Auto merge of #121400 - fmease:rollup-8m29g7a, r=fmease

Rollup of 8 pull requests

Successful merges:

 - #121044 (Support async trait bounds in macros)
 - #121175 (match lowering: test one or pattern at a time)
 - #121340 (bootstrap: apply most of clippy's suggestions)
 - #121347 (compiletest: support auxiliaries with auxiliaries)
 - #121359 (miscellaneous type system improvements)
 - #121366 (Remove `diagnostic_builder.rs`)
 - #121379 (Remove an `unchecked_error_guaranteed` call.)
 - #121396 (make it possible for outside crates to inspect a mir::ConstValue with the interpreter)

r? `@ghost`
`@rustbot` modify labels: rollup

69 files changed, 1202 insertions, 973 deletions

diff --git a/compiler/rustc_ast_passes/src/ast_validation.rs b/compiler/rustc_ast_passes/src/ast_validation.rs
index fa0f5326196..8c9ad836087 100644
--- a/compiler/rustc_ast_passes/src/ast_validation.rs
+++ b/compiler/rustc_ast_passes/src/ast_validation.rs
@@ -881,9 +881,10 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                         &item.vis,
                         errors::VisibilityNotPermittedNote::TraitImpl,
                     );
-                    // njn: use Dummy here
-                    if let TyKind::Err(_) = self_ty.kind {
-                        this.dcx().emit_err(errors::ObsoleteAuto { span: item.span });
+                    if let TyKind::Dummy = self_ty.kind {
+                        // Abort immediately otherwise the `TyKind::Dummy` will reach HIR lowering,
+                        // which isn't allowed. Not a problem for this obscure, obsolete syntax.
+                        this.dcx().emit_fatal(errors::ObsoleteAuto { span: item.span });
                     }
                     if let (&Unsafe::Yes(span), &ImplPolarity::Negative(sp)) = (unsafety, polarity)
                     {
diff --git a/compiler/rustc_const_eval/src/const_eval/eval_queries.rs b/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
index 7099cdd5a75..9e4e7911c3a 100644
--- a/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
+++ b/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
@@ -3,10 +3,11 @@ use either::{Left, Right};
 use rustc_hir::def::DefKind;
 use rustc_middle::mir::interpret::{AllocId, ErrorHandled, InterpErrorInfo};
 use rustc_middle::mir::{self, ConstAlloc, ConstValue};
+use rustc_middle::query::TyCtxtAt;
 use rustc_middle::traits::Reveal;
 use rustc_middle::ty::layout::LayoutOf;
 use rustc_middle::ty::print::with_no_trimmed_paths;
-use rustc_middle::ty::{self, TyCtxt};
+use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_span::def_id::LocalDefId;
 use rustc_span::Span;
 use rustc_target::abi::{self, Abi};
@@ -87,13 +88,16 @@ fn eval_body_using_ecx<'mir, 'tcx>(
 }
 
 /// The `InterpCx` is only meant to be used to do field and index projections into constants for
-/// `simd_shuffle` and const patterns in match arms. It never performs alignment checks.
+/// `simd_shuffle` and const patterns in match arms.
+///
+/// This should *not* be used to do any actual interpretation. In particular, alignment checks are
+/// turned off!
 ///
 /// The function containing the `match` that is currently being analyzed may have generic bounds
 /// that inform us about the generic bounds of the constant. E.g., using an associated constant
 /// of a function's generic parameter will require knowledge about the bounds on the generic
 /// parameter. These bounds are passed to `mk_eval_cx` via the `ParamEnv` argument.
-pub(crate) fn mk_eval_cx<'mir, 'tcx>(
+pub(crate) fn mk_eval_cx_to_read_const_val<'mir, 'tcx>(
     tcx: TyCtxt<'tcx>,
     root_span: Span,
     param_env: ty::ParamEnv<'tcx>,
@@ -108,6 +112,19 @@ pub(crate) fn mk_eval_cx<'mir, 'tcx>(
     )
 }
 
+/// Create an interpreter context to inspect the given `ConstValue`.
+/// Returns both the context and an `OpTy` that represents the constant.
+pub fn mk_eval_cx_for_const_val<'mir, 'tcx>(
+    tcx: TyCtxtAt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    val: mir::ConstValue<'tcx>,
+    ty: Ty<'tcx>,
+) -> Option<(CompileTimeEvalContext<'mir, 'tcx>, OpTy<'tcx>)> {
+    let ecx = mk_eval_cx_to_read_const_val(tcx.tcx, tcx.span, param_env, CanAccessMutGlobal::No);
+    let op = ecx.const_val_to_op(val, ty, None).ok()?;
+    Some((ecx, op))
+}
+
 /// This function converts an interpreter value into a MIR constant.
 ///
 /// The `for_diagnostics` flag turns the usual rules for returning `ConstValue::Scalar` into a
@@ -203,7 +220,7 @@ pub(crate) fn turn_into_const_value<'tcx>(
     let def_id = cid.instance.def.def_id();
     let is_static = tcx.is_static(def_id);
     // This is just accessing an already computed constant, so no need to check alignment here.
-    let ecx = mk_eval_cx(
+    let ecx = mk_eval_cx_to_read_const_val(
         tcx,
         tcx.def_span(key.value.instance.def_id()),
         key.param_env,
diff --git a/compiler/rustc_const_eval/src/const_eval/mod.rs b/compiler/rustc_const_eval/src/const_eval/mod.rs
index cd50701040e..289dcb7d01d 100644
--- a/compiler/rustc_const_eval/src/const_eval/mod.rs
+++ b/compiler/rustc_const_eval/src/const_eval/mod.rs
@@ -47,8 +47,7 @@ pub(crate) fn try_destructure_mir_constant_for_user_output<'tcx>(
     ty: Ty<'tcx>,
 ) -> Option<mir::DestructuredConstant<'tcx>> {
     let param_env = ty::ParamEnv::reveal_all();
-    let ecx = mk_eval_cx(tcx.tcx, tcx.span, param_env, CanAccessMutGlobal::No);
-    let op = ecx.const_val_to_op(val, ty, None).ok()?;
+    let (ecx, op) = mk_eval_cx_for_const_val(tcx, param_env, val, ty)?;
 
     // We go to `usize` as we cannot allocate anything bigger anyway.
     let (field_count, variant, down) = match ty.kind() {
diff --git a/compiler/rustc_const_eval/src/const_eval/valtrees.rs b/compiler/rustc_const_eval/src/const_eval/valtrees.rs
index 514a6a7df76..d3428d27d52 100644
--- a/compiler/rustc_const_eval/src/const_eval/valtrees.rs
+++ b/compiler/rustc_const_eval/src/const_eval/valtrees.rs
@@ -5,7 +5,7 @@ use rustc_middle::ty::{self, ScalarInt, Ty, TyCtxt};
 use rustc_span::DUMMY_SP;
 use rustc_target::abi::{Abi, VariantIdx};
 
-use super::eval_queries::{mk_eval_cx, op_to_const};
+use super::eval_queries::{mk_eval_cx_to_read_const_val, op_to_const};
 use super::machine::CompileTimeEvalContext;
 use super::{ValTreeCreationError, ValTreeCreationResult, VALTREE_MAX_NODES};
 use crate::const_eval::CanAccessMutGlobal;
@@ -223,7 +223,7 @@ pub(crate) fn eval_to_valtree<'tcx>(
     let const_alloc = tcx.eval_to_allocation_raw(param_env.and(cid))?;
 
     // FIXME Need to provide a span to `eval_to_valtree`
-    let ecx = mk_eval_cx(
+    let ecx = mk_eval_cx_to_read_const_val(
         tcx,
         DUMMY_SP,
         param_env,
@@ -287,7 +287,8 @@ pub fn valtree_to_const_value<'tcx>(
             }
         }
         ty::Ref(_, inner_ty, _) => {
-            let mut ecx = mk_eval_cx(tcx, DUMMY_SP, param_env, CanAccessMutGlobal::No);
+            let mut ecx =
+                mk_eval_cx_to_read_const_val(tcx, DUMMY_SP, param_env, CanAccessMutGlobal::No);
             let imm = valtree_to_ref(&mut ecx, valtree, *inner_ty);
             let imm = ImmTy::from_immediate(imm, tcx.layout_of(param_env_ty).unwrap());
             op_to_const(&ecx, &imm.into(), /* for diagnostics */ false)
@@ -314,7 +315,8 @@ pub fn valtree_to_const_value<'tcx>(
                 bug!("could not find non-ZST field during in {layout:#?}");
             }
 
-            let mut ecx = mk_eval_cx(tcx, DUMMY_SP, param_env, CanAccessMutGlobal::No);
+            let mut ecx =
+                mk_eval_cx_to_read_const_val(tcx, DUMMY_SP, param_env, CanAccessMutGlobal::No);
 
             // Need to create a place for this valtree.
             let place = create_valtree_place(&mut ecx, layout, valtree);
diff --git a/compiler/rustc_const_eval/src/util/caller_location.rs b/compiler/rustc_const_eval/src/util/caller_location.rs
index d1c2d22b5a9..b8e15c485f5 100644
--- a/compiler/rustc_const_eval/src/util/caller_location.rs
+++ b/compiler/rustc_const_eval/src/util/caller_location.rs
@@ -6,7 +6,7 @@ use rustc_middle::ty::layout::LayoutOf;
 use rustc_span::symbol::Symbol;
 use rustc_type_ir::Mutability;
 
-use crate::const_eval::{mk_eval_cx, CanAccessMutGlobal, CompileTimeEvalContext};
+use crate::const_eval::{mk_eval_cx_to_read_const_val, CanAccessMutGlobal, CompileTimeEvalContext};
 use crate::interpret::*;
 
 /// Allocate a `const core::panic::Location` with the provided filename and line/column numbers.
@@ -57,7 +57,12 @@ pub(crate) fn const_caller_location_provider(
     col: u32,
 ) -> mir::ConstValue<'_> {
     trace!("const_caller_location: {}:{}:{}", file, line, col);
-    let mut ecx = mk_eval_cx(tcx.tcx, tcx.span, ty::ParamEnv::reveal_all(), CanAccessMutGlobal::No);
+    let mut ecx = mk_eval_cx_to_read_const_val(
+        tcx.tcx,
+        tcx.span,
+        ty::ParamEnv::reveal_all(),
+        CanAccessMutGlobal::No,
+    );
 
     let loc_place = alloc_caller_location(&mut ecx, file, line, col);
     if intern_const_alloc_recursive(&mut ecx, InternKind::Constant, &loc_place).is_err() {
diff --git a/compiler/rustc_errors/src/diagnostic.rs b/compiler/rustc_errors/src/diagnostic.rs
index 57610635ee6..f096f015910 100644
--- a/compiler/rustc_errors/src/diagnostic.rs
+++ b/compiler/rustc_errors/src/diagnostic.rs
@@ -1,19 +1,22 @@
 use crate::snippet::Style;
 use crate::{
-    CodeSuggestion, DiagnosticBuilder, DiagnosticMessage, EmissionGuarantee, ErrCode, Level,
-    MultiSpan, SubdiagnosticMessage, Substitution, SubstitutionPart, SuggestionStyle,
+    CodeSuggestion, DiagCtxt, DiagnosticMessage, ErrCode, ErrorGuaranteed, ExplicitBug, Level,
+    MultiSpan, StashKey, SubdiagnosticMessage, Substitution, SubstitutionPart, SuggestionStyle,
 };
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_error_messages::fluent_value_from_str_list_sep_by_and;
 use rustc_error_messages::FluentValue;
 use rustc_lint_defs::{Applicability, LintExpectationId};
+use rustc_span::source_map::Spanned;
 use rustc_span::symbol::Symbol;
 use rustc_span::{Span, DUMMY_SP};
 use std::borrow::Cow;
 use std::fmt::{self, Debug};
 use std::hash::{Hash, Hasher};
+use std::marker::PhantomData;
 use std::ops::{Deref, DerefMut};
-use std::panic::Location;
+use std::panic;
+use std::thread::panicking;
 
 /// Error type for `Diagnostic`'s `suggestions` field, indicating that
 /// `.disable_suggestions()` was called on the `Diagnostic`.
@@ -40,6 +43,86 @@ pub enum DiagnosticArgValue {
     StrListSepByAnd(Vec<Cow<'static, str>>),
 }
 
+/// Trait for types that `DiagnosticBuilder::emit` can return as a "guarantee"
+/// (or "proof") token that the emission happened.
+pub trait EmissionGuarantee: Sized {
+    /// This exists so that bugs and fatal errors can both result in `!` (an
+    /// abort) when emitted, but have different aborting behaviour.
+    type EmitResult = Self;
+
+    /// Implementation of `DiagnosticBuilder::emit`, fully controlled by each
+    /// `impl` of `EmissionGuarantee`, to make it impossible to create a value
+    /// of `Self::EmitResult` without actually performing the emission.
+    #[track_caller]
+    fn emit_producing_guarantee(db: DiagnosticBuilder<'_, Self>) -> Self::EmitResult;
+}
+
+impl EmissionGuarantee for ErrorGuaranteed {
+    fn emit_producing_guarantee(db: DiagnosticBuilder<'_, Self>) -> Self::EmitResult {
+        db.emit_producing_error_guaranteed()
+    }
+}
+
+impl EmissionGuarantee for () {
+    fn emit_producing_guarantee(db: DiagnosticBuilder<'_, Self>) -> Self::EmitResult {
+        db.emit_producing_nothing();
+    }
+}
+
+/// Marker type which enables implementation of `create_bug` and `emit_bug` functions for
+/// bug diagnostics.
+#[derive(Copy, Clone)]
+pub struct BugAbort;
+
+impl EmissionGuarantee for BugAbort {
+    type EmitResult = !;
+
+    fn emit_producing_guarantee(db: DiagnosticBuilder<'_, Self>) -> Self::EmitResult {
+        db.emit_producing_nothing();
+        panic::panic_any(ExplicitBug);
+    }
+}
+
+/// Marker type which enables implementation of `create_fatal` and `emit_fatal` functions for
+/// fatal diagnostics.
+#[derive(Copy, Clone)]
+pub struct FatalAbort;
+
+impl EmissionGuarantee for FatalAbort {
+    type EmitResult = !;
+
+    fn emit_producing_guarantee(db: DiagnosticBuilder<'_, Self>) -> Self::EmitResult {
+        db.emit_producing_nothing();
+        crate::FatalError.raise()
+    }
+}
+
+impl EmissionGuarantee for rustc_span::fatal_error::FatalError {
+    fn emit_producing_guarantee(db: DiagnosticBuilder<'_, Self>) -> Self::EmitResult {
+        db.emit_producing_nothing();
+        rustc_span::fatal_error::FatalError
+    }
+}
+
+/// Trait implemented by error types. This is rarely implemented manually. Instead, use
+/// `#[derive(Diagnostic)]` -- see [rustc_macros::Diagnostic].
+#[rustc_diagnostic_item = "IntoDiagnostic"]
+pub trait IntoDiagnostic<'a, G: EmissionGuarantee = ErrorGuaranteed> {
+    /// Write out as a diagnostic out of `DiagCtxt`.
+    #[must_use]
+    fn into_diagnostic(self, dcx: &'a DiagCtxt, level: Level) -> DiagnosticBuilder<'a, G>;
+}
+
+impl<'a, T, G> IntoDiagnostic<'a, G> for Spanned<T>
+where
+    T: IntoDiagnostic<'a, G>,
+    G: EmissionGuarantee,
+{
+    fn into_diagnostic(self, dcx: &'a DiagCtxt, level: Level) -> DiagnosticBuilder<'a, G> {
+        self.node.into_diagnostic(dcx, level).with_span(self.span)
+    }
+}
+
 /// Converts a value of a type into a `DiagnosticArg` (typically a field of an `IntoDiagnostic`
 /// struct). Implemented as a custom trait rather than `From` so that it is implemented on the type
 /// being converted rather than on `DiagnosticArgValue`, which enables types from other `rustc_*`
@@ -98,36 +181,6 @@ pub trait DecorateLint<'a, G: EmissionGuarantee> {
     fn msg(&self) -> DiagnosticMessage;
 }
 
