitizerSet::THREAD;
+    base.supports_xray = true;
+
+    Target {
+        llvm_target: "x86_64-unknown-unknown-gnu".into(),
+        metadata: crate::spec::TargetMetadata {
+            description: Some("LynxOS-178".into()),
+            tier: Some(3),
+            host_tools: Some(false),
+            std: Some(false),
+        },
+        pointer_width: 64,
+        data_layout:
+            "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128".into(),
+        arch: "x86_64".into(),
+        options: base,
+    }
+}
diff --git a/compiler/rustc_target/src/target_features.rs b/compiler/rustc_target/src/target_features.rs
index b4ec1879fed..aeace6a40c7 100644
--- a/compiler/rustc_target/src/target_features.rs
+++ b/compiler/rustc_target/src/target_features.rs
@@ -491,7 +491,8 @@ const MIPS_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
 static RISCV_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     // tidy-alphabetical-start
     ("a", Stable, &["zaamo", "zalrsc"]),
-    ("c", Stable, &[]),
+    ("b", Unstable(sym::riscv_target_feature), &["zba", "zbb", "zbs"]),
+    ("c", Stable, &["zca"]),
     ("d", Unstable(sym::riscv_target_feature), &["f"]),
     ("e", Unstable(sym::riscv_target_feature), &[]),
     ("f", Unstable(sym::riscv_target_feature), &["zicsr"]),
@@ -520,17 +521,25 @@ static RISCV_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("zbkc", Stable, &[]),
     ("zbkx", Stable, &[]),
     ("zbs", Stable, &[]),
+    ("zca", Unstable(sym::riscv_target_feature), &[]),
+    ("zcb", Unstable(sym::riscv_target_feature), &["zca"]),
+    ("zcmop", Unstable(sym::riscv_target_feature), &["zca"]),
     ("zdinx", Unstable(sym::riscv_target_feature), &["zfinx"]),
+    ("zfa", Unstable(sym::riscv_target_feature), &["f"]),
     ("zfh", Unstable(sym::riscv_target_feature), &["zfhmin"]),
     ("zfhmin", Unstable(sym::riscv_target_feature), &["f"]),
     ("zfinx", Unstable(sym::riscv_target_feature), &["zicsr"]),
     ("zhinx", Unstable(sym::riscv_target_feature), &["zhinxmin"]),
     ("zhinxmin", Unstable(sym::riscv_target_feature), &["zfinx"]),
+    ("zicboz", Unstable(sym::riscv_target_feature), &[]),
     ("zicntr", Unstable(sym::riscv_target_feature), &["zicsr"]),
+    ("zicond", Unstable(sym::riscv_target_feature), &[]),
     ("zicsr", Unstable(sym::riscv_target_feature), &[]),
     ("zifencei", Unstable(sym::riscv_target_feature), &[]),
+    ("zihintntl", Unstable(sym::riscv_target_feature), &[]),
     ("zihintpause", Unstable(sym::riscv_target_feature), &[]),
     ("zihpm", Unstable(sym::riscv_target_feature), &["zicsr"]),
+    ("zimop", Unstable(sym::riscv_target_feature), &[]),
     ("zk", Stable, &["zkn", "zkr", "zkt"]),
     ("zkn", Stable, &["zbkb", "zbkc", "zbkx", "zkne", "zknd", "zknh"]),
     ("zknd", Stable, &[]),
@@ -541,6 +550,7 @@ static RISCV_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("zksed", Stable, &[]),
     ("zksh", Stable, &[]),
     ("zkt", Stable, &[]),
+    ("ztso", Unstable(sym::riscv_target_feature), &[]),
     ("zvbb", Unstable(sym::riscv_target_feature), &["zvkb"]),
     ("zvbc", Unstable(sym::riscv_target_feature), &["zve64x"]),
     ("zve32f", Unstable(sym::riscv_target_feature), &["zve32x", "f"]),
diff --git a/compiler/rustc_trait_selection/messages.ftl b/compiler/rustc_trait_selection/messages.ftl
index 05bbb42fb7c..74e38f525c8 100644
--- a/compiler/rustc_trait_selection/messages.ftl
+++ b/compiler/rustc_trait_selection/messages.ftl
@@ -268,8 +268,8 @@ trait_selection_oc_type_compat = type not compatible with trait
 trait_selection_opaque_captures_lifetime = hidden type for `{$opaque_ty}` captures lifetime that does not appear in bounds
     .label = opaque type defined here
 trait_selection_opaque_type_non_generic_param =
-    expected generic {$kind} parameter, found `{$ty}`
-    .label = {STREQ($ty, "'static") ->
+    expected generic {$kind} parameter, found `{$arg}`
+    .label = {STREQ($arg, "'static") ->
         [true] cannot use static lifetime; use a bound lifetime instead or remove the lifetime parameter from the opaque type
         *[other] this generic parameter must be used with a generic {$kind} parameter
     }
diff --git a/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs b/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs
index 40f8af1f691..fdd547448f0 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs
@@ -2334,13 +2334,13 @@ impl<'tcx> ObligationCause<'tcx> {
         subdiags: Vec<TypeErrorAdditionalDiags>,
     ) -> ObligationCauseFailureCode {
         match self.code() {
-            ObligationCauseCode::CompareImplItem { kind: ty::AssocKind::Fn, .. } => {
+            ObligationCauseCode::CompareImplItem { kind: ty::AssocKind::Fn { .. }, .. } => {
                 ObligationCauseFailureCode::MethodCompat { span, subdiags }
             }
-            ObligationCauseCode::CompareImplItem { kind: ty::AssocKind::Type, .. } => {
+            ObligationCauseCode::CompareImplItem { kind: ty::AssocKind::Type { .. }, .. } => {
                 ObligationCauseFailureCode::TypeCompat { span, subdiags }
             }
-            ObligationCauseCode::CompareImplItem { kind: ty::AssocKind::Const, .. } => {
+            ObligationCauseCode::CompareImplItem { kind: ty::AssocKind::Const { .. }, .. } => {
                 ObligationCauseFailureCode::ConstCompat { span, subdiags }
             }
             ObligationCauseCode::BlockTailExpression(.., hir::MatchSource::TryDesugar(_)) => {
@@ -2398,13 +2398,13 @@ impl<'tcx> ObligationCause<'tcx> {
 
     fn as_requirement_str(&self) -> &'static str {
         match self.code() {
-            ObligationCauseCode::CompareImplItem { kind: ty::AssocKind::Fn, .. } => {
+            ObligationCauseCode::CompareImplItem { kind: ty::AssocKind::Fn { .. }, .. } => {
                 "method type is compatible with trait"
             }
-            ObligationCauseCode::CompareImplItem { kind: ty::AssocKind::Type, .. } => {
+            ObligationCauseCode::CompareImplItem { kind: ty::AssocKind::Type { .. }, .. } => {
                 "associated type is compatible with trait"
             }
-            ObligationCauseCode::CompareImplItem { kind: ty::AssocKind::Const, .. } => {
+            ObligationCauseCode::CompareImplItem { kind: ty::AssocKind::Const { .. }, .. } => {
                 "const is compatible with trait"
             }
             ObligationCauseCode::MainFunctionType => "`main` function has the correct type",
@@ -2422,9 +2422,13 @@ pub struct ObligationCauseAsDiagArg<'tcx>(pub ObligationCause<'tcx>);
 impl IntoDiagArg for ObligationCauseAsDiagArg<'_> {
     fn into_diag_arg(self, _: &mut Option<std::path::PathBuf>) -> rustc_errors::DiagArgValue {
         let kind = match self.0.code() {
-            ObligationCauseCode::CompareImplItem { kind: ty::AssocKind::Fn, .. } => "method_compat",
-            ObligationCauseCode::CompareImplItem { kind: ty::AssocKind::Type, .. } => "type_compat",
-            ObligationCauseCode::CompareImplItem { kind: ty::AssocKind::Const, .. } => {
+            ObligationCauseCode::CompareImplItem { kind: ty::AssocKind::Fn { .. }, .. } => {
+                "method_compat"
+            }
+            ObligationCauseCode::CompareImplItem { kind: ty::AssocKind::Type { .. }, .. } => {
+                "type_compat"
+            }
+            ObligationCauseCode::CompareImplItem { kind: ty::AssocKind::Const { .. }, .. } => {
                 "const_compat"
             }
             ObligationCauseCode::MainFunctionType => "fn_main_correct_type",
diff --git a/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/static_impl_trait.rs b/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/static_impl_trait.rs
index 3559c660ee2..eaa06d8e8b0 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/static_impl_trait.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/static_impl_trait.rs
@@ -6,7 +6,7 @@ use rustc_hir::def_id::DefId;
 use rustc_hir::intravisit::{Visitor, VisitorExt, walk_ty};
 use rustc_hir::{
     self as hir, AmbigArg, GenericBound, GenericParam, GenericParamKind, Item, ItemKind, Lifetime,
-    LifetimeName, LifetimeParamKind, MissingLifetimeKind, Node, TyKind,
+    LifetimeKind, LifetimeParamKind, MissingLifetimeKind, Node, TyKind,
 };
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeSuperVisitable, TypeVisitor};
 use rustc_span::def_id::LocalDefId;
@@ -165,7 +165,7 @@ pub fn suggest_new_region_bound(
 
                 if let Some(span) = opaque.bounds.iter().find_map(|arg| match arg {
                     GenericBound::Outlives(Lifetime {
-                        res: LifetimeName::Static, ident, ..
+                        kind: LifetimeKind::Static, ident, ..
                     }) => Some(ident.span),
                     _ => None,
                 }) {
@@ -253,7 +253,7 @@ pub fn suggest_new_region_bound(
                 }
             }
             TyKind::TraitObject(_, lt) => {
-                if let LifetimeName::ImplicitObjectLifetimeDefault = lt.res {
+                if let LifetimeKind::ImplicitObjectLifetimeDefault = lt.kind {
                     err.span_suggestion_verbose(
                         fn_return.span.shrink_to_hi(),
                         format!("{declare} the trait object {captures}, {explicit}",),
@@ -414,7 +414,7 @@ pub struct HirTraitObjectVisitor<'a>(pub &'a mut Vec<Span>, pub DefId);
 impl<'a, 'tcx> Visitor<'tcx> for HirTraitObjectVisitor<'a> {
     fn visit_ty(&mut self, t: &'tcx hir::Ty<'tcx, AmbigArg>) {
         if let TyKind::TraitObject(poly_trait_refs, lifetime_ptr) = t.kind
-            && let Lifetime { res: LifetimeName::ImplicitObjectLifetimeDefault, .. } =
+            && let Lifetime { kind: LifetimeKind::ImplicitObjectLifetimeDefault, .. } =
                 lifetime_ptr.pointer()
         {
             for ptr in poly_trait_refs {
diff --git a/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/trait_impl_difference.rs b/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/trait_impl_difference.rs
index 74205922851..b66bd2c6ab7 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/trait_impl_difference.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/trait_impl_difference.rs
@@ -98,7 +98,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
         let assoc_item = self.tcx().associated_item(trait_item_def_id);
         let mut visitor = TypeParamSpanVisitor { tcx: self.tcx(), types: vec![] };
         match assoc_item.kind {
-            ty::AssocKind::Fn => {
+            ty::AssocKind::Fn { .. } => {
                 if let Some(hir_id) =
                     assoc_item.def_id.as_local().map(|id| self.tcx().local_def_id_to_hir_id(id))
                 {
diff --git a/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/util.rs b/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/util.rs
index fd2d2fa7210..4a71ab4e06a 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/util.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/util.rs
@@ -158,6 +158,6 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
             && self
                 .tcx()
                 .opt_associated_item(scope_def_id.to_def_id())
-                .is_some_and(|i| i.fn_has_self_parameter)
+                .is_some_and(|i| i.is_method())
     }
 }
diff --git a/compiler/rustc_trait_selection/src/error_reporting/infer/note_and_explain.rs b/compiler/rustc_trait_selection/src/error_reporting/infer/note_and_explain.rs
index 5583deda99a..be508c8cee1 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/infer/note_and_explain.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/infer/note_and_explain.rs
@@ -782,8 +782,8 @@ fn foo(&self) -> Self::T { String::new() }
         let methods: Vec<(Span, String)> = items
             .in_definition_order()
             .filter(|item| {
-                ty::AssocKind::Fn == item.kind
-                    && Some(item.name) != current_method_ident
+                item.is_fn()
+                    && Some(item.name()) != current_method_ident
                     && !tcx.is_doc_hidden(item.def_id)
             })
             .filter_map(|item| {
diff --git a/compiler/rustc_trait_selection/src/error_reporting/infer/region.rs b/compiler/rustc_trait_selection/src/error_reporting/infer/region.rs
index 49c6acedcfa..1cf1ac5403f 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/infer/region.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/infer/region.rs
@@ -850,14 +850,14 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         add_lt_suggs: &mut Vec<(Span, String)>,
     ) -> String {
         struct LifetimeReplaceVisitor<'a> {
-            needle: hir::LifetimeName,
+            needle: hir::LifetimeKind,
             new_lt: &'a str,
             add_lt_suggs: &'a mut Vec<(Span, String)>,
         }
 
         impl<'hir> hir::intravisit::Visitor<'hir> for LifetimeReplaceVisitor<'_> {
             fn visit_lifetime(&mut self, lt: &'hir hir::Lifetime) {
-                if lt.res == self.needle {
+                if lt.kind == self.needle {
                     self.add_lt_suggs.push(lt.suggestion(self.new_lt));
                 }
             }
@@ -894,7 +894,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         };
 
         let mut visitor = LifetimeReplaceVisitor {
-            needle: hir::LifetimeName::Param(lifetime_def_id),
+            needle: hir::LifetimeKind::Param(lifetime_def_id),
             add_lt_suggs,
             new_lt: &new_lt,
         };
@@ -1017,7 +1017,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             infer::BoundRegion(_, br, infer::AssocTypeProjection(def_id)) => format!(
                 " for lifetime parameter {}in trait containing associated type `{}`",
                 br_string(br),
-                self.tcx.associated_item(def_id).name
+                self.tcx.associated_item(def_id).name()
             ),
             infer::RegionParameterDefinition(_, name) => {
                 format!(" for lifetime parameter `{name}`")
diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/ambiguity.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/ambiguity.rs
index 59c93db9c8f..54b50851b74 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/ambiguity.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/ambiguity.rs
@@ -348,11 +348,11 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                         && let None = self.tainted_by_errors()
                     {
                         let (verb, noun) = match self.tcx.associated_item(item_id).kind {
-                            ty::AssocKind::Const => ("refer to the", "constant"),
-                            ty::AssocKind::Fn => ("call", "function"),
+                            ty::AssocKind::Const { .. } => ("refer to the", "constant"),
+                            ty::AssocKind::Fn { .. } => ("call", "function"),
                             // This is already covered by E0223, but this following single match
                             // arm doesn't hurt here.
-                            ty::AssocKind::Type => ("refer to the", "type"),
+                            ty::AssocKind::Type { .. } => ("refer to the", "type"),
                         };
 
