Merge ref 'caccb4d0368b' from rust-lang/rust

Pull recent changes from https://github.com/rust-lang/rust via Josh.

Upstream ref: caccb4d0368bd918ef6668af8e13834d07040417
Filtered ref: 0f345ed05d559bbfb754f1403b16199366cda2e0
Upstream diff: https://github.com/rust-lang/rust/compare/21a19c297d4f5a03501d92ca251bd7a17073c08a...caccb4d0368bd918ef6668af8e13834d07040417

This merge was created using https://github.com/rust-lang/josh-sync.

13 files changed, 519 insertions, 257 deletions

diff --git a/compiler/rustc_parse/src/parser/attr.rs b/compiler/rustc_parse/src/parser/attr.rs
index 41d3889c448..acd338156ce 100644
--- a/compiler/rustc_parse/src/parser/attr.rs
+++ b/compiler/rustc_parse/src/parser/attr.rs
@@ -11,6 +11,7 @@ use tracing::debug;
 use super::{
     AttrWrapper, Capturing, FnParseMode, ForceCollect, Parser, PathStyle, Trailing, UsePreAttrPos,
 };
+use crate::parser::FnContext;
 use crate::{errors, exp, fluent_generated as fluent};
 
 // Public for rustfmt usage
@@ -200,7 +201,7 @@ impl<'a> Parser<'a> {
             AttrWrapper::empty(),
             true,
             false,
-            FnParseMode { req_name: |_| true, req_body: true },
+            FnParseMode { req_name: |_| true, context: FnContext::Free, req_body: true },
             ForceCollect::No,
         ) {
             Ok(Some(item)) => {
@@ -398,7 +399,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Matches `COMMASEP(meta_item_inner)`.
-    pub(crate) fn parse_meta_seq_top(&mut self) -> PResult<'a, ThinVec<ast::MetaItemInner>> {
+    pub fn parse_meta_seq_top(&mut self) -> PResult<'a, ThinVec<ast::MetaItemInner>> {
         // Presumably, the majority of the time there will only be one attr.
         let mut nmis = ThinVec::with_capacity(1);
         while self.token != token::Eof {
diff --git a/compiler/rustc_parse/src/parser/cfg_select.rs b/compiler/rustc_parse/src/parser/cfg_select.rs
index 2c6fb224d70..08a71db4de8 100644
--- a/compiler/rustc_parse/src/parser/cfg_select.rs
+++ b/compiler/rustc_parse/src/parser/cfg_select.rs
@@ -1,11 +1,12 @@
 use rustc_ast::token::Token;
 use rustc_ast::tokenstream::{TokenStream, TokenTree};
+use rustc_ast::util::classify;
 use rustc_ast::{MetaItemInner, token};
 use rustc_errors::PResult;
 use rustc_span::Span;
 
 use crate::exp;
-use crate::parser::Parser;
+use crate::parser::{AttrWrapper, ForceCollect, Parser, Restrictions, Trailing, UsePreAttrPos};
 
 pub enum CfgSelectPredicate {
     Cfg(MetaItemInner),
@@ -23,19 +24,26 @@ pub struct CfgSelectBranches {
     pub unreachable: Vec<(CfgSelectPredicate, TokenStream, Span)>,
 }
 
-/// Parses a `TokenTree` that must be of the form `{ /* ... */ }`, and returns a `TokenStream` where
-/// the surrounding braces are stripped.
+/// Parses a `TokenTree` consisting either of `{ /* ... */ }` (and strip the braces) or an
+/// expression followed by a comma (and strip the comma).
 fn parse_token_tree<'a>(p: &mut Parser<'a>) -> PResult<'a, TokenStream> {
-    // Generate an error if the `=>` is not followed by `{`.
-    if p.token != token::OpenBrace {
-        p.expect(exp!(OpenBrace))?;
+    if p.token == token::OpenBrace {
+        // Strip the outer '{' and '}'.
+        match p.parse_token_tree() {
+            TokenTree::Token(..) => unreachable!("because of the expect above"),
+            TokenTree::Delimited(.., tts) => return Ok(tts),
+        }
     }
-
-    // Strip the outer '{' and '}'.
-    match p.parse_token_tree() {
-        TokenTree::Token(..) => unreachable!("because of the expect above"),
-        TokenTree::Delimited(.., tts) => Ok(tts),
+    let expr = p.collect_tokens(None, AttrWrapper::empty(), ForceCollect::Yes, |p, _| {
+        p.parse_expr_res(Restrictions::STMT_EXPR, AttrWrapper::empty())
+            .map(|(expr, _)| (expr, Trailing::No, UsePreAttrPos::No))
+    })?;
+    if !classify::expr_is_complete(&expr) && p.token != token::CloseBrace && p.token != token::Eof {
+        p.expect(exp!(Comma))?;
+    } else {
+        let _ = p.eat(exp!(Comma));
     }
+    Ok(TokenStream::from_ast(&expr))
 }
 
 pub fn parse_cfg_select<'a>(p: &mut Parser<'a>) -> PResult<'a, CfgSelectBranches> {
diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index a32cd33a260..a28af7833c3 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/compiler/rustc_parse/src/parser/diagnostics.rs
@@ -44,6 +44,7 @@ use crate::errors::{
     UnexpectedConstParamDeclaration, UnexpectedConstParamDeclarationSugg, UnmatchedAngleBrackets,
     UseEqInstead, WrapType,
 };
+use crate::parser::FnContext;
 use crate::parser::attr::InnerAttrPolicy;
 use crate::{exp, fluent_generated as fluent};
 
@@ -462,8 +463,8 @@ impl<'a> Parser<'a> {
 
     pub(super) fn expected_one_of_not_found(
         &mut self,
-        edible: &[ExpTokenPair<'_>],
-        inedible: &[ExpTokenPair<'_>],
+        edible: &[ExpTokenPair],
+        inedible: &[ExpTokenPair],
     ) -> PResult<'a, ErrorGuaranteed> {
         debug!("expected_one_of_not_found(edible: {:?}, inedible: {:?})", edible, inedible);
         fn tokens_to_string(tokens: &[TokenType]) -> String {
@@ -1091,7 +1092,7 @@ impl<'a> Parser<'a> {
 
     /// Eats and discards tokens until one of `closes` is encountered. Respects token trees,
     /// passes through any errors encountered. Used for error recovery.
-    pub(super) fn eat_to_tokens(&mut self, closes: &[ExpTokenPair<'_>]) {
+    pub(super) fn eat_to_tokens(&mut self, closes: &[ExpTokenPair]) {
         if let Err(err) = self
             .parse_seq_to_before_tokens(closes, &[], SeqSep::none(), |p| Ok(p.parse_token_tree()))
         {
@@ -1112,7 +1113,7 @@ impl<'a> Parser<'a> {
     pub(super) fn check_trailing_angle_brackets(
         &mut self,
         segment: &PathSegment,
-        end: &[ExpTokenPair<'_>],
+        end: &[ExpTokenPair],
     ) -> Option<ErrorGuaranteed> {
         if !self.may_recover() {
             return None;
@@ -1195,7 +1196,7 @@ impl<'a> Parser<'a> {
         // second case.
         if self.look_ahead(position, |t| {
             trace!("check_trailing_angle_brackets: t={:?}", t);
-            end.iter().any(|exp| exp.tok == &t.kind)
+            end.iter().any(|exp| exp.tok == t.kind)
         }) {
             // Eat from where we started until the end token so that parsing can continue
             // as if we didn't have those extra angle brackets.
@@ -2119,8 +2120,8 @@ impl<'a> Parser<'a> {
 
     pub(super) fn recover_seq_parse_error(
         &mut self,
-        open: ExpTokenPair<'_>,
-        close: ExpTokenPair<'_>,
+        open: ExpTokenPair,
+        close: ExpTokenPair,
         lo: Span,
         err: Diag<'a>,
     ) -> Box<Expr> {
@@ -2246,6 +2247,7 @@ impl<'a> Parser<'a> {
         pat: Box<ast::Pat>,
         require_name: bool,
         first_param: bool,
+        fn_parse_mode: &crate::parser::item::FnParseMode,
     ) -> Option<Ident> {
         // If we find a pattern followed by an identifier, it could be an (incorrect)
         // C-style parameter declaration.
@@ -2268,7 +2270,14 @@ impl<'a> Parser<'a> {
                 || self.token == token::Lt
                 || self.token == token::CloseParen)
         {
-            let rfc_note = "anonymous parameters are removed in the 2018 edition (see RFC 1685)";
+            let maybe_emit_anon_params_note = |this: &mut Self, err: &mut Diag<'_>| {
+                let ed = this.token.span.with_neighbor(this.prev_token.span).edition();
+                if matches!(fn_parse_mode.context, crate::parser::item::FnContext::Trait)
+                    && (fn_parse_mode.req_name)(ed)
+                {
+                    err.note("anonymous parameters are removed in the 2018 edition (see RFC 1685)");
+                }
+            };
 
             let (ident, self_sugg, param_sugg, type_sugg, self_span, param_span, type_span) =
                 match pat.kind {
@@ -2305,7 +2314,7 @@ impl<'a> Parser<'a> {
                                 "_: ".to_string(),
                                 Applicability::MachineApplicable,
                             );
-                            err.note(rfc_note);
+                            maybe_emit_anon_params_note(self, err);
                         }
 
                         return None;
@@ -2313,7 +2322,13 @@ impl<'a> Parser<'a> {
                 };
 
             // `fn foo(a, b) {}`, `fn foo(a<x>, b<y>) {}` or `fn foo(usize, usize) {}`
-            if first_param {
+            if first_param
+                // Only when the fn is a method, we emit this suggestion.
+                && matches!(
+                    fn_parse_mode.context,
+                    FnContext::Trait | FnContext::Impl
+                )
+            {
                 err.span_suggestion_verbose(
                     self_span,
                     "if this is a `self` type, give it a parameter name",
@@ -2337,7 +2352,7 @@ impl<'a> Parser<'a> {
                 type_sugg,
                 Applicability::MachineApplicable,
             );
-            err.note(rfc_note);
+            maybe_emit_anon_params_note(self, err);
 