-/// The main part of a diagnostic. Note that `DiagnosticBuilder`, which wraps
-/// this type, is used for most operations, and should be used instead whenever
-/// possible. This type should only be used when `DiagnosticBuilder`'s lifetime
-/// causes difficulties, e.g. when storing diagnostics within `DiagCtxt`.
-#[must_use]
-#[derive(Clone, Debug, Encodable, Decodable)]
-pub struct Diagnostic {
-    // NOTE(eddyb) this is private to disallow arbitrary after-the-fact changes,
-    // outside of what methods in this crate themselves allow.
-    pub(crate) level: Level,
-
-    pub messages: Vec<(DiagnosticMessage, Style)>,
-    pub code: Option<ErrCode>,
-    pub span: MultiSpan,
-    pub children: Vec<SubDiagnostic>,
-    pub suggestions: Result<Vec<CodeSuggestion>, SuggestionsDisabled>,
-    args: FxIndexMap<DiagnosticArgName, DiagnosticArgValue>,
-
-    /// This is not used for highlighting or rendering any error message. Rather, it can be used
-    /// as a sort key to sort a buffer of diagnostics. By default, it is the primary span of
-    /// `span` if there is one. Otherwise, it is `DUMMY_SP`.
-    pub sort_span: Span,
-
-    pub is_lint: Option<IsLint>,
-
-    /// With `-Ztrack_diagnostics` enabled,
-    /// we print where in rustc this error was emitted.
-    pub(crate) emitted_at: DiagnosticLocation,
-}
-
 #[derive(Clone, Debug, Encodable, Decodable)]
 pub struct DiagnosticLocation {
     file: Cow<'static, str>,
@@ -138,7 +191,7 @@ pub struct DiagnosticLocation {
 impl DiagnosticLocation {
     #[track_caller]
     fn caller() -> Self {
-        let loc = Location::caller();
+        let loc = panic::Location::caller();
         DiagnosticLocation { file: loc.file().into(), line: loc.line(), col: loc.column() }
     }
 }
@@ -157,15 +210,6 @@ pub struct IsLint {
     has_future_breakage: bool,
 }
 
-/// A "sub"-diagnostic attached to a parent diagnostic.
-/// For example, a note attached to an error.
-#[derive(Clone, Debug, PartialEq, Hash, Encodable, Decodable)]
-pub struct SubDiagnostic {
-    pub level: Level,
-    pub messages: Vec<(DiagnosticMessage, Style)>,
-    pub span: MultiSpan,
-}
-
 #[derive(Debug, PartialEq, Eq)]
 pub struct DiagnosticStyledString(pub Vec<StringPart>);
 
@@ -215,6 +259,36 @@ impl StringPart {
     }
 }
 
+/// The main part of a diagnostic. Note that `DiagnosticBuilder`, which wraps
+/// this type, is used for most operations, and should be used instead whenever
+/// possible. This type should only be used when `DiagnosticBuilder`'s lifetime
+/// causes difficulties, e.g. when storing diagnostics within `DiagCtxt`.
+#[must_use]
+#[derive(Clone, Debug, Encodable, Decodable)]
+pub struct Diagnostic {
+    // NOTE(eddyb) this is private to disallow arbitrary after-the-fact changes,
+    // outside of what methods in this crate themselves allow.
+    pub(crate) level: Level,
+
+    pub messages: Vec<(DiagnosticMessage, Style)>,
+    pub code: Option<ErrCode>,
+    pub span: MultiSpan,
+    pub children: Vec<SubDiagnostic>,
+    pub suggestions: Result<Vec<CodeSuggestion>, SuggestionsDisabled>,
+    args: FxIndexMap<DiagnosticArgName, DiagnosticArgValue>,
+
+    /// This is not used for highlighting or rendering any error message. Rather, it can be used
+    /// as a sort key to sort a buffer of diagnostics. By default, it is the primary span of
+    /// `span` if there is one. Otherwise, it is `DUMMY_SP`.
+    pub sort_span: Span,
+
+    pub is_lint: Option<IsLint>,
+
+    /// With `-Ztrack_diagnostics` enabled,
+    /// we print where in rustc this error was emitted.
+    pub(crate) emitted_at: DiagnosticLocation,
+}
+
 impl Diagnostic {
     #[track_caller]
     pub fn new<M: Into<DiagnosticMessage>>(level: Level, message: M) -> Self {
@@ -336,6 +410,118 @@ impl Diagnostic {
     pub fn replace_args(&mut self, args: FxIndexMap<DiagnosticArgName, DiagnosticArgValue>) {
         self.args = args;
     }
+
+    /// Fields used for Hash, and PartialEq trait.
+    fn keys(
+        &self,
+    ) -> (
+        &Level,
+        &[(DiagnosticMessage, Style)],
+        &Option<ErrCode>,
+        &MultiSpan,
+        &[SubDiagnostic],
+        &Result<Vec<CodeSuggestion>, SuggestionsDisabled>,
+        Vec<(&DiagnosticArgName, &DiagnosticArgValue)>,
+        &Option<IsLint>,
+    ) {
+        (
+            &self.level,
+            &self.messages,
+            &self.code,
+            &self.span,
+            &self.children,
+            &self.suggestions,
+            self.args().collect(),
+            // omit self.sort_span
+            &self.is_lint,
+            // omit self.emitted_at
+        )
+    }
+}
+
+impl Hash for Diagnostic {
+    fn hash<H>(&self, state: &mut H)
+    where
+        H: Hasher,
+    {
+        self.keys().hash(state);
+    }
+}
+
+impl PartialEq for Diagnostic {
+    fn eq(&self, other: &Self) -> bool {
+        self.keys() == other.keys()
+    }
+}
+
+/// A "sub"-diagnostic attached to a parent diagnostic.
+/// For example, a note attached to an error.
+#[derive(Clone, Debug, PartialEq, Hash, Encodable, Decodable)]
+pub struct SubDiagnostic {
+    pub level: Level,
+    pub messages: Vec<(DiagnosticMessage, Style)>,
+    pub span: MultiSpan,
+}
+
+/// Used for emitting structured error messages and other diagnostic information.
+/// Wraps a `Diagnostic`, adding some useful things.
+/// - The `dcx` field, allowing it to (a) emit itself, and (b) do a drop check
+///   that it has been emitted or cancelled.
+/// - The `EmissionGuarantee`, which determines the type returned from `emit`.
+///
+/// Each constructed `DiagnosticBuilder` must be consumed by a function such as
+/// `emit`, `cancel`, `delay_as_bug`, or `into_diagnostic`. A panic occurrs if a
+/// `DiagnosticBuilder` is dropped without being consumed by one of these
+/// functions.
+///
+/// If there is some state in a downstream crate you would like to
+/// access in the methods of `DiagnosticBuilder` here, consider
+/// extending `DiagCtxtFlags`.
+#[must_use]
+pub struct DiagnosticBuilder<'a, G: EmissionGuarantee = ErrorGuaranteed> {
+    pub dcx: &'a DiagCtxt,
+
+    /// Why the `Option`? It is always `Some` until the `DiagnosticBuilder` is
+    /// consumed via `emit`, `cancel`, etc. At that point it is consumed and
+    /// replaced with `None`. Then `drop` checks that it is `None`; if not, it
+    /// panics because a diagnostic was built but not used.
+    ///
+    /// Why the Box? `Diagnostic` is a large type, and `DiagnosticBuilder` is
+    /// often used as a return value, especially within the frequently-used
+    /// `PResult` type. In theory, return value optimization (RVO) should avoid
+    /// unnecessary copying. In practice, it does not (at the time of writing).
+    diag: Option<Box<Diagnostic>>,
+
+    _marker: PhantomData<G>,
+}
+
+// Cloning a `DiagnosticBuilder` is a recipe for a diagnostic being emitted
+// twice, which would be bad.
+impl<G> !Clone for DiagnosticBuilder<'_, G> {}
+
+rustc_data_structures::static_assert_size!(
+    DiagnosticBuilder<'_, ()>,
+    2 * std::mem::size_of::<usize>()
+);
+
+impl<G: EmissionGuarantee> Deref for DiagnosticBuilder<'_, G> {
+    type Target = Diagnostic;
+
+    fn deref(&self) -> &Diagnostic {
+        self.diag.as_ref().unwrap()
+    }
+}
+
+impl<G: EmissionGuarantee> DerefMut for DiagnosticBuilder<'_, G> {
+    fn deref_mut(&mut self) -> &mut Diagnostic {
+        self.diag.as_mut().unwrap()
+    }
+}
+
+impl<G: EmissionGuarantee> Debug for DiagnosticBuilder<'_, G> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.diag.fmt(f)
+    }
 }
 
 /// `DiagnosticBuilder` impls many `&mut self -> &mut Self` methods. Each one
@@ -382,6 +568,20 @@ macro_rules! with_fn {
 }
 
 impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
+    #[rustc_lint_diagnostics]
+    #[track_caller]
+    pub fn new<M: Into<DiagnosticMessage>>(dcx: &'a DiagCtxt, level: Level, message: M) -> Self {
+        Self::new_diagnostic(dcx, Diagnostic::new(level, message))
+    }
+
+    /// Creates a new `DiagnosticBuilder` with an already constructed
+    /// diagnostic.
+    #[track_caller]
+    pub(crate) fn new_diagnostic(dcx: &'a DiagCtxt, diag: Diagnostic) -> Self {
+        debug!("Created new diagnostic");
+        Self { dcx, diag: Some(Box::new(diag)), _marker: PhantomData }
+    }
+
     /// Delay emission of this diagnostic as a bug.
     ///
     /// This can be useful in contexts where an error indicates a bug but
@@ -1040,48 +1240,112 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
         let sub = SubDiagnostic { level, messages, span };
         self.children.push(sub);
     }
-}
 
-impl Diagnostic {
-    /// Fields used for Hash, and PartialEq trait
-    fn keys(
-        &self,
-    ) -> (
-        &Level,
-        &[(DiagnosticMessage, Style)],
-        &Option<ErrCode>,
-        &MultiSpan,
-        &[SubDiagnostic],
-        &Result<Vec<CodeSuggestion>, SuggestionsDisabled>,
-        Vec<(&DiagnosticArgName, &DiagnosticArgValue)>,
-        &Option<IsLint>,
-    ) {
-        (
-            &self.level,
-            &self.messages,
-            &self.code,
-            &self.span,
-            &self.children,
-            &self.suggestions,
-            self.args().collect(),
-            // omit self.sort_span
-            &self.is_lint,
-            // omit self.emitted_at
-        )
+    /// Takes the diagnostic. For use by methods that consume the
+    /// DiagnosticBuilder: `emit`, `cancel`, etc. Afterwards, `drop` is the
+    /// only code that will be run on `self`.
+    fn take_diag(&mut self) -> Diagnostic {
+        Box::into_inner(self.diag.take().unwrap())
     }
-}
 
-impl Hash for Diagnostic {
-    fn hash<H>(&self, state: &mut H)
-    where
-        H: Hasher,
-    {
-        self.keys().hash(state);
+    /// Most `emit_producing_guarantee` functions use this as a starting point.
+    fn emit_producing_nothing(mut self) {
+        let diag = self.take_diag();
+        self.dcx.emit_diagnostic(diag);
+    }
+
+    /// `ErrorGuaranteed::emit_producing_guarantee` uses this.
+    fn emit_producing_error_guaranteed(mut self) -> ErrorGuaranteed {
+        let diag = self.take_diag();
+
+        // The only error levels that produce `ErrorGuaranteed` are
+        // `Error` and `DelayedBug`. But `DelayedBug` should never occur here
+        // because delayed bugs have their level changed to `Bug` when they are
+        // actually printed, so they produce an ICE.
+        //
+        // (Also, even though `level` isn't `pub`, the whole `Diagnostic` could
+        // be overwritten with a new one thanks to `DerefMut`. So this assert
+        // protects against that, too.)
+        assert!(
+            matches!(diag.level, Level::Error | Level::DelayedBug),
+            "invalid diagnostic level ({:?})",
+            diag.level,
+        );
+
+        let guar = self.dcx.emit_diagnostic(diag);
+        guar.unwrap()
+    }
+
+    /// Emit and consume the diagnostic.
+    #[track_caller]
+    pub fn emit(self) -> G::EmitResult {
+        G::emit_producing_guarantee(self)
+    }
+
+    /// Emit the diagnostic unless `delay` is true,
+    /// in which case the emission will be delayed as a bug.
+    ///
+    /// See `emit` and `delay_as_bug` for details.
+    #[track_caller]
+    pub fn emit_unless(mut self, delay: bool) -> G::EmitResult {
+        if delay {
+            self.downgrade_to_delayed_bug();
+        }
+        self.emit()
+    }
+
+    /// Cancel and consume the diagnostic. (A diagnostic must either be emitted or
+    /// cancelled or it will panic when dropped).
+    pub fn cancel(mut self) {
+        self.diag = None;
+        drop(self);
+    }
+
+    /// Stashes diagnostic for possible later improvement in a different,
+    /// later stage of the compiler. The diagnostic can be accessed with
+    /// the provided `span` and `key` through [`DiagCtxt::steal_diagnostic()`].
+    pub fn stash(mut self, span: Span, key: StashKey) {
+        self.dcx.stash_diagnostic(span, key, self.take_diag());
+    }
+
+    /// Delay emission of this diagnostic as a bug.
+    ///
+    /// This can be useful in contexts where an error indicates a bug but
+    /// typically this only happens when other compilation errors have already
+    /// happened. In those cases this can be used to defer emission of this
+    /// diagnostic as a bug in the compiler only if no other errors have been
+    /// emitted.
+    ///
+    /// In the meantime, though, callsites are required to deal with the "bug"
+    /// locally in whichever way makes the most sense.
+    #[track_caller]
+    pub fn delay_as_bug(mut self) -> G::EmitResult {
+        self.downgrade_to_delayed_bug();
+        self.emit()
     }
 }
 