                         // Replace the more general E0283 with a more specific error
diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
index b963e4a2c7c..7d95a7b3fed 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
@@ -2112,16 +2112,20 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         trait_ref: DefId,
     ) {
         if let Some(assoc_item) = self.tcx.opt_associated_item(item_def_id) {
-            if let ty::AssocKind::Const | ty::AssocKind::Type = assoc_item.kind {
+            if let ty::AssocKind::Const { .. } | ty::AssocKind::Type { .. } = assoc_item.kind {
                 err.note(format!(
                     "{}s cannot be accessed directly on a `trait`, they can only be \
                         accessed through a specific `impl`",
-                    self.tcx.def_kind_descr(assoc_item.kind.as_def_kind(), item_def_id)
+                    self.tcx.def_kind_descr(assoc_item.as_def_kind(), item_def_id)
                 ));
                 err.span_suggestion(
                     span,
                     "use the fully qualified path to an implementation",
-                    format!("<Type as {}>::{}", self.tcx.def_path_str(trait_ref), assoc_item.name),
+                    format!(
+                        "<Type as {}>::{}",
+                        self.tcx.def_path_str(trait_ref),
+                        assoc_item.name()
+                    ),
                     Applicability::HasPlaceholders,
                 );
             }
@@ -5411,7 +5415,7 @@ fn point_at_assoc_type_restriction<G: EmissionGuarantee>(
                 tcx.associated_items(data.impl_or_alias_def_id).find_by_ident_and_kind(
                     tcx,
                     Ident::with_dummy_span(name),
-                    ty::AssocKind::Type,
+                    ty::AssocTag::Type,
                     data.impl_or_alias_def_id,
                 )
             {
diff --git a/compiler/rustc_trait_selection/src/errors.rs b/compiler/rustc_trait_selection/src/errors.rs
index bb4aba9d29e..756d9a57b93 100644
--- a/compiler/rustc_trait_selection/src/errors.rs
+++ b/compiler/rustc_trait_selection/src/errors.rs
@@ -4,7 +4,7 @@ use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
 use rustc_errors::codes::*;
 use rustc_errors::{
     Applicability, Diag, DiagCtxtHandle, DiagMessage, DiagStyledString, Diagnostic,
-    EmissionGuarantee, IntoDiagArg, Level, MultiSpan, SubdiagMessageOp, Subdiagnostic,
+    EmissionGuarantee, IntoDiagArg, Level, MultiSpan, Subdiagnostic,
 };
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LocalDefId};
@@ -107,11 +107,7 @@ pub enum AdjustSignatureBorrow {
 }
 
 impl Subdiagnostic for AdjustSignatureBorrow {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         match self {
             AdjustSignatureBorrow::Borrow { to_borrow } => {
                 diag.arg("len", to_borrow.len());
@@ -381,11 +377,7 @@ pub enum RegionOriginNote<'a> {
 }
 
 impl Subdiagnostic for RegionOriginNote<'_> {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         let mut label_or_note = |span, msg: DiagMessage| {
             let sub_count = diag.children.iter().filter(|d| d.span.is_dummy()).count();
             let expanded_sub_count = diag.children.iter().filter(|d| !d.span.is_dummy()).count();
@@ -446,11 +438,7 @@ pub enum LifetimeMismatchLabels {
 }
 
 impl Subdiagnostic for LifetimeMismatchLabels {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         match self {
             LifetimeMismatchLabels::InRet { param_span, ret_span, span, label_var1 } => {
                 diag.span_label(param_span, fluent::trait_selection_declared_different);
@@ -495,11 +483,7 @@ pub struct AddLifetimeParamsSuggestion<'a> {
 }
 
 impl Subdiagnostic for AddLifetimeParamsSuggestion<'_> {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         let mut mk_suggestion = || {
             let Some(anon_reg) = self.tcx.is_suitable_region(self.generic_param_scope, self.sub)
             else {
@@ -689,11 +673,7 @@ pub struct IntroducesStaticBecauseUnmetLifetimeReq {
 }
 
 impl Subdiagnostic for IntroducesStaticBecauseUnmetLifetimeReq {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        mut self,
-        diag: &mut Diag<'_, G>,
-        _f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(mut self, diag: &mut Diag<'_, G>) {
         self.unmet_requirements
             .push_span_label(self.binding_span, fluent::trait_selection_msl_introduces_static);
         diag.span_note(self.unmet_requirements, fluent::trait_selection_msl_unmet_req);
@@ -1008,17 +988,13 @@ pub struct ConsiderBorrowingParamHelp {
 }
 
 impl Subdiagnostic for ConsiderBorrowingParamHelp {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         let mut type_param_span: MultiSpan = self.spans.clone().into();
         for &span in &self.spans {
             // Seems like we can't call f() here as Into<DiagMessage> is required
             type_param_span.push_span_label(span, fluent::trait_selection_tid_consider_borrowing);
         }
-        let msg = f(diag, fluent::trait_selection_tid_param_help.into());
+        let msg = diag.eagerly_translate(fluent::trait_selection_tid_param_help);
         diag.span_help(type_param_span, msg);
     }
 }
@@ -1053,18 +1029,14 @@ pub struct DynTraitConstraintSuggestion {
 }
 
 impl Subdiagnostic for DynTraitConstraintSuggestion {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         let mut multi_span: MultiSpan = vec![self.span].into();
         multi_span.push_span_label(self.span, fluent::trait_selection_dtcs_has_lifetime_req_label);
         multi_span
             .push_span_label(self.ident.span, fluent::trait_selection_dtcs_introduces_requirement);
-        let msg = f(diag, fluent::trait_selection_dtcs_has_req_note.into());
+        let msg = diag.eagerly_translate(fluent::trait_selection_dtcs_has_req_note);
         diag.span_note(multi_span, msg);
-        let msg = f(diag, fluent::trait_selection_dtcs_suggestion.into());
+        let msg = diag.eagerly_translate(fluent::trait_selection_dtcs_suggestion);
         diag.span_suggestion_verbose(
             self.span.shrink_to_hi(),
             msg,
@@ -1101,11 +1073,7 @@ pub struct ReqIntroducedLocations {
 }
 
 impl Subdiagnostic for ReqIntroducedLocations {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        mut self,
-        diag: &mut Diag<'_, G>,
-        f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(mut self, diag: &mut Diag<'_, G>) {
         for sp in self.spans {
             self.span.push_span_label(sp, fluent::trait_selection_ril_introduced_here);
         }
@@ -1114,7 +1082,7 @@ impl Subdiagnostic for ReqIntroducedLocations {
             self.span.push_span_label(self.fn_decl_span, fluent::trait_selection_ril_introduced_by);
         }
         self.span.push_span_label(self.cause_span, fluent::trait_selection_ril_because_of);
-        let msg = f(diag, fluent::trait_selection_ril_static_introduced_by.into());
+        let msg = diag.eagerly_translate(fluent::trait_selection_ril_static_introduced_by);
         diag.span_note(self.span, msg);
     }
 }
@@ -1513,13 +1481,9 @@ pub struct SuggestTuplePatternMany {
 }
 
 impl Subdiagnostic for SuggestTuplePatternMany {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         diag.arg("path", self.path);
-        let message = f(diag, crate::fluent_generated::trait_selection_stp_wrap_many.into());
+        let message = diag.eagerly_translate(fluent::trait_selection_stp_wrap_many);
         diag.multipart_suggestions(
             message,
             self.compatible_variants.into_iter().map(|variant| {
@@ -1752,11 +1716,7 @@ pub struct AddPreciseCapturingAndParams {
 }
 
 impl Subdiagnostic for AddPreciseCapturingAndParams {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         diag.arg("new_lifetime", self.new_lifetime);
         diag.multipart_suggestion_verbose(
             fluent::trait_selection_precise_capturing_new_but_apit,
@@ -1896,11 +1856,7 @@ pub struct AddPreciseCapturingForOvercapture {
 }
 
 impl Subdiagnostic for AddPreciseCapturingForOvercapture {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        _f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         let applicability = if self.apit_spans.is_empty() {
             Applicability::MachineApplicable
         } else {
@@ -1926,7 +1882,7 @@ impl Subdiagnostic for AddPreciseCapturingForOvercapture {
 #[derive(Diagnostic)]
 #[diag(trait_selection_opaque_type_non_generic_param, code = E0792)]
 pub(crate) struct NonGenericOpaqueTypeParam<'a, 'tcx> {
-    pub ty: GenericArg<'tcx>,
+    pub arg: GenericArg<'tcx>,
     pub kind: &'a str,
     #[primary_span]
     pub span: Span,
diff --git a/compiler/rustc_trait_selection/src/errors/note_and_explain.rs b/compiler/rustc_trait_selection/src/errors/note_and_explain.rs
index e4ab78b6247..84e7686fdd3 100644
--- a/compiler/rustc_trait_selection/src/errors/note_and_explain.rs
+++ b/compiler/rustc_trait_selection/src/errors/note_and_explain.rs
@@ -1,4 +1,4 @@
-use rustc_errors::{Diag, EmissionGuarantee, IntoDiagArg, SubdiagMessageOp, Subdiagnostic};
+use rustc_errors::{Diag, EmissionGuarantee, IntoDiagArg, Subdiagnostic};
 use rustc_hir::def_id::LocalDefId;
 use rustc_middle::bug;
 use rustc_middle::ty::{self, TyCtxt};
@@ -162,17 +162,13 @@ impl RegionExplanation<'_> {
 }
 
 impl Subdiagnostic for RegionExplanation<'_> {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        f: &F,
-    ) {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
         diag.arg("pref_kind", self.prefix);
         diag.arg("suff_kind", self.suffix);
         diag.arg("desc_kind", self.desc.kind);
         diag.arg("desc_arg", self.desc.arg);
 
-        let msg = f(diag, fluent::trait_selection_region_explanation.into());
+        let msg = diag.eagerly_translate(fluent::trait_selection_region_explanation);
         if let Some(span) = self.desc.span {
             diag.span_note(span, msg);
         } else {
diff --git a/compiler/rustc_trait_selection/src/opaque_types.rs b/compiler/rustc_trait_selection/src/opaque_types.rs
index 309bf4dda3d..cce67b066dd 100644
--- a/compiler/rustc_trait_selection/src/opaque_types.rs
+++ b/compiler/rustc_trait_selection/src/opaque_types.rs
@@ -70,7 +70,7 @@ pub fn check_opaque_type_parameter_valid<'tcx>(
             opaque_env.param_is_error(i)?;
 
             return Err(infcx.dcx().emit_err(NonGenericOpaqueTypeParam {
-                ty: arg,
+                arg,
                 kind,
                 span,
                 param_span: tcx.def_span(opaque_param.def_id),
diff --git a/compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs b/compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs
index bf9fcb0915a..519394685a8 100644
--- a/compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs
+++ b/compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs
@@ -188,7 +188,7 @@ fn bounds_reference_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span
     tcx.associated_items(trait_def_id)
         .in_definition_order()
         // We're only looking at associated type bounds
-        .filter(|item| item.kind == ty::AssocKind::Type)
+        .filter(|item| item.is_type())
         // Ignore GATs with `Self: Sized`
         .filter(|item| !tcx.generics_require_sized_self(item.def_id))
         .flat_map(|item| tcx.explicit_item_bounds(item.def_id).iter_identity_copied())
@@ -298,31 +298,33 @@ pub fn dyn_compatibility_violations_for_assoc_item(
     match item.kind {
         // Associated consts are never dyn-compatible, as they can't have `where` bounds yet at all,
         // and associated const bounds in trait objects aren't a thing yet either.
-        ty::AssocKind::Const => {
-            vec![DynCompatibilityViolation::AssocConst(item.name, item.ident(tcx).span)]
+        ty::AssocKind::Const { name } => {
+            vec![DynCompatibilityViolation::AssocConst(name, item.ident(tcx).span)]
         }
-        ty::AssocKind::Fn => virtual_call_violations_for_method(tcx, trait_def_id, item)
-            .into_iter()
-            .map(|v| {
-                let node = tcx.hir_get_if_local(item.def_id);
-                // Get an accurate span depending on the violation.
-                let span = match (&v, node) {
-                    (MethodViolationCode::ReferencesSelfInput(Some(span)), _) => *span,
-                    (MethodViolationCode::UndispatchableReceiver(Some(span)), _) => *span,
-                    (MethodViolationCode::ReferencesImplTraitInTrait(span), _) => *span,
-                    (MethodViolationCode::ReferencesSelfOutput, Some(node)) => {
-                        node.fn_decl().map_or(item.ident(tcx).span, |decl| decl.output.span())
-                    }
-                    _ => item.ident(tcx).span,
-                };
+        ty::AssocKind::Fn { name, .. } => {
+            virtual_call_violations_for_method(tcx, trait_def_id, item)
+                .into_iter()
+                .map(|v| {
+                    let node = tcx.hir_get_if_local(item.def_id);
+                    // Get an accurate span depending on the violation.
+                    let span = match (&v, node) {
+                        (MethodViolationCode::ReferencesSelfInput(Some(span)), _) => *span,
+                        (MethodViolationCode::UndispatchableReceiver(Some(span)), _) => *span,
+                        (MethodViolationCode::ReferencesImplTraitInTrait(span), _) => *span,
+                        (MethodViolationCode::ReferencesSelfOutput, Some(node)) => {
+                            node.fn_decl().map_or(item.ident(tcx).span, |decl| decl.output.span())
+                        }
+                        _ => item.ident(tcx).span,
+                    };
 