             // Don't attempt to recover by using the `X` in `X<Y>` as the parameter name.
             return if self.token == token::Lt { None } else { Some(ident) };
@@ -2371,8 +2386,8 @@ impl<'a> Parser<'a> {
 
     pub(super) fn consume_block(
         &mut self,
-        open: ExpTokenPair<'_>,
-        close: ExpTokenPair<'_>,
+        open: ExpTokenPair,
+        close: ExpTokenPair,
         consume_close: ConsumeClosingDelim,
     ) {
         let mut brace_depth = 0;
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index d0604f76317..81a5d48d94e 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -1163,7 +1163,10 @@ impl<'a> Parser<'a> {
                         suffix,
                     }) => {
                         if let Some(suffix) = suffix {
-                            self.expect_no_tuple_index_suffix(current.span, suffix);
+                            self.dcx().emit_err(errors::InvalidLiteralSuffixOnTupleIndex {
+                                span: current.span,
+                                suffix,
+                            });
                         }
                         match self.break_up_float(symbol, current.span) {
                             // 1e2
@@ -1239,7 +1242,8 @@ impl<'a> Parser<'a> {
         suffix: Option<Symbol>,
     ) -> Box<Expr> {
         if let Some(suffix) = suffix {
-            self.expect_no_tuple_index_suffix(ident_span, suffix);
+            self.dcx()
+                .emit_err(errors::InvalidLiteralSuffixOnTupleIndex { span: ident_span, suffix });
         }
         self.mk_expr(lo.to(ident_span), ExprKind::Field(base, Ident::new(field, ident_span)))
     }
@@ -1598,7 +1602,7 @@ impl<'a> Parser<'a> {
         self.maybe_recover_from_bad_qpath(expr)
     }
 
-    fn parse_expr_array_or_repeat(&mut self, close: ExpTokenPair<'_>) -> PResult<'a, Box<Expr>> {
+    fn parse_expr_array_or_repeat(&mut self, close: ExpTokenPair) -> PResult<'a, Box<Expr>> {
         let lo = self.token.span;
         self.bump(); // `[` or other open delim
 
@@ -2156,7 +2160,7 @@ impl<'a> Parser<'a> {
 
     /// Keep this in sync with `Token::can_begin_literal_maybe_minus` and
     /// `Lit::from_token` (excluding unary negation).
-    fn eat_token_lit(&mut self) -> Option<token::Lit> {
+    pub fn eat_token_lit(&mut self) -> Option<token::Lit> {
         let check_expr = |expr: Box<Expr>| {
             if let ast::ExprKind::Lit(token_lit) = expr.kind {
                 Some(token_lit)
@@ -2225,24 +2229,6 @@ impl<'a> Parser<'a> {
         })
     }
 
-    pub(super) fn expect_no_tuple_index_suffix(&self, span: Span, suffix: Symbol) {
-        if [sym::i32, sym::u32, sym::isize, sym::usize].contains(&suffix) {
-            // #59553: warn instead of reject out of hand to allow the fix to percolate
-            // through the ecosystem when people fix their macros
-            self.dcx().emit_warn(errors::InvalidLiteralSuffixOnTupleIndex {
-                span,
-                suffix,
-                exception: true,
-            });
-        } else {
-            self.dcx().emit_err(errors::InvalidLiteralSuffixOnTupleIndex {
-                span,
-                suffix,
-                exception: false,
-            });
-        }
-    }
-
     /// Matches `'-' lit | lit` (cf. `ast_validation::AstValidator::check_expr_within_pat`).
     /// Keep this in sync with `Token::can_begin_literal_maybe_minus`.
     pub fn parse_literal_maybe_minus(&mut self) -> PResult<'a, Box<Expr>> {
@@ -2397,20 +2383,24 @@ impl<'a> Parser<'a> {
         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()?;
+            let (bound_vars, _) = self.parse_higher_ranked_binder()?;
             let span = lo.to(self.prev_token.span);
 
             self.psess.gated_spans.gate(sym::closure_lifetime_binder, span);
 
-            ClosureBinder::For { span, generic_params: lifetime_defs }
+            ClosureBinder::For { span, generic_params: bound_vars }
         } else {
             ClosureBinder::NotPresent
         };
 
         let constness = self.parse_closure_constness();
 
-        let movability =
-            if self.eat_keyword(exp!(Static)) { Movability::Static } else { Movability::Movable };
+        let movability = if self.eat_keyword(exp!(Static)) {
+            self.psess.gated_spans.gate(sym::coroutines, self.prev_token.span);
+            Movability::Static
+        } else {
+            Movability::Movable
+        };
 
         let coroutine_kind = if self.token_uninterpolated_span().at_least_rust_2018() {
             self.parse_coroutine_kind(Case::Sensitive)
@@ -2738,7 +2728,7 @@ impl<'a> Parser<'a> {
     /// The specified `edition` in `let_chains_policy` should be that of the whole `if` construct,
     /// i.e. the same span we use to later decide whether the drop behaviour should be that of
     /// edition `..=2021` or that of `2024..`.
-    // Public because it is used in rustfmt forks such as https://github.com/tucant/rustfmt/blob/30c83df9e1db10007bdd16dafce8a86b404329b2/src/parse/macros/html.rs#L57 for custom if expressions.
+    // Public to use it for custom `if` expressions in rustfmt forks like https://github.com/tucant/rustfmt
     pub fn parse_expr_cond(
         &mut self,
         let_chains_policy: LetChainsPolicy,
@@ -2906,7 +2896,8 @@ impl<'a> Parser<'a> {
         }
     }
 
-    fn parse_for_head(&mut self) -> PResult<'a, (Box<Pat>, Box<Expr>)> {
+    // Public to use it for custom `for` expressions in rustfmt forks like https://github.com/tucant/rustfmt
+    pub fn parse_for_head(&mut self) -> PResult<'a, (Box<Pat>, Box<Expr>)> {
         let begin_paren = if self.token == token::OpenParen {
             // Record whether we are about to parse `for (`.
             // This is used below for recovery in case of `for ( $stuff ) $block`
@@ -3090,7 +3081,7 @@ impl<'a> Parser<'a> {
         if let Some((ident, is_raw)) = self.token.lifetime() {
             // Disallow `'fn`, but with a better error message than `expect_lifetime`.
             if matches!(is_raw, IdentIsRaw::No) && ident.without_first_quote().is_reserved() {
-                self.dcx().emit_err(errors::InvalidLabel { span: ident.span, name: ident.name });
+                self.dcx().emit_err(errors::KeywordLabel { span: ident.span });
             }
 
             self.bump();
@@ -3657,7 +3648,7 @@ impl<'a> Parser<'a> {
         &mut self,
         pth: ast::Path,
         recover: bool,
-        close: ExpTokenPair<'_>,
+        close: ExpTokenPair,
     ) -> PResult<
         'a,
         (
@@ -3676,8 +3667,8 @@ impl<'a> Parser<'a> {
             errors::HelpUseLatestEdition::new().add_to_diag(e);
         };
 
-        while self.token != *close.tok {
-            if self.eat(exp!(DotDot)) || self.recover_struct_field_dots(close.tok) {
+        while self.token != close.tok {
+            if self.eat(exp!(DotDot)) || self.recover_struct_field_dots(&close.tok) {
                 let exp_span = self.prev_token.span;
                 // We permit `.. }` on the left-hand side of a destructuring assignment.
                 if self.check(close) {
diff --git a/compiler/rustc_parse/src/parser/generics.rs b/compiler/rustc_parse/src/parser/generics.rs
index 4a8530a2b38..eb684c3a62f 100644
--- a/compiler/rustc_parse/src/parser/generics.rs
+++ b/compiler/rustc_parse/src/parser/generics.rs
@@ -172,8 +172,11 @@ impl<'a> Parser<'a> {
         })
     }
 
-    /// Parses a (possibly empty) list of lifetime and type parameters, possibly including
-    /// a trailing comma and erroneous trailing attributes.
+    /// Parse a (possibly empty) list of generic (lifetime, type, const) parameters.
+    ///
+    /// ```ebnf
+    /// GenericParams = (GenericParam ("," GenericParam)* ","?)?
+    /// ```
     pub(super) fn parse_generic_params(&mut self) -> PResult<'a, ThinVec<ast::GenericParam>> {
         let mut params = ThinVec::new();
         let mut done = false;
@@ -520,7 +523,7 @@ impl<'a> Parser<'a> {
         // * `for<'a> Trait1<'a>: Trait2<'a /* ok */>`
         // * `(for<'a> Trait1<'a>): Trait2<'a /* not ok */>`
         // * `for<'a> for<'b> Trait1<'a, 'b>: Trait2<'a /* ok */, 'b /* not ok */>`
-        let (lifetime_defs, _) = self.parse_late_bound_lifetime_defs()?;
+        let (bound_vars, _) = self.parse_higher_ranked_binder()?;
 
         // Parse type with mandatory colon and (possibly empty) bounds,
         // or with mandatory equality sign and the second type.
@@ -528,7 +531,7 @@ impl<'a> Parser<'a> {
         if self.eat(exp!(Colon)) {
             let bounds = self.parse_generic_bounds()?;
             Ok(ast::WherePredicateKind::BoundPredicate(ast::WhereBoundPredicate {
-                bound_generic_params: lifetime_defs,
+                bound_generic_params: bound_vars,
                 bounded_ty: ty,
                 bounds,
             }))
diff --git a/compiler/rustc_parse/src/parser/item.rs b/compiler/rustc_parse/src/parser/item.rs
index 14a90e74049..eb264f59fed 100644
--- a/compiler/rustc_parse/src/parser/item.rs
+++ b/compiler/rustc_parse/src/parser/item.rs
@@ -43,7 +43,7 @@ impl<'a> Parser<'a> {
             self.expect(exp!(OpenBrace))?;
             let (inner_attrs, items, inner_span) = self.parse_mod(exp!(CloseBrace))?;
             attrs.extend(inner_attrs);
-            ModKind::Loaded(items, Inline::Yes, inner_span, Ok(()))
+            ModKind::Loaded(items, Inline::Yes, inner_span)
         };
         Ok(ItemKind::Mod(safety, ident, mod_kind))
     }
@@ -54,7 +54,7 @@ impl<'a> Parser<'a> {
     /// - `}` for mod items
     pub fn parse_mod(
         &mut self,
-        term: ExpTokenPair<'_>,
+        term: ExpTokenPair,
     ) -> PResult<'a, (AttrVec, ThinVec<Box<Item>>, ModSpans)> {
         let lo = self.token.span;
         let attrs = self.parse_inner_attributes()?;
@@ -116,7 +116,8 @@ impl<'a> Parser<'a> {
 
 impl<'a> Parser<'a> {
     pub fn parse_item(&mut self, force_collect: ForceCollect) -> PResult<'a, Option<Box<Item>>> {
-        let fn_parse_mode = FnParseMode { req_name: |_| true, req_body: true };
+        let fn_parse_mode =
+            FnParseMode { req_name: |_| true, context: FnContext::Free, req_body: true };
         self.parse_item_(fn_parse_mode, force_collect).map(|i| i.map(Box::new))
     }
 