-impl PartialEq for Diagnostic {
-    fn eq(&self, other: &Self) -> bool {
-        self.keys() == other.keys()
+/// Destructor bomb: every `DiagnosticBuilder` must be consumed (emitted,
+/// cancelled, etc.) or we emit a bug.
+impl<G: EmissionGuarantee> Drop for DiagnosticBuilder<'_, G> {
+    fn drop(&mut self) {
+        match self.diag.take() {
+            Some(diag) if !panicking() => {
+                self.dcx.emit_diagnostic(Diagnostic::new(
+                    Level::Bug,
+                    DiagnosticMessage::from("the following error was constructed but not emitted"),
+                ));
+                self.dcx.emit_diagnostic(*diag);
+                panic!("error was constructed but not emitted");
+            }
+            _ => {}
+        }
     }
 }
+
+#[macro_export]
+macro_rules! struct_span_code_err {
+    ($dcx:expr, $span:expr, $code:expr, $($message:tt)*) => ({
+        $dcx.struct_span_err($span, format!($($message)*)).with_code($code)
+    })
+}
diff --git a/compiler/rustc_errors/src/diagnostic_builder.rs b/compiler/rustc_errors/src/diagnostic_builder.rs
deleted file mode 100644
index 3a6a494af95..00000000000
--- a/compiler/rustc_errors/src/diagnostic_builder.rs
+++ /dev/null
@@ -1,282 +0,0 @@
-use crate::{
-    DiagCtxt, Diagnostic, DiagnosticMessage, ErrorGuaranteed, ExplicitBug, Level, StashKey,
-};
-use rustc_span::source_map::Spanned;
-use rustc_span::Span;
-use std::fmt::{self, Debug};
-use std::marker::PhantomData;
-use std::ops::{Deref, DerefMut};
-use std::panic;
-use std::thread::panicking;
-
-/// Trait implemented by error types. This is rarely implemented manually. Instead, use
-/// `#[derive(Diagnostic)]` -- see [rustc_macros::Diagnostic].
-#[rustc_diagnostic_item = "IntoDiagnostic"]
-pub trait IntoDiagnostic<'a, G: EmissionGuarantee = ErrorGuaranteed> {
-    /// Write out as a diagnostic out of `DiagCtxt`.
-    #[must_use]
-    fn into_diagnostic(self, dcx: &'a DiagCtxt, level: Level) -> DiagnosticBuilder<'a, G>;
-}
-
-impl<'a, T, G> IntoDiagnostic<'a, G> for Spanned<T>
-where
-    T: IntoDiagnostic<'a, G>,
-    G: EmissionGuarantee,
-{
-    fn into_diagnostic(self, dcx: &'a DiagCtxt, level: Level) -> DiagnosticBuilder<'a, G> {
-        self.node.into_diagnostic(dcx, level).with_span(self.span)
-    }
-}
-
-/// Used for emitting structured error messages and other diagnostic information.
-/// Wraps a `Diagnostic`, adding some useful things.
-/// - The `dcx` field, allowing it to (a) emit itself, and (b) do a drop check
-///   that it has been emitted or cancelled.
-/// - The `EmissionGuarantee`, which determines the type returned from `emit`.
-///
-/// Each constructed `DiagnosticBuilder` must be consumed by a function such as
-/// `emit`, `cancel`, `delay_as_bug`, or `into_diagnostic`. A panic occurrs if a
-/// `DiagnosticBuilder` is dropped without being consumed by one of these
-/// functions.
-///
-/// If there is some state in a downstream crate you would like to
-/// access in the methods of `DiagnosticBuilder` here, consider
-/// extending `DiagCtxtFlags`.
-#[must_use]
-pub struct DiagnosticBuilder<'a, G: EmissionGuarantee = ErrorGuaranteed> {
-    pub dcx: &'a DiagCtxt,
-
-    /// Why the `Option`? It is always `Some` until the `DiagnosticBuilder` is
-    /// consumed via `emit`, `cancel`, etc. At that point it is consumed and
-    /// replaced with `None`. Then `drop` checks that it is `None`; if not, it
-    /// panics because a diagnostic was built but not used.
-    ///
-    /// Why the Box? `Diagnostic` is a large type, and `DiagnosticBuilder` is
-    /// often used as a return value, especially within the frequently-used
-    /// `PResult` type. In theory, return value optimization (RVO) should avoid
-    /// unnecessary copying. In practice, it does not (at the time of writing).
-    // FIXME(nnethercote) Make private once this moves to diagnostic.rs.
-    pub(crate) diag: Option<Box<Diagnostic>>,
-
-    // FIXME(nnethercote) Make private once this moves to diagnostic.rs.
-    pub(crate) _marker: PhantomData<G>,
-}
-
-// Cloning a `DiagnosticBuilder` is a recipe for a diagnostic being emitted
-// twice, which would be bad.
-impl<G> !Clone for DiagnosticBuilder<'_, G> {}
-
-rustc_data_structures::static_assert_size!(
-    DiagnosticBuilder<'_, ()>,
-    2 * std::mem::size_of::<usize>()
-);
-
-/// Trait for types that `DiagnosticBuilder::emit` can return as a "guarantee"
-/// (or "proof") token that the emission happened.
-pub trait EmissionGuarantee: Sized {
-    /// This exists so that bugs and fatal errors can both result in `!` (an
-    /// abort) when emitted, but have different aborting behaviour.
-    type EmitResult = Self;
-
-    /// Implementation of `DiagnosticBuilder::emit`, fully controlled by each
-    /// `impl` of `EmissionGuarantee`, to make it impossible to create a value
-    /// of `Self::EmitResult` without actually performing the emission.
-    #[track_caller]
-    fn emit_producing_guarantee(db: DiagnosticBuilder<'_, Self>) -> Self::EmitResult;
-}
-
-impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
-    /// Takes the diagnostic. For use by methods that consume the
-    /// DiagnosticBuilder: `emit`, `cancel`, etc. Afterwards, `drop` is the
-    /// only code that will be run on `self`.
-    // FIXME(nnethercote) Make private once this moves to diagnostic.rs.
-    pub(crate) fn take_diag(&mut self) -> Diagnostic {
-        Box::into_inner(self.diag.take().unwrap())
-    }
-
-    /// Most `emit_producing_guarantee` functions use this as a starting point.
-    // FIXME(nnethercote) Make private once this moves to diagnostic.rs.
-    pub(crate) fn emit_producing_nothing(mut self) {
-        let diag = self.take_diag();
-        self.dcx.emit_diagnostic(diag);
-    }
-
-    /// `ErrorGuaranteed::emit_producing_guarantee` uses this.
-    // FIXME(nnethercote) Make private once this moves to diagnostic.rs.
-    pub(crate) fn emit_producing_error_guaranteed(mut self) -> ErrorGuaranteed {
-        let diag = self.take_diag();
-
-        // The only error levels that produce `ErrorGuaranteed` are
-        // `Error` and `DelayedBug`. But `DelayedBug` should never occur here
-        // because delayed bugs have their level changed to `Bug` when they are
-        // actually printed, so they produce an ICE.
-        //
-        // (Also, even though `level` isn't `pub`, the whole `Diagnostic` could
-        // be overwritten with a new one thanks to `DerefMut`. So this assert
-        // protects against that, too.)
-        assert!(
-            matches!(diag.level, Level::Error | Level::DelayedBug),
-            "invalid diagnostic level ({:?})",
-            diag.level,
-        );
-
-        let guar = self.dcx.emit_diagnostic(diag);
-        guar.unwrap()
-    }
-}
-
-impl EmissionGuarantee for ErrorGuaranteed {
-    fn emit_producing_guarantee(db: DiagnosticBuilder<'_, Self>) -> Self::EmitResult {
-        db.emit_producing_error_guaranteed()
-    }
-}
-
-impl EmissionGuarantee for () {
-    fn emit_producing_guarantee(db: DiagnosticBuilder<'_, Self>) -> Self::EmitResult {
-        db.emit_producing_nothing();
-    }
-}
-
-/// Marker type which enables implementation of `create_bug` and `emit_bug` functions for
-/// bug diagnostics.
-#[derive(Copy, Clone)]
-pub struct BugAbort;
-
-impl EmissionGuarantee for BugAbort {
-    type EmitResult = !;
-
-    fn emit_producing_guarantee(db: DiagnosticBuilder<'_, Self>) -> Self::EmitResult {
-        db.emit_producing_nothing();
-        panic::panic_any(ExplicitBug);
-    }
-}
-
-/// Marker type which enables implementation of `create_fatal` and `emit_fatal` functions for
-/// fatal diagnostics.
-#[derive(Copy, Clone)]
-pub struct FatalAbort;
-
-impl EmissionGuarantee for FatalAbort {
-    type EmitResult = !;
-
-    fn emit_producing_guarantee(db: DiagnosticBuilder<'_, Self>) -> Self::EmitResult {
-        db.emit_producing_nothing();
-        crate::FatalError.raise()
-    }
-}
-
-impl EmissionGuarantee for rustc_span::fatal_error::FatalError {
-    fn emit_producing_guarantee(db: DiagnosticBuilder<'_, Self>) -> Self::EmitResult {
-        db.emit_producing_nothing();
-        rustc_span::fatal_error::FatalError
-    }
-}
-
-impl<G: EmissionGuarantee> Deref for DiagnosticBuilder<'_, G> {
-    type Target = Diagnostic;
-
-    fn deref(&self) -> &Diagnostic {
-        self.diag.as_ref().unwrap()
-    }
-}
-
-impl<G: EmissionGuarantee> DerefMut for DiagnosticBuilder<'_, G> {
-    fn deref_mut(&mut self) -> &mut Diagnostic {
-        self.diag.as_mut().unwrap()
-    }
-}
-
-impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
-    #[rustc_lint_diagnostics]
-    #[track_caller]
-    pub fn new<M: Into<DiagnosticMessage>>(dcx: &'a DiagCtxt, level: Level, message: M) -> Self {
-        Self::new_diagnostic(dcx, Diagnostic::new(level, message))
-    }
-
-    /// Creates a new `DiagnosticBuilder` with an already constructed
-    /// diagnostic.
-    #[track_caller]
-    pub(crate) fn new_diagnostic(dcx: &'a DiagCtxt, diag: Diagnostic) -> Self {
-        debug!("Created new diagnostic");
-        Self { dcx, diag: Some(Box::new(diag)), _marker: PhantomData }
-    }
-
-    /// Emit and consume the diagnostic.
-    #[track_caller]
-    pub fn emit(self) -> G::EmitResult {
-        G::emit_producing_guarantee(self)
-    }
-
-    /// Emit the diagnostic unless `delay` is true,
-    /// in which case the emission will be delayed as a bug.
-    ///
-    /// See `emit` and `delay_as_bug` for details.
-    #[track_caller]
-    pub fn emit_unless(mut self, delay: bool) -> G::EmitResult {
-        if delay {
-            self.downgrade_to_delayed_bug();
-        }
-        self.emit()
-    }
-
-    /// Cancel and consume the diagnostic. (A diagnostic must either be emitted or
-    /// cancelled or it will panic when dropped).
-    pub fn cancel(mut self) {
-        self.diag = None;
-        drop(self);
-    }
-
-    /// Stashes diagnostic for possible later improvement in a different,
-    /// later stage of the compiler. The diagnostic can be accessed with
-    /// the provided `span` and `key` through [`DiagCtxt::steal_diagnostic()`].
-    pub fn stash(mut self, span: Span, key: StashKey) {
-        self.dcx.stash_diagnostic(span, key, self.take_diag());
-    }
-
-    /// Delay emission of this diagnostic as a bug.
-    ///
-    /// This can be useful in contexts where an error indicates a bug but
-    /// typically this only happens when other compilation errors have already
-    /// happened. In those cases this can be used to defer emission of this
-    /// diagnostic as a bug in the compiler only if no other errors have been
-    /// emitted.
-    ///
-    /// In the meantime, though, callsites are required to deal with the "bug"
-    /// locally in whichever way makes the most sense.
-    #[track_caller]
-    pub fn delay_as_bug(mut self) -> G::EmitResult {
-        self.downgrade_to_delayed_bug();
-        self.emit()
-    }
-}
-
-impl<G: EmissionGuarantee> Debug for DiagnosticBuilder<'_, G> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        self.diag.fmt(f)
-    }
-}
-
-/// Destructor bomb: every `DiagnosticBuilder` must be consumed (emitted,
-/// cancelled, etc.) or we emit a bug.
-impl<G: EmissionGuarantee> Drop for DiagnosticBuilder<'_, G> {
-    fn drop(&mut self) {
-        match self.diag.take() {
-            Some(diag) if !panicking() => {
-                self.dcx.emit_diagnostic(Diagnostic::new(
-                    Level::Bug,
-                    DiagnosticMessage::from("the following error was constructed but not emitted"),
-                ));
-                self.dcx.emit_diagnostic(*diag);
-                panic!("error was constructed but not emitted");
-            }
-            _ => {}
-        }
-    }
-}
-
-#[macro_export]
-macro_rules! struct_span_code_err {
-    ($dcx:expr, $span:expr, $code:expr, $($message:tt)*) => ({
-        $dcx.struct_span_err($span, format!($($message)*)).with_code($code)
-    })
-}
diff --git a/compiler/rustc_errors/src/lib.rs b/compiler/rustc_errors/src/lib.rs
index 73cda64f1cc..052d9b3a783 100644
--- a/compiler/rustc_errors/src/lib.rs
+++ b/compiler/rustc_errors/src/lib.rs
@@ -37,12 +37,9 @@ extern crate self as rustc_errors;
 
 pub use codes::*;
 pub use diagnostic::{
-    AddToDiagnostic, DecorateLint, Diagnostic, DiagnosticArg, DiagnosticArgName,
-    DiagnosticArgValue, DiagnosticStyledString, IntoDiagnosticArg, StringPart, SubDiagnostic,
-    SubdiagnosticMessageOp,
-};
-pub use diagnostic_builder::{
-    BugAbort, DiagnosticBuilder, EmissionGuarantee, FatalAbort, IntoDiagnostic,
+    AddToDiagnostic, BugAbort, DecorateLint, Diagnostic, DiagnosticArg, DiagnosticArgName,
+    DiagnosticArgValue, DiagnosticBuilder, DiagnosticStyledString, EmissionGuarantee, FatalAbort,
+    IntoDiagnostic, IntoDiagnosticArg, StringPart, SubDiagnostic, SubdiagnosticMessageOp,
 };
 pub use diagnostic_impls::{
     DiagnosticArgFromDisplay, DiagnosticSymbolList, ExpectedLifetimeParameter,
@@ -87,7 +84,6 @@ use Level::*;
 pub mod annotate_snippet_emitter_writer;
 pub mod codes;
 mod diagnostic;
-mod diagnostic_builder;
 mod diagnostic_impls;
 pub mod emitter;
 pub mod error;
diff --git a/compiler/rustc_infer/src/infer/relate/combine.rs b/compiler/rustc_infer/src/infer/relate/combine.rs
index 454de4f9785..f7690831c2a 100644
--- a/compiler/rustc_infer/src/infer/relate/combine.rs
+++ b/compiler/rustc_infer/src/infer/relate/combine.rs
@@ -194,7 +194,7 @@ impl<'tcx> InferCtxt<'tcx> {
                 ty::ConstKind::Infer(InferConst::Var(b_vid)),
             ) => {
                 self.inner.borrow_mut().const_unification_table().union(a_vid, b_vid);
-                return Ok(a);
+                Ok(a)
             }
 
             (
@@ -202,7 +202,7 @@ impl<'tcx> InferCtxt<'tcx> {
                 ty::ConstKind::Infer(InferConst::EffectVar(b_vid)),
             ) => {
                 self.inner.borrow_mut().effect_unification_table().union(a_vid, b_vid);
-                return Ok(a);
+                Ok(a)
             }
 
             // All other cases of inference with other variables are errors.
@@ -220,19 +220,21 @@ impl<'tcx> InferCtxt<'tcx> {
             }
 
             (ty::ConstKind::Infer(InferConst::Var(vid)), _) => {
-                return self.instantiate_const_var(vid, b);
+                self.instantiate_const_var(relation, relation.a_is_expected(), vid, b)?;
+                Ok(b)
             }
 
             (_, ty::ConstKind::Infer(InferConst::Var(vid))) => {
-                return self.instantiate_const_var(vid, a);
+                self.instantiate_const_var(relation, !relation.a_is_expected(), vid, a)?;
+                Ok(a)
             }
 
             (ty::ConstKind::Infer(InferConst::EffectVar(vid)), _) => {
-                return Ok(self.unify_effect_variable(vid, b));
+                Ok(self.unify_effect_variable(vid, b))
             }
 
             (_, ty::ConstKind::Infer(InferConst::EffectVar(vid))) => {
-                return Ok(self.unify_effect_variable(vid, a));
+                Ok(self.unify_effect_variable(vid, a))
             }
 
             (ty::ConstKind::Unevaluated(..), _) | (_, ty::ConstKind::Unevaluated(..))
@@ -240,7 +242,7 @@ impl<'tcx> InferCtxt<'tcx> {
             {
                 let (a, b) = if relation.a_is_expected() { (a, b) } else { (b, a) };
 
-                relation.register_predicates([ty::Binder::dummy(if self.next_trait_solver() {
+                relation.register_predicates([if self.next_trait_solver() {
                     ty::PredicateKind::AliasRelate(
                         a.into(),
                         b.into(),
@@ -248,14 +250,12 @@ impl<'tcx> InferCtxt<'tcx> {
                     )
                 } else {
                     ty::PredicateKind::ConstEquate(a, b)
-                })]);
+                }]);
 
-                return Ok(b);
+                Ok(b)
             }
-            _ => {}
+            _ => ty::relate::structurally_relate_consts(relation, a, b),
         }
-
-        ty::relate::structurally_relate_consts(relation, a, b)
     }
 
     fn unify_integral_variable(
diff --git a/compiler/rustc_infer/src/infer/relate/generalize.rs b/compiler/rustc_infer/src/infer/relate/generalize.rs
index c0cb02916fe..371340c5bbf 100644
--- a/compiler/rustc_infer/src/infer/relate/generalize.rs
+++ b/compiler/rustc_infer/src/infer/relate/generalize.rs
@@ -22,7 +22,7 @@ impl<'tcx> InferCtxt<'tcx> {
     /// subtyping could occur. This also does the occurs checks, detecting whether
     /// instantiating `target_vid` would result in a cyclic type. We eagerly error
     /// in this case.
-    #[instrument(skip(self, relation, target_is_expected), level = "debug")]
+    #[instrument(level = "debug", skip(self, relation, target_is_expected))]
     pub(super) fn instantiate_ty_var<R: ObligationEmittingRelation<'tcx>>(
         &self,
         relation: &mut R,
@@ -158,26 +158,22 @@ impl<'tcx> InferCtxt<'tcx> {
     /// As `3 + 4` contains `N` in its args, this must not succeed.
     ///
     /// See `tests/ui/const-generics/occurs-check/` for more examples where this is relevant.
-    #[instrument(level = "debug", skip(self))]
-    pub(super) fn instantiate_const_var(
+    #[instrument(level = "debug", skip(self, relation))]
+    pub(super) fn instantiate_const_var<R: ObligationEmittingRelation<'tcx>>(
         &self,
+        relation: &mut R,
+        target_is_expected: bool,
         target_vid: ty::ConstVid,
         source_ct: ty::Const<'tcx>,
-    ) -> RelateResult<'tcx, ty::Const<'tcx>> {
-        let span = match self.inner.borrow_mut().const_unification_table().probe_value(target_vid) {
-            ConstVariableValue::Known { value } => {
-                bug!("instantiating a known const var: {target_vid:?} {value} {source_ct}")
-            }
-            ConstVariableValue::Unknown { origin, universe: _ } => origin.span,
-        };
+    ) -> RelateResult<'tcx, ()> {
         // FIXME(generic_const_exprs): Occurs check failures for unevaluated
         // constants and generic expressions are not yet handled correctly.
         let Generalization { value_may_be_infer: generalized_ct, has_unconstrained_ty_var } =
-            self.generalize(span, target_vid, ty::Variance::Invariant, source_ct)?;
+            self.generalize(relation.span(), target_vid, ty::Variance::Invariant, source_ct)?;
 
         debug_assert!(!generalized_ct.is_ct_infer());
         if has_unconstrained_ty_var {
-            span_bug!(span, "unconstrained ty var when generalizing `{source_ct:?}`");
+            bug!("unconstrained ty var when generalizing `{source_ct:?}`");
         }
 
         self.inner
@@ -185,9 +181,25 @@ impl<'tcx> InferCtxt<'tcx> {
             .const_unification_table()
             .union_value(target_vid, ConstVariableValue::Known { value: generalized_ct });
 