-                DynCompatibilityViolation::Method(item.name, v, span)
-            })
-            .collect(),
+                    DynCompatibilityViolation::Method(name, v, span)
+                })
+                .collect()
+        }
         // Associated types can only be dyn-compatible if they have `Self: Sized` bounds.
-        ty::AssocKind::Type => {
+        ty::AssocKind::Type { .. } => {
             if !tcx.generics_of(item.def_id).is_own_empty() && !item.is_impl_trait_in_trait() {
-                vec![DynCompatibilityViolation::GAT(item.name, item.ident(tcx).span)]
+                vec![DynCompatibilityViolation::GAT(item.name(), item.ident(tcx).span)]
             } else {
                 // We will permit associated types if they are explicitly mentioned in the trait object.
                 // We can't check this here, as here we only check if it is guaranteed to not be possible.
@@ -344,7 +346,7 @@ fn virtual_call_violations_for_method<'tcx>(
     let sig = tcx.fn_sig(method.def_id).instantiate_identity();
 
     // The method's first parameter must be named `self`
-    if !method.fn_has_self_parameter {
+    if !method.is_method() {
         let sugg = if let Some(hir::Node::TraitItem(hir::TraitItem {
             generics,
             kind: hir::TraitItemKind::Fn(sig, _),
diff --git a/compiler/rustc_trait_selection/src/traits/fulfill.rs b/compiler/rustc_trait_selection/src/traits/fulfill.rs
index e98a240a53f..1b76d48e431 100644
--- a/compiler/rustc_trait_selection/src/traits/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/traits/fulfill.rs
@@ -24,10 +24,10 @@ use super::{
 };
 use crate::error_reporting::InferCtxtErrorExt;
 use crate::infer::{InferCtxt, TyOrConstInferVar};
-use crate::traits::EvaluateConstErr;
 use crate::traits::normalize::normalize_with_depth_to;
 use crate::traits::project::{PolyProjectionObligation, ProjectionCacheKeyExt as _};
 use crate::traits::query::evaluate_obligation::InferCtxtExt;
+use crate::traits::{EvaluateConstErr, sizedness_fast_path};
 
 pub(crate) type PendingPredicateObligations<'tcx> = ThinVec<PendingPredicateObligation<'tcx>>;
 
@@ -335,6 +335,10 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
 
         let infcx = self.selcx.infcx;
 
+        if sizedness_fast_path(infcx.tcx, obligation.predicate) {
+            return ProcessResult::Changed(thin_vec::thin_vec![]);
+        }
+
         if obligation.predicate.has_aliases() {
             let mut obligations = PredicateObligations::new();
             let predicate = normalize_with_depth_to(
diff --git a/compiler/rustc_trait_selection/src/traits/mod.rs b/compiler/rustc_trait_selection/src/traits/mod.rs
index de337710b5e..0987c5b42d8 100644
--- a/compiler/rustc_trait_selection/src/traits/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/mod.rs
@@ -65,8 +65,8 @@ pub use self::specialize::{
 pub use self::structural_normalize::StructurallyNormalizeExt;
 pub use self::util::{
     BoundVarReplacer, PlaceholderReplacer, elaborate, expand_trait_aliases, impl_item_is_final,
-    supertrait_def_ids, supertraits, transitive_bounds_that_define_assoc_item, upcast_choices,
-    with_replaced_escaping_bound_vars,
+    sizedness_fast_path, supertrait_def_ids, supertraits, transitive_bounds_that_define_assoc_item,
+    upcast_choices, with_replaced_escaping_bound_vars,
 };
 use crate::error_reporting::InferCtxtErrorExt;
 use crate::infer::outlives::env::OutlivesEnvironment;
@@ -643,7 +643,7 @@ fn replace_param_and_infer_args_with_placeholder<'tcx>(
 ) -> GenericArgsRef<'tcx> {
     struct ReplaceParamAndInferWithPlaceholder<'tcx> {
         tcx: TyCtxt<'tcx>,
-        idx: u32,
+        idx: ty::BoundVar,
     }
 
     impl<'tcx> TypeFolder<TyCtxt<'tcx>> for ReplaceParamAndInferWithPlaceholder<'tcx> {
@@ -653,19 +653,13 @@ fn replace_param_and_infer_args_with_placeholder<'tcx>(
 
         fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
             if let ty::Infer(_) = t.kind() {
-                let idx = {
-                    let idx = self.idx;
-                    self.idx += 1;
-                    idx
-                };
+                let idx = self.idx;
+                self.idx += 1;
                 Ty::new_placeholder(
                     self.tcx,
                     ty::PlaceholderType {
                         universe: ty::UniverseIndex::ROOT,
-                        bound: ty::BoundTy {
-                            var: ty::BoundVar::from_u32(idx),
-                            kind: ty::BoundTyKind::Anon,
-                        },
+                        bound: ty::BoundTy { var: idx, kind: ty::BoundTyKind::Anon },
                     },
                 )
             } else {
@@ -675,16 +669,11 @@ fn replace_param_and_infer_args_with_placeholder<'tcx>(
 
         fn fold_const(&mut self, c: ty::Const<'tcx>) -> ty::Const<'tcx> {
             if let ty::ConstKind::Infer(_) = c.kind() {
+                let idx = self.idx;
+                self.idx += 1;
                 ty::Const::new_placeholder(
                     self.tcx,
-                    ty::PlaceholderConst {
-                        universe: ty::UniverseIndex::ROOT,
-                        bound: ty::BoundVar::from_u32({
-                            let idx = self.idx;
-                            self.idx += 1;
-                            idx
-                        }),
-                    },
+                    ty::PlaceholderConst { universe: ty::UniverseIndex::ROOT, bound: idx },
                 )
             } else {
                 c.super_fold_with(self)
@@ -692,7 +681,7 @@ fn replace_param_and_infer_args_with_placeholder<'tcx>(
         }
     }
 
-    args.fold_with(&mut ReplaceParamAndInferWithPlaceholder { tcx, idx: 0 })
+    args.fold_with(&mut ReplaceParamAndInferWithPlaceholder { tcx, idx: ty::BoundVar::ZERO })
 }
 
 /// Normalizes the predicates and checks whether they hold in an empty environment. If this
diff --git a/compiler/rustc_trait_selection/src/traits/project.rs b/compiler/rustc_trait_selection/src/traits/project.rs
index 83591219b14..0dce504903c 100644
--- a/compiler/rustc_trait_selection/src/traits/project.rs
+++ b/compiler/rustc_trait_selection/src/traits/project.rs
@@ -1393,7 +1393,7 @@ fn confirm_future_candidate<'cx, 'tcx>(
         coroutine_sig,
     );
 
-    debug_assert_eq!(tcx.associated_item(obligation.predicate.def_id).name, sym::Output);
+    debug_assert_eq!(tcx.associated_item(obligation.predicate.def_id).name(), sym::Output);
 
     let predicate = ty::ProjectionPredicate {
         projection_term: ty::AliasTerm::new_from_args(
@@ -1439,7 +1439,7 @@ fn confirm_iterator_candidate<'cx, 'tcx>(
         gen_sig,
     );
 
-    debug_assert_eq!(tcx.associated_item(obligation.predicate.def_id).name, sym::Item);
+    debug_assert_eq!(tcx.associated_item(obligation.predicate.def_id).name(), sym::Item);
 
     let predicate = ty::ProjectionPredicate {
         projection_term: ty::AliasTerm::new_from_args(
@@ -1485,7 +1485,7 @@ fn confirm_async_iterator_candidate<'cx, 'tcx>(
         gen_sig,
     );
 
-    debug_assert_eq!(tcx.associated_item(obligation.predicate.def_id).name, sym::Item);
+    debug_assert_eq!(tcx.associated_item(obligation.predicate.def_id).name(), sym::Item);
 
     let ty::Adt(_poll_adt, args) = *yield_ty.kind() else {
         bug!();
@@ -2005,7 +2005,8 @@ fn confirm_impl_candidate<'cx, 'tcx>(
     if !assoc_ty.item.defaultness(tcx).has_value() {
         debug!(
             "confirm_impl_candidate: no associated type {:?} for {:?}",
-            assoc_ty.item.name, obligation.predicate
+            assoc_ty.item.name(),
+            obligation.predicate
         );
         if tcx.impl_self_is_guaranteed_unsized(impl_def_id) {
             // We treat this projection as rigid here, which is represented via
diff --git a/compiler/rustc_trait_selection/src/traits/query/type_op/prove_predicate.rs b/compiler/rustc_trait_selection/src/traits/query/type_op/prove_predicate.rs
index 4f9e2e79d62..87fc532adaf 100644
--- a/compiler/rustc_trait_selection/src/traits/query/type_op/prove_predicate.rs
+++ b/compiler/rustc_trait_selection/src/traits/query/type_op/prove_predicate.rs
@@ -1,4 +1,3 @@
-use rustc_hir::LangItem;
 use rustc_infer::traits::Obligation;
 use rustc_middle::traits::ObligationCause;
 use rustc_middle::traits::query::NoSolution;
@@ -7,7 +6,7 @@ use rustc_middle::ty::{self, ParamEnvAnd, TyCtxt};
 use rustc_span::Span;
 
 use crate::infer::canonical::{CanonicalQueryInput, CanonicalQueryResponse};
-use crate::traits::ObligationCtxt;
+use crate::traits::{ObligationCtxt, sizedness_fast_path};
 
 impl<'tcx> super::QueryTypeOp<'tcx> for ProvePredicate<'tcx> {
     type QueryResponse = ();
@@ -16,15 +15,7 @@ impl<'tcx> super::QueryTypeOp<'tcx> for ProvePredicate<'tcx> {
         tcx: TyCtxt<'tcx>,
         key: &ParamEnvAnd<'tcx, Self>,
     ) -> Option<Self::QueryResponse> {
-        // Proving Sized, very often on "obviously sized" types like
-        // `&T`, accounts for about 60% percentage of the predicates
-        // we have to prove. No need to canonicalize and all that for
-        // such cases.
-        if let ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_ref)) =
-            key.value.predicate.kind().skip_binder()
-            && tcx.is_lang_item(trait_ref.def_id(), LangItem::Sized)
-            && trait_ref.self_ty().is_trivially_sized(tcx)
-        {
+        if sizedness_fast_path(tcx, key.value.predicate) {
             return Some(());
         }
 
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index 69e7b2a43ff..d71d1e9ae0f 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -594,9 +594,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             // Associated types that require `Self: Sized` do not show up in the built-in
             // implementation of `Trait for dyn Trait`, and can be dropped here.
             .filter(|item| !tcx.generics_require_sized_self(item.def_id))
-            .filter_map(
-                |item| if item.kind == ty::AssocKind::Type { Some(item.def_id) } else { None },
-            )
+            .filter_map(|item| if item.is_type() { Some(item.def_id) } else { None })
             .collect();
 
         for assoc_type in assoc_types {
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 56ff46e89e7..4d88a23250a 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -49,7 +49,9 @@ use crate::infer::{InferCtxt, InferOk, TypeFreshener};
 use crate::solve::InferCtxtSelectExt as _;
 use crate::traits::normalize::{normalize_with_depth, normalize_with_depth_to};
 use crate::traits::project::{ProjectAndUnifyResult, ProjectionCacheKeyExt};
-use crate::traits::{EvaluateConstErr, ProjectionCacheKey, Unimplemented, effects};
+use crate::traits::{
+    EvaluateConstErr, ProjectionCacheKey, Unimplemented, effects, sizedness_fast_path,
+};
 
 mod _match;
 mod candidate_assembly;
@@ -603,6 +605,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             None => self.check_recursion_limit(&obligation, &obligation)?,
         }
 