@@ -663,20 +664,44 @@ impl<'a> Parser<'a> {
                 };
                 let trait_ref = TraitRef { path, ref_id: ty_first.id };
 
-                (Some(trait_ref), ty_second)
+                let of_trait = Some(Box::new(TraitImplHeader {
+                    defaultness,
+                    safety,
+                    constness,
+                    polarity,
+                    trait_ref,
+                }));
+                (of_trait, ty_second)
+            }
+            None => {
+                let self_ty = ty_first;
+                let error = |modifier, modifier_name, modifier_span| {
+                    self.dcx().create_err(errors::TraitImplModifierInInherentImpl {
+                        span: self_ty.span,
+                        modifier,
+                        modifier_name,
+                        modifier_span,
+                        self_ty: self_ty.span,
+                    })
+                };
+
+                if let Safety::Unsafe(span) = safety {
+                    error("unsafe", "unsafe", span).with_code(E0197).emit();
+                }
+                if let ImplPolarity::Negative(span) = polarity {
+                    error("!", "negative", span).emit();
+                }
+                if let Defaultness::Default(def_span) = defaultness {
+                    error("default", "default", def_span).emit();
+                }
+                if let Const::Yes(span) = constness {
+                    error("const", "const", span).emit();
+                }
+                (None, self_ty)
             }
-            None => (None, ty_first), // impl Type
         };
-        Ok(ItemKind::Impl(Box::new(Impl {
-            safety,
-            polarity,
-            defaultness,
-            constness,
-            generics,
-            of_trait,
-            self_ty,
-            items: impl_items,
-        })))
+
+        Ok(ItemKind::Impl(Impl { generics, of_trait, self_ty, items: impl_items }))
     }
 
     fn parse_item_delegation(&mut self) -> PResult<'a, ItemKind> {
@@ -951,7 +976,8 @@ impl<'a> Parser<'a> {
         &mut self,
         force_collect: ForceCollect,
     ) -> PResult<'a, Option<Option<Box<AssocItem>>>> {
-        let fn_parse_mode = FnParseMode { req_name: |_| true, req_body: true };
+        let fn_parse_mode =
+            FnParseMode { req_name: |_| true, context: FnContext::Impl, req_body: true };
         self.parse_assoc_item(fn_parse_mode, force_collect)
     }
 
@@ -959,8 +985,11 @@ impl<'a> Parser<'a> {
         &mut self,
         force_collect: ForceCollect,
     ) -> PResult<'a, Option<Option<Box<AssocItem>>>> {
-        let fn_parse_mode =
-            FnParseMode { req_name: |edition| edition >= Edition::Edition2018, req_body: false };
+        let fn_parse_mode = FnParseMode {
+            req_name: |edition| edition >= Edition::Edition2018,
+            context: FnContext::Trait,
+            req_body: false,
+        };
         self.parse_assoc_item(fn_parse_mode, force_collect)
     }
 
@@ -1172,7 +1201,7 @@ impl<'a> Parser<'a> {
         }?;
 
         let dash = exp!(Minus);
-        if self.token != *dash.tok {
+        if self.token != dash.tok {
             return Ok(ident);
         }
 
@@ -1237,7 +1266,8 @@ impl<'a> Parser<'a> {
         &mut self,
         force_collect: ForceCollect,
     ) -> PResult<'a, Option<Option<Box<ForeignItem>>>> {
-        let fn_parse_mode = FnParseMode { req_name: |_| true, req_body: false };
+        let fn_parse_mode =
+            FnParseMode { req_name: |_| true, context: FnContext::Free, req_body: false };
         Ok(self.parse_item_(fn_parse_mode, force_collect)?.map(
             |Item { attrs, id, span, vis, kind, tokens }| {
                 let kind = match ForeignItemKind::try_from(kind) {
@@ -1364,10 +1394,10 @@ impl<'a> Parser<'a> {
         };
 
         match &mut item_kind {
-            ItemKind::Impl(box Impl { of_trait: Some(trai), constness, .. }) => {
-                *constness = Const::Yes(const_span);
+            ItemKind::Impl(Impl { of_trait: Some(of_trait), .. }) => {
+                of_trait.constness = Const::Yes(const_span);
 
-                let before_trait = trai.path.span.shrink_to_lo();
+                let before_trait = of_trait.trait_ref.path.span.shrink_to_lo();
                 let const_up_to_impl = const_span.with_hi(impl_span.lo());
                 err.with_multipart_suggestion(
                     "you might have meant to write a const trait impl",
@@ -2111,7 +2141,8 @@ impl<'a> Parser<'a> {
                 let inherited_vis =
                     Visibility { span: DUMMY_SP, kind: VisibilityKind::Inherited, tokens: None };
                 // We use `parse_fn` to get a span for the function
-                let fn_parse_mode = FnParseMode { req_name: |_| true, req_body: true };
+                let fn_parse_mode =
+                    FnParseMode { req_name: |_| true, context: FnContext::Free, req_body: true };
                 match self.parse_fn(
                     &mut AttrVec::new(),
                     fn_parse_mode,
@@ -2379,6 +2410,9 @@ pub(crate) struct FnParseMode {
     ///   * The span is from Edition 2015. In particular, you can get a
     ///     2015 span inside a 2021 crate using macros.
     pub(super) req_name: ReqName,
+    /// The context in which this function is parsed, used for diagnostics.
+    /// This indicates the fn is a free function or method and so on.
+    pub(super) context: FnContext,
     /// If this flag is set to `true`, then plain, semicolon-terminated function
     /// prototypes are not allowed here.
     ///
@@ -2400,6 +2434,18 @@ pub(crate) struct FnParseMode {
     pub(super) req_body: bool,
 }
 
+/// The context in which a function is parsed.
+/// FIXME(estebank, xizheyin): Use more variants.
+#[derive(Clone, Copy, PartialEq, Eq)]
+pub(crate) enum FnContext {
+    /// Free context.
+    Free,
+    /// A Trait context.
+    Trait,
+    /// An Impl block.
+    Impl,
+}
+
 /// Parsing of functions and methods.
 impl<'a> Parser<'a> {
     /// Parse a function starting from the front matter (`const ...`) to the body `{ ... }` or `;`.
@@ -2415,11 +2461,8 @@ impl<'a> Parser<'a> {
         let header = self.parse_fn_front_matter(vis, case, FrontMatterParsingMode::Function)?; // `const ... fn`
         let ident = self.parse_ident()?; // `foo`
         let mut generics = self.parse_generics()?; // `<'a, T, ...>`
-        let decl = match self.parse_fn_decl(
-            fn_parse_mode.req_name,
-            AllowPlus::Yes,
-            RecoverReturnSign::Yes,
-        ) {
+        let decl = match self.parse_fn_decl(&fn_parse_mode, AllowPlus::Yes, RecoverReturnSign::Yes)
+        {
             Ok(decl) => decl,
             Err(old_err) => {
                 // If we see `for Ty ...` then user probably meant `impl` item.
@@ -2937,18 +2980,21 @@ impl<'a> Parser<'a> {
     /// Parses the parameter list and result type of a function declaration.
     pub(super) fn parse_fn_decl(
         &mut self,
-        req_name: ReqName,
+        fn_parse_mode: &FnParseMode,
         ret_allow_plus: AllowPlus,
         recover_return_sign: RecoverReturnSign,
     ) -> PResult<'a, Box<FnDecl>> {
         Ok(Box::new(FnDecl {
-            inputs: self.parse_fn_params(req_name)?,
+            inputs: self.parse_fn_params(fn_parse_mode)?,
             output: self.parse_ret_ty(ret_allow_plus, RecoverQPath::Yes, recover_return_sign)?,
         }))
     }
 
     /// Parses the parameter list of a function, including the `(` and `)` delimiters.
-    pub(super) fn parse_fn_params(&mut self, req_name: ReqName) -> PResult<'a, ThinVec<Param>> {
+    pub(super) fn parse_fn_params(
+        &mut self,
+        fn_parse_mode: &FnParseMode,
+    ) -> PResult<'a, ThinVec<Param>> {
         let mut first_param = true;
         // Parse the arguments, starting out with `self` being allowed...
         if self.token != TokenKind::OpenParen
@@ -2964,7 +3010,7 @@ impl<'a> Parser<'a> {
         let (mut params, _) = self.parse_paren_comma_seq(|p| {
             p.recover_vcs_conflict_marker();
             let snapshot = p.create_snapshot_for_diagnostic();
-            let param = p.parse_param_general(req_name, first_param, true).or_else(|e| {
+            let param = p.parse_param_general(fn_parse_mode, first_param, true).or_else(|e| {
                 let guar = e.emit();
                 // When parsing a param failed, we should check to make the span of the param
                 // not contain '(' before it.
@@ -2995,7 +3041,7 @@ impl<'a> Parser<'a> {
     /// - `recover_arg_parse` is used to recover from a failed argument parse.
     pub(super) fn parse_param_general(
         &mut self,
-        req_name: ReqName,
+        fn_parse_mode: &FnParseMode,
         first_param: bool,
         recover_arg_parse: bool,
     ) -> PResult<'a, Param> {
@@ -3011,16 +3057,22 @@ impl<'a> Parser<'a> {
 