-        // FIXME(generic_const_exprs): We have to make sure we actually equate
-        // `generalized_ct` and `source_ct` here.
-        Ok(generalized_ct)
+        // HACK: make sure that we `a_is_expected` continues to be
+        // correct when relating the generalized type with the source.
+        if target_is_expected == relation.a_is_expected() {
+            relation.relate_with_variance(
+                ty::Variance::Invariant,
+                ty::VarianceDiagInfo::default(),
+                generalized_ct,
+                source_ct,
+            )?;
+        } else {
+            relation.relate_with_variance(
+                ty::Variance::Invariant,
+                ty::VarianceDiagInfo::default(),
+                source_ct,
+                generalized_ct,
+            )?;
+        }
+
+        Ok(())
     }
 
     /// Attempts to generalize `source_term` for the type variable `target_vid`.
@@ -287,6 +299,49 @@ impl<'tcx> Generalizer<'_, 'tcx> {
             ty::TermKind::Const(ct) => TypeError::CyclicConst(ct),
         }
     }
+
+    /// An occurs check failure inside of an alias does not mean
+    /// that the types definitely don't unify. We may be able
+    /// to normalize the alias after all.
+    ///
+    /// We handle this by lazily equating the alias and generalizing
+    /// it to an inference variable.
+    ///
+    /// This is incomplete and will hopefully soon get fixed by #119106.
+    fn generalize_alias_ty(
+        &mut self,
+        alias: ty::AliasTy<'tcx>,
+    ) -> Result<Ty<'tcx>, TypeError<'tcx>> {
+        let is_nested_alias = mem::replace(&mut self.in_alias, true);
+        let result = match self.relate(alias, alias) {
+            Ok(alias) => Ok(alias.to_ty(self.tcx())),
+            Err(e) => {
+                if is_nested_alias {
+                    return Err(e);
+                } else {
+                    let mut visitor = MaxUniverse::new();
+                    alias.visit_with(&mut visitor);
+                    let infer_replacement_is_complete =
+                        self.for_universe.can_name(visitor.max_universe())
+                            && !alias.has_escaping_bound_vars();
+                    if !infer_replacement_is_complete {
+                        warn!("may incompletely handle alias type: {alias:?}");
+                    }
+
+                    debug!("generalization failure in alias");
+                    Ok(self.infcx.next_ty_var_in_universe(
+                        TypeVariableOrigin {
+                            kind: TypeVariableOriginKind::MiscVariable,
+                            span: self.span,
+                        },
+                        self.for_universe,
+                    ))
+                }
+            }
+        };
+        self.in_alias = is_nested_alias;
+        result
+    }
 }
 
 impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
@@ -433,43 +488,7 @@ impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
                 }
             }
 
-            ty::Alias(kind, data) => {
-                // An occurs check failure inside of an alias does not mean
-                // that the types definitely don't unify. We may be able
-                // to normalize the alias after all.
-                //
-                // We handle this by lazily equating the alias and generalizing
-                // it to an inference variable.
-                let is_nested_alias = mem::replace(&mut self.in_alias, true);
-                let result = match self.relate(data, data) {
-                    Ok(data) => Ok(Ty::new_alias(self.tcx(), kind, data)),
-                    Err(e) => {
-                        if is_nested_alias {
-                            return Err(e);
-                        } else {
-                            let mut visitor = MaxUniverse::new();
-                            t.visit_with(&mut visitor);
-                            let infer_replacement_is_complete =
-                                self.for_universe.can_name(visitor.max_universe())
-                                    && !t.has_escaping_bound_vars();
-                            if !infer_replacement_is_complete {
-                                warn!("may incompletely handle alias type: {t:?}");
-                            }
-
-                            debug!("generalization failure in alias");
-                            Ok(self.infcx.next_ty_var_in_universe(
-                                TypeVariableOrigin {
-                                    kind: TypeVariableOriginKind::MiscVariable,
-                                    span: self.span,
-                                },
-                                self.for_universe,
-                            ))
-                        }
-                    }
-                };
-                self.in_alias = is_nested_alias;
-                result
-            }
+            ty::Alias(_, data) => self.generalize_alias_ty(data),
 
             _ => relate::structurally_relate_tys(self, t, t),
         }?;
diff --git a/compiler/rustc_mir_build/src/build/matches/mod.rs b/compiler/rustc_mir_build/src/build/matches/mod.rs
index ccf299649cf..88a5eae281b 100644
--- a/compiler/rustc_mir_build/src/build/matches/mod.rs
+++ b/compiler/rustc_mir_build/src/build/matches/mod.rs
@@ -1052,7 +1052,7 @@ struct Ascription<'tcx> {
     variance: ty::Variance,
 }
 
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub(crate) struct MatchPair<'pat, 'tcx> {
     // This place...
     place: PlaceBuilder<'tcx>,
@@ -1408,51 +1408,66 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         span: Span,
         scrutinee_span: Span,
         candidates: &mut [&mut Candidate<'_, 'tcx>],
-        block: BasicBlock,
+        start_block: BasicBlock,
         otherwise_block: BasicBlock,
         fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
     ) {
         let (first_candidate, remaining_candidates) = candidates.split_first_mut().unwrap();
-
-        // All of the or-patterns have been sorted to the end, so if the first
-        // pattern is an or-pattern we only have or-patterns.
-        match first_candidate.match_pairs[0].pattern.kind {
-            PatKind::Or { .. } => (),
-            _ => {
-                self.test_candidates(
-                    span,
-                    scrutinee_span,
-                    candidates,
-                    block,
-                    otherwise_block,
-                    fake_borrows,
-                );
-                return;
-            }
+        assert!(first_candidate.subcandidates.is_empty());
+        if !matches!(first_candidate.match_pairs[0].pattern.kind, PatKind::Or { .. }) {
+            self.test_candidates(
+                span,
+                scrutinee_span,
+                candidates,
+                start_block,
+                otherwise_block,
+                fake_borrows,
+            );
+            return;
         }
 
         let match_pairs = mem::take(&mut first_candidate.match_pairs);
-        first_candidate.pre_binding_block = Some(block);
+        let (first_match_pair, remaining_match_pairs) = match_pairs.split_first().unwrap();
+        let PatKind::Or { ref pats } = &first_match_pair.pattern.kind else { unreachable!() };
 
         let remainder_start = self.cfg.start_new_block();
-        for match_pair in match_pairs {
-            let PatKind::Or { ref pats } = &match_pair.pattern.kind else {
-                bug!("Or-patterns should have been sorted to the end");
-            };
-            let or_span = match_pair.pattern.span;
+        let or_span = first_match_pair.pattern.span;
+        // Test the alternatives of this or-pattern.
+        self.test_or_pattern(
+            first_candidate,
+            start_block,
+            remainder_start,
+            pats,
+            or_span,
+            &first_match_pair.place,
+            fake_borrows,
+        );
 
+        if !remaining_match_pairs.is_empty() {
+            // If more match pairs remain, test them after each subcandidate.
+            // We could add them to the or-candidates before the call to `test_or_pattern` but this
+            // would make it impossible to detect simplifiable or-patterns. That would guarantee
+            // exponentially large CFGs for cases like `(1 | 2, 3 | 4, ...)`.
             first_candidate.visit_leaves(|leaf_candidate| {
-                self.test_or_pattern(
-                    leaf_candidate,
-                    remainder_start,
-                    pats,
-                    or_span,
-                    &match_pair.place,
+                assert!(leaf_candidate.match_pairs.is_empty());
+                leaf_candidate.match_pairs.extend(remaining_match_pairs.iter().cloned());
+                let or_start = leaf_candidate.pre_binding_block.unwrap();
+                // In a case like `(a | b, c | d)`, if `a` succeeds and `c | d` fails, we know `(b,
+                // c | d)` will fail too. If there is no guard, we skip testing of `b` by branching
+                // directly to `remainder_start`. If there is a guard, we have to try `(b, c | d)`.
+                let or_otherwise = leaf_candidate.otherwise_block.unwrap_or(remainder_start);
+                self.test_candidates_with_or(
+                    span,
+                    scrutinee_span,
+                    &mut [leaf_candidate],
+                    or_start,
+                    or_otherwise,
                     fake_borrows,
                 );
             });
         }
 
+        // Test the remaining candidates.
         self.match_candidates(
             span,
             scrutinee_span,
@@ -1460,17 +1475,18 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             otherwise_block,
             remaining_candidates,
             fake_borrows,
-        )
+        );
     }
 
     #[instrument(
-        skip(self, otherwise, or_span, place, fake_borrows, candidate, pats),
+        skip(self, start_block, otherwise_block, or_span, place, fake_borrows, candidate, pats),
         level = "debug"
     )]
     fn test_or_pattern<'pat>(
         &mut self,
         candidate: &mut Candidate<'pat, 'tcx>,
-        otherwise: BasicBlock,
+        start_block: BasicBlock,
+        otherwise_block: BasicBlock,
         pats: &'pat [Box<Pat<'tcx>>],
         or_span: Span,
         place: &PlaceBuilder<'tcx>,
@@ -1482,16 +1498,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             .map(|pat| Candidate::new(place.clone(), pat, candidate.has_guard, self))
             .collect();
         let mut or_candidate_refs: Vec<_> = or_candidates.iter_mut().collect();
-        let otherwise = if let Some(otherwise_block) = candidate.otherwise_block {
-            otherwise_block
-        } else {
-            otherwise
-        };
         self.match_candidates(
             or_span,
             or_span,
-            candidate.pre_binding_block.unwrap(),
-            otherwise,
+            start_block,
+            otherwise_block,
             &mut or_candidate_refs,
             fake_borrows,
         );
diff --git a/compiler/rustc_parse/messages.ftl b/compiler/rustc_parse/messages.ftl
index 7c2ecf34c17..55baf6f9f2e 100644
--- a/compiler/rustc_parse/messages.ftl
+++ b/compiler/rustc_parse/messages.ftl
@@ -27,6 +27,8 @@ parse_async_bound_modifier_in_2015 = `async` trait bounds are only allowed in Ru
 parse_async_fn_in_2015 = `async fn` is not permitted in Rust 2015
     .label = to use `async fn`, switch to Rust 2018 or later
 
+parse_async_impl = `async` trait implementations are unsupported
+
 parse_async_move_block_in_2015 = `async move` blocks are only allowed in Rust 2018 or later
 
 parse_async_move_order_incorrect = the order of `move` and `async` is incorrect
diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index fde67ac089a..2d4447a42c2 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -2975,3 +2975,10 @@ pub(crate) struct ArrayIndexInOffsetOf(#[primary_span] pub Span);
 #[derive(Diagnostic)]
 #[diag(parse_invalid_offset_of)]
 pub(crate) struct InvalidOffsetOf(#[primary_span] pub Span);
+
+#[derive(Diagnostic)]
+#[diag(parse_async_impl)]
+pub(crate) struct AsyncImpl {
+    #[primary_span]
+    pub span: Span,
+}
diff --git a/compiler/rustc_parse/src/parser/item.rs b/compiler/rustc_parse/src/parser/item.rs
index e7b9076bd3c..77381ef4626 100644
--- a/compiler/rustc_parse/src/parser/item.rs
+++ b/compiler/rustc_parse/src/parser/item.rs
@@ -562,6 +562,15 @@ impl<'a> Parser<'a> {
             self.sess.gated_spans.gate(sym::const_trait_impl, span);
         }
 
+        // Parse stray `impl async Trait`
+        if (self.token.uninterpolated_span().at_least_rust_2018()
+            && self.token.is_keyword(kw::Async))
+            || self.is_kw_followed_by_ident(kw::Async)
+        {
+            self.bump();
+            self.dcx().emit_err(errors::AsyncImpl { span: self.prev_token.span });
+        }
+
         let polarity = self.parse_polarity();
 
         // Parse both types and traits as a type, then reinterpret if necessary.
@@ -592,22 +601,10 @@ impl<'a> Parser<'a> {
             // We need to report this error after `cfg` expansion for compatibility reasons
             self.bump(); // `..`, do not add it to expected tokens
 
-            // FIXME(nnethercote): AST validation later detects this
-            // `TyKind::Err` and emits an errors. So why the unchecked
-            // ErrorGuaranteed?
-            // - A `span_delayed_bug` doesn't work here, because rustfmt can
-            //   hit this path but then not hit the follow-up path in the AST
-            //   validator that issues the error, which results in ICEs.
-            // - `TyKind::Dummy` doesn't work, because it ends up reaching HIR
-            //   lowering, which results in ICEs. Changing `TyKind::Dummy` to
-            //   `TyKind::Err` during AST validation might fix that, but that's
-            //   not possible because AST validation doesn't allow mutability.
-            //
-            // #121072 will hopefully remove all this special handling of the
-            // obsolete `impl Trait for ..` and then this can go away.
-            #[allow(deprecated)]
-            let guar = rustc_errors::ErrorGuaranteed::unchecked_error_guaranteed();
-            Some(self.mk_ty(self.prev_token.span, TyKind::Err(guar)))
+            // AST validation later detects this `TyKind::Dummy` and emits an
+            // error. (#121072 will hopefully remove all this special handling
+            // of the obsolete `impl Trait for ..` and then this can go away.)
+            Some(self.mk_ty(self.prev_token.span, TyKind::Dummy))
         } else if has_for || self.token.can_begin_type() {
             Some(self.parse_ty()?)
         } else {
diff --git a/compiler/rustc_parse/src/parser/ty.rs b/compiler/rustc_parse/src/parser/ty.rs
index f79f2a813b2..23a92e6dd3d 100644
--- a/compiler/rustc_parse/src/parser/ty.rs
+++ b/compiler/rustc_parse/src/parser/ty.rs
@@ -778,9 +778,10 @@ impl<'a> Parser<'a> {
             || self.check(&token::Not)
             || self.check(&token::Question)
             || self.check(&token::Tilde)
-            || self.check_keyword(kw::Const)
             || self.check_keyword(kw::For)
             || self.check(&token::OpenDelim(Delimiter::Parenthesis))
+            || self.check_keyword(kw::Const)
+            || self.check_keyword(kw::Async)
     }
 
     /// Parses a bound according to the grammar:
@@ -882,11 +883,13 @@ impl<'a> Parser<'a> {
             BoundConstness::Never
         };
 
-        let asyncness = if self.token.span.at_least_rust_2018() && self.eat_keyword(kw::Async) {
+        let asyncness = if self.token.uninterpolated_span().at_least_rust_2018()
+            && self.eat_keyword(kw::Async)
+        {
             self.sess.gated_spans.gate(sym::async_closure, self.prev_token.span);
             BoundAsyncness::Async(self.prev_token.span)
         } else if self.may_recover()
-            && self.token.span.is_rust_2015()
+            && self.token.uninterpolated_span().is_rust_2015()
             && self.is_kw_followed_by_ident(kw::Async)
         {
             self.bump(); // eat `async`
diff --git a/compiler/rustc_trait_selection/src/solve/normalize.rs b/compiler/rustc_trait_selection/src/solve/normalize.rs
index b07702e8421..91312c9fdd6 100644
--- a/compiler/rustc_trait_selection/src/solve/normalize.rs
+++ b/compiler/rustc_trait_selection/src/solve/normalize.rs
@@ -85,25 +85,16 @@ impl<'tcx> NormalizationFolder<'_, 'tcx> {
             ),
         );
 