+        if sizedness_fast_path(self.tcx(), obligation.predicate) {
+            return Ok(EvaluatedToOk);
+        }
+
         ensure_sufficient_stack(|| {
             let bound_predicate = obligation.predicate.kind();
             match bound_predicate.skip_binder() {
diff --git a/compiler/rustc_trait_selection/src/traits/util.rs b/compiler/rustc_trait_selection/src/traits/util.rs
index 9f20cd7eacb..035fd38c48a 100644
--- a/compiler/rustc_trait_selection/src/traits/util.rs
+++ b/compiler/rustc_trait_selection/src/traits/util.rs
@@ -1,6 +1,7 @@
 use std::collections::{BTreeMap, VecDeque};
 
 use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
+use rustc_hir::LangItem;
 use rustc_hir::def_id::DefId;
 use rustc_infer::infer::InferCtxt;
 pub use rustc_infer::traits::util::*;
@@ -504,3 +505,21 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for PlaceholderReplacer<'_, 'tcx> {
         }
     }
 }
+
+pub fn sizedness_fast_path<'tcx>(tcx: TyCtxt<'tcx>, predicate: ty::Predicate<'tcx>) -> bool {
+    // Proving `Sized` very often on "obviously sized" types like `&T`, accounts for about 60%
+    // percentage of the predicates we have to prove. No need to canonicalize and all that for
+    // such cases.
+    if let ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_ref)) =
+        predicate.kind().skip_binder()
+    {
+        if tcx.is_lang_item(trait_ref.def_id(), LangItem::Sized)
+            && trait_ref.self_ty().is_trivially_sized(tcx)
+        {
+            debug!("fast path -- trivial sizedness");
+            return true;
+        }
+    }
+
+    false
+}
diff --git a/compiler/rustc_trait_selection/src/traits/vtable.rs b/compiler/rustc_trait_selection/src/traits/vtable.rs
index 165174c0bcc..3565c11249a 100644
--- a/compiler/rustc_trait_selection/src/traits/vtable.rs
+++ b/compiler/rustc_trait_selection/src/traits/vtable.rs
@@ -197,10 +197,8 @@ fn own_existential_vtable_entries_iter(
     tcx: TyCtxt<'_>,
     trait_def_id: DefId,
 ) -> impl Iterator<Item = DefId> {
-    let trait_methods = tcx
-        .associated_items(trait_def_id)
-        .in_definition_order()
-        .filter(|item| item.kind == ty::AssocKind::Fn);
+    let trait_methods =
+        tcx.associated_items(trait_def_id).in_definition_order().filter(|item| item.is_fn());
 
     // Now list each method's DefId (for within its trait).
     let own_entries = trait_methods.filter_map(move |&trait_method| {
diff --git a/compiler/rustc_traits/src/codegen.rs b/compiler/rustc_traits/src/codegen.rs
index 4a889abfc28..88f02d16c7d 100644
--- a/compiler/rustc_traits/src/codegen.rs
+++ b/compiler/rustc_traits/src/codegen.rs
@@ -6,11 +6,11 @@
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::bug;
 use rustc_middle::traits::CodegenObligationError;
-use rustc_middle::ty::{self, PseudoCanonicalInput, TyCtxt, TypeVisitableExt};
+use rustc_middle::ty::{self, PseudoCanonicalInput, TyCtxt, TypeVisitableExt, Upcast};
 use rustc_trait_selection::error_reporting::InferCtxtErrorExt;
 use rustc_trait_selection::traits::{
     ImplSource, Obligation, ObligationCause, ObligationCtxt, ScrubbedTraitError, SelectionContext,
-    Unimplemented,
+    Unimplemented, sizedness_fast_path,
 };
 use tracing::debug;
 
@@ -34,6 +34,13 @@ pub(crate) fn codegen_select_candidate<'tcx>(
     let (infcx, param_env) = tcx.infer_ctxt().ignoring_regions().build_with_typing_env(typing_env);
     let mut selcx = SelectionContext::new(&infcx);
 
+    if sizedness_fast_path(tcx, trait_ref.upcast(tcx)) {
+        return Ok(&*tcx.arena.alloc(ImplSource::Builtin(
+            ty::solve::BuiltinImplSource::Trivial,
+            Default::default(),
+        )));
+    }
+
     let obligation_cause = ObligationCause::dummy();
     let obligation = Obligation::new(tcx, obligation_cause, param_env, trait_ref);
 
diff --git a/compiler/rustc_traits/src/evaluate_obligation.rs b/compiler/rustc_traits/src/evaluate_obligation.rs
index c9ad096c6e9..7771db855d7 100644
--- a/compiler/rustc_traits/src/evaluate_obligation.rs
+++ b/compiler/rustc_traits/src/evaluate_obligation.rs
@@ -5,6 +5,7 @@ use rustc_span::DUMMY_SP;
 use rustc_trait_selection::traits::query::CanonicalPredicateGoal;
 use rustc_trait_selection::traits::{
     EvaluationResult, Obligation, ObligationCause, OverflowError, SelectionContext, TraitQueryMode,
+    sizedness_fast_path,
 };
 use tracing::debug;
 
@@ -23,6 +24,10 @@ fn evaluate_obligation<'tcx>(
     debug!("evaluate_obligation: goal={:#?}", goal);
     let ParamEnvAnd { param_env, value: predicate } = goal;
 
+    if sizedness_fast_path(tcx, predicate) {
+        return Ok(EvaluationResult::EvaluatedToOk);
+    }
+
     let mut selcx = SelectionContext::with_query_mode(infcx, TraitQueryMode::Canonical);
     let obligation = Obligation::new(tcx, ObligationCause::dummy(), param_env, predicate);
 
diff --git a/compiler/rustc_transmute/src/lib.rs b/compiler/rustc_transmute/src/lib.rs
index 7d800e49ff4..00928137d29 100644
--- a/compiler/rustc_transmute/src/lib.rs
+++ b/compiler/rustc_transmute/src/lib.rs
@@ -1,5 +1,4 @@
 // tidy-alphabetical-start
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![feature(never_type)]
 // tidy-alphabetical-end
 
diff --git a/compiler/rustc_transmute/src/maybe_transmutable/tests.rs b/compiler/rustc_transmute/src/maybe_transmutable/tests.rs
index 4d81382eba0..69a6b1b77f4 100644
--- a/compiler/rustc_transmute/src/maybe_transmutable/tests.rs
+++ b/compiler/rustc_transmute/src/maybe_transmutable/tests.rs
@@ -1,93 +1,115 @@
 use itertools::Itertools;
 
 use super::query_context::test::{Def, UltraMinimal};
-use crate::maybe_transmutable::MaybeTransmutableQuery;
-use crate::{Reason, layout};
+use crate::{Answer, Assume, Reason, layout};
 
-mod safety {
-    use super::*;
-    use crate::Answer;
+type Tree = layout::Tree<Def, !>;
+type Dfa = layout::Dfa<!>;
 
-    type Tree = layout::Tree<Def, !>;
+trait Representation {
+    fn is_transmutable(src: Self, dst: Self, assume: Assume) -> Answer<!>;
+}
 
-    const DST_HAS_SAFETY_INVARIANTS: Answer<!> =
-        Answer::No(crate::Reason::DstMayHaveSafetyInvariants);
+impl Representation for Tree {
+    fn is_transmutable(src: Self, dst: Self, assume: Assume) -> Answer<!> {
+        crate::maybe_transmutable::MaybeTransmutableQuery::new(src, dst, assume, UltraMinimal)
+            .answer()
+    }
+}
 
-    fn is_transmutable(src: &Tree, dst: &Tree, assume_safety: bool) -> crate::Answer<!> {
-        let src = src.clone();
-        let dst = dst.clone();
-        // The only dimension of the transmutability analysis we want to test
-        // here is the safety analysis. To ensure this, we disable all other
-        // toggleable aspects of the transmutability analysis.
-        let assume = crate::Assume {
-            alignment: true,
-            lifetimes: true,
-            validity: true,
-            safety: assume_safety,
-        };
+impl Representation for Dfa {
+    fn is_transmutable(src: Self, dst: Self, assume: Assume) -> Answer<!> {
         crate::maybe_transmutable::MaybeTransmutableQuery::new(src, dst, assume, UltraMinimal)
             .answer()
     }
+}
+
+fn is_transmutable<R: Representation + Clone>(
+    src: &R,
+    dst: &R,
+    assume: Assume,
+) -> crate::Answer<!> {
+    let src = src.clone();
+    let dst = dst.clone();
+    // The only dimension of the transmutability analysis we want to test
+    // here is the safety analysis. To ensure this, we disable all other
+    // toggleable aspects of the transmutability analysis.
+    R::is_transmutable(src, dst, assume)
+}
+
+mod safety {
+    use super::*;
+    use crate::Answer;
+
+    const DST_HAS_SAFETY_INVARIANTS: Answer<!> =
+        Answer::No(crate::Reason::DstMayHaveSafetyInvariants);
 
     #[test]
     fn src_safe_dst_safe() {
         let src = Tree::Def(Def::NoSafetyInvariants).then(Tree::u8());
         let dst = Tree::Def(Def::NoSafetyInvariants).then(Tree::u8());
-        assert_eq!(is_transmutable(&src, &dst, false), Answer::Yes);
-        assert_eq!(is_transmutable(&src, &dst, true), Answer::Yes);
+        assert_eq!(is_transmutable(&src, &dst, Assume::default()), Answer::Yes);
+        assert_eq!(
+            is_transmutable(&src, &dst, Assume { safety: true, ..Assume::default() }),
+            Answer::Yes
+        );
     }
 
     #[test]
     fn src_safe_dst_unsafe() {
         let src = Tree::Def(Def::NoSafetyInvariants).then(Tree::u8());
         let dst = Tree::Def(Def::HasSafetyInvariants).then(Tree::u8());
-        assert_eq!(is_transmutable(&src, &dst, false), DST_HAS_SAFETY_INVARIANTS);
-        assert_eq!(is_transmutable(&src, &dst, true), Answer::Yes);
+        assert_eq!(is_transmutable(&src, &dst, Assume::default()), DST_HAS_SAFETY_INVARIANTS);
+        assert_eq!(
+            is_transmutable(&src, &dst, Assume { safety: true, ..Assume::default() }),
+            Answer::Yes
+        );
     }
 
     #[test]
     fn src_unsafe_dst_safe() {
         let src = Tree::Def(Def::HasSafetyInvariants).then(Tree::u8());
         let dst = Tree::Def(Def::NoSafetyInvariants).then(Tree::u8());
-        assert_eq!(is_transmutable(&src, &dst, false), Answer::Yes);
-        assert_eq!(is_transmutable(&src, &dst, true), Answer::Yes);
+        assert_eq!(is_transmutable(&src, &dst, Assume::default()), Answer::Yes);
+        assert_eq!(
+            is_transmutable(&src, &dst, Assume { safety: true, ..Assume::default() }),
+            Answer::Yes
+        );
     }
 
     #[test]
     fn src_unsafe_dst_unsafe() {
         let src = Tree::Def(Def::HasSafetyInvariants).then(Tree::u8());
         let dst = Tree::Def(Def::HasSafetyInvariants).then(Tree::u8());
-        assert_eq!(is_transmutable(&src, &dst, false), DST_HAS_SAFETY_INVARIANTS);
-        assert_eq!(is_transmutable(&src, &dst, true), Answer::Yes);
+        assert_eq!(is_transmutable(&src, &dst, Assume::default()), DST_HAS_SAFETY_INVARIANTS);
+        assert_eq!(
+            is_transmutable(&src, &dst, Assume { safety: true, ..Assume::default() }),
+            Answer::Yes
+        );
     }
 }
 
 mod bool {
     use super::*;
-    use crate::Answer;
 
     #[test]
     fn should_permit_identity_transmutation_tree() {
-        let answer = crate::maybe_transmutable::MaybeTransmutableQuery::new(
-            layout::Tree::<Def, !>::bool(),
-            layout::Tree::<Def, !>::bool(),
-            crate::Assume { alignment: false, lifetimes: false, validity: true, safety: false },
-            UltraMinimal,
-        )
-        .answer();
-        assert_eq!(answer, Answer::Yes);
+        let src = Tree::bool();
+        assert_eq!(is_transmutable(&src, &src, Assume::default()), Answer::Yes);
+        assert_eq!(
+            is_transmutable(&src, &src, Assume { validity: true, ..Assume::default() }),
+            Answer::Yes
+        );
     }
 
     #[test]
     fn should_permit_identity_transmutation_dfa() {
-        let answer = crate::maybe_transmutable::MaybeTransmutableQuery::new(
-            layout::Dfa::<!>::bool(),
-            layout::Dfa::<!>::bool(),
-            crate::Assume { alignment: false, lifetimes: false, validity: true, safety: false },
-            UltraMinimal,
-        )
-        .answer();
-        assert_eq!(answer, Answer::Yes);
+        let src = Dfa::bool();
+        assert_eq!(is_transmutable(&src, &src, Assume::default()), Answer::Yes);
+        assert_eq!(
+            is_transmutable(&src, &src, Assume { validity: true, ..Assume::default() }),
+            Answer::Yes
+        );
     }
 