             let is_name_required = match this.token.kind {
                 token::DotDotDot => false,
-                _ => req_name(this.token.span.with_neighbor(this.prev_token.span).edition()),
+                _ => (fn_parse_mode.req_name)(
+                    this.token.span.with_neighbor(this.prev_token.span).edition(),
+                ),
             };
             let (pat, ty) = if is_name_required || this.is_named_param() {
                 debug!("parse_param_general parse_pat (is_name_required:{})", is_name_required);
                 let (pat, colon) = this.parse_fn_param_pat_colon()?;
                 if !colon {
                     let mut err = this.unexpected().unwrap_err();
-                    return if let Some(ident) =
-                        this.parameter_without_type(&mut err, pat, is_name_required, first_param)
-                    {
+                    return if let Some(ident) = this.parameter_without_type(
+                        &mut err,
+                        pat,
+                        is_name_required,
+                        first_param,
+                        fn_parse_mode,
+                    ) {
                         let guar = err.emit();
                         Ok((dummy_arg(ident, guar), Trailing::No, UsePreAttrPos::No))
                     } else {
diff --git a/compiler/rustc_parse/src/parser/mod.rs b/compiler/rustc_parse/src/parser/mod.rs
index 0a8a0203013..ed4069dae93 100644
--- a/compiler/rustc_parse/src/parser/mod.rs
+++ b/compiler/rustc_parse/src/parser/mod.rs
@@ -22,9 +22,11 @@ use std::{fmt, mem, slice};
 use attr_wrapper::{AttrWrapper, UsePreAttrPos};
 pub use diagnostics::AttemptLocalParseRecovery;
 pub(crate) use expr::ForbiddenLetReason;
-pub(crate) use item::FnParseMode;
+// Public to use it for custom `if` expressions in rustfmt forks like https://github.com/tucant/rustfmt
+pub use expr::LetChainsPolicy;
+pub(crate) use item::{FnContext, FnParseMode};
 pub use pat::{CommaRecoveryMode, RecoverColon, RecoverComma};
-use path::PathStyle;
+pub use path::PathStyle;
 use rustc_ast::token::{
     self, IdentIsRaw, InvisibleOrigin, MetaVarKind, NtExprKind, NtPatKind, Token, TokenKind,
 };
@@ -261,19 +263,19 @@ struct CaptureState {
 
 /// A sequence separator.
 #[derive(Debug)]
-struct SeqSep<'a> {
+struct SeqSep {
     /// The separator token.
-    sep: Option<ExpTokenPair<'a>>,
+    sep: Option<ExpTokenPair>,
     /// `true` if a trailing separator is allowed.
     trailing_sep_allowed: bool,
 }
 
-impl<'a> SeqSep<'a> {
-    fn trailing_allowed(sep: ExpTokenPair<'a>) -> SeqSep<'a> {
+impl SeqSep {
+    fn trailing_allowed(sep: ExpTokenPair) -> SeqSep {
         SeqSep { sep: Some(sep), trailing_sep_allowed: true }
     }
 
-    fn none() -> SeqSep<'a> {
+    fn none() -> SeqSep {
         SeqSep { sep: None, trailing_sep_allowed: false }
     }
 }
@@ -285,7 +287,7 @@ pub enum FollowedByType {
 }
 
 #[derive(Copy, Clone, Debug)]
-enum Trailing {
+pub enum Trailing {
     No,
     Yes,
 }
@@ -425,13 +427,13 @@ impl<'a> Parser<'a> {
     }
 
     /// Expects and consumes the token `t`. Signals an error if the next token is not `t`.
-    pub fn expect(&mut self, exp: ExpTokenPair<'_>) -> PResult<'a, Recovered> {
+    pub fn expect(&mut self, exp: ExpTokenPair) -> PResult<'a, Recovered> {
         if self.expected_token_types.is_empty() {
-            if self.token == *exp.tok {
+            if self.token == exp.tok {
                 self.bump();
                 Ok(Recovered::No)
             } else {
-                self.unexpected_try_recover(exp.tok)
+                self.unexpected_try_recover(&exp.tok)
             }
         } else {
             self.expect_one_of(slice::from_ref(&exp), &[])
@@ -443,13 +445,13 @@ impl<'a> Parser<'a> {
     /// anything. Signal a fatal error if next token is unexpected.
     fn expect_one_of(
         &mut self,
-        edible: &[ExpTokenPair<'_>],
-        inedible: &[ExpTokenPair<'_>],
+        edible: &[ExpTokenPair],
+        inedible: &[ExpTokenPair],
     ) -> PResult<'a, Recovered> {
-        if edible.iter().any(|exp| exp.tok == &self.token.kind) {
+        if edible.iter().any(|exp| exp.tok == self.token.kind) {
             self.bump();
             Ok(Recovered::No)
-        } else if inedible.iter().any(|exp| exp.tok == &self.token.kind) {
+        } else if inedible.iter().any(|exp| exp.tok == self.token.kind) {
             // leave it in the input
             Ok(Recovered::No)
         } else if self.token != token::Eof
@@ -494,8 +496,8 @@ impl<'a> Parser<'a> {
     /// This method will automatically add `tok` to `expected_token_types` if `tok` is not
     /// encountered.
     #[inline]
-    fn check(&mut self, exp: ExpTokenPair<'_>) -> bool {
-        let is_present = self.token == *exp.tok;
+    pub fn check(&mut self, exp: ExpTokenPair) -> bool {
+        let is_present = self.token == exp.tok;
         if !is_present {
             self.expected_token_types.insert(exp.token_type);
         }
@@ -542,7 +544,7 @@ impl<'a> Parser<'a> {
     /// Consumes a token 'tok' if it exists. Returns whether the given token was present.
     #[inline]
     #[must_use]
-    pub fn eat(&mut self, exp: ExpTokenPair<'_>) -> bool {
+    pub fn eat(&mut self, exp: ExpTokenPair) -> bool {
         let is_present = self.check(exp);
         if is_present {
             self.bump()
@@ -633,7 +635,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Consume a sequence produced by a metavar expansion, if present.
-    fn eat_metavar_seq<T>(
+    pub fn eat_metavar_seq<T>(
         &mut self,
         mv_kind: MetaVarKind,
         f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
@@ -745,13 +747,13 @@ impl<'a> Parser<'a> {
     /// Eats the expected token if it's present possibly breaking
     /// compound tokens like multi-character operators in process.
     /// Returns `true` if the token was eaten.
-    fn break_and_eat(&mut self, exp: ExpTokenPair<'_>) -> bool {
-        if self.token == *exp.tok {
+    fn break_and_eat(&mut self, exp: ExpTokenPair) -> bool {
+        if self.token == exp.tok {
             self.bump();
             return true;
         }
         match self.token.kind.break_two_token_op(1) {
-            Some((first, second)) if first == *exp.tok => {
+            Some((first, second)) if first == exp.tok => {
                 let first_span = self.psess.source_map().start_point(self.token.span);
                 let second_span = self.token.span.with_lo(first_span.hi());
                 self.token = Token::new(first, first_span);
@@ -826,7 +828,7 @@ impl<'a> Parser<'a> {
     /// Checks if the next token is contained within `closes`, and returns `true` if so.
     fn expect_any_with_type(
         &mut self,
-        closes_expected: &[ExpTokenPair<'_>],
+        closes_expected: &[ExpTokenPair],
         closes_not_expected: &[&TokenKind],
     ) -> bool {
         closes_expected.iter().any(|&close| self.check(close))
@@ -838,9 +840,9 @@ impl<'a> Parser<'a> {
     /// closing bracket.
     fn parse_seq_to_before_tokens<T>(
         &mut self,
-        closes_expected: &[ExpTokenPair<'_>],
+        closes_expected: &[ExpTokenPair],
         closes_not_expected: &[&TokenKind],
-        sep: SeqSep<'_>,
+        sep: SeqSep,
         mut f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
     ) -> PResult<'a, (ThinVec<T>, Trailing, Recovered)> {
         let mut first = true;
@@ -869,7 +871,7 @@ impl<'a> Parser<'a> {
                         }
                         Err(mut expect_err) => {
                             let sp = self.prev_token.span.shrink_to_hi();
-                            let token_str = pprust::token_kind_to_string(exp.tok);
+                            let token_str = pprust::token_kind_to_string(&exp.tok);
 