-        // Do not emit an error if normalization is known to fail but instead
-        // keep the projection unnormalized. This is the case for projections
-        // with a `T: Trait` where-clause and opaque types outside of the defining
-        // scope.
-        let result = if infcx.predicate_may_hold(&obligation) {
-            self.fulfill_cx.register_predicate_obligation(infcx, obligation);
-            let errors = self.fulfill_cx.select_all_or_error(infcx);
-            if !errors.is_empty() {
-                return Err(errors);
-            }
-            let ty = infcx.resolve_vars_if_possible(new_infer_ty);
-
-            // Alias is guaranteed to be fully structurally resolved,
-            // so we can super fold here.
-            ty.try_super_fold_with(self)?
-        } else {
-            alias_ty.try_super_fold_with(self)?
-        };
+        self.fulfill_cx.register_predicate_obligation(infcx, obligation);
+        let errors = self.fulfill_cx.select_all_or_error(infcx);
+        if !errors.is_empty() {
+            return Err(errors);
+        }
 
+        // Alias is guaranteed to be fully structurally resolved,
+        // so we can super fold here.
+        let ty = infcx.resolve_vars_if_possible(new_infer_ty);
+        let result = ty.try_super_fold_with(self)?;
         self.depth -= 1;
         Ok(result)
     }
@@ -178,6 +169,7 @@ impl<'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for NormalizationFolder<'_, 'tcx> {
         Ok(t)
     }
 
+    #[instrument(level = "debug", skip(self), ret)]
     fn try_fold_ty(&mut self, ty: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
         let infcx = self.at.infcx;
         debug_assert_eq!(ty, infcx.shallow_resolve(ty));
@@ -204,6 +196,7 @@ impl<'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for NormalizationFolder<'_, 'tcx> {
         }
     }
 
+    #[instrument(level = "debug", skip(self), ret)]
     fn try_fold_const(&mut self, ct: ty::Const<'tcx>) -> Result<ty::Const<'tcx>, Self::Error> {
         let infcx = self.at.infcx;
         debug_assert_eq!(ct, infcx.shallow_resolve(ct));
diff --git a/compiler/rustc_trait_selection/src/solve/normalizes_to/anon_const.rs b/compiler/rustc_trait_selection/src/solve/normalizes_to/anon_const.rs
new file mode 100644
index 00000000000..911462f4b9a
--- /dev/null
+++ b/compiler/rustc_trait_selection/src/solve/normalizes_to/anon_const.rs
@@ -0,0 +1,25 @@
+use crate::solve::EvalCtxt;
+use rustc_middle::traits::solve::{Certainty, Goal, QueryResult};
+use rustc_middle::ty;
+
+impl<'tcx> EvalCtxt<'_, 'tcx> {
+    #[instrument(level = "debug", skip(self), ret)]
+    pub(super) fn normalize_anon_const(
+        &mut self,
+        goal: Goal<'tcx, ty::NormalizesTo<'tcx>>,
+    ) -> QueryResult<'tcx> {
+        if let Some(normalized_const) = self.try_const_eval_resolve(
+            goal.param_env,
+            ty::UnevaluatedConst::new(goal.predicate.alias.def_id, goal.predicate.alias.args),
+            self.tcx()
+                .type_of(goal.predicate.alias.def_id)
+                .no_bound_vars()
+                .expect("const ty should not rely on other generics"),
+        ) {
+            self.eq(goal.param_env, normalized_const, goal.predicate.term.ct().unwrap())?;
+            self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+        } else {
+            self.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
+        }
+    }
+}
diff --git a/compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs b/compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs
index d177109c420..5f625831156 100644
--- a/compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs
+++ b/compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs
@@ -18,8 +18,9 @@ use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_middle::ty::{ToPredicate, TypeVisitableExt};
 use rustc_span::{sym, ErrorGuaranteed, DUMMY_SP};
 
+mod anon_const;
 mod inherent;
-mod opaques;
+mod opaque_types;
 mod weak_types;
 
 impl<'tcx> EvalCtxt<'_, 'tcx> {
@@ -31,34 +32,34 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         let def_id = goal.predicate.def_id();
         match self.tcx().def_kind(def_id) {
             DefKind::AssocTy | DefKind::AssocConst => {
-                // To only compute normalization once for each projection we only
-                // assemble normalization candidates if the expected term is an
-                // unconstrained inference variable.
-                //
-                // Why: For better cache hits, since if we have an unconstrained RHS then
-                // there are only as many cache keys as there are (canonicalized) alias
-                // types in each normalizes-to goal. This also weakens inference in a
-                // forwards-compatible way so we don't use the value of the RHS term to
-                // affect candidate assembly for projections.
-                //
-                // E.g. for `<T as Trait>::Assoc == u32` we recursively compute the goal
-                // `exists<U> <T as Trait>::Assoc == U` and then take the resulting type for
-                // `U` and equate it with `u32`. This means that we don't need a separate
-                // projection cache in the solver, since we're piggybacking off of regular
-                // goal caching.
-                if self.term_is_fully_unconstrained(goal) {
-                    match self.tcx().associated_item(def_id).container {
-                        ty::AssocItemContainer::TraitContainer => {
+                match self.tcx().associated_item(def_id).container {
+                    ty::AssocItemContainer::TraitContainer => {
+                        // To only compute normalization once for each projection we only
+                        // assemble normalization candidates if the expected term is an
+                        // unconstrained inference variable.
+                        //
+                        // Why: For better cache hits, since if we have an unconstrained RHS then
+                        // there are only as many cache keys as there are (canonicalized) alias
+                        // types in each normalizes-to goal. This also weakens inference in a
+                        // forwards-compatible way so we don't use the value of the RHS term to
+                        // affect candidate assembly for projections.
+                        //
+                        // E.g. for `<T as Trait>::Assoc == u32` we recursively compute the goal
+                        // `exists<U> <T as Trait>::Assoc == U` and then take the resulting type for
+                        // `U` and equate it with `u32`. This means that we don't need a separate
+                        // projection cache in the solver, since we're piggybacking off of regular
+                        // goal caching.
+                        if self.term_is_fully_unconstrained(goal) {
                             let candidates = self.assemble_and_evaluate_candidates(goal);
                             self.merge_candidates(candidates)
+                        } else {
+                            self.set_normalizes_to_hack_goal(goal);
+                            self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
                         }
-                        ty::AssocItemContainer::ImplContainer => {
-                            self.normalize_inherent_associated_type(goal)
-                        }
                     }
-                } else {
-                    self.set_normalizes_to_hack_goal(goal);
-                    self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+                    ty::AssocItemContainer::ImplContainer => {
+                        self.normalize_inherent_associated_type(goal)
+                    }
                 }
             }
             DefKind::AnonConst => self.normalize_anon_const(goal),
@@ -67,26 +68,6 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             kind => bug!("unknown DefKind {} in projection goal: {goal:#?}", kind.descr(def_id)),
         }
     }
-
-    #[instrument(level = "debug", skip(self), ret)]
-    fn normalize_anon_const(
-        &mut self,
-        goal: Goal<'tcx, ty::NormalizesTo<'tcx>>,
-    ) -> QueryResult<'tcx> {
-        if let Some(normalized_const) = self.try_const_eval_resolve(
-            goal.param_env,
-            ty::UnevaluatedConst::new(goal.predicate.alias.def_id, goal.predicate.alias.args),
-            self.tcx()
-                .type_of(goal.predicate.alias.def_id)
-                .no_bound_vars()
-                .expect("const ty should not rely on other generics"),
-        ) {
-            self.eq(goal.param_env, normalized_const, goal.predicate.term.ct().unwrap())?;
-            self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
-        } else {
-            self.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
-        }
-    }
 }
 
 impl<'tcx> assembly::GoalKind<'tcx> for NormalizesTo<'tcx> {
diff --git a/compiler/rustc_trait_selection/src/solve/normalizes_to/opaques.rs b/compiler/rustc_trait_selection/src/solve/normalizes_to/opaque_types.rs
index 356c3776c04..356c3776c04 100644
--- a/compiler/rustc_trait_selection/src/solve/normalizes_to/opaques.rs
+++ b/compiler/rustc_trait_selection/src/solve/normalizes_to/opaque_types.rs
diff --git a/src/bootstrap/src/bin/main.rs b/src/bootstrap/src/bin/main.rs
index b97f73aa652..070d951dba9 100644
--- a/src/bootstrap/src/bin/main.rs
+++ b/src/bootstrap/src/bin/main.rs
@@ -39,14 +39,14 @@ fn main() {
             .open(&lock_path)));
         _build_lock_guard = match build_lock.try_write() {
             Ok(mut lock) => {
-                t!(lock.write(&process::id().to_string().as_ref()));
+                t!(lock.write(process::id().to_string().as_ref()));
                 lock
             }
             err => {
                 drop(err);
                 println!("WARNING: build directory locked by process {pid}, waiting for lock");
                 let mut lock = t!(build_lock.write());
-                t!(lock.write(&process::id().to_string().as_ref()));
+                t!(lock.write(process::id().to_string().as_ref()));
                 lock
             }
         };
@@ -113,14 +113,14 @@ fn main() {
                 continue;
             }
 
-            let file = t!(fs::File::open(&entry.path()));
+            let file = t!(fs::File::open(entry.path()));
 
             // To ensure deterministic results we must sort the dump lines.
             // This is necessary because the order of rustc invocations different
             // almost all the time.
             let mut lines: Vec<String> = t!(BufReader::new(&file).lines().collect());
             lines.sort_by_key(|t| t.to_lowercase());
-            let mut file = t!(OpenOptions::new().write(true).truncate(true).open(&entry.path()));
+            let mut file = t!(OpenOptions::new().write(true).truncate(true).open(entry.path()));
             t!(file.write_all(lines.join("\n").as_bytes()));
         }
     }
@@ -156,7 +156,7 @@ fn check_version(config: &Config) -> Option<String> {
         msg.push_str("There have been changes to x.py since you last updated:\n");
 
         for change in changes {
-            msg.push_str(&format!("  [{}] {}\n", change.severity.to_string(), change.summary));
+            msg.push_str(&format!("  [{}] {}\n", change.severity, change.summary));
             msg.push_str(&format!(
                 "    - PR Link https://github.com/rust-lang/rust/pull/{}\n",
                 change.change_id
diff --git a/src/bootstrap/src/bin/rustc.rs b/src/bootstrap/src/bin/rustc.rs
index 38c55b20344..74a924d86c7 100644
--- a/src/bootstrap/src/bin/rustc.rs
+++ b/src/bootstrap/src/bin/rustc.rs
@@ -276,7 +276,7 @@ fn main() {
                 dur.as_secs(),
                 dur.subsec_millis(),
                 if rusage_data.is_some() { " " } else { "" },
-                rusage_data.unwrap_or(String::new()),
+                rusage_data.unwrap_or_default(),
             );
         }
     }
@@ -440,5 +440,5 @@ fn format_rusage_data(_child: Child) -> Option<String> {
         ));
     }
 
-    return Some(init_str);
+    Some(init_str)
 }
diff --git a/src/bootstrap/src/bin/sccache-plus-cl.rs b/src/bootstrap/src/bin/sccache-plus-cl.rs
index 554c2dd4d81..6e87d4222e8 100644
--- a/src/bootstrap/src/bin/sccache-plus-cl.rs
+++ b/src/bootstrap/src/bin/sccache-plus-cl.rs
@@ -18,9 +18,9 @@ fn main() {
 
     // Invoke sccache with said compiler
     let sccache_path = env::var_os("SCCACHE_PATH").unwrap();
-    let mut cmd = Command::new(&sccache_path);
+    let mut cmd = Command::new(sccache_path);
     cmd.arg(compiler.path());
-    for &(ref k, ref v) in compiler.env() {
+    for (k, v) in compiler.env() {
         cmd.env(k, v);
     }
     for arg in env::args().skip(1) {
diff --git a/src/bootstrap/src/core/build_steps/check.rs b/src/bootstrap/src/core/build_steps/check.rs
index 4a04dbf44aa..3ac60f15ef6 100644
--- a/src/bootstrap/src/core/build_steps/check.rs
+++ b/src/bootstrap/src/core/build_steps/check.rs
@@ -34,7 +34,7 @@ fn args(builder: &Builder<'_>) -> Vec<String> {
         &builder.config.cmd
     {
         // disable the most spammy clippy lints
-        let ignored_lints = vec![
+        let ignored_lints = [
             "many_single_char_names", // there are a lot in stdarch
             "collapsible_if",
             "type_complexity",
@@ -150,7 +150,7 @@ impl Step for Std {
         if compiler.stage == 0 {
             let libdir = builder.sysroot_libdir(compiler, target);
             let hostdir = builder.sysroot_libdir(compiler, compiler.host);
-            add_to_sysroot(&builder, &libdir, &hostdir, &libstd_stamp(builder, compiler, target));
+            add_to_sysroot(builder, &libdir, &hostdir, &libstd_stamp(builder, compiler, target));
         }
         drop(_guard);
 
@@ -301,7 +301,7 @@ impl Step for Rustc {
 
         let libdir = builder.sysroot_libdir(compiler, target);
         let hostdir = builder.sysroot_libdir(compiler, compiler.host);
-        add_to_sysroot(&builder, &libdir, &hostdir, &librustc_stamp(builder, compiler, target));
+        add_to_sysroot(builder, &libdir, &hostdir, &librustc_stamp(builder, compiler, target));
     }
 }
 
@@ -353,7 +353,7 @@ impl Step for CodegenBackend {
             .arg(builder.src.join(format!("compiler/rustc_codegen_{backend}/Cargo.toml")));
         rustc_cargo_env(builder, &mut cargo, target, compiler.stage);
 
-        let _guard = builder.msg_check(&backend, target);
+        let _guard = builder.msg_check(backend, target);
 
         run_cargo(
             builder,
diff --git a/src/bootstrap/src/core/build_steps/compile.rs b/src/bootstrap/src/core/build_steps/compile.rs
index 9d7f88a9d42..d349cd67fed 100644
--- a/src/bootstrap/src/core/build_steps/compile.rs
+++ b/src/bootstrap/src/core/build_steps/compile.rs
@@ -107,8 +107,8 @@ impl Std {
     ) -> Vec<(PathBuf, DependencyType)> {
         let mut deps = Vec::new();
         if !self.is_for_mir_opt_tests {
-            deps.extend(copy_third_party_objects(builder, &compiler, target));
-            deps.extend(copy_self_contained_objects(builder, &compiler, target));
+            deps.extend(copy_third_party_objects(builder, compiler, target));
+            deps.extend(copy_self_contained_objects(builder, compiler, target));
         }
         deps
     }
@@ -186,7 +186,7 @@ impl Step for Std {
 
         // Profiler information requires LLVM's compiler-rt
         if builder.config.profiler {
-            builder.update_submodule(&Path::new("src/llvm-project"));
+            builder.update_submodule(Path::new("src/llvm-project"));
         }
 
         let mut target_deps = builder.ensure(StartupObjects { compiler, target });
@@ -271,7 +271,7 @@ impl Step for Std {
         if target.is_synthetic() {
             cargo.env("RUSTC_BOOTSTRAP_SYNTHETIC_TARGET", "1");
         }
-        for rustflag in self.extra_rust_args.into_iter() {
+        for rustflag in self.extra_rust_args.iter() {
             cargo.rustflag(rustflag);
         }
 
@@ -333,7 +333,7 @@ fn copy_third_party_objects(
         // The sanitizers are only copied in stage1 or above,
         // to avoid creating dependency on LLVM.
         target_deps.extend(
-            copy_sanitizers(builder, &compiler, target)
+            copy_sanitizers(builder, compiler, target)
                 .into_iter()
                 .map(|d| (d, DependencyType::Target)),
         );
@@ -487,7 +487,7 @@ pub fn std_cargo(builder: &Builder<'_>, target: TargetSelection, stage: u32, car
 
         // for no-std targets we only compile a few no_std crates
         cargo
-            .args(&["-p", "alloc"])
+            .args(["-p", "alloc"])
             .arg("--manifest-path")
             .arg(builder.src.join("library/alloc/Cargo.toml"))
             .arg("--features")
@@ -626,20 +626,20 @@ impl Step for StdLink {
                 .build
                 .config
                 .initial_rustc
-                .starts_with(builder.out.join(&compiler.host.triple).join("stage0/bin"))
+                .starts_with(builder.out.join(compiler.host.triple).join("stage0/bin"))
         {
             // Copy bin files from stage0/bin to stage0-sysroot/bin
-            let sysroot = builder.out.join(&compiler.host.triple).join("stage0-sysroot");
+            let sysroot = builder.out.join(compiler.host.triple).join("stage0-sysroot");
 
             let host = compiler.host.triple;
-            let stage0_bin_dir = builder.out.join(&host).join("stage0/bin");
+            let stage0_bin_dir = builder.out.join(host).join("stage0/bin");
             let sysroot_bin_dir = sysroot.join("bin");
             t!(fs::create_dir_all(&sysroot_bin_dir));
             builder.cp_r(&stage0_bin_dir, &sysroot_bin_dir);
 