     #[test]
@@ -122,13 +144,7 @@ mod bool {
                 if src_set.is_subset(&dst_set) {
                     assert_eq!(
                         Answer::Yes,
-                        MaybeTransmutableQuery::new(
-                            src_layout.clone(),
-                            dst_layout.clone(),
-                            crate::Assume { validity: false, ..crate::Assume::default() },
-                            UltraMinimal,
-                        )
-                        .answer(),
+                        is_transmutable(&src_layout, &dst_layout, Assume::default()),
                         "{:?} SHOULD be transmutable into {:?}",
                         src_layout,
                         dst_layout
@@ -136,13 +152,11 @@ mod bool {
                 } else if !src_set.is_disjoint(&dst_set) {
                     assert_eq!(
                         Answer::Yes,
-                        MaybeTransmutableQuery::new(
-                            src_layout.clone(),
-                            dst_layout.clone(),
-                            crate::Assume { validity: true, ..crate::Assume::default() },
-                            UltraMinimal,
-                        )
-                        .answer(),
+                        is_transmutable(
+                            &src_layout,
+                            &dst_layout,
+                            Assume { validity: true, ..Assume::default() }
+                        ),
                         "{:?} SHOULD be transmutable (assuming validity) into {:?}",
                         src_layout,
                         dst_layout
@@ -150,13 +164,7 @@ mod bool {
                 } else {
                     assert_eq!(
                         Answer::No(Reason::DstIsBitIncompatible),
-                        MaybeTransmutableQuery::new(
-                            src_layout.clone(),
-                            dst_layout.clone(),
-                            crate::Assume { validity: false, ..crate::Assume::default() },
-                            UltraMinimal,
-                        )
-                        .answer(),
+                        is_transmutable(&src_layout, &dst_layout, Assume::default()),
                         "{:?} should NOT be transmutable into {:?}",
                         src_layout,
                         dst_layout
diff --git a/compiler/rustc_ty_utils/src/assoc.rs b/compiler/rustc_ty_utils/src/assoc.rs
index 9520d948f51..6cb9fdc6f93 100644
--- a/compiler/rustc_ty_utils/src/assoc.rs
+++ b/compiler/rustc_ty_utils/src/assoc.rs
@@ -6,7 +6,6 @@ use rustc_hir::{self as hir, AmbigArg};
 use rustc_middle::query::Providers;
 use rustc_middle::ty::{self, ImplTraitInTraitData, TyCtxt};
 use rustc_middle::{bug, span_bug};
-use rustc_span::kw;
 
 pub(crate) fn provide(providers: &mut Providers) {
     *providers = Providers {
@@ -129,39 +128,35 @@ fn associated_item(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::AssocItem {
 
 fn associated_item_from_trait_item_ref(trait_item_ref: &hir::TraitItemRef) -> ty::AssocItem {
     let owner_id = trait_item_ref.id.owner_id;
-    let (kind, has_self) = match trait_item_ref.kind {
-        hir::AssocItemKind::Const => (ty::AssocKind::Const, false),
-        hir::AssocItemKind::Fn { has_self } => (ty::AssocKind::Fn, has_self),
-        hir::AssocItemKind::Type => (ty::AssocKind::Type, false),
+    let name = trait_item_ref.ident.name;
+    let kind = match trait_item_ref.kind {
+        hir::AssocItemKind::Const => ty::AssocKind::Const { name },
+        hir::AssocItemKind::Fn { has_self } => ty::AssocKind::Fn { name, has_self },
+        hir::AssocItemKind::Type => ty::AssocKind::Type { data: ty::AssocTypeData::Normal(name) },
     };
 
     ty::AssocItem {
-        name: trait_item_ref.ident.name,
         kind,
         def_id: owner_id.to_def_id(),
         trait_item_def_id: Some(owner_id.to_def_id()),
         container: ty::AssocItemContainer::Trait,
-        fn_has_self_parameter: has_self,
-        opt_rpitit_info: None,
     }
 }
 
 fn associated_item_from_impl_item_ref(impl_item_ref: &hir::ImplItemRef) -> ty::AssocItem {
     let def_id = impl_item_ref.id.owner_id;
-    let (kind, has_self) = match impl_item_ref.kind {
-        hir::AssocItemKind::Const => (ty::AssocKind::Const, false),
-        hir::AssocItemKind::Fn { has_self } => (ty::AssocKind::Fn, has_self),
-        hir::AssocItemKind::Type => (ty::AssocKind::Type, false),
+    let name = impl_item_ref.ident.name;
+    let kind = match impl_item_ref.kind {
+        hir::AssocItemKind::Const => ty::AssocKind::Const { name },
+        hir::AssocItemKind::Fn { has_self } => ty::AssocKind::Fn { name, has_self },
+        hir::AssocItemKind::Type => ty::AssocKind::Type { data: ty::AssocTypeData::Normal(name) },
     };
 
     ty::AssocItem {
-        name: impl_item_ref.ident.name,
         kind,
         def_id: def_id.to_def_id(),
         trait_item_def_id: impl_item_ref.trait_item_def_id,
         container: ty::AssocItemContainer::Impl,
-        fn_has_self_parameter: has_self,
-        opt_rpitit_info: None,
     }
 }
 
@@ -264,16 +259,15 @@ fn associated_type_for_impl_trait_in_trait(
     trait_assoc_ty.def_ident_span(Some(span));
 
     trait_assoc_ty.associated_item(ty::AssocItem {
-        name: kw::Empty,
-        kind: ty::AssocKind::Type,
+        kind: ty::AssocKind::Type {
+            data: ty::AssocTypeData::Rpitit(ImplTraitInTraitData::Trait {
+                fn_def_id: fn_def_id.to_def_id(),
+                opaque_def_id: opaque_ty_def_id.to_def_id(),
+            }),
+        },
         def_id,
         trait_item_def_id: None,
         container: ty::AssocItemContainer::Trait,
-        fn_has_self_parameter: false,
-        opt_rpitit_info: Some(ImplTraitInTraitData::Trait {
-            fn_def_id: fn_def_id.to_def_id(),
-            opaque_def_id: opaque_ty_def_id.to_def_id(),
-        }),
     });
 
     // Copy visility of the containing function.
@@ -317,13 +311,14 @@ fn associated_type_for_impl_trait_in_impl(
     impl_assoc_ty.def_ident_span(Some(span));
 
     impl_assoc_ty.associated_item(ty::AssocItem {
-        name: kw::Empty,
-        kind: ty::AssocKind::Type,
+        kind: ty::AssocKind::Type {
+            data: ty::AssocTypeData::Rpitit(ImplTraitInTraitData::Impl {
+                fn_def_id: impl_fn_def_id.to_def_id(),
+            }),
+        },
         def_id,
         trait_item_def_id: Some(trait_assoc_def_id),
         container: ty::AssocItemContainer::Impl,
-        fn_has_self_parameter: false,
-        opt_rpitit_info: Some(ImplTraitInTraitData::Impl { fn_def_id: impl_fn_def_id.to_def_id() }),
     });
 
     // Copy visility of the containing function.
diff --git a/compiler/rustc_ty_utils/src/layout.rs b/compiler/rustc_ty_utils/src/layout.rs
index 0017186c1b0..1915ba623cb 100644
--- a/compiler/rustc_ty_utils/src/layout.rs
+++ b/compiler/rustc_ty_utils/src/layout.rs
@@ -514,6 +514,9 @@ fn layout_of_uncached<'tcx>(
                 return map_layout(cx.calc.layout_of_union(&def.repr(), &variants));
             }
 
+            // UnsafeCell and UnsafePinned both disable niche optimizations
+            let is_special_no_niche = def.is_unsafe_cell() || def.is_unsafe_pinned();
+
             let get_discriminant_type =
                 |min, max| abi::Integer::repr_discr(tcx, ty, &def.repr(), min, max);
 
@@ -542,7 +545,7 @@ fn layout_of_uncached<'tcx>(
                     &def.repr(),
                     &variants,
                     def.is_enum(),
-                    def.is_unsafe_cell(),
+                    is_special_no_niche,
                     tcx.layout_scalar_valid_range(def.did()),
                     get_discriminant_type,
                     discriminants_iter(),
@@ -568,7 +571,7 @@ fn layout_of_uncached<'tcx>(
                     &def.repr(),
                     &variants,
                     def.is_enum(),
-                    def.is_unsafe_cell(),
+                    is_special_no_niche,
                     tcx.layout_scalar_valid_range(def.did()),
                     get_discriminant_type,
                     discriminants_iter(),
diff --git a/compiler/rustc_ty_utils/src/lib.rs b/compiler/rustc_ty_utils/src/lib.rs
index 143b7d53880..35cc6f39856 100644
--- a/compiler/rustc_ty_utils/src/lib.rs
+++ b/compiler/rustc_ty_utils/src/lib.rs
@@ -6,7 +6,6 @@
 
 // tidy-alphabetical-start
 #![allow(internal_features)]
-#![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(assert_matches)]
diff --git a/compiler/rustc_type_ir/src/binder.rs b/compiler/rustc_type_ir/src/binder.rs
index e9055940310..27ea4e211fe 100644
--- a/compiler/rustc_type_ir/src/binder.rs
+++ b/compiler/rustc_type_ir/src/binder.rs
@@ -122,6 +122,10 @@ impl<I: Interner, T: TypeFoldable<I>> TypeFoldable<I> for Binder<I, T> {
     fn try_fold_with<F: FallibleTypeFolder<I>>(self, folder: &mut F) -> Result<Self, F::Error> {
         folder.try_fold_binder(self)
     }
+
+    fn fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self {
+        folder.fold_binder(self)
+    }
 }
 
 impl<I: Interner, T: TypeVisitable<I>> TypeVisitable<I> for Binder<I, T> {
@@ -135,7 +139,11 @@ impl<I: Interner, T: TypeFoldable<I>> TypeSuperFoldable<I> for Binder<I, T> {
         self,
         folder: &mut F,
     ) -> Result<Self, F::Error> {
-        self.try_map_bound(|ty| ty.try_fold_with(folder))
+        self.try_map_bound(|t| t.try_fold_with(folder))
+    }
+
+    fn super_fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self {
+        self.map_bound(|t| t.fold_with(folder))
     }
 }
 