                             match self.current_closure.take() {
                                 Some(closure_spans) if self.token == TokenKind::Semi => {
@@ -1039,8 +1041,8 @@ impl<'a> Parser<'a> {
     /// closing bracket.
     fn parse_seq_to_before_end<T>(
         &mut self,
-        close: ExpTokenPair<'_>,
-        sep: SeqSep<'_>,
+        close: ExpTokenPair,
+        sep: SeqSep,
         f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
     ) -> PResult<'a, (ThinVec<T>, Trailing, Recovered)> {
         self.parse_seq_to_before_tokens(&[close], &[], sep, f)
@@ -1051,8 +1053,8 @@ impl<'a> Parser<'a> {
     /// closing bracket.
     fn parse_seq_to_end<T>(
         &mut self,
-        close: ExpTokenPair<'_>,
-        sep: SeqSep<'_>,
+        close: ExpTokenPair,
+        sep: SeqSep,
         f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
     ) -> PResult<'a, (ThinVec<T>, Trailing)> {
         let (val, trailing, recovered) = self.parse_seq_to_before_end(close, sep, f)?;
@@ -1070,9 +1072,9 @@ impl<'a> Parser<'a> {
     /// closing bracket.
     fn parse_unspanned_seq<T>(
         &mut self,
-        open: ExpTokenPair<'_>,
-        close: ExpTokenPair<'_>,
-        sep: SeqSep<'_>,
+        open: ExpTokenPair,
+        close: ExpTokenPair,
+        sep: SeqSep,
         f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
     ) -> PResult<'a, (ThinVec<T>, Trailing)> {
         self.expect(open)?;
@@ -1084,8 +1086,8 @@ impl<'a> Parser<'a> {
     /// closing bracket.
     fn parse_delim_comma_seq<T>(
         &mut self,
-        open: ExpTokenPair<'_>,
-        close: ExpTokenPair<'_>,
+        open: ExpTokenPair,
+        close: ExpTokenPair,
         f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
     ) -> PResult<'a, (ThinVec<T>, Trailing)> {
         self.parse_unspanned_seq(open, close, SeqSep::trailing_allowed(exp!(Comma)), f)
@@ -1094,7 +1096,7 @@ impl<'a> Parser<'a> {
     /// Parses a comma-separated sequence delimited by parentheses (e.g. `(x, y)`).
     /// The function `f` must consume tokens until reaching the next separator or
     /// closing bracket.
-    fn parse_paren_comma_seq<T>(
+    pub fn parse_paren_comma_seq<T>(
         &mut self,
         f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
     ) -> PResult<'a, (ThinVec<T>, Trailing)> {
@@ -1333,7 +1335,10 @@ impl<'a> Parser<'a> {
         if let token::Literal(token::Lit { kind: token::Integer, symbol, suffix }) = self.token.kind
         {
             if let Some(suffix) = suffix {
-                self.expect_no_tuple_index_suffix(self.token.span, suffix);
+                self.dcx().emit_err(errors::InvalidLiteralSuffixOnTupleIndex {
+                    span: self.token.span,
+                    suffix,
+                });
             }
             self.bump();
             Ok(Ident::new(symbol, self.prev_token.span))
@@ -1355,7 +1360,8 @@ impl<'a> Parser<'a> {
             AttrArgs::Delimited(args)
         } else if self.eat(exp!(Eq)) {
             let eq_span = self.prev_token.span;
-            AttrArgs::Eq { eq_span, expr: self.parse_expr_force_collect()? }
+            let expr = self.parse_expr_force_collect()?;
+            AttrArgs::Eq { eq_span, expr }
         } else {
             AttrArgs::Empty
         })
@@ -1388,15 +1394,26 @@ impl<'a> Parser<'a> {
             // matching `CloseDelim` we are *after* the delimited sequence,
             // i.e. at depth `d - 1`.
             let target_depth = self.token_cursor.stack.len() - 1;
-            loop {
-                // Advance one token at a time, so `TokenCursor::next()`
-                // can capture these tokens if necessary.
+
+            if let Capturing::No = self.capture_state.capturing {
+                // We are not capturing tokens, so skip to the end of the
+                // delimited sequence. This is a perf win when dealing with
+                // declarative macros that pass large `tt` fragments through
+                // multiple rules, as seen in the uom-0.37.0 crate.
+                self.token_cursor.curr.bump_to_end();
                 self.bump();
-                if self.token_cursor.stack.len() == target_depth {
-                    debug_assert!(self.token.kind.close_delim().is_some());
-                    break;
+                debug_assert_eq!(self.token_cursor.stack.len(), target_depth);
+            } else {
+                loop {
+                    // Advance one token at a time, so `TokenCursor::next()`
+                    // can capture these tokens if necessary.
+                    self.bump();
+                    if self.token_cursor.stack.len() == target_depth {
+                        break;
+                    }
                 }
             }
+            debug_assert!(self.token.kind.close_delim().is_some());
 
             // Consume close delimiter
             self.bump();
diff --git a/compiler/rustc_parse/src/parser/pat.rs b/compiler/rustc_parse/src/parser/pat.rs
index a415849b915..fda19d62bc7 100644
--- a/compiler/rustc_parse/src/parser/pat.rs
+++ b/compiler/rustc_parse/src/parser/pat.rs
@@ -24,10 +24,10 @@ use crate::errors::{
     GenericArgsInPatRequireTurbofishSyntax, InclusiveRangeExtraEquals, InclusiveRangeMatchArrow,
     InclusiveRangeNoEnd, InvalidMutInPattern, ParenRangeSuggestion, PatternOnWrongSideOfAt,
     RemoveLet, RepeatedMutInPattern, SwitchRefBoxOrder, TopLevelOrPatternNotAllowed,
-    TopLevelOrPatternNotAllowedSugg, TrailingVertNotAllowed, UnexpectedExpressionInPattern,
-    UnexpectedExpressionInPatternSugg, UnexpectedLifetimeInPattern, UnexpectedParenInRangePat,
-    UnexpectedParenInRangePatSugg, UnexpectedVertVertBeforeFunctionParam,
-    UnexpectedVertVertInPattern, WrapInParens,
+    TopLevelOrPatternNotAllowedSugg, TrailingVertNotAllowed, TrailingVertSuggestion,
+    UnexpectedExpressionInPattern, UnexpectedExpressionInPatternSugg, UnexpectedLifetimeInPattern,
+    UnexpectedParenInRangePat, UnexpectedParenInRangePatSugg,
+    UnexpectedVertVertBeforeFunctionParam, UnexpectedVertVertInPattern, WrapInParens,
 };
 use crate::parser::expr::{DestructuredFloat, could_be_unclosed_char_literal};
 use crate::{exp, maybe_recover_from_interpolated_ty_qpath};
@@ -267,10 +267,9 @@ impl<'a> Parser<'a> {
 
         if let PatKind::Or(pats) = &pat.kind {
             let span = pat.span;
-            let sub = if pats.len() == 1 {
-                Some(TopLevelOrPatternNotAllowedSugg::RemoveLeadingVert {
-                    span: span.with_hi(span.lo() + BytePos(1)),
-                })
+            let sub = if let [_] = &pats[..] {
+                let span = span.with_hi(span.lo() + BytePos(1));
+                Some(TopLevelOrPatternNotAllowedSugg::RemoveLeadingVert { span })
             } else {
                 Some(TopLevelOrPatternNotAllowedSugg::WrapInParens {
                     span,
@@ -362,6 +361,9 @@ impl<'a> Parser<'a> {
                 self.dcx().emit_err(TrailingVertNotAllowed {
                     span: self.token.span,
                     start: lo,
+                    suggestion: TrailingVertSuggestion {
+                        span: self.prev_token.span.shrink_to_hi().with_hi(self.token.span.hi()),
+                    },
                     token: self.token,
                     note_double_vert: self.token.kind == token::OrOr,
                 });
@@ -754,7 +756,7 @@ impl<'a> Parser<'a> {
             self.bump(); // `..`
             PatKind::Rest
         } else if self.check(exp!(DotDotDot)) && !self.is_pat_range_end_start(1) {
-            self.recover_dotdotdot_rest_pat(lo)
+            self.recover_dotdotdot_rest_pat(lo, expected)
         } else if let Some(form) = self.parse_range_end() {
             self.parse_pat_range_to(form)? // `..=X`, `...X`, or `..X`.
         } else if self.eat(exp!(Bang)) {
@@ -884,16 +886,27 @@ impl<'a> Parser<'a> {
 
     /// Recover from a typoed `...` pattern that was encountered
     /// Ref: Issue #70388
-    fn recover_dotdotdot_rest_pat(&mut self, lo: Span) -> PatKind {
+    fn recover_dotdotdot_rest_pat(&mut self, lo: Span, expected: Option<Expected>) -> PatKind {
         // A typoed rest pattern `...`.
         self.bump(); // `...`
 
-        // The user probably mistook `...` for a rest pattern `..`.
-        self.dcx().emit_err(DotDotDotRestPattern {
-            span: lo,
-            suggestion: lo.with_lo(lo.hi() - BytePos(1)),
-        });
-        PatKind::Rest
+        if let Some(Expected::ParameterName) = expected {
+            // We have `...` in a closure argument, likely meant to be var-arg, which aren't
+            // supported in closures (#146489).
+            PatKind::Err(self.dcx().emit_err(DotDotDotRestPattern {
+                span: lo,
+                suggestion: None,
+                var_args: Some(()),
+            }))
+        } else {
+            // The user probably mistook `...` for a rest pattern `..`.
+            self.dcx().emit_err(DotDotDotRestPattern {
+                span: lo,
+                suggestion: Some(lo.with_lo(lo.hi() - BytePos(1))),
+                var_args: None,
+            });
+            PatKind::Rest
+        }
     }
 
     /// Try to recover the more general form `intersect ::= $pat_lhs @ $pat_rhs`.
@@ -1514,7 +1527,7 @@ impl<'a> Parser<'a> {
                 || self.check_noexpect(&token::DotDotDot)
                 || self.check_keyword(exp!(Underscore))
             {
-                etc = PatFieldsRest::Rest;
+                etc = PatFieldsRest::Rest(self.token.span);
                 let mut etc_sp = self.token.span;
                 if first_etc_and_maybe_comma_span.is_none() {
                     if let Some(comma_tok) =
diff --git a/compiler/rustc_parse/src/parser/path.rs b/compiler/rustc_parse/src/parser/path.rs
index a6ec3ea4245..86045648859 100644
--- a/compiler/rustc_parse/src/parser/path.rs
+++ b/compiler/rustc_parse/src/parser/path.rs
@@ -20,11 +20,13 @@ use crate::errors::{
     PathFoundAttributeInParams, PathFoundCVariadicParams, PathSingleColon, PathTripleColon,
 };
 use crate::exp;
-use crate::parser::{CommaRecoveryMode, ExprKind, RecoverColon, RecoverComma};
+use crate::parser::{
+    CommaRecoveryMode, ExprKind, FnContext, FnParseMode, RecoverColon, RecoverComma,
+};
 
 /// Specifies how to parse a path.
 #[derive(Copy, Clone, PartialEq)]
-pub(super) enum PathStyle {
+pub enum PathStyle {
     /// In some contexts, notably in expressions, paths with generic arguments are ambiguous
     /// with something else. For example, in expressions `segment < ....` can be interpreted
     /// as a comparison and `segment ( ....` can be interpreted as a function call.
@@ -148,7 +150,7 @@ impl<'a> Parser<'a> {
         true
     }
 
-    pub(super) fn parse_path(&mut self, style: PathStyle) -> PResult<'a, Path> {
+    pub fn parse_path(&mut self, style: PathStyle) -> PResult<'a, Path> {
         self.parse_path_inner(style, None)
     }
 