             // Copy all *.so files from stage0/lib to stage0-sysroot/lib
-            let stage0_lib_dir = builder.out.join(&host).join("stage0/lib");
-            if let Ok(files) = fs::read_dir(&stage0_lib_dir) {
+            let stage0_lib_dir = builder.out.join(host).join("stage0/lib");
+            if let Ok(files) = fs::read_dir(stage0_lib_dir) {
                 for file in files {
                     let file = t!(file);
                     let path = file.path();
@@ -654,9 +654,9 @@ impl Step for StdLink {
             t!(fs::create_dir_all(&sysroot_codegen_backends));
             let stage0_codegen_backends = builder
                 .out
-                .join(&host)
+                .join(host)
                 .join("stage0/lib/rustlib")
-                .join(&host)
+                .join(host)
                 .join("codegen-backends");
             if stage0_codegen_backends.exists() {
                 builder.cp_r(&stage0_codegen_backends, &sysroot_codegen_backends);
@@ -1179,7 +1179,7 @@ fn rustc_llvm_env(builder: &Builder<'_>, cargo: &mut Cargo, target: TargetSelect
     // The config can also specify its own llvm linker flags.
     if let Some(ref s) = builder.config.llvm_ldflags {
         if !llvm_linker_flags.is_empty() {
-            llvm_linker_flags.push_str(" ");
+            llvm_linker_flags.push(' ');
         }
         llvm_linker_flags.push_str(s);
     }
@@ -1270,7 +1270,7 @@ fn needs_codegen_config(run: &RunConfig<'_>) -> bool {
     for path_set in &run.paths {
         needs_codegen_cfg = match path_set {
             PathSet::Set(set) => set.iter().any(|p| is_codegen_cfg_needed(p, run)),
-            PathSet::Suite(suite) => is_codegen_cfg_needed(&suite, run),
+            PathSet::Suite(suite) => is_codegen_cfg_needed(suite, run),
         }
     }
     needs_codegen_cfg
@@ -1279,7 +1279,7 @@ fn needs_codegen_config(run: &RunConfig<'_>) -> bool {
 pub(crate) const CODEGEN_BACKEND_PREFIX: &str = "rustc_codegen_";
 
 fn is_codegen_cfg_needed(path: &TaskPath, run: &RunConfig<'_>) -> bool {
-    if path.path.to_str().unwrap().contains(&CODEGEN_BACKEND_PREFIX) {
+    if path.path.to_str().unwrap().contains(CODEGEN_BACKEND_PREFIX) {
         let mut needs_codegen_backend_config = true;
         for &backend in run.builder.config.codegen_backends(run.target) {
             if path
@@ -1300,7 +1300,7 @@ fn is_codegen_cfg_needed(path: &TaskPath, run: &RunConfig<'_>) -> bool {
         }
     }
 
-    return false;
+    false
 }
 
 impl Step for CodegenBackend {
@@ -1393,7 +1393,7 @@ impl Step for CodegenBackend {
         }
         let stamp = codegen_backend_stamp(builder, compiler, target, backend);
         let codegen_backend = codegen_backend.to_str().unwrap();
-        t!(fs::write(&stamp, &codegen_backend));
+        t!(fs::write(stamp, codegen_backend));
     }
 }
 
@@ -1441,7 +1441,7 @@ fn copy_codegen_backends_to_sysroot(
             let dot = filename.find('.').unwrap();
             format!("{}-{}{}", &filename[..dash], builder.rust_release(), &filename[dot..])
         };
-        builder.copy(&file, &dst.join(target_filename));
+        builder.copy(file, &dst.join(target_filename));
     }
 }
 
@@ -1519,7 +1519,7 @@ impl Step for Sysroot {
     /// 1-3.
     fn run(self, builder: &Builder<'_>) -> Interned<PathBuf> {
         let compiler = self.compiler;
-        let host_dir = builder.out.join(&compiler.host.triple);
+        let host_dir = builder.out.join(compiler.host.triple);
 
         let sysroot_dir = |stage| {
             if stage == 0 {
@@ -1578,7 +1578,7 @@ impl Step for Sysroot {
             let mut add_filtered_files = |suffix, contents| {
                 for path in contents {
                     let path = Path::new(&path);
-                    if path.parent().map_or(false, |parent| parent.ends_with(&suffix)) {
+                    if path.parent().map_or(false, |parent| parent.ends_with(suffix)) {
                         filtered_files.push(path.file_name().unwrap().to_owned());
                     }
                 }
@@ -1802,7 +1802,7 @@ impl Step for Assemble {
         if let Some(lld_install) = lld_install {
             let src_exe = exe("lld", target_compiler.host);
             let dst_exe = exe("rust-lld", target_compiler.host);
-            builder.copy(&lld_install.join("bin").join(&src_exe), &libdir_bin.join(&dst_exe));
+            builder.copy(&lld_install.join("bin").join(src_exe), &libdir_bin.join(dst_exe));
             let self_contained_lld_dir = libdir_bin.join("gcc-ld");
             t!(fs::create_dir_all(&self_contained_lld_dir));
             let lld_wrapper_exe = builder.ensure(crate::core::build_steps::tool::LldWrapper {
@@ -1850,7 +1850,7 @@ impl Step for Assemble {
         let out_dir = builder.cargo_out(build_compiler, Mode::Rustc, host);
         let rustc = out_dir.join(exe("rustc-main", host));
         let bindir = sysroot.join("bin");
-        t!(fs::create_dir_all(&bindir));
+        t!(fs::create_dir_all(bindir));
         let compiler = builder.rustc(target_compiler);
         builder.copy(&rustc, &compiler);
 
@@ -1869,9 +1869,9 @@ pub fn add_to_sysroot(
     stamp: &Path,
 ) {
     let self_contained_dst = &sysroot_dst.join("self-contained");
-    t!(fs::create_dir_all(&sysroot_dst));
-    t!(fs::create_dir_all(&sysroot_host_dst));
-    t!(fs::create_dir_all(&self_contained_dst));
+    t!(fs::create_dir_all(sysroot_dst));
+    t!(fs::create_dir_all(sysroot_host_dst));
+    t!(fs::create_dir_all(self_contained_dst));
     for (path, dependency_type) in builder.read_stamp_file(stamp) {
         let dst = match dependency_type {
             DependencyType::Host => sysroot_host_dst,
@@ -2009,14 +2009,14 @@ pub fn run_cargo(
         .map(|e| (e.path(), e.file_name().into_string().unwrap(), t!(e.metadata())))
         .collect::<Vec<_>>();
     for (prefix, extension, expected_len) in toplevel {
-        let candidates = contents.iter().filter(|&&(_, ref filename, ref meta)| {
+        let candidates = contents.iter().filter(|&(_, filename, meta)| {
             meta.len() == expected_len
                 && filename
                     .strip_prefix(&prefix[..])
                     .map(|s| s.starts_with('-') && s.ends_with(&extension[..]))
                     .unwrap_or(false)
         });
-        let max = candidates.max_by_key(|&&(_, _, ref metadata)| {
+        let max = candidates.max_by_key(|&(_, _, metadata)| {
             metadata.modified().expect("mtime should be available on all relevant OSes")
         });
         let path_to_add = match max {
@@ -2045,7 +2045,7 @@ pub fn run_cargo(
         new_contents.extend(dep.to_str().unwrap().as_bytes());
         new_contents.extend(b"\0");
     }
-    t!(fs::write(&stamp, &new_contents));
+    t!(fs::write(stamp, &new_contents));
     deps.into_iter().map(|(d, _)| d).collect()
 }
 
diff --git a/src/bootstrap/src/core/build_steps/dist.rs b/src/bootstrap/src/core/build_steps/dist.rs
index 750d3095ff6..d9ab18e7250 100644
--- a/src/bootstrap/src/core/build_steps/dist.rs
+++ b/src/bootstrap/src/core/build_steps/dist.rs
@@ -78,7 +78,7 @@ impl Step for Docs {
         let mut tarball = Tarball::new(builder, "rust-docs", &host.triple);
         tarball.set_product_name("Rust Documentation");
         tarball.add_bulk_dir(&builder.doc_out(host), dest);
-        tarball.add_file(&builder.src.join("src/doc/robots.txt"), dest, 0o644);
+        tarball.add_file(builder.src.join("src/doc/robots.txt"), dest, 0o644);
         Some(tarball.generate())
     }
 }
@@ -342,7 +342,7 @@ impl Step for Mingw {
         // thrown away (this contains the runtime DLLs included in the rustc package
         // above) and the second argument is where to place all the MinGW components
         // (which is what we want).
-        make_win_dist(&tmpdir(builder), tarball.image_dir(), host, &builder);
+        make_win_dist(&tmpdir(builder), tarball.image_dir(), host, builder);
 
         Some(tarball.generate())
     }
@@ -658,7 +658,7 @@ impl Step for Std {
         let compiler_to_use = builder.compiler_for(compiler.stage, compiler.host, target);
         let stamp = compile::libstd_stamp(builder, compiler_to_use, target);
         verify_uefi_rlib_format(builder, target, &stamp);
-        copy_target_libs(builder, target, &tarball.image_dir(), &stamp);
+        copy_target_libs(builder, target, tarball.image_dir(), &stamp);
 
         Some(tarball.generate())
     }
@@ -734,7 +734,7 @@ impl Step for Analysis {
     const DEFAULT: bool = true;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        let default = should_build_extended_tool(&run.builder, "analysis");
+        let default = should_build_extended_tool(run.builder, "analysis");
         run.alias("rust-analysis").default_condition(default)
     }
 
@@ -890,7 +890,7 @@ impl Step for Src {
     /// Creates the `rust-src` installer component
     fn run(self, builder: &Builder<'_>) -> GeneratedTarball {
         if !builder.config.dry_run() {
-            builder.update_submodule(&Path::new("src/llvm-project"));
+            builder.update_submodule(Path::new("src/llvm-project"));
         }
 
         let tarball = Tarball::new_targetless(builder, "rust-src");
@@ -976,7 +976,7 @@ impl Step for PlainSourceTarball {
         ];
         let src_dirs = ["src", "compiler", "library", "tests"];
 
-        copy_src_dirs(builder, &builder.src, &src_dirs, &[], &plain_dst_src);
+        copy_src_dirs(builder, &builder.src, &src_dirs, &[], plain_dst_src);
 
         // Copy the files normally
         for item in &src_files {
@@ -986,8 +986,8 @@ impl Step for PlainSourceTarball {
         // Create the version file
         builder.create(&plain_dst_src.join("version"), &builder.rust_version());
         if let Some(info) = builder.rust_info().info() {
-            channel::write_commit_hash_file(&plain_dst_src, &info.sha);
-            channel::write_commit_info_file(&plain_dst_src, info);
+            channel::write_commit_hash_file(plain_dst_src, &info.sha);
+            channel::write_commit_info_file(plain_dst_src, info);
         }
 
         // If we're building from git or tarball sources, we need to vendor
@@ -1014,7 +1014,7 @@ impl Step for PlainSourceTarball {
                 // Will read the libstd Cargo.toml
                 // which uses the unstable `public-dependency` feature.
                 .env("RUSTC_BOOTSTRAP", "1")
-                .current_dir(&plain_dst_src);
+                .current_dir(plain_dst_src);
 
             let config = if !builder.config.dry_run() {
                 t!(String::from_utf8(t!(cmd.output()).stdout))
@@ -1043,7 +1043,7 @@ impl Step for Cargo {
     const ONLY_HOSTS: bool = true;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        let default = should_build_extended_tool(&run.builder, "cargo");
+        let default = should_build_extended_tool(run.builder, "cargo");
         run.alias("cargo").default_condition(default)
     }
 
@@ -1070,7 +1070,7 @@ impl Step for Cargo {
         let mut tarball = Tarball::new(builder, "cargo", &target.triple);
         tarball.set_overlay(OverlayKind::Cargo);
 
-        tarball.add_file(&cargo, "bin", 0o755);
+        tarball.add_file(cargo, "bin", 0o755);
         tarball.add_file(etc.join("_cargo"), "share/zsh/site-functions", 0o644);
         tarball.add_renamed_file(etc.join("cargo.bashcomp.sh"), "etc/bash_completion.d", "cargo");
         tarball.add_dir(etc.join("man"), "share/man/man1");
@@ -1092,7 +1092,7 @@ impl Step for Rls {
     const DEFAULT: bool = true;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        let default = should_build_extended_tool(&run.builder, "rls");
+        let default = should_build_extended_tool(run.builder, "rls");
         run.alias("rls").default_condition(default)
     }
 
@@ -1134,7 +1134,7 @@ impl Step for RustAnalyzer {
     const ONLY_HOSTS: bool = true;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        let default = should_build_extended_tool(&run.builder, "rust-analyzer");
+        let default = should_build_extended_tool(run.builder, "rust-analyzer");
         run.alias("rust-analyzer").default_condition(default)
     }
 
@@ -1176,7 +1176,7 @@ impl Step for Clippy {
     const ONLY_HOSTS: bool = true;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        let default = should_build_extended_tool(&run.builder, "clippy");
+        let default = should_build_extended_tool(run.builder, "clippy");
         run.alias("clippy").default_condition(default)
     }
 
@@ -1224,7 +1224,7 @@ impl Step for Miri {
     const ONLY_HOSTS: bool = true;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        let default = should_build_extended_tool(&run.builder, "miri");
+        let default = should_build_extended_tool(run.builder, "miri");
         run.alias("miri").default_condition(default)
     }
 
@@ -1337,12 +1337,12 @@ impl Step for CodegenBackend {
         let src = builder.sysroot(compiler);
         let backends_src = builder.sysroot_codegen_backends(compiler);
         let backends_rel = backends_src
-            .strip_prefix(&src)
+            .strip_prefix(src)
             .unwrap()
             .strip_prefix(builder.sysroot_libdir_relative(compiler))
             .unwrap();
         // Don't use custom libdir here because ^lib/ will be resolved again with installer
-        let backends_dst = PathBuf::from("lib").join(&backends_rel);
+        let backends_dst = PathBuf::from("lib").join(backends_rel);
 
         let backend_name = format!("rustc_codegen_{}", backend);
         let mut found_backend = false;
@@ -1371,7 +1371,7 @@ impl Step for Rustfmt {
     const ONLY_HOSTS: bool = true;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        let default = should_build_extended_tool(&run.builder, "rustfmt");
+        let default = should_build_extended_tool(run.builder, "rustfmt");
         run.alias("rustfmt").default_condition(default)
     }
 