diff --git a/compiler/rustc_type_ir/src/flags.rs b/compiler/rustc_type_ir/src/flags.rs
index 6a2498242fe..e9d3a149a73 100644
--- a/compiler/rustc_type_ir/src/flags.rs
+++ b/compiler/rustc_type_ir/src/flags.rs
@@ -1,3 +1,9 @@
+use std::slice;
+
+use crate::inherent::*;
+use crate::visit::Flags;
+use crate::{self as ty, Interner};
+
 bitflags::bitflags! {
     /// Flags that we track on types. These flags are propagated upwards
     /// through the type during type construction, so that we can quickly check
@@ -128,3 +134,362 @@ bitflags::bitflags! {
         const HAS_BINDER_VARS             = 1 << 23;
     }
 }
+
+#[derive(Debug)]
+pub struct FlagComputation<I> {
+    pub flags: TypeFlags,
+
+    /// see `Ty::outer_exclusive_binder` for details
+    pub outer_exclusive_binder: ty::DebruijnIndex,
+
+    interner: std::marker::PhantomData<I>,
+}
+
+impl<I: Interner> FlagComputation<I> {
+    fn new() -> FlagComputation<I> {
+        FlagComputation {
+            flags: TypeFlags::empty(),
+            outer_exclusive_binder: ty::INNERMOST,
+            interner: std::marker::PhantomData,
+        }
+    }
+
+    #[allow(rustc::usage_of_ty_tykind)]
+    pub fn for_kind(kind: &ty::TyKind<I>) -> FlagComputation<I> {
+        let mut result = FlagComputation::new();
+        result.add_kind(kind);
+        result
+    }
+
+    pub fn for_predicate(binder: ty::Binder<I, ty::PredicateKind<I>>) -> FlagComputation<I> {
+        let mut result = FlagComputation::new();
+        result.add_predicate(binder);
+        result
+    }
+
+    pub fn for_const_kind(kind: &ty::ConstKind<I>) -> FlagComputation<I> {
+        let mut result = FlagComputation::new();
+        result.add_const_kind(kind);
+        result
+    }
+
+    pub fn for_clauses(clauses: &[I::Clause]) -> FlagComputation<I> {
+        let mut result = FlagComputation::new();
+        for c in clauses {
+            result.add_flags(c.as_predicate().flags());
+            result.add_exclusive_binder(c.as_predicate().outer_exclusive_binder());
+        }
+        result
+    }
+
+    fn add_flags(&mut self, flags: TypeFlags) {
+        self.flags = self.flags | flags;
+    }
+
+    /// indicates that `self` refers to something at binding level `binder`
+    fn add_bound_var(&mut self, binder: ty::DebruijnIndex) {
+        let exclusive_binder = binder.shifted_in(1);
+        self.add_exclusive_binder(exclusive_binder);
+    }
+
+    /// indicates that `self` refers to something *inside* binding
+    /// level `binder` -- not bound by `binder`, but bound by the next
+    /// binder internal to it
+    fn add_exclusive_binder(&mut self, exclusive_binder: ty::DebruijnIndex) {
+        self.outer_exclusive_binder = self.outer_exclusive_binder.max(exclusive_binder);
+    }
+
+    /// Adds the flags/depth from a set of types that appear within the current type, but within a
+    /// region binder.
+    fn bound_computation<T, F>(&mut self, value: ty::Binder<I, T>, f: F)
+    where
+        F: FnOnce(&mut Self, T),
+    {
+        let mut computation = FlagComputation::new();
+
+        if !value.bound_vars().is_empty() {
+            computation.add_flags(TypeFlags::HAS_BINDER_VARS);
+        }
+
+        f(&mut computation, value.skip_binder());
+
+        self.add_flags(computation.flags);
+
+        // The types that contributed to `computation` occurred within
+        // a region binder, so subtract one from the region depth
+        // within when adding the depth to `self`.
+        let outer_exclusive_binder = computation.outer_exclusive_binder;
+        if outer_exclusive_binder > ty::INNERMOST {
+            self.add_exclusive_binder(outer_exclusive_binder.shifted_out(1));
+        } // otherwise, this binder captures nothing
+    }
+
+    #[allow(rustc::usage_of_ty_tykind)]
+    fn add_kind(&mut self, kind: &ty::TyKind<I>) {
+        match *kind {
+            ty::Bool
+            | ty::Char
+            | ty::Int(_)
+            | ty::Float(_)
+            | ty::Uint(_)
+            | ty::Never
+            | ty::Str
+            | ty::Foreign(..) => {}
+
+            ty::Error(_) => self.add_flags(TypeFlags::HAS_ERROR),
+
+            ty::Param(_) => {
+                self.add_flags(TypeFlags::HAS_TY_PARAM);
+            }
+
+            ty::Closure(_, args)
+            | ty::Coroutine(_, args)
+            | ty::CoroutineClosure(_, args)
+            | ty::CoroutineWitness(_, args) => {
+                self.add_args(args.as_slice());
+            }
+
+            ty::Bound(debruijn, _) => {
+                self.add_bound_var(debruijn);
+                self.add_flags(TypeFlags::HAS_TY_BOUND);
+            }
+
+            ty::Placeholder(..) => {
+                self.add_flags(TypeFlags::HAS_TY_PLACEHOLDER);
+            }
+
+            ty::Infer(infer) => match infer {
+                ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_) => {
+                    self.add_flags(TypeFlags::HAS_TY_FRESH)
+                }
+
+                ty::TyVar(_) | ty::IntVar(_) | ty::FloatVar(_) => {
+                    self.add_flags(TypeFlags::HAS_TY_INFER)
+                }
+            },
+
+            ty::Adt(_, args) => {
+                self.add_args(args.as_slice());
+            }
+
+            ty::Alias(kind, data) => {
+                self.add_flags(match kind {
+                    ty::Projection => TypeFlags::HAS_TY_PROJECTION,
+                    ty::Weak => TypeFlags::HAS_TY_WEAK,
+                    ty::Opaque => TypeFlags::HAS_TY_OPAQUE,
+                    ty::Inherent => TypeFlags::HAS_TY_INHERENT,
+                });
+
+                self.add_alias_ty(data);
+            }
+
+            ty::Dynamic(obj, r, _) => {
+                for predicate in obj.iter() {
+                    self.bound_computation(predicate, |computation, predicate| match predicate {
+                        ty::ExistentialPredicate::Trait(tr) => {
+                            computation.add_args(tr.args.as_slice())
+                        }
+                        ty::ExistentialPredicate::Projection(p) => {
+                            computation.add_existential_projection(&p);
+                        }
+                        ty::ExistentialPredicate::AutoTrait(_) => {}
+                    });
+                }
+
+                self.add_region(r);
+            }
+
+            ty::Array(tt, len) => {
+                self.add_ty(tt);
+                self.add_const(len);
+            }
+
+            ty::Pat(ty, pat) => {
+                self.add_ty(ty);
+                self.add_flags(pat.flags());
+            }
+
+            ty::Slice(tt) => self.add_ty(tt),
+
+            ty::RawPtr(ty, _) => {
+                self.add_ty(ty);
+            }
+
+            ty::Ref(r, ty, _) => {
+                self.add_region(r);
+                self.add_ty(ty);
+            }
+
+            ty::Tuple(types) => {
+                self.add_tys(types);
+            }
+
+            ty::FnDef(_, args) => {
+                self.add_args(args.as_slice());
+            }
+
+            ty::FnPtr(sig_tys, _) => self.bound_computation(sig_tys, |computation, sig_tys| {
+                computation.add_tys(sig_tys.inputs_and_output);
+            }),
+
+            ty::UnsafeBinder(bound_ty) => {
+                self.bound_computation(bound_ty.into(), |computation, ty| {
+                    computation.add_ty(ty);
+                })
+            }
+        }
+    }
+
+    fn add_predicate(&mut self, binder: ty::Binder<I, ty::PredicateKind<I>>) {
+        self.bound_computation(binder, |computation, atom| computation.add_predicate_atom(atom));
+    }
+
+    fn add_predicate_atom(&mut self, atom: ty::PredicateKind<I>) {
+        match atom {
+            ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_pred)) => {
+                self.add_args(trait_pred.trait_ref.args.as_slice());
+            }
+            ty::PredicateKind::Clause(ty::ClauseKind::HostEffect(ty::HostEffectPredicate {
+                trait_ref,
+                constness: _,
+            })) => {
+                self.add_args(trait_ref.args.as_slice());
+            }
+            ty::PredicateKind::Clause(ty::ClauseKind::RegionOutlives(ty::OutlivesPredicate(
+                a,
+                b,
+            ))) => {
+                self.add_region(a);
+                self.add_region(b);
+            }
+            ty::PredicateKind::Clause(ty::ClauseKind::TypeOutlives(ty::OutlivesPredicate(
+                ty,
+                region,
+            ))) => {
+                self.add_ty(ty);
+                self.add_region(region);
+            }
+            ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(ct, ty)) => {
+                self.add_const(ct);
+                self.add_ty(ty);
+            }
+            ty::PredicateKind::Subtype(ty::SubtypePredicate { a_is_expected: _, a, b }) => {
+                self.add_ty(a);
+                self.add_ty(b);
+            }
+            ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) => {
+                self.add_ty(a);
+                self.add_ty(b);
+            }
+            ty::PredicateKind::Clause(ty::ClauseKind::Projection(ty::ProjectionPredicate {
+                projection_term,
+                term,
+            })) => {
+                self.add_alias_term(projection_term);
+                self.add_term(term);
+            }
+            ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(arg)) => {
+                self.add_args(slice::from_ref(&arg));
+            }
+            ty::PredicateKind::DynCompatible(_def_id) => {}
+            ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(uv)) => {
+                self.add_const(uv);
+            }
+            ty::PredicateKind::ConstEquate(expected, found) => {
+                self.add_const(expected);
+                self.add_const(found);
+            }
+            ty::PredicateKind::Ambiguous => {}
+            ty::PredicateKind::NormalizesTo(ty::NormalizesTo { alias, term }) => {
+                self.add_alias_term(alias);
+                self.add_term(term);
+            }
+            ty::PredicateKind::AliasRelate(t1, t2, _) => {
+                self.add_term(t1);
+                self.add_term(t2);
+            }
+        }
+    }
+
+    fn add_ty(&mut self, ty: I::Ty) {
+        self.add_flags(ty.flags());
+        self.add_exclusive_binder(ty.outer_exclusive_binder());
+    }
+
+    fn add_tys(&mut self, tys: I::Tys) {
+        for ty in tys.iter() {
+            self.add_ty(ty);
+        }
+    }
+
+    fn add_region(&mut self, r: I::Region) {
+        self.add_flags(r.flags());
+        if let ty::ReBound(debruijn, _) = r.kind() {
+            self.add_bound_var(debruijn);
+        }
+    }
+
+    fn add_const(&mut self, c: I::Const) {
+        self.add_flags(c.flags());
+        self.add_exclusive_binder(c.outer_exclusive_binder());
+    }
+
+    fn add_const_kind(&mut self, c: &ty::ConstKind<I>) {
+        match *c {
+            ty::ConstKind::Unevaluated(uv) => {
+                self.add_args(uv.args.as_slice());
+                self.add_flags(TypeFlags::HAS_CT_PROJECTION);
+            }
+            ty::ConstKind::Infer(infer) => match infer {
+                ty::InferConst::Fresh(_) => self.add_flags(TypeFlags::HAS_CT_FRESH),
+                ty::InferConst::Var(_) => self.add_flags(TypeFlags::HAS_CT_INFER),
+            },
+            ty::ConstKind::Bound(debruijn, _) => {
+                self.add_bound_var(debruijn);
+                self.add_flags(TypeFlags::HAS_CT_BOUND);
+            }
+            ty::ConstKind::Param(_) => {
+                self.add_flags(TypeFlags::HAS_CT_PARAM);
+            }
+            ty::ConstKind::Placeholder(_) => {
+                self.add_flags(TypeFlags::HAS_CT_PLACEHOLDER);
+            }
+            ty::ConstKind::Value(cv) => self.add_ty(cv.ty()),
+            ty::ConstKind::Expr(e) => self.add_args(e.args().as_slice()),
+            ty::ConstKind::Error(_) => self.add_flags(TypeFlags::HAS_ERROR),
+        }
+    }
+
+    fn add_existential_projection(&mut self, projection: &ty::ExistentialProjection<I>) {
+        self.add_args(projection.args.as_slice());
+        match projection.term.kind() {
+            ty::TermKind::Ty(ty) => self.add_ty(ty),
+            ty::TermKind::Const(ct) => self.add_const(ct),
+        }
+    }
+
+    fn add_alias_ty(&mut self, alias_ty: ty::AliasTy<I>) {
+        self.add_args(alias_ty.args.as_slice());
+    }
+
+    fn add_alias_term(&mut self, alias_term: ty::AliasTerm<I>) {
+        self.add_args(alias_term.args.as_slice());
+    }
+
+    fn add_args(&mut self, args: &[I::GenericArg]) {
+        for kind in args {
+            match kind.kind() {
+                ty::GenericArgKind::Type(ty) => self.add_ty(ty),
+                ty::GenericArgKind::Lifetime(lt) => self.add_region(lt),
+                ty::GenericArgKind::Const(ct) => self.add_const(ct),
+            }
+        }
+    }
+
+    fn add_term(&mut self, term: I::Term) {
+        match term.kind() {
+            ty::TermKind::Ty(ty) => self.add_ty(ty),
+            ty::TermKind::Const(ct) => self.add_const(ct),
+        }
+    }
+}
diff --git a/compiler/rustc_type_ir/src/fold.rs b/compiler/rustc_type_ir/src/fold.rs
index e58f25f4ce7..ce1188070ca 100644
--- a/compiler/rustc_type_ir/src/fold.rs
+++ b/compiler/rustc_type_ir/src/fold.rs
@@ -45,6 +45,7 @@
 //! - u.fold_with(folder)
 //! ```
 
+use std::convert::Infallible;
 use std::mem;
 use std::sync::Arc;
 
@@ -56,12 +57,6 @@ use crate::inherent::*;
 use crate::visit::{TypeVisitable, TypeVisitableExt as _};
 use crate::{self as ty, Interner, TypeFlags};
 
-#[cfg(feature = "nightly")]
-type Never = !;
-
-#[cfg(not(feature = "nightly"))]
-type Never = std::convert::Infallible;
-
 /// This trait is implemented for every type that can be folded,
 /// providing the skeleton of the traversal.
 ///
@@ -82,18 +77,24 @@ pub trait TypeFoldable<I: Interner>: TypeVisitable<I> + Clone {
     ///
     /// For types of interest (such as `Ty`), the implementation of this method
     /// calls a folder method specifically for that type (such as
-    /// `F::try_fold_ty`). This is where control transfers from `TypeFoldable`
-    /// to `TypeFolder`.
+    /// `F::try_fold_ty`). This is where control transfers from [`TypeFoldable`]
+    /// to [`FallibleTypeFolder`].
     fn try_fold_with<F: FallibleTypeFolder<I>>(self, folder: &mut F) -> Result<Self, F::Error>;
 
-    /// A convenient alternative to `try_fold_with` for use with infallible
-    /// folders. Do not override this method, to ensure coherence with
-    /// `try_fold_with`.
-    fn fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self {
-        match self.try_fold_with(folder) {
-            Ok(t) => t,
-        }
-    }
+    /// The entry point for folding. To fold a value `t` with a folder `f`
+    /// call: `t.fold_with(f)`.
+    ///
+    /// For most types, this just traverses the value, calling `fold_with`
+    /// on each field/element.
+    ///
+    /// For types of interest (such as `Ty`), the implementation of this method
+    /// calls a folder method specifically for that type (such as
+    /// `F::fold_ty`). This is where control transfers from `TypeFoldable`
+    /// to `TypeFolder`.
+    ///
+    /// Same as [`TypeFoldable::try_fold_with`], but not fallible. Make sure to keep
+    /// the behavior in sync across functions.
+    fn fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self;
 }
 
 // This trait is implemented for types of interest.
@@ -112,11 +113,7 @@ pub trait TypeSuperFoldable<I: Interner>: TypeFoldable<I> {
     /// A convenient alternative to `try_super_fold_with` for use with
     /// infallible folders. Do not override this method, to ensure coherence
     /// with `try_super_fold_with`.
-    fn super_fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self {
-        match self.try_super_fold_with(folder) {
-            Ok(t) => t,
-        }
-    }
+    fn super_fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self;
 }
 
 /// This trait is implemented for every infallible folding traversal. There is
@@ -128,7 +125,7 @@ pub trait TypeSuperFoldable<I: Interner>: TypeFoldable<I> {
 /// A blanket implementation of [`FallibleTypeFolder`] will defer to
 /// the infallible methods of this trait to ensure that the two APIs
 /// are coherent.
-pub trait TypeFolder<I: Interner>: FallibleTypeFolder<I, Error = Never> {
+pub trait TypeFolder<I: Interner>: Sized {
     fn cx(&self) -> I;
 
     fn fold_binder<T>(&mut self, t: ty::Binder<I, T>) -> ty::Binder<I, T>
@@ -195,42 +192,6 @@ pub trait FallibleTypeFolder<I: Interner>: Sized {
     }
 }
 
-// This blanket implementation of the fallible trait for infallible folders
-// delegates to infallible methods to ensure coherence.
-impl<I: Interner, F> FallibleTypeFolder<I> for F
-where
-    F: TypeFolder<I>,
-{
-    type Error = Never;
-
-    fn cx(&self) -> I {
-        TypeFolder::cx(self)
-    }
-
-    fn try_fold_binder<T>(&mut self, t: ty::Binder<I, T>) -> Result<ty::Binder<I, T>, Never>
-    where
-        T: TypeFoldable<I>,
-    {
-        Ok(self.fold_binder(t))
-    }
-
-    fn try_fold_ty(&mut self, t: I::Ty) -> Result<I::Ty, Never> {
-        Ok(self.fold_ty(t))
-    }
-
-    fn try_fold_region(&mut self, r: I::Region) -> Result<I::Region, Never> {
-        Ok(self.fold_region(r))
-    }
-
-    fn try_fold_const(&mut self, c: I::Const) -> Result<I::Const, Never> {
-        Ok(self.fold_const(c))
-    }
-
-    fn try_fold_predicate(&mut self, p: I::Predicate) -> Result<I::Predicate, Never> {
-        Ok(self.fold_predicate(p))
-    }
-}
-
 ///////////////////////////////////////////////////////////////////////////
 // Traversal implementations.
 