@@ -399,7 +401,13 @@ impl<'a> Parser<'a> {
 
                     let dcx = self.dcx();
                     let parse_params_result = self.parse_paren_comma_seq(|p| {
-                        let param = p.parse_param_general(|_| false, false, false);
+                        // Inside parenthesized type arguments, we want types only, not names.
+                        let mode = FnParseMode {
+                            context: FnContext::Free,
+                            req_name: |_| false,
+                            req_body: false,
+                        };
+                        let param = p.parse_param_general(&mode, false, false);
                         param.map(move |param| {
                             if !matches!(param.pat.kind, PatKind::Missing) {
                                 dcx.emit_err(FnPathFoundNamedParams {
diff --git a/compiler/rustc_parse/src/parser/stmt.rs b/compiler/rustc_parse/src/parser/stmt.rs
index 7aacb674253..ad5ab6e6b77 100644
--- a/compiler/rustc_parse/src/parser/stmt.rs
+++ b/compiler/rustc_parse/src/parser/stmt.rs
@@ -6,6 +6,7 @@ use ast::Label;
 use rustc_ast as ast;
 use rustc_ast::token::{self, Delimiter, InvisibleOrigin, MetaVarKind, TokenKind};
 use rustc_ast::util::classify::{self, TrailingBrace};
+use rustc_ast::visit::{Visitor, walk_expr};
 use rustc_ast::{
     AttrStyle, AttrVec, Block, BlockCheckMode, DUMMY_NODE_ID, Expr, ExprKind, HasAttrs, Local,
     LocalKind, MacCall, MacCallStmt, MacStmtStyle, Recovered, Stmt, StmtKind,
@@ -19,8 +20,8 @@ use super::diagnostics::AttemptLocalParseRecovery;
 use super::pat::{PatternLocation, RecoverComma};
 use super::path::PathStyle;
 use super::{
-    AttrWrapper, BlockMode, FnParseMode, ForceCollect, Parser, Restrictions, SemiColonMode,
-    Trailing, UsePreAttrPos,
+    AttrWrapper, BlockMode, FnContext, FnParseMode, ForceCollect, Parser, Restrictions,
+    SemiColonMode, Trailing, UsePreAttrPos,
 };
 use crate::errors::{self, MalformedLoopLabel};
 use crate::exp;
@@ -153,7 +154,7 @@ impl<'a> Parser<'a> {
             attrs.clone(), // FIXME: unwanted clone of attrs
             false,
             true,
-            FnParseMode { req_name: |_| true, req_body: true },
+            FnParseMode { req_name: |_| true, context: FnContext::Free, req_body: true },
             force_collect,
         )? {
             self.mk_stmt(lo.to(item.span), StmtKind::Item(Box::new(item)))
@@ -783,6 +784,100 @@ impl<'a> Parser<'a> {
         Ok(self.mk_block(stmts, s, lo.to(self.prev_token.span)))
     }
 
+    fn recover_missing_let_else(&mut self, err: &mut Diag<'_>, pat: &ast::Pat, stmt_span: Span) {
+        if self.token.kind != token::OpenBrace {
+            return;
+        }
+        match pat.kind {
+            ast::PatKind::Ident(..) | ast::PatKind::Missing | ast::PatKind::Wild => {
+                // Not if let or let else
+                return;
+            }
+            _ => {}
+        }
+        let snapshot = self.create_snapshot_for_diagnostic();
+        let block_span = self.token.span;
+        let (if_let, let_else) = match self.parse_block() {
+            Ok(block) => {
+                let mut idents = vec![];
+                pat.walk(&mut |pat: &ast::Pat| {
+                    if let ast::PatKind::Ident(_, ident, _) = pat.kind {
+                        idents.push(ident);
+                    }
+                    true
+                });
+
+                struct IdentFinder {
+                    idents: Vec<Ident>,
+                    /// If a block references one of the bindings introduced by the let pattern,
+                    /// we likely meant to use `if let`.
+                    /// This is pre-expansion, so if we encounter
+                    /// `let Some(x) = foo() { println!("{x}") }` we won't find it.
+                    references_ident: bool = false,
+                    /// If a block has a `return`, then we know with high certainty that it was
+                    /// meant to be let-else.
+                    has_return: bool = false,
+                }
+
+                impl<'a> Visitor<'a> for IdentFinder {
+                    fn visit_ident(&mut self, ident: &Ident) {
+                        for i in &self.idents {
+                            if ident.name == i.name {
+                                self.references_ident = true;
+                            }
+                        }
+                    }
+                    fn visit_expr(&mut self, node: &'a Expr) {
+                        if let ExprKind::Ret(..) = node.kind {
+                            self.has_return = true;
+                        }
+                        walk_expr(self, node);
+                    }
+                }
+
+                // Collect all bindings in pattern and see if they appear in the block. Likely meant
+                // to write `if let`. See if the block has a return. Likely meant to write
+                // `let else`.
+                let mut visitor = IdentFinder { idents, .. };
+                visitor.visit_block(&block);
+
+                (visitor.references_ident, visitor.has_return)
+            }
+            Err(e) => {
+                e.cancel();
+                self.restore_snapshot(snapshot);
+                (false, false)
+            }
+        };
+
+        let mut alternatively = "";
+        if if_let || !let_else {
+            alternatively = "alternatively, ";
+            err.span_suggestion_verbose(
+                stmt_span.shrink_to_lo(),
+                "you might have meant to use `if let`",
+                "if ".to_string(),
+                if if_let {
+                    Applicability::MachineApplicable
+                } else {
+                    Applicability::MaybeIncorrect
+                },
+            );
+        }
+        if let_else || !if_let {
+            err.span_suggestion_verbose(
+                block_span.shrink_to_lo(),
+                format!("{alternatively}you might have meant to use `let else`"),
+                "else ".to_string(),
+                if let_else {
+                    Applicability::MachineApplicable
+                } else {
+                    Applicability::MaybeIncorrect
+                },
+            );
+        }
+    }
+
     fn recover_missing_dot(&mut self, err: &mut Diag<'_>) {
         let Some((ident, _)) = self.token.ident() else {
             return;
@@ -977,6 +1072,7 @@ impl<'a> Parser<'a> {
                         self.check_mistyped_turbofish_with_multiple_type_params(e, expr).map_err(
                             |mut e| {
                                 self.recover_missing_dot(&mut e);
+                                self.recover_missing_let_else(&mut e, &local.pat, stmt.span);
                                 e
                             },
                         )?;
diff --git a/compiler/rustc_parse/src/parser/tests.rs b/compiler/rustc_parse/src/parser/tests.rs
index a6e7266e71b..e645fb47b9e 100644
--- a/compiler/rustc_parse/src/parser/tests.rs
+++ b/compiler/rustc_parse/src/parser/tests.rs
@@ -22,6 +22,7 @@ use rustc_span::{
 };
 use termcolor::WriteColor;
 
+use crate::lexer::StripTokens;
 use crate::parser::{ForceCollect, Parser};
 use crate::{new_parser_from_source_str, source_str_to_stream, unwrap_or_emit_fatal};
 
@@ -35,6 +36,7 @@ fn string_to_parser(psess: &ParseSess, source_str: String) -> Parser<'_> {
         psess,
         PathBuf::from("bogofile").into(),
         source_str,
+        StripTokens::Nothing,
     ))
 }
 
@@ -2240,7 +2242,7 @@ fn parse_item_from_source_str(
     source: String,
     psess: &ParseSess,
 ) -> PResult<'_, Option<Box<ast::Item>>> {
-    unwrap_or_emit_fatal(new_parser_from_source_str(psess, name, source))
+    unwrap_or_emit_fatal(new_parser_from_source_str(psess, name, source, StripTokens::Nothing))
         .parse_item(ForceCollect::No)
 }
 
@@ -2520,7 +2522,8 @@ fn ttdelim_span() {
         source: String,
         psess: &ParseSess,
     ) -> PResult<'_, Box<ast::Expr>> {
-        unwrap_or_emit_fatal(new_parser_from_source_str(psess, name, source)).parse_expr()
+        unwrap_or_emit_fatal(new_parser_from_source_str(psess, name, source, StripTokens::Nothing))
+            .parse_expr()
     }
 
     create_default_session_globals_then(|| {
diff --git a/compiler/rustc_parse/src/parser/token_type.rs b/compiler/rustc_parse/src/parser/token_type.rs
index b91548196a3..bd4bb368df0 100644
--- a/compiler/rustc_parse/src/parser/token_type.rs
+++ b/compiler/rustc_parse/src/parser/token_type.rs
@@ -416,8 +416,8 @@ impl TokenType {
 /// is always by used those methods. The second field is only used when the
 /// first field doesn't match.
 #[derive(Clone, Copy, Debug)]
-pub struct ExpTokenPair<'a> {
-    pub tok: &'a TokenKind,
+pub struct ExpTokenPair {
+    pub tok: TokenKind,
     pub token_type: TokenType,
 }
 
@@ -444,7 +444,7 @@ macro_rules! exp {
     // `ExpTokenPair` helper rules.
     (@tok, $tok:ident) => {
         $crate::parser::token_type::ExpTokenPair {
-            tok: &rustc_ast::token::$tok,
+            tok: rustc_ast::token::$tok,
             token_type: $crate::parser::token_type::TokenType::$tok
         }
     };
diff --git a/compiler/rustc_parse/src/parser/ty.rs b/compiler/rustc_parse/src/parser/ty.rs
index 0d479731e73..65347496599 100644
--- a/compiler/rustc_parse/src/parser/ty.rs
+++ b/compiler/rustc_parse/src/parser/ty.rs
@@ -15,10 +15,11 @@ use super::{Parser, PathStyle, SeqSep, TokenType, Trailing};
 use crate::errors::{
     self, AttributeOnEmptyType, AttributeOnType, DynAfterMut, ExpectedFnPathFoundFnKeyword,
     ExpectedMutOrConstInRawPointerType, FnPtrWithGenerics, FnPtrWithGenericsSugg,
-    HelpUseLatestEdition, InvalidDynKeyword, LifetimeAfterMut, NeedPlusAfterTraitObjectLifetime,
-    NestedCVariadicType, ReturnTypesUseThinArrow,
+    HelpUseLatestEdition, InvalidCVariadicType, InvalidDynKeyword, LifetimeAfterMut,
+    NeedPlusAfterTraitObjectLifetime, NestedCVariadicType, ReturnTypesUseThinArrow,
 };
 use crate::parser::item::FrontMatterParsingMode;
+use crate::parser::{FnContext, FnParseMode};
 use crate::{exp, maybe_recover_from_interpolated_ty_qpath};
 