@@ -1454,7 +1454,7 @@ impl Step for RustDemangler {
         let mut tarball = Tarball::new(builder, "rust-demangler", &target.triple);
         tarball.set_overlay(OverlayKind::RustDemangler);
         tarball.is_preview(true);
-        tarball.add_file(&rust_demangler, "bin", 0o755);
+        tarball.add_file(rust_demangler, "bin", 0o755);
         tarball.add_legal_and_readme_to("share/doc/rust-demangler");
         Some(tarball.generate())
     }
@@ -1609,7 +1609,7 @@ impl Step for Extended {
             let prepare = |name: &str| {
                 builder.create_dir(&pkg.join(name));
                 builder.cp_r(
-                    &work.join(&format!("{}-{}", pkgname(builder, name), target.triple)),
+                    &work.join(format!("{}-{}", pkgname(builder, name), target.triple)),
                     &pkg.join(name),
                 );
                 builder.install(&etc.join("pkg/postinstall"), &pkg.join(name), 0o755);
@@ -1673,7 +1673,7 @@ impl Step for Extended {
                     name.to_string()
                 };
                 builder.cp_r(
-                    &work.join(&format!("{}-{}", pkgname(builder, name), target.triple)).join(dir),
+                    &work.join(format!("{}-{}", pkgname(builder, name), target.triple)).join(dir),
                     &exe.join(name),
                 );
                 builder.remove(&exe.join(name).join("manifest.in"));
@@ -1707,7 +1707,7 @@ impl Step for Extended {
                     .current_dir(&exe)
                     .arg("dir")
                     .arg("rustc")
-                    .args(&heat_flags)
+                    .args(heat_flags)
                     .arg("-cg")
                     .arg("RustcGroup")
                     .arg("-dr")
@@ -1723,7 +1723,7 @@ impl Step for Extended {
                         .current_dir(&exe)
                         .arg("dir")
                         .arg("rust-docs")
-                        .args(&heat_flags)
+                        .args(heat_flags)
                         .arg("-cg")
                         .arg("DocsGroup")
                         .arg("-dr")
@@ -1741,7 +1741,7 @@ impl Step for Extended {
                     .current_dir(&exe)
                     .arg("dir")
                     .arg("cargo")
-                    .args(&heat_flags)
+                    .args(heat_flags)
                     .arg("-cg")
                     .arg("CargoGroup")
                     .arg("-dr")
@@ -1758,7 +1758,7 @@ impl Step for Extended {
                     .current_dir(&exe)
                     .arg("dir")
                     .arg("rust-std")
-                    .args(&heat_flags)
+                    .args(heat_flags)
                     .arg("-cg")
                     .arg("StdGroup")
                     .arg("-dr")
@@ -1774,7 +1774,7 @@ impl Step for Extended {
                         .current_dir(&exe)
                         .arg("dir")
                         .arg("rust-analyzer")
-                        .args(&heat_flags)
+                        .args(heat_flags)
                         .arg("-cg")
                         .arg("RustAnalyzerGroup")
                         .arg("-dr")
@@ -1793,7 +1793,7 @@ impl Step for Extended {
                         .current_dir(&exe)
                         .arg("dir")
                         .arg("clippy")
-                        .args(&heat_flags)
+                        .args(heat_flags)
                         .arg("-cg")
                         .arg("ClippyGroup")
                         .arg("-dr")
@@ -1812,7 +1812,7 @@ impl Step for Extended {
                         .current_dir(&exe)
                         .arg("dir")
                         .arg("rust-demangler")
-                        .args(&heat_flags)
+                        .args(heat_flags)
                         .arg("-cg")
                         .arg("RustDemanglerGroup")
                         .arg("-dr")
@@ -1831,7 +1831,7 @@ impl Step for Extended {
                         .current_dir(&exe)
                         .arg("dir")
                         .arg("miri")
-                        .args(&heat_flags)
+                        .args(heat_flags)
                         .arg("-cg")
                         .arg("MiriGroup")
                         .arg("-dr")
@@ -1849,7 +1849,7 @@ impl Step for Extended {
                     .current_dir(&exe)
                     .arg("dir")
                     .arg("rust-analysis")
-                    .args(&heat_flags)
+                    .args(heat_flags)
                     .arg("-cg")
                     .arg("AnalysisGroup")
                     .arg("-dr")
@@ -1867,7 +1867,7 @@ impl Step for Extended {
                         .current_dir(&exe)
                         .arg("dir")
                         .arg("rust-mingw")
-                        .args(&heat_flags)
+                        .args(heat_flags)
                         .arg("-cg")
                         .arg("GccGroup")
                         .arg("-dr")
@@ -1890,10 +1890,10 @@ impl Step for Extended {
                     .arg("-dStdDir=rust-std")
                     .arg("-dAnalysisDir=rust-analysis")
                     .arg("-arch")
-                    .arg(&arch)
+                    .arg(arch)
                     .arg("-out")
                     .arg(&output)
-                    .arg(&input);
+                    .arg(input);
                 add_env(builder, &mut cmd, target);
 
                 if built_tools.contains("clippy") {
@@ -2026,7 +2026,7 @@ fn install_llvm_file(builder: &Builder<'_>, source: &Path, destination: &Path) {
         return;
     }
 
-    builder.install(&source, destination, 0o644);
+    builder.install(source, destination, 0o644);
 }
 
 /// Maybe add LLVM object files to the given destination lib-dir. Allows either static or dynamic linking.
@@ -2123,7 +2123,7 @@ impl Step for LlvmTools {
     const DEFAULT: bool = true;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        let default = should_build_extended_tool(&run.builder, "llvm-tools");
+        let default = should_build_extended_tool(run.builder, "llvm-tools");
         // FIXME: allow using the names of the tools themselves?
         run.alias("llvm-tools").default_condition(default)
     }
@@ -2231,12 +2231,12 @@ impl Step for RustDev {
             tarball.add_file(lld_path, "bin", 0o755);
         }
 
-        tarball.add_file(&builder.llvm_filecheck(target), "bin", 0o755);
+        tarball.add_file(builder.llvm_filecheck(target), "bin", 0o755);
 
         // Copy the include directory as well; needed mostly to build
         // librustc_llvm properly (e.g., llvm-config.h is in here). But also
         // just broadly useful to be able to link against the bundled LLVM.
-        tarball.add_dir(&builder.llvm_out(target).join("include"), "include");
+        tarball.add_dir(builder.llvm_out(target).join("include"), "include");
 
         // Copy libLLVM.so to the target lib dir as well, so the RPATH like
         // `$ORIGIN/../lib` can find it. It may also be used as a dependency
@@ -2312,7 +2312,7 @@ impl Step for BuildManifest {
         let build_manifest = builder.tool_exe(Tool::BuildManifest);
 
         let tarball = Tarball::new(builder, "build-manifest", &self.target.triple);
-        tarball.add_file(&build_manifest, "bin", 0o755);
+        tarball.add_file(build_manifest, "bin", 0o755);
         tarball.generate()
     }
 }
diff --git a/src/bootstrap/src/core/build_steps/doc.rs b/src/bootstrap/src/core/build_steps/doc.rs
index fdb099e4ab1..7a122a8676b 100644
--- a/src/bootstrap/src/core/build_steps/doc.rs
+++ b/src/bootstrap/src/core/build_steps/doc.rs
@@ -151,7 +151,7 @@ impl<P: Step> Step for RustbookSrc<P> {
             builder.info(&format!("Rustbook ({target}) - {name}"));
             let _ = fs::remove_dir_all(&out);
 
-            builder.run(rustbook_cmd.arg("build").arg(&src).arg("-d").arg(out));
+            builder.run(rustbook_cmd.arg("build").arg(src).arg("-d").arg(out));
         }
 
         if self.parent.is_some() {
@@ -384,7 +384,7 @@ impl Step for Standalone {
         // with no particular explicit doc requested (e.g. library/core).
         if builder.paths.is_empty() || builder.was_invoked_explicitly::<Self>(Kind::Doc) {
             let index = out.join("index.html");
-            builder.open_in_browser(&index);
+            builder.open_in_browser(index);
         }
     }
 }
@@ -517,7 +517,7 @@ impl Step for SharedAssets {
                 .replace("VERSION", &builder.rust_release())
                 .replace("SHORT_HASH", builder.rust_info().sha_short().unwrap_or(""))
                 .replace("STAMP", builder.rust_info().sha().unwrap_or(""));
-            t!(fs::write(&version_info, &info));
+            t!(fs::write(&version_info, info));
         }
 
         builder.copy(&builder.src.join("src").join("doc").join("rust.css"), &out.join("rust.css"));
@@ -714,11 +714,11 @@ fn doc_std(
     }
 
     let description =
-        format!("library{} in {} format", crate_description(&requested_crates), format.as_str());
-    let _guard = builder.msg_doc(compiler, &description, target);
+        format!("library{} in {} format", crate_description(requested_crates), format.as_str());
+    let _guard = builder.msg_doc(compiler, description, target);
 
     builder.run(&mut cargo.into());
-    builder.cp_r(&out_dir, &out);
+    builder.cp_r(&out_dir, out);
 }
 
 #[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
@@ -781,7 +781,7 @@ impl Step for Rustc {
         let _guard = builder.msg_sysroot_tool(
             Kind::Doc,
             stage,
-            &format!("compiler{}", crate_description(&self.crates)),
+            format!("compiler{}", crate_description(&self.crates)),
             compiler.host,
             target,
         );
@@ -819,7 +819,7 @@ impl Step for Rustc {
             // Create all crate output directories first to make sure rustdoc uses
             // relative links.
             // FIXME: Cargo should probably do this itself.
-            let dir_name = krate.replace("-", "_");
+            let dir_name = krate.replace('-', "_");
             t!(fs::create_dir_all(out_dir.join(&*dir_name)));
             cargo.arg("-p").arg(krate);
             if to_open.is_none() {
@@ -844,7 +844,7 @@ impl Step for Rustc {
         if !builder.config.dry_run() {
             // Sanity check on linked compiler crates
             for krate in &*self.crates {
-                let dir_name = krate.replace("-", "_");
+                let dir_name = krate.replace('-', "_");
                 // Making sure the directory exists and is not empty.
                 assert!(out.join(&*dir_name).read_dir().unwrap().next().is_some());
             }
@@ -1160,7 +1160,7 @@ impl Step for RustcBook {
         cmd.arg(&out_listing);
         cmd.arg("--rustc");
         cmd.arg(&rustc);
-        cmd.arg("--rustc-target").arg(&self.target.rustc_target_arg());
+        cmd.arg("--rustc-target").arg(self.target.rustc_target_arg());
         if builder.is_verbose() {
             cmd.arg("--verbose");
         }
diff --git a/src/bootstrap/src/core/build_steps/format.rs b/src/bootstrap/src/core/build_steps/format.rs
index e792d38b7ea..69c8792b031 100644
--- a/src/bootstrap/src/core/build_steps/format.rs
+++ b/src/bootstrap/src/core/build_steps/format.rs
@@ -11,7 +11,7 @@ use std::process::{Command, Stdio};
 use std::sync::mpsc::SyncSender;
 
 fn rustfmt(src: &Path, rustfmt: &Path, paths: &[PathBuf], check: bool) -> impl FnMut(bool) -> bool {
-    let mut cmd = Command::new(&rustfmt);
+    let mut cmd = Command::new(rustfmt);
     // avoid the submodule config paths from coming into play,
     // we only allow a single global config for the workspace for now
     cmd.arg("--config-path").arg(&src.canonicalize().unwrap());
@@ -162,7 +162,7 @@ pub fn format(build: &Builder<'_>, check: bool, paths: &[PathBuf]) {
                 // against anything like `compiler/rustc_foo/src/foo.rs`,
                 // preventing the latter from being formatted.
                 untracked_count += 1;
-                fmt_override.add(&format!("!/{untracked_path}")).expect(&untracked_path);
+                fmt_override.add(&format!("!/{untracked_path}")).expect(untracked_path);
             }
             // Only check modified files locally to speed up runtime.
             // We still check all files in CI to avoid bugs in `get_modified_rs_files` letting regressions slip through;
@@ -221,7 +221,7 @@ pub fn format(build: &Builder<'_>, check: bool, paths: &[PathBuf]) {
     assert!(rustfmt_path.exists(), "{}", rustfmt_path.display());
     let src = build.src.clone();
     let (tx, rx): (SyncSender<PathBuf>, _) = std::sync::mpsc::sync_channel(128);
-    let walker = match paths.get(0) {
+    let walker = match paths.first() {
         Some(first) => {
             let find_shortcut_candidates = |p: &PathBuf| {
                 let mut candidates = Vec::new();
diff --git a/src/bootstrap/src/core/build_steps/install.rs b/src/bootstrap/src/core/build_steps/install.rs
index 0225f8f24a5..29238b90225 100644
--- a/src/bootstrap/src/core/build_steps/install.rs
+++ b/src/bootstrap/src/core/build_steps/install.rs
@@ -24,7 +24,7 @@ const SHELL: &str = "sh";
 // We have to run a few shell scripts, which choke quite a bit on both `\`
 // characters and on `C:\` paths, so normalize both of them away.
 fn sanitize_sh(path: &Path) -> String {
-    let path = path.to_str().unwrap().replace("\\", "/");
+    let path = path.to_str().unwrap().replace('\\', "/");
     return change_drive(unc_to_lfs(&path)).unwrap_or(path);
 
     fn unc_to_lfs(s: &str) -> &str {
@@ -44,7 +44,7 @@ fn sanitize_sh(path: &Path) -> String {
     }
 }
 
-fn is_dir_writable_for_user(dir: &PathBuf) -> bool {
+fn is_dir_writable_for_user(dir: &Path) -> bool {
     let tmp = dir.join(".tmp");
     match fs::create_dir_all(&tmp) {
         Ok(_) => {
diff --git a/src/bootstrap/src/core/build_steps/llvm.rs b/src/bootstrap/src/core/build_steps/llvm.rs
index 836ac3de94d..9622321a74e 100644
--- a/src/bootstrap/src/core/build_steps/llvm.rs
+++ b/src/bootstrap/src/core/build_steps/llvm.rs
@@ -110,7 +110,7 @@ pub fn prebuilt_llvm_config(
     let smart_stamp_hash = STAMP_HASH_MEMO.get_or_init(|| {
         generate_smart_stamp_hash(
             &builder.config.src.join("src/llvm-project"),
-            &builder.in_tree_llvm_info.sha().unwrap_or_default(),
+            builder.in_tree_llvm_info.sha().unwrap_or_default(),
         )
     });
 
@@ -289,7 +289,7 @@ impl Step for Llvm {
 
         let _guard = builder.msg_unstaged(Kind::Build, "LLVM", target);
         t!(stamp.remove());
-        let _time = helpers::timeit(&builder);
+        let _time = helpers::timeit(builder);
         t!(fs::create_dir_all(&out_dir));
 
         // https://llvm.org/docs/CMake.html
@@ -355,7 +355,7 @@ impl Step for Llvm {
             cfg.define("LLVM_BUILD_RUNTIME", "No");
         }
         if let Some(path) = builder.config.llvm_profile_use.as_ref() {
-            cfg.define("LLVM_PROFDATA_FILE", &path);
+            cfg.define("LLVM_PROFDATA_FILE", path);
         }
 
         // Disable zstd to avoid a dependency on libzstd.so.
@@ -643,7 +643,7 @@ fn configure_cmake(
 
     let sanitize_cc = |cc: &Path| {
         if target.is_msvc() {
-            OsString::from(cc.to_str().unwrap().replace("\\", "/"))
+            OsString::from(cc.to_str().unwrap().replace('\\', "/"))
         } else {
             cc.as_os_str().to_owned()
         }
@@ -808,10 +808,10 @@ fn configure_llvm(builder: &Builder<'_>, target: TargetSelection, cfg: &mut cmak
 // Adapted from https://github.com/alexcrichton/cc-rs/blob/fba7feded71ee4f63cfe885673ead6d7b4f2f454/src/lib.rs#L2347-L2365
 fn get_var(var_base: &str, host: &str, target: &str) -> Option<OsString> {
     let kind = if host == target { "HOST" } else { "TARGET" };
-    let target_u = target.replace("-", "_");
-    env::var_os(&format!("{var_base}_{target}"))
-        .or_else(|| env::var_os(&format!("{}_{}", var_base, target_u)))
-        .or_else(|| env::var_os(&format!("{}_{}", kind, var_base)))
+    let target_u = target.replace('-', "_");
+    env::var_os(format!("{var_base}_{target}"))
+        .or_else(|| env::var_os(format!("{}_{}", var_base, target_u)))
+        .or_else(|| env::var_os(format!("{}_{}", kind, var_base)))
         .or_else(|| env::var_os(var_base))
 }
 
@@ -862,7 +862,7 @@ impl Step for Lld {
         }
 
         let _guard = builder.msg_unstaged(Kind::Build, "LLD", target);
-        let _time = helpers::timeit(&builder);
+        let _time = helpers::timeit(builder);
         t!(fs::create_dir_all(&out_dir));
 
         let mut cfg = cmake::Config::new(builder.src.join("src/llvm-project/lld"));
@@ -986,7 +986,7 @@ impl Step for Sanitizers {
 
         let _guard = builder.msg_unstaged(Kind::Build, "sanitizers", self.target);
         t!(stamp.remove());
-        let _time = helpers::timeit(&builder);
+        let _time = helpers::timeit(builder);
 
         let mut cfg = cmake::Config::new(&compiler_rt_dir);
         cfg.profile("Release");
@@ -1051,7 +1051,7 @@ fn supported_sanitizers(
             .map(move |c| SanitizerRuntime {
                 cmake_target: format!("clang_rt.{}_{}_dynamic", c, os),
                 path: out_dir
-                    .join(&format!("build/lib/darwin/libclang_rt.{}_{}_dynamic.dylib", c, os)),
+                    .join(format!("build/lib/darwin/libclang_rt.{}_{}_dynamic.dylib", c, os)),
                 name: format!("librustc-{}_rt.{}.dylib", channel, c),
             })
             .collect()
@@ -1062,7 +1062,7 @@ fn supported_sanitizers(
             .iter()
             .map(move |c| SanitizerRuntime {
                 cmake_target: format!("clang_rt.{}-{}", c, arch),
-                path: out_dir.join(&format!("build/lib/{}/libclang_rt.{}-{}.a", os, c, arch)),
+                path: out_dir.join(format!("build/lib/{}/libclang_rt.{}-{}.a", os, c, arch)),
                 name: format!("librustc-{}_rt.{}.a", channel, c),
             })
             .collect()
@@ -1165,7 +1165,7 @@ impl Step for CrtBeginEnd {
 