@@ -238,6 +199,10 @@ impl<I: Interner, T: TypeFoldable<I>, U: TypeFoldable<I>> TypeFoldable<I> for (T
     fn try_fold_with<F: FallibleTypeFolder<I>>(self, folder: &mut F) -> Result<(T, U), F::Error> {
         Ok((self.0.try_fold_with(folder)?, self.1.try_fold_with(folder)?))
     }
+
+    fn fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self {
+        (self.0.fold_with(folder), self.1.fold_with(folder))
+    }
 }
 
 impl<I: Interner, A: TypeFoldable<I>, B: TypeFoldable<I>, C: TypeFoldable<I>> TypeFoldable<I>
@@ -253,6 +218,10 @@ impl<I: Interner, A: TypeFoldable<I>, B: TypeFoldable<I>, C: TypeFoldable<I>> Ty
             self.2.try_fold_with(folder)?,
         ))
     }
+
+    fn fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self {
+        (self.0.fold_with(folder), self.1.fold_with(folder), self.2.fold_with(folder))
+    }
 }
 
 impl<I: Interner, T: TypeFoldable<I>> TypeFoldable<I> for Option<T> {
@@ -262,6 +231,10 @@ impl<I: Interner, T: TypeFoldable<I>> TypeFoldable<I> for Option<T> {
             None => None,
         })
     }
+
+    fn fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self {
+        Some(self?.fold_with(folder))
+    }
 }
 
 impl<I: Interner, T: TypeFoldable<I>, E: TypeFoldable<I>> TypeFoldable<I> for Result<T, E> {
@@ -271,41 +244,61 @@ impl<I: Interner, T: TypeFoldable<I>, E: TypeFoldable<I>> TypeFoldable<I> for Re
             Err(e) => Err(e.try_fold_with(folder)?),
         })
     }
+
+    fn fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self {
+        match self {
+            Ok(v) => Ok(v.fold_with(folder)),
+            Err(e) => Err(e.fold_with(folder)),
+        }
+    }
+}
+
+fn fold_arc<T: Clone, E>(
+    mut arc: Arc<T>,
+    fold: impl FnOnce(T) -> Result<T, E>,
+) -> Result<Arc<T>, E> {
+    // We merely want to replace the contained `T`, if at all possible,
+    // so that we don't needlessly allocate a new `Arc` or indeed clone
+    // the contained type.
+    unsafe {
+        // First step is to ensure that we have a unique reference to
+        // the contained type, which `Arc::make_mut` will accomplish (by
+        // allocating a new `Arc` and cloning the `T` only if required).
+        // This is done *before* casting to `Arc<ManuallyDrop<T>>` so that
+        // panicking during `make_mut` does not leak the `T`.
+        Arc::make_mut(&mut arc);
+
+        // Casting to `Arc<ManuallyDrop<T>>` is safe because `ManuallyDrop`
+        // is `repr(transparent)`.
+        let ptr = Arc::into_raw(arc).cast::<mem::ManuallyDrop<T>>();
+        let mut unique = Arc::from_raw(ptr);
+
+        // Call to `Arc::make_mut` above guarantees that `unique` is the
+        // sole reference to the contained value, so we can avoid doing
+        // a checked `get_mut` here.
+        let slot = Arc::get_mut(&mut unique).unwrap_unchecked();
+
+        // Semantically move the contained type out from `unique`, fold
+        // it, then move the folded value back into `unique`. Should
+        // folding fail, `ManuallyDrop` ensures that the "moved-out"
+        // value is not re-dropped.
+        let owned = mem::ManuallyDrop::take(slot);
+        let folded = fold(owned)?;
+        *slot = mem::ManuallyDrop::new(folded);
+
+        // Cast back to `Arc<T>`.
+        Ok(Arc::from_raw(Arc::into_raw(unique).cast()))
+    }
 }
 
 impl<I: Interner, T: TypeFoldable<I>> TypeFoldable<I> for Arc<T> {
-    fn try_fold_with<F: FallibleTypeFolder<I>>(mut self, folder: &mut F) -> Result<Self, F::Error> {
-        // We merely want to replace the contained `T`, if at all possible,
-        // so that we don't needlessly allocate a new `Arc` or indeed clone
-        // the contained type.
-        unsafe {
-            // First step is to ensure that we have a unique reference to
-            // the contained type, which `Arc::make_mut` will accomplish (by
-            // allocating a new `Arc` and cloning the `T` only if required).
-            // This is done *before* casting to `Arc<ManuallyDrop<T>>` so that
-            // panicking during `make_mut` does not leak the `T`.
-            Arc::make_mut(&mut self);
-
-            // Casting to `Arc<ManuallyDrop<T>>` is safe because `ManuallyDrop`
-            // is `repr(transparent)`.
-            let ptr = Arc::into_raw(self).cast::<mem::ManuallyDrop<T>>();
-            let mut unique = Arc::from_raw(ptr);
-
-            // Call to `Arc::make_mut` above guarantees that `unique` is the
-            // sole reference to the contained value, so we can avoid doing
-            // a checked `get_mut` here.
-            let slot = Arc::get_mut(&mut unique).unwrap_unchecked();
-
-            // Semantically move the contained type out from `unique`, fold
-            // it, then move the folded value back into `unique`. Should
-            // folding fail, `ManuallyDrop` ensures that the "moved-out"
-            // value is not re-dropped.
-            let owned = mem::ManuallyDrop::take(slot);
-            let folded = owned.try_fold_with(folder)?;
-            *slot = mem::ManuallyDrop::new(folded);
-
-            // Cast back to `Arc<T>`.
-            Ok(Arc::from_raw(Arc::into_raw(unique).cast()))
+    fn try_fold_with<F: FallibleTypeFolder<I>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        fold_arc(self, |t| t.try_fold_with(folder))
+    }
+
+    fn fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self {
+        match fold_arc::<T, Infallible>(self, |t| Ok(t.fold_with(folder))) {
+            Ok(t) => t,
         }
     }
 }
@@ -315,30 +308,51 @@ impl<I: Interner, T: TypeFoldable<I>> TypeFoldable<I> for Box<T> {
         *self = (*self).try_fold_with(folder)?;
         Ok(self)
     }
+
+    fn fold_with<F: TypeFolder<I>>(mut self, folder: &mut F) -> Self {
+        *self = (*self).fold_with(folder);
+        self
+    }
 }
 
 impl<I: Interner, T: TypeFoldable<I>> TypeFoldable<I> for Vec<T> {
     fn try_fold_with<F: FallibleTypeFolder<I>>(self, folder: &mut F) -> Result<Self, F::Error> {
         self.into_iter().map(|t| t.try_fold_with(folder)).collect()
     }
+
+    fn fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self {
+        self.into_iter().map(|t| t.fold_with(folder)).collect()
+    }
 }
 
 impl<I: Interner, T: TypeFoldable<I>> TypeFoldable<I> for ThinVec<T> {
     fn try_fold_with<F: FallibleTypeFolder<I>>(self, folder: &mut F) -> Result<Self, F::Error> {
         self.into_iter().map(|t| t.try_fold_with(folder)).collect()
     }
+
+    fn fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self {
+        self.into_iter().map(|t| t.fold_with(folder)).collect()
+    }
 }
 
 impl<I: Interner, T: TypeFoldable<I>> TypeFoldable<I> for Box<[T]> {
     fn try_fold_with<F: FallibleTypeFolder<I>>(self, folder: &mut F) -> Result<Self, F::Error> {
         Vec::from(self).try_fold_with(folder).map(Vec::into_boxed_slice)
     }
+
+    fn fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self {
+        Vec::into_boxed_slice(Vec::from(self).fold_with(folder))
+    }
 }
 
 impl<I: Interner, T: TypeFoldable<I>, Ix: Idx> TypeFoldable<I> for IndexVec<Ix, T> {
     fn try_fold_with<F: FallibleTypeFolder<I>>(self, folder: &mut F) -> Result<Self, F::Error> {
         self.raw.try_fold_with(folder).map(IndexVec::from_raw)
     }
+
+    fn fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self {
+        IndexVec::from_raw(self.raw.fold_with(folder))
+    }
 }
 
 ///////////////////////////////////////////////////////////////////////////
diff --git a/compiler/rustc_type_ir/src/inherent.rs b/compiler/rustc_type_ir/src/inherent.rs
index 6e6c40580d8..417803e75ea 100644
--- a/compiler/rustc_type_ir/src/inherent.rs
+++ b/compiler/rustc_type_ir/src/inherent.rs
@@ -583,7 +583,7 @@ pub trait Span<I: Interner>: Copy + Debug + Hash + Eq + TypeFoldable<I> {
 
 pub trait SliceLike: Sized + Copy {
     type Item: Copy;
-    type IntoIter: Iterator<Item = Self::Item>;
+    type IntoIter: Iterator<Item = Self::Item> + DoubleEndedIterator;
 
     fn iter(self) -> Self::IntoIter;
 
diff --git a/compiler/rustc_type_ir/src/interner.rs b/compiler/rustc_type_ir/src/interner.rs
index a9e6764e218..71bfeabfda8 100644
--- a/compiler/rustc_type_ir/src/interner.rs
+++ b/compiler/rustc_type_ir/src/interner.rs
@@ -31,6 +31,7 @@ pub trait Interner:
     + IrPrint<ty::SubtypePredicate<Self>>
     + IrPrint<ty::CoercePredicate<Self>>
     + IrPrint<ty::FnSig<Self>>
+    + IrPrint<ty::PatternKind<Self>>
 {
     type DefId: DefId<Self>;
     type LocalDefId: Copy + Debug + Hash + Eq + Into<Self::DefId> + TypeFoldable<Self>;
@@ -104,7 +105,14 @@ pub trait Interner:
     type ErrorGuaranteed: Copy + Debug + Hash + Eq;
     type BoundExistentialPredicates: BoundExistentialPredicates<Self>;
     type AllocId: Copy + Debug + Hash + Eq;
-    type Pat: Copy + Debug + Hash + Eq + Debug + Relate<Self>;
+    type Pat: Copy
+        + Debug
+        + Hash
+        + Eq
+        + Debug
+        + Relate<Self>
+        + Flags
+        + IntoKind<Kind = ty::PatternKind<Self>>;
     type Safety: Safety<Self>;
     type Abi: Abi<Self>;
 
diff --git a/compiler/rustc_type_ir/src/ir_print.rs b/compiler/rustc_type_ir/src/ir_print.rs
index 0c71f3a3df2..388ad09cb20 100644
--- a/compiler/rustc_type_ir/src/ir_print.rs
+++ b/compiler/rustc_type_ir/src/ir_print.rs
@@ -2,8 +2,8 @@ use std::fmt;
 
 use crate::{
     AliasTerm, AliasTy, Binder, CoercePredicate, ExistentialProjection, ExistentialTraitRef, FnSig,
-    HostEffectPredicate, Interner, NormalizesTo, OutlivesPredicate, ProjectionPredicate,
-    SubtypePredicate, TraitPredicate, TraitRef,
+    HostEffectPredicate, Interner, NormalizesTo, OutlivesPredicate, PatternKind,
+    ProjectionPredicate, SubtypePredicate, TraitPredicate, TraitRef,
 };
 
 pub trait IrPrint<T> {
@@ -57,9 +57,10 @@ define_display_via_print!(
     AliasTy,
     AliasTerm,
     FnSig,
+    PatternKind,
 );
 
-define_debug_via_print!(TraitRef, ExistentialTraitRef, ExistentialProjection);
+define_debug_via_print!(TraitRef, ExistentialTraitRef, PatternKind);
 
 impl<I: Interner, T> fmt::Display for OutlivesPredicate<I, T>
 where
diff --git a/compiler/rustc_type_ir/src/lib.rs b/compiler/rustc_type_ir/src/lib.rs
index bdc61e956f8..792090effcf 100644
--- a/compiler/rustc_type_ir/src/lib.rs
+++ b/compiler/rustc_type_ir/src/lib.rs
@@ -31,6 +31,7 @@ pub mod outlives;
 pub mod relate;
 pub mod search_graph;
 pub mod solve;
+pub mod walk;
 