 /// Signals whether parsing a type should allow `+`.
@@ -91,10 +92,10 @@ fn can_continue_type_after_non_fn_ident(t: &Token) -> bool {
 }
 
 fn can_begin_dyn_bound_in_edition_2015(t: &Token) -> bool {
-    // `Not`, `Tilde` & `Const` are deliberately not part of this list to
+    // `!`, `const`, `[`, `async` are deliberately not part of this list to
     // contain the number of potential regressions esp. in MBE code.
-    // `Const` would regress `rfc-2632-const-trait-impl/mbe-dyn-const-2015.rs`.
-    // `Not` would regress `dyn!(...)` macro calls in Rust 2015.
+    // `const` and `[` would regress UI test `macro-dyn-const-2015.rs`.
+    // `!` would regress `dyn!(...)` macro calls in Rust 2015.
     t.is_path_start()
         || t.is_lifetime()
         || t == &TokenKind::Question
@@ -105,6 +106,15 @@ fn can_begin_dyn_bound_in_edition_2015(t: &Token) -> bool {
 impl<'a> Parser<'a> {
     /// Parses a type.
     pub fn parse_ty(&mut self) -> PResult<'a, Box<Ty>> {
+        if self.token == token::DotDotDot {
+            // We special case this so that we don't talk about "nested C-variadics" in types.
+            // We still pass in `AllowCVariadic::No` so that `parse_ty_common` can complain about
+            // things like `Vec<...>`.
+            let span = self.token.span;
+            self.bump();
+            let kind = TyKind::Err(self.dcx().emit_err(InvalidCVariadicType { span }));
+            return Ok(self.mk_ty(span, kind));
+        }
         // Make sure deeply nested types don't overflow the stack.
         ensure_sufficient_stack(|| {
             self.parse_ty_common(
@@ -136,14 +146,37 @@ impl<'a> Parser<'a> {
     /// The difference from `parse_ty` is that this version allows `...`
     /// (`CVarArgs`) at the top level of the type.
     pub(super) fn parse_ty_for_param(&mut self) -> PResult<'a, Box<Ty>> {
-        self.parse_ty_common(
+        let ty = self.parse_ty_common(
             AllowPlus::Yes,
             AllowCVariadic::Yes,
             RecoverQPath::Yes,
             RecoverReturnSign::Yes,
             None,
             RecoverQuestionMark::Yes,
-        )
+        )?;
+
+        // Recover a trailing `= EXPR` if present.
+        if self.may_recover()
+            && self.check_noexpect(&token::Eq)
+            && self.look_ahead(1, |tok| tok.can_begin_expr())
+        {
+            let snapshot = self.create_snapshot_for_diagnostic();
+            self.bump();
+            let eq_span = self.prev_token.span;
+            match self.parse_expr() {
+                Ok(e) => {
+                    self.dcx()
+                        .struct_span_err(eq_span.to(e.span), "parameter defaults are not supported")
+                        .emit();
+                }
+                Err(diag) => {
+                    diag.cancel();
+                    self.restore_snapshot(snapshot);
+                }
+            }
+        }
+
+        Ok(ty)
     }
 
     /// Parses a type in restricted contexts where `+` is not permitted.
@@ -307,11 +340,11 @@ impl<'a> Parser<'a> {
             // Function pointer type or bound list (trait object type) starting with a poly-trait.
             //   `for<'lt> [unsafe] [extern "ABI"] fn (&'lt S) -> T`
             //   `for<'lt> Trait1<'lt> + Trait2 + 'a`
-            let (lifetime_defs, _) = self.parse_late_bound_lifetime_defs()?;
+            let (bound_vars, _) = self.parse_higher_ranked_binder()?;
             if self.check_fn_front_matter(false, Case::Sensitive) {
                 self.parse_ty_fn_ptr(
                     lo,
-                    lifetime_defs,
+                    bound_vars,
                     Some(self.prev_token.span.shrink_to_lo()),
                     recover_return_sign,
                 )?
@@ -325,7 +358,7 @@ impl<'a> Parser<'a> {
                     let path = self.parse_path(PathStyle::Type)?;
                     let parse_plus = allow_plus == AllowPlus::Yes && self.check_plus();
                     let kind = self.parse_remaining_bounds_path(
-                        lifetime_defs,
+                        bound_vars,
                         path,
                         lo,
                         parse_plus,
@@ -358,7 +391,7 @@ impl<'a> Parser<'a> {
                     let path = self.parse_path(PathStyle::Type)?;
                     let parse_plus = allow_plus == AllowPlus::Yes && self.check_plus();
                     self.parse_remaining_bounds_path(
-                        lifetime_defs,
+                        bound_vars,
                         path,
                         lo,
                         parse_plus,
@@ -442,7 +475,7 @@ impl<'a> Parser<'a> {
             let ty = ts.into_iter().next().unwrap();
             let maybe_bounds = allow_plus == AllowPlus::Yes && self.token.is_like_plus();
             match ty.kind {
-                // `(TY_BOUND_NOPAREN) + BOUND + ...`.
+                // `"(" BareTraitBound ")" "+" Bound "+" ...`.
                 TyKind::Path(None, path) if maybe_bounds => self.parse_remaining_bounds_path(
                     ThinVec::new(),
                     path,
@@ -769,7 +802,12 @@ impl<'a> Parser<'a> {
         if self.may_recover() && self.token == TokenKind::Lt {
             self.recover_fn_ptr_with_generics(lo, &mut params, param_insertion_point)?;
         }
-        let decl = self.parse_fn_decl(|_| false, AllowPlus::No, recover_return_sign)?;
+        let mode = crate::parser::item::FnParseMode {
+            req_name: |_| false,
+            context: FnContext::Free,
+            req_body: false,
+        };
+        let decl = self.parse_fn_decl(&mode, AllowPlus::No, recover_return_sign)?;
 
         let decl_span = span_start.to(self.prev_token.span);
         Ok(TyKind::FnPtr(Box::new(FnPtrTy {
@@ -847,10 +885,13 @@ impl<'a> Parser<'a> {
         Ok(TyKind::ImplTrait(ast::DUMMY_NODE_ID, bounds))
     }
 
-    fn parse_precise_capturing_args(
-        &mut self,
-    ) -> PResult<'a, (ThinVec<PreciseCapturingArg>, Span)> {
-        let lo = self.token.span;
+    /// Parse a use-bound aka precise capturing list.
+    ///
+    /// ```ebnf
+    /// UseBound = "use" "<" (PreciseCapture ("," PreciseCapture)* ","?)? ">"
+    /// PreciseCapture = "Self" | Ident | Lifetime
+    /// ```
+    fn parse_use_bound(&mut self, lo: Span, parens: ast::Parens) -> PResult<'a, GenericBound> {
         self.expect_lt()?;
         let (args, _, _) = self.parse_seq_to_before_tokens(
             &[exp!(Gt)],
@@ -876,7 +917,13 @@ impl<'a> Parser<'a> {
             },
         )?;
         self.expect_gt()?;
-        Ok((args, lo.to(self.prev_token.span)))
+
+        if let ast::Parens::Yes = parens {
+            self.expect(exp!(CloseParen))?;
+            self.report_parenthesized_bound(lo, self.prev_token.span, "precise capturing lists");
+        }
+
+        Ok(GenericBound::Use(args, lo.to(self.prev_token.span)))
     }
 
     /// Is a `dyn B0 + ... + Bn` type allowed here?
@@ -934,9 +981,10 @@ impl<'a> Parser<'a> {
         self.parse_generic_bounds_common(AllowPlus::Yes)
     }
 
-    /// Parses bounds of a type parameter `BOUND + BOUND + ...`, possibly with trailing `+`.
+    /// Parse generic bounds.
     ///
-    /// See `parse_generic_bound` for the `BOUND` grammar.
+    /// Only if `allow_plus` this parses a `+`-separated list of bounds (trailing `+` is admitted).
+    /// Otherwise, this only parses a single bound or none.
     fn parse_generic_bounds_common(&mut self, allow_plus: AllowPlus) -> PResult<'a, GenericBounds> {
         let mut bounds = Vec::new();
 
@@ -976,54 +1024,68 @@ impl<'a> Parser<'a> {
             || self.check(exp!(Tilde))
             || self.check_keyword(exp!(For))
             || self.check(exp!(OpenParen))
-            || self.check(exp!(OpenBracket))
+            || self.can_begin_maybe_const_bound()
             || self.check_keyword(exp!(Const))
             || self.check_keyword(exp!(Async))
             || self.check_keyword(exp!(Use))
     }
 
-    /// Parses a bound according to the grammar:
+    fn can_begin_maybe_const_bound(&mut self) -> bool {
+        self.check(exp!(OpenBracket))
+            && self.look_ahead(1, |t| t.is_keyword(kw::Const))
+            && self.look_ahead(2, |t| *t == token::CloseBracket)
+    }
+
+    /// Parse a bound.
+    ///
     /// ```ebnf
-    /// BOUND = TY_BOUND | LT_BOUND
+    /// Bound = LifetimeBound | UseBound | TraitBound
     /// ```
     fn parse_generic_bound(&mut self) -> PResult<'a, GenericBound> {
-        let lo = self.token.span;
         let leading_token = self.prev_token;
+        let lo = self.token.span;
+
+        // We only admit parenthesized *trait* bounds. However, we want to gracefully recover from
+        // other kinds of parenthesized bounds, so parse the opening parenthesis *here*.
+        //
+        // In the future we might want to lift this syntactic restriction and
+        // introduce "`GenericBound::Paren(Box<GenericBound>)`".
         let parens = if self.eat(exp!(OpenParen)) { ast::Parens::Yes } else { ast::Parens::No };
 