 // These modules are not `pub` since they are glob-imported.
 #[macro_use]
@@ -44,6 +45,7 @@ mod generic_arg;
 mod infer_ctxt;
 mod interner;
 mod opaque_ty;
+mod pattern;
 mod predicate;
 mod predicate_kind;
 mod region_kind;
@@ -67,6 +69,7 @@ pub use generic_arg::*;
 pub use infer_ctxt::*;
 pub use interner::*;
 pub use opaque_ty::*;
+pub use pattern::*;
 pub use predicate::*;
 pub use predicate_kind::*;
 pub use region_kind::*;
diff --git a/compiler/rustc_type_ir/src/pattern.rs b/compiler/rustc_type_ir/src/pattern.rs
new file mode 100644
index 00000000000..d74a82da1f9
--- /dev/null
+++ b/compiler/rustc_type_ir/src/pattern.rs
@@ -0,0 +1,16 @@
+use derive_where::derive_where;
+#[cfg(feature = "nightly")]
+use rustc_macros::{Decodable_NoContext, Encodable_NoContext, HashStable_NoContext};
+use rustc_type_ir_macros::{Lift_Generic, TypeFoldable_Generic, TypeVisitable_Generic};
+
+use crate::Interner;
+
+#[derive_where(Clone, Copy, Hash, PartialEq, Eq; I: Interner)]
+#[derive(TypeVisitable_Generic, TypeFoldable_Generic, Lift_Generic)]
+#[cfg_attr(
+    feature = "nightly",
+    derive(Decodable_NoContext, Encodable_NoContext, HashStable_NoContext)
+)]
+pub enum PatternKind<I: Interner> {
+    Range { start: I::Const, end: I::Const },
+}
diff --git a/compiler/rustc_type_ir/src/predicate.rs b/compiler/rustc_type_ir/src/predicate.rs
index 22d0fa23d0c..8e10636ff65 100644
--- a/compiler/rustc_type_ir/src/predicate.rs
+++ b/compiler/rustc_type_ir/src/predicate.rs
@@ -374,7 +374,7 @@ impl<I: Interner> ty::Binder<I, ExistentialTraitRef<I>> {
 }
 
 /// A `ProjectionPredicate` for an `ExistentialTraitRef`.
-#[derive_where(Clone, Copy, Hash, PartialEq, Eq; I: Interner)]
+#[derive_where(Clone, Copy, Hash, PartialEq, Eq, Debug; I: Interner)]
 #[derive(TypeVisitable_Generic, TypeFoldable_Generic, Lift_Generic)]
 #[cfg_attr(
     feature = "nightly",
diff --git a/compiler/rustc_middle/src/ty/walk.rs b/compiler/rustc_type_ir/src/walk.rs
index a23316ae6fc..5683e1f1712 100644
--- a/compiler/rustc_middle/src/ty/walk.rs
+++ b/compiler/rustc_type_ir/src/walk.rs
@@ -1,20 +1,21 @@
 //! An iterator over the type substructure.
 //! WARNING: this does not keep track of the region depth.
 
-use rustc_data_structures::sso::SsoHashSet;
 use smallvec::{SmallVec, smallvec};
 use tracing::debug;
 
-use crate::ty::{self, GenericArg, GenericArgKind, Ty};
+use crate::data_structures::SsoHashSet;
+use crate::inherent::*;
+use crate::{self as ty, Interner};
 
 // The TypeWalker's stack is hot enough that it's worth going to some effort to
 // avoid heap allocations.
-type TypeWalkerStack<'tcx> = SmallVec<[GenericArg<'tcx>; 8]>;
+type TypeWalkerStack<I> = SmallVec<[<I as Interner>::GenericArg; 8]>;
 
-pub struct TypeWalker<'tcx> {
-    stack: TypeWalkerStack<'tcx>,
+pub struct TypeWalker<I: Interner> {
+    stack: TypeWalkerStack<I>,
     last_subtree: usize,
-    pub visited: SsoHashSet<GenericArg<'tcx>>,
+    pub visited: SsoHashSet<I::GenericArg>,
 }
 
 /// An iterator for walking the type tree.
@@ -25,8 +26,8 @@ pub struct TypeWalker<'tcx> {
 /// in this situation walker only visits each type once.
 /// It maintains a set of visited types and
 /// skips any types that are already there.
-impl<'tcx> TypeWalker<'tcx> {
-    pub fn new(root: GenericArg<'tcx>) -> Self {
+impl<I: Interner> TypeWalker<I> {
+    pub fn new(root: I::GenericArg) -> Self {
         Self { stack: smallvec![root], last_subtree: 1, visited: SsoHashSet::new() }
     }
 
@@ -47,16 +48,16 @@ impl<'tcx> TypeWalker<'tcx> {
     }
 }
 
-impl<'tcx> Iterator for TypeWalker<'tcx> {
-    type Item = GenericArg<'tcx>;
+impl<I: Interner> Iterator for TypeWalker<I> {
+    type Item = I::GenericArg;
 
-    fn next(&mut self) -> Option<GenericArg<'tcx>> {
+    fn next(&mut self) -> Option<I::GenericArg> {
         debug!("next(): stack={:?}", self.stack);
         loop {
             let next = self.stack.pop()?;
             self.last_subtree = self.stack.len();
             if self.visited.insert(next) {
-                push_inner(&mut self.stack, next);
+                push_inner::<I>(&mut self.stack, next);
                 debug!("next: stack={:?}", self.stack);
                 return Some(next);
             }
@@ -64,63 +65,15 @@ impl<'tcx> Iterator for TypeWalker<'tcx> {
     }
 }
 
-impl<'tcx> GenericArg<'tcx> {
-    /// Iterator that walks `self` and any types reachable from
-    /// `self`, in depth-first order. Note that just walks the types
-    /// that appear in `self`, it does not descend into the fields of
-    /// structs or variants. For example:
-    ///
-    /// ```text
-    /// isize => { isize }
-    /// Foo<Bar<isize>> => { Foo<Bar<isize>>, Bar<isize>, isize }
-    /// [isize] => { [isize], isize }
-    /// ```
-    pub fn walk(self) -> TypeWalker<'tcx> {
-        TypeWalker::new(self)
-    }
-}
-
-impl<'tcx> Ty<'tcx> {
-    /// Iterator that walks `self` and any types reachable from
-    /// `self`, in depth-first order. Note that just walks the types
-    /// that appear in `self`, it does not descend into the fields of
-    /// structs or variants. For example:
-    ///
-    /// ```text
-    /// isize => { isize }
-    /// Foo<Bar<isize>> => { Foo<Bar<isize>>, Bar<isize>, isize }
-    /// [isize] => { [isize], isize }
-    /// ```
-    pub fn walk(self) -> TypeWalker<'tcx> {
-        TypeWalker::new(self.into())
-    }
-}
-
-impl<'tcx> ty::Const<'tcx> {
-    /// Iterator that walks `self` and any types reachable from
-    /// `self`, in depth-first order. Note that just walks the types
-    /// that appear in `self`, it does not descend into the fields of
-    /// structs or variants. For example:
-    ///
-    /// ```text
-    /// isize => { isize }
-    /// Foo<Bar<isize>> => { Foo<Bar<isize>>, Bar<isize>, isize }
-    /// [isize] => { [isize], isize }
-    /// ```
-    pub fn walk(self) -> TypeWalker<'tcx> {
-        TypeWalker::new(self.into())
-    }
-}
-
 /// We push `GenericArg`s on the stack in reverse order so as to
 /// maintain a pre-order traversal. As of the time of this
 /// writing, the fact that the traversal is pre-order is not
 /// known to be significant to any code, but it seems like the
 /// natural order one would expect (basically, the order of the
 /// types as they are written).
-fn push_inner<'tcx>(stack: &mut TypeWalkerStack<'tcx>, parent: GenericArg<'tcx>) {
-    match parent.unpack() {
-        GenericArgKind::Type(parent_ty) => match *parent_ty.kind() {
+fn push_inner<I: Interner>(stack: &mut TypeWalkerStack<I>, parent: I::GenericArg) {
+    match parent.kind() {
+        ty::GenericArgKind::Type(parent_ty) => match parent_ty.kind() {
             ty::Bool
             | ty::Char
             | ty::Int(_)
@@ -136,7 +89,7 @@ fn push_inner<'tcx>(stack: &mut TypeWalkerStack<'tcx>, parent: GenericArg<'tcx>)
             | ty::Foreign(..) => {}
 
             ty::Pat(ty, pat) => {
-                match *pat {
+                match pat.kind() {
                     ty::PatternKind::Range { start, end } => {
                         stack.push(end.into());
                         stack.push(start.into());
@@ -163,22 +116,25 @@ fn push_inner<'tcx>(stack: &mut TypeWalkerStack<'tcx>, parent: GenericArg<'tcx>)
             }
             ty::Dynamic(obj, lt, _) => {
                 stack.push(lt.into());
-                stack.extend(obj.iter().rev().flat_map(|predicate| {
-                    let (args, opt_ty) = match predicate.skip_binder() {
-                        ty::ExistentialPredicate::Trait(tr) => (tr.args, None),
-                        ty::ExistentialPredicate::Projection(p) => (p.args, Some(p.term)),
-                        ty::ExistentialPredicate::AutoTrait(_) =>
-                        // Empty iterator
-                        {
-                            (ty::GenericArgs::empty(), None)
-                        }
-                    };
-
-                    args.iter().rev().chain(opt_ty.map(|term| match term.unpack() {
-                        ty::TermKind::Ty(ty) => ty.into(),
-                        ty::TermKind::Const(ct) => ct.into(),
-                    }))
-                }));
+                stack.extend(
+                    obj.iter()
+                        .rev()
+                        .filter_map(|predicate| {
+                            let (args, opt_ty) = match predicate.skip_binder() {
+                                ty::ExistentialPredicate::Trait(tr) => (tr.args, None),
+                                ty::ExistentialPredicate::Projection(p) => (p.args, Some(p.term)),
+                                ty::ExistentialPredicate::AutoTrait(_) => {
+                                    return None;
+                                }
+                            };
+
+                            Some(args.iter().rev().chain(opt_ty.map(|term| match term.kind() {
+                                ty::TermKind::Ty(ty) => ty.into(),
+                                ty::TermKind::Const(ct) => ct.into(),
+                            })))
+                        })
+                        .flatten(),
+                );
             }
             ty::Adt(_, args)
             | ty::Closure(_, args)
@@ -188,7 +144,7 @@ fn push_inner<'tcx>(stack: &mut TypeWalkerStack<'tcx>, parent: GenericArg<'tcx>)
             | ty::FnDef(_, args) => {
                 stack.extend(args.iter().rev());
             }
-            ty::Tuple(ts) => stack.extend(ts.iter().rev().map(GenericArg::from)),
+            ty::Tuple(ts) => stack.extend(ts.iter().rev().map(|ty| ty.into())),
             ty::FnPtr(sig_tys, _hdr) => {
                 stack.extend(
                     sig_tys.skip_binder().inputs_and_output.iter().rev().map(|ty| ty.into()),
@@ -198,15 +154,15 @@ fn push_inner<'tcx>(stack: &mut TypeWalkerStack<'tcx>, parent: GenericArg<'tcx>)
                 stack.push(bound_ty.skip_binder().into());
             }
         },
-        GenericArgKind::Lifetime(_) => {}
-        GenericArgKind::Const(parent_ct) => match parent_ct.kind() {
+        ty::GenericArgKind::Lifetime(_) => {}
+        ty::GenericArgKind::Const(parent_ct) => match parent_ct.kind() {
             ty::ConstKind::Infer(_)
             | ty::ConstKind::Param(_)
             | ty::ConstKind::Placeholder(_)
             | ty::ConstKind::Bound(..)
             | ty::ConstKind::Error(_) => {}
 
-            ty::ConstKind::Value(cv) => stack.push(cv.ty.into()),
+            ty::ConstKind::Value(cv) => stack.push(cv.ty().into()),
 
             ty::ConstKind::Expr(expr) => stack.extend(expr.args().iter().rev()),
             ty::ConstKind::Unevaluated(ct) => {
diff --git a/compiler/rustc_type_ir_macros/src/lib.rs b/compiler/rustc_type_ir_macros/src/lib.rs
index 8eefecdc980..3a10d0d41ef 100644
--- a/compiler/rustc_type_ir_macros/src/lib.rs
+++ b/compiler/rustc_type_ir_macros/src/lib.rs
@@ -83,7 +83,7 @@ fn type_foldable_derive(mut s: synstructure::Structure<'_>) -> proc_macro2::Toke
     s.add_where_predicate(parse_quote! { I: Interner });
     s.add_bounds(synstructure::AddBounds::Fields);
     s.bind_with(|_| synstructure::BindStyle::Move);
-    let body_fold = s.each_variant(|vi| {
+    let body_try_fold = s.each_variant(|vi| {
         let bindings = vi.bindings();
         vi.construct(|_, index| {
             let bind = &bindings[index];
@@ -99,6 +99,22 @@ fn type_foldable_derive(mut s: synstructure::Structure<'_>) -> proc_macro2::Toke
         })
     });
 
+    let body_fold = s.each_variant(|vi| {
+        let bindings = vi.bindings();
+        vi.construct(|_, index| {
+            let bind = &bindings[index];
+
+            // retain value of fields with #[type_foldable(identity)]
+            if has_ignore_attr(&bind.ast().attrs, "type_foldable", "identity") {
+                bind.to_token_stream()
+            } else {
+                quote! {
+                    ::rustc_type_ir::TypeFoldable::fold_with(#bind, __folder)
+                }
+            }
+        })
+    });
+
     // We filter fields which get ignored and don't require them to implement
     // `TypeFoldable`. We do so after generating `body_fold` as we still need
     // to generate code for them.
@@ -111,7 +127,14 @@ fn type_foldable_derive(mut s: synstructure::Structure<'_>) -> proc_macro2::Toke
                 self,
                 __folder: &mut __F
             ) -> Result<Self, __F::Error> {
-                Ok(match self { #body_fold })
+                Ok(match self { #body_try_fold })
+            }
+
+            fn fold_with<__F: ::rustc_type_ir::TypeFolder<I>>(
+                self,
+                __folder: &mut __F
+            ) -> Self {
+                match self { #body_fold }
             }
         },
     )