-        let bound = if self.token.is_lifetime() {
-            self.parse_generic_lt_bound(lo, parens)?
+        if self.token.is_lifetime() {
+            self.parse_lifetime_bound(lo, parens)
         } else if self.eat_keyword(exp!(Use)) {
-            // parse precise captures, if any. This is `use<'lt, 'lt, P, P>`; a list of
-            // lifetimes and ident params (including SelfUpper). These are validated later
-            // for order, duplication, and whether they actually reference params.
-            let use_span = self.prev_token.span;
-            let (args, args_span) = self.parse_precise_capturing_args()?;
-            GenericBound::Use(args, use_span.to(args_span))
+            self.parse_use_bound(lo, parens)
         } else {
-            self.parse_generic_ty_bound(lo, parens, &leading_token)?
-        };
-
-        Ok(bound)
+            self.parse_trait_bound(lo, parens, &leading_token)
+        }
     }
 
-    /// Parses a lifetime ("outlives") bound, e.g. `'a`, according to:
+    /// Parse a lifetime-bound aka outlives-bound.
+    ///
     /// ```ebnf
-    /// LT_BOUND = LIFETIME
+    /// LifetimeBound = Lifetime
     /// ```
-    fn parse_generic_lt_bound(
-        &mut self,
-        lo: Span,
-        parens: ast::Parens,
-    ) -> PResult<'a, GenericBound> {
+    fn parse_lifetime_bound(&mut self, lo: Span, parens: ast::Parens) -> PResult<'a, GenericBound> {
         let lt = self.expect_lifetime();
-        let bound = GenericBound::Outlives(lt);
+
         if let ast::Parens::Yes = parens {
-            // FIXME(Centril): Consider not erroring here and accepting `('lt)` instead,
-            // possibly introducing `GenericBound::Paren(Box<GenericBound>)`?
-            self.recover_paren_lifetime(lo)?;
+            self.expect(exp!(CloseParen))?;
+            self.report_parenthesized_bound(lo, self.prev_token.span, "lifetime bounds");
         }
-        Ok(bound)
+
+        Ok(GenericBound::Outlives(lt))
+    }
+
+    fn report_parenthesized_bound(&self, lo: Span, hi: Span, kind: &str) -> ErrorGuaranteed {
+        let mut diag =
+            self.dcx().struct_span_err(lo.to(hi), format!("{kind} may not be parenthesized"));
+        diag.multipart_suggestion(
+            "remove the parentheses",
+            vec![(lo, String::new()), (hi, String::new())],
+            Applicability::MachineApplicable,
+        );
+        diag.emit()
     }
 
     /// Emits an error if any trait bound modifiers were present.
@@ -1068,27 +1130,17 @@ impl<'a> Parser<'a> {
         unreachable!("lifetime bound intercepted in `parse_generic_ty_bound` but no modifiers?")
     }
 
-    /// Recover on `('lifetime)` with `(` already eaten.
-    fn recover_paren_lifetime(&mut self, lo: Span) -> PResult<'a, ()> {
-        self.expect(exp!(CloseParen))?;
-        let span = lo.to(self.prev_token.span);
-        let sugg = errors::RemoveParens { lo, hi: self.prev_token.span };
-
-        self.dcx().emit_err(errors::ParenthesizedLifetime { span, sugg });
-        Ok(())
-    }
-
     /// Parses the modifiers that may precede a trait in a bound, e.g. `?Trait` or `[const] Trait`.
     ///
     /// If no modifiers are present, this does not consume any tokens.
     ///
     /// ```ebnf
-    /// CONSTNESS = [["["] "const" ["]"]]
-    /// ASYNCNESS = ["async"]
-    /// POLARITY = ["?" | "!"]
+    /// Constness = ("const" | "[" "const" "]")?
+    /// Asyncness = "async"?
+    /// Polarity = ("?" | "!")?
     /// ```
     ///
-    /// See `parse_generic_ty_bound` for the complete grammar of trait bound modifiers.
+    /// See `parse_trait_bound` for more context.
     fn parse_trait_bound_modifiers(&mut self) -> PResult<'a, TraitBoundModifiers> {
         let modifier_lo = self.token.span;
         let constness = self.parse_bound_constness()?;
@@ -1162,10 +1214,7 @@ impl<'a> Parser<'a> {
             let span = tilde.to(self.prev_token.span);
             self.psess.gated_spans.gate(sym::const_trait_impl, span);
             BoundConstness::Maybe(span)
-        } else if self.check(exp!(OpenBracket))
-            && self.look_ahead(1, |t| t.is_keyword(kw::Const))
-            && self.look_ahead(2, |t| *t == token::CloseBracket)
-        {
+        } else if self.can_begin_maybe_const_bound() {
             let start = self.token.span;
             self.bump();
             self.expect_keyword(exp!(Const)).unwrap();
@@ -1181,20 +1230,21 @@ impl<'a> Parser<'a> {
         })
     }
 
-    /// Parses a type bound according to:
+    /// Parse a trait bound.
+    ///
     /// ```ebnf
-    /// TY_BOUND = TY_BOUND_NOPAREN | (TY_BOUND_NOPAREN)
-    /// TY_BOUND_NOPAREN = [for<GENERIC_PARAMS> CONSTNESS ASYNCNESS | POLARITY] SIMPLE_PATH
+    /// TraitBound = BareTraitBound | "(" BareTraitBound ")"
+    /// BareTraitBound =
+    ///     (HigherRankedBinder Constness Asyncness | Polarity)
+    ///     TypePath
     /// ```
-    ///
-    /// For example, this grammar accepts `for<'a: 'b> [const] ?m::Trait<'a>`.
-    fn parse_generic_ty_bound(
+    fn parse_trait_bound(
         &mut self,
         lo: Span,
         parens: ast::Parens,
         leading_token: &Token,
     ) -> PResult<'a, GenericBound> {
-        let (mut lifetime_defs, binder_span) = self.parse_late_bound_lifetime_defs()?;
+        let (mut bound_vars, binder_span) = self.parse_higher_ranked_binder()?;
 
         let modifiers_lo = self.token.span;
         let modifiers = self.parse_trait_bound_modifiers()?;
@@ -1217,11 +1267,11 @@ impl<'a> Parser<'a> {
         // e.g. `T: for<'a> 'a` or `T: [const] 'a`.
         if self.token.is_lifetime() {
             let _: ErrorGuaranteed = self.error_lt_bound_with_modifiers(modifiers, binder_span);
-            return self.parse_generic_lt_bound(lo, parens);
+            return self.parse_lifetime_bound(lo, parens);
         }
 
-        if let (more_lifetime_defs, Some(binder_span)) = self.parse_late_bound_lifetime_defs()? {
-            lifetime_defs.extend(more_lifetime_defs);
+        if let (more_bound_vars, Some(binder_span)) = self.parse_higher_ranked_binder()? {
+            bound_vars.extend(more_bound_vars);
             self.dcx().emit_err(errors::BinderBeforeModifiers { binder_span, modifiers_span });
         }
 
@@ -1281,7 +1331,7 @@ impl<'a> Parser<'a> {
         };
 
         if self.may_recover() && self.token == TokenKind::OpenParen {
-            self.recover_fn_trait_with_lifetime_params(&mut path, &mut lifetime_defs)?;
+            self.recover_fn_trait_with_lifetime_params(&mut path, &mut bound_vars)?;
         }
 
         if let ast::Parens::Yes = parens {
@@ -1304,7 +1354,7 @@ impl<'a> Parser<'a> {
         }
 
         let poly_trait =
-            PolyTraitRef::new(lifetime_defs, path, modifiers, lo.to(self.prev_token.span), parens);
+            PolyTraitRef::new(bound_vars, path, modifiers, lo.to(self.prev_token.span), parens);
         Ok(GenericBound::Trait(poly_trait))
     }
 
@@ -1314,7 +1364,8 @@ impl<'a> Parser<'a> {
         self.bump();
         let args_lo = self.token.span;
         let snapshot = self.create_snapshot_for_diagnostic();
-        match self.parse_fn_decl(|_| false, AllowPlus::No, RecoverReturnSign::OnlyFatArrow) {
+        let mode = FnParseMode { req_name: |_| false, context: FnContext::Free, req_body: false };
+        match self.parse_fn_decl(&mode, AllowPlus::No, RecoverReturnSign::OnlyFatArrow) {
             Ok(decl) => {
                 self.dcx().emit_err(ExpectedFnPathFoundFnKeyword { fn_token_span });
                 Some(ast::Path {
@@ -1342,8 +1393,12 @@ impl<'a> Parser<'a> {
         }
     }
 
-    /// Optionally parses `for<$generic_params>`.
-    pub(super) fn parse_late_bound_lifetime_defs(
+    /// Parse an optional higher-ranked binder.
+    ///
+    /// ```ebnf
+    /// HigherRankedBinder = ("for" "<" GenericParams ">")?
+    /// ```
+    pub(super) fn parse_higher_ranked_binder(
         &mut self,
     ) -> PResult<'a, (ThinVec<GenericParam>, Option<Span>)> {
         if self.eat_keyword(exp!(For)) {
@@ -1400,8 +1455,9 @@ impl<'a> Parser<'a> {
 
         // Parse `(T, U) -> R`.
         let inputs_lo = self.token.span;
+        let mode = FnParseMode { req_name: |_| false, context: FnContext::Free, req_body: false };
         let inputs: ThinVec<_> =
-            self.parse_fn_params(|_| false)?.into_iter().map(|input| input.ty).collect();
+            self.parse_fn_params(&mode)?.into_iter().map(|input| input.ty).collect();
         let inputs_span = inputs_lo.to(self.prev_token.span);
         let output = self.parse_ret_ty(AllowPlus::No, RecoverQPath::No, RecoverReturnSign::No)?;
         let args = ast::ParenthesizedArgs {
@@ -1458,8 +1514,7 @@ impl<'a> Parser<'a> {
     pub(super) fn expect_lifetime(&mut self) -> Lifetime {
         if let Some((ident, is_raw)) = self.token.lifetime() {
             if matches!(is_raw, IdentIsRaw::No)
-                && ident.without_first_quote().is_reserved()
-                && ![kw::UnderscoreLifetime, kw::StaticLifetime].contains(&ident.name)
+                && ident.without_first_quote().is_reserved_lifetime()
             {
                 self.dcx().emit_err(errors::KeywordLifetime { span: ident.span });
             }