     /// Build crtbegin.o/crtend.o for musl target.
     fn run(self, builder: &Builder<'_>) -> Self::Output {
-        builder.update_submodule(&Path::new("src/llvm-project"));
+        builder.update_submodule(Path::new("src/llvm-project"));
 
         let out_dir = builder.native_dir(self.target).join("crt");
 
@@ -1233,7 +1233,7 @@ impl Step for Libunwind {
 
     /// Build libunwind.a
     fn run(self, builder: &Builder<'_>) -> Self::Output {
-        builder.update_submodule(&Path::new("src/llvm-project"));
+        builder.update_submodule(Path::new("src/llvm-project"));
 
         if builder.config.dry_run() {
             return PathBuf::new();
diff --git a/src/bootstrap/src/core/build_steps/run.rs b/src/bootstrap/src/core/build_steps/run.rs
index 5fa5f2d4794..27b0c7760f0 100644
--- a/src/bootstrap/src/core/build_steps/run.rs
+++ b/src/bootstrap/src/core/build_steps/run.rs
@@ -23,7 +23,7 @@ impl Step for ExpandYamlAnchors {
     fn run(self, builder: &Builder<'_>) {
         builder.info("Expanding YAML anchors in the GitHub Actions configuration");
         builder.run_delaying_failure(
-            &mut builder.tool_cmd(Tool::ExpandYamlAnchors).arg("generate").arg(&builder.src),
+            builder.tool_cmd(Tool::ExpandYamlAnchors).arg("generate").arg(&builder.src),
         );
     }
 
diff --git a/src/bootstrap/src/core/build_steps/setup.rs b/src/bootstrap/src/core/build_steps/setup.rs
index f7747e66dd9..74a5578b43e 100644
--- a/src/bootstrap/src/core/build_steps/setup.rs
+++ b/src/bootstrap/src/core/build_steps/setup.rs
@@ -8,7 +8,7 @@ use std::env::consts::EXE_SUFFIX;
 use std::fmt::Write as _;
 use std::fs::File;
 use std::io::Write;
-use std::path::{Path, PathBuf, MAIN_SEPARATOR};
+use std::path::{Path, PathBuf, MAIN_SEPARATOR_STR};
 use std::process::Command;
 use std::str::FromStr;
 use std::{fmt, fs, io};
@@ -257,8 +257,7 @@ impl Step for Link {
             return;
         }
         let stage_path =
-            ["build", config.build.rustc_target_arg(), "stage1"].join(&MAIN_SEPARATOR.to_string());
-
+            ["build", config.build.rustc_target_arg(), "stage1"].join(MAIN_SEPARATOR_STR);
         if !rustup_installed() {
             eprintln!("`rustup` is not installed; cannot link `stage1` toolchain");
         } else if stage_dir_exists(&stage_path[..]) && !config.dry_run() {
@@ -276,7 +275,7 @@ fn rustup_installed() -> bool {
 }
 
 fn stage_dir_exists(stage_path: &str) -> bool {
-    match fs::create_dir(&stage_path) {
+    match fs::create_dir(stage_path) {
         Ok(_) => true,
         Err(_) => Path::new(&stage_path).exists(),
     }
@@ -294,7 +293,7 @@ fn attempt_toolchain_link(stage_path: &str) {
         return;
     }
 
-    if try_link_toolchain(&stage_path) {
+    if try_link_toolchain(stage_path) {
         println!(
             "Added `stage1` rustup toolchain; try `cargo +stage1 build` on a separate rust project to run a newly-built toolchain"
         );
@@ -310,7 +309,7 @@ fn attempt_toolchain_link(stage_path: &str) {
 
 fn toolchain_is_linked() -> bool {
     match Command::new("rustup")
-        .args(&["toolchain", "list"])
+        .args(["toolchain", "list"])
         .stdout(std::process::Stdio::piped())
         .output()
     {
@@ -337,7 +336,7 @@ fn toolchain_is_linked() -> bool {
 fn try_link_toolchain(stage_path: &str) -> bool {
     Command::new("rustup")
         .stdout(std::process::Stdio::null())
-        .args(&["toolchain", "link", "stage1", &stage_path])
+        .args(["toolchain", "link", "stage1", stage_path])
         .output()
         .map_or(false, |output| output.status.success())
 }
@@ -366,7 +365,7 @@ fn ensure_stage1_toolchain_placeholder_exists(stage_path: &str) -> bool {
         return false;
     }
 
-    return true;
+    true
 }
 
 // Used to get the path for `Subcommand::Setup`
@@ -469,13 +468,13 @@ impl Step for Hook {
         if config.dry_run() {
             return;
         }
-        t!(install_git_hook_maybe(&config));
+        t!(install_git_hook_maybe(config));
     }
 }
 
 // install a git hook to automatically run tidy, if they want
 fn install_git_hook_maybe(config: &Config) -> io::Result<()> {
-    let git = t!(config.git().args(&["rev-parse", "--git-common-dir"]).output().map(|output| {
+    let git = t!(config.git().args(["rev-parse", "--git-common-dir"]).output().map(|output| {
         assert!(output.status.success(), "failed to run `git`");
         PathBuf::from(t!(String::from_utf8(output.stdout)).trim())
     }));
@@ -541,7 +540,7 @@ impl Step for Vscode {
         if config.dry_run() {
             return;
         }
-        while !t!(create_vscode_settings_maybe(&config)) {}
+        while !t!(create_vscode_settings_maybe(config)) {}
     }
 }
 
@@ -608,7 +607,7 @@ fn create_vscode_settings_maybe(config: &Config) -> io::Result<bool> {
             }
             _ => "Created",
         };
-        fs::write(&vscode_settings, &RUST_ANALYZER_SETTINGS)?;
+        fs::write(&vscode_settings, RUST_ANALYZER_SETTINGS)?;
         println!("{verb} `.vscode/settings.json`");
     } else {
         println!("\n{RUST_ANALYZER_SETTINGS}");
diff --git a/src/bootstrap/src/core/build_steps/suggest.rs b/src/bootstrap/src/core/build_steps/suggest.rs
index 93da27560c6..c057fa9a566 100644
--- a/src/bootstrap/src/core/build_steps/suggest.rs
+++ b/src/bootstrap/src/core/build_steps/suggest.rs
@@ -36,7 +36,7 @@ pub fn suggest(builder: &Builder<'_>, run: bool) {
             // this code expects one suggestion per line in the following format:
             // <x_subcommand> {some number of flags} [optional stage number]
             let cmd = sections.next().unwrap();
-            let stage = sections.next_back().map(|s| str::parse(s).ok()).flatten();
+            let stage = sections.next_back().and_then(|s| str::parse(s).ok());
             let paths: Vec<PathBuf> = sections.map(|p| PathBuf::from_str(p).unwrap()).collect();
 
             (cmd, stage, paths)
diff --git a/src/bootstrap/src/core/build_steps/synthetic_targets.rs b/src/bootstrap/src/core/build_steps/synthetic_targets.rs
index 9acdcaeb517..a00835402ec 100644
--- a/src/bootstrap/src/core/build_steps/synthetic_targets.rs
+++ b/src/bootstrap/src/core/build_steps/synthetic_targets.rs
@@ -79,7 +79,7 @@ fn create_synthetic_target(
 
     customize(spec_map);
 
-    std::fs::write(&path, &serde_json::to_vec_pretty(&spec).unwrap()).unwrap();
+    std::fs::write(&path, serde_json::to_vec_pretty(&spec).unwrap()).unwrap();
     let target = TargetSelection::create_synthetic(&name, path.to_str().unwrap());
     crate::utils::cc_detect::find_target(builder, target);
 
diff --git a/src/bootstrap/src/core/build_steps/test.rs b/src/bootstrap/src/core/build_steps/test.rs
index 0c7e751c8da..791f847a866 100644
--- a/src/bootstrap/src/core/build_steps/test.rs
+++ b/src/bootstrap/src/core/build_steps/test.rs
@@ -156,7 +156,7 @@ You can skip linkcheck with --skip src/tools/linkchecker"
         // Run the linkchecker.
         let _guard =
             builder.msg(Kind::Test, compiler.stage, "Linkcheck", bootstrap_host, bootstrap_host);
-        let _time = helpers::timeit(&builder);
+        let _time = helpers::timeit(builder);
         builder.run_delaying_failure(linkchecker.arg(builder.out.join(host.triple).join("doc")));
     }
 
@@ -253,15 +253,15 @@ impl Step for Cargotest {
         let out_dir = builder.out.join("ct");
         t!(fs::create_dir_all(&out_dir));
 
-        let _time = helpers::timeit(&builder);
+        let _time = helpers::timeit(builder);
         let mut cmd = builder.tool_cmd(Tool::CargoTest);
-        let mut cmd = cmd
+        let cmd = cmd
             .arg(&cargo)
             .arg(&out_dir)
             .args(builder.config.test_args())
             .env("RUSTC", builder.rustc(compiler))
             .env("RUSTDOC", builder.rustdoc(compiler));
-        add_rustdoc_cargo_linker_args(&mut cmd, builder, compiler.host, LldThreads::No);
+        add_rustdoc_cargo_linker_args(cmd, builder, compiler.host, LldThreads::No);
         builder.run_delaying_failure(cmd);
     }
 }
@@ -322,7 +322,7 @@ impl Step for Cargo {
             builder,
         );
 
-        let _time = helpers::timeit(&builder);
+        let _time = helpers::timeit(builder);
         add_flags_and_try_run_tests(builder, &mut cargo);
     }
 }
@@ -474,7 +474,7 @@ impl Step for RustDemangler {
         );
 
         let dir = testdir(builder, compiler.host);
-        t!(fs::create_dir_all(&dir));
+        t!(fs::create_dir_all(dir));
 
         cargo.env("RUST_DEMANGLER_DRIVER_PATH", rust_demangler);
         cargo.add_rustc_lib_path(builder);
@@ -525,7 +525,7 @@ impl Miri {
         // Tell `cargo miri setup` where to find the sources.
         cargo.env("MIRI_LIB_SRC", builder.src.join("library"));
         // Tell it where to find Miri.
-        cargo.env("MIRI", &miri);
+        cargo.env("MIRI", miri);
         // Tell it where to put the sysroot.
         cargo.env("MIRI_SYSROOT", &miri_sysroot);
         // Debug things.
@@ -637,7 +637,7 @@ impl Step for Miri {
         // does not understand the flags added by `add_flags_and_try_run_test`.
         let mut cargo = prepare_cargo_test(cargo, &[], &[], "miri", compiler, target, builder);
         {
-            let _time = helpers::timeit(&builder);
+            let _time = helpers::timeit(builder);
             builder.run(&mut cargo);
         }
 
@@ -649,11 +649,11 @@ impl Step for Miri {
             // `MIRI_SKIP_UI_CHECKS` and `RUSTC_BLESS` are incompatible
             cargo.env_remove("RUSTC_BLESS");
             // Optimizations can change error locations and remove UB so don't run `fail` tests.
-            cargo.args(&["tests/pass", "tests/panic"]);
+            cargo.args(["tests/pass", "tests/panic"]);
 
             let mut cargo = prepare_cargo_test(cargo, &[], &[], "miri", compiler, target, builder);
             {
-                let _time = helpers::timeit(&builder);
+                let _time = helpers::timeit(builder);
                 builder.run(&mut cargo);
             }
         }
@@ -693,7 +693,7 @@ impl Step for Miri {
 
         let mut cargo = Command::from(cargo);
         {
-            let _time = helpers::timeit(&builder);
+            let _time = helpers::timeit(builder);
             builder.run(&mut cargo);
         }
     }
@@ -946,7 +946,7 @@ impl Step for RustdocJSNotStd {
 }
 
 fn get_browser_ui_test_version_inner(npm: &Path, global: bool) -> Option<String> {
-    let mut command = Command::new(&npm);
+    let mut command = Command::new(npm);
     command.arg("list").arg("--parseable").arg("--long").arg("--depth=0");
     if global {
         command.arg("--global");
@@ -954,7 +954,7 @@ fn get_browser_ui_test_version_inner(npm: &Path, global: bool) -> Option<String>
     let lines = command
         .output()
         .map(|output| String::from_utf8_lossy(&output.stdout).into_owned())
-        .unwrap_or(String::new());
+        .unwrap_or_default();
     lines
         .lines()
         .find_map(|l| l.split(':').nth(1)?.strip_prefix("browser-ui-test@"))
@@ -1048,7 +1048,7 @@ impl Step for RustdocGUI {
             cmd.arg("--npm").arg(npm);
         }
 
-        let _time = helpers::timeit(&builder);
+        let _time = helpers::timeit(builder);
         let _guard = builder.msg_sysroot_tool(
             Kind::Test,
             self.compiler.stage,
@@ -1096,7 +1096,7 @@ impl Step for Tidy {
             cmd.arg(format!("--extra-checks={s}"));
         }
         let mut args = std::env::args_os();
-        if let Some(_) = args.find(|arg| arg == OsStr::new("--")) {
+        if args.any(|arg| arg == OsStr::new("--")) {
             cmd.arg("--");
             cmd.args(args);
         }
@@ -1116,7 +1116,7 @@ HELP: to skip test's attempt to check tidiness, pass `--skip src/tools/tidy` to
                 );
                 crate::exit!(1);
             }
-            crate::core::build_steps::format::format(&builder, !builder.config.cmd.bless(), &[]);
+            crate::core::build_steps::format::format(builder, !builder.config.cmd.bless(), &[]);
         }
 
         builder.info("tidy check");
@@ -1171,7 +1171,7 @@ impl Step for ExpandYamlAnchors {
         }
         builder.info("Ensuring the YAML anchors in the GitHub Actions config were expanded");
         builder.run_delaying_failure(
-            &mut builder.tool_cmd(Tool::ExpandYamlAnchors).arg("check").arg(&builder.src),
+            builder.tool_cmd(Tool::ExpandYamlAnchors).arg("check").arg(&builder.src),
         );
     }
 
@@ -1759,7 +1759,7 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
 
         for exclude in &builder.config.skip {
             cmd.arg("--skip");
-            cmd.arg(&exclude);
+            cmd.arg(exclude);
         }
 
         // Get paths from cmd args
@@ -1780,7 +1780,7 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
         // so the correct filters are passed to libtest
         if cfg!(windows) {
             let test_args_win: Vec<String> =
-                test_args.iter().map(|s| s.replace("/", "\\")).collect();
+                test_args.iter().map(|s| s.replace('/', "\\")).collect();
             cmd.args(&test_args_win);
         } else {
             cmd.args(&test_args);
@@ -1900,7 +1900,7 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
             // Note that if we encounter `PATH` we make sure to append to our own `PATH`
             // rather than stomp over it.
             if !builder.config.dry_run() && target.is_msvc() {
-                for &(ref k, ref v) in builder.cc.borrow()[&target].env() {
+                for (k, v) in builder.cc.borrow()[&target].env() {
                     if k != "PATH" {
                         cmd.env(k, v);
                     }
@@ -1996,7 +1996,7 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
         let _group = builder.msg(
             Kind::Test,
             compiler.stage,
-            &format!("compiletest suite={suite} mode={mode}"),
+            format!("compiletest suite={suite} mode={mode}"),
             compiler.host,
             target,
         );
@@ -2022,7 +2022,7 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
                 "Check compiletest suite={} mode={} compare_mode={} ({} -> {})",
                 suite, mode, compare_mode, &compiler.host, target
             ));
-            let _time = helpers::timeit(&builder);
+            let _time = helpers::timeit(builder);
             try_run_tests(builder, &mut cmd, false);
         }
     }
@@ -2094,7 +2094,7 @@ impl BookTest {
             compiler.host,
             compiler.host,
         );
-        let _time = helpers::timeit(&builder);
+        let _time = helpers::timeit(builder);
         let toolstate = if builder.run_delaying_failure(&mut rustbook_cmd) {
             ToolState::TestPass
         } else {
@@ -2111,12 +2111,12 @@ impl BookTest {
         builder.ensure(compile::Std::new(compiler, host));
 
         let _guard =
-            builder.msg(Kind::Test, compiler.stage, &format!("book {}", self.name), host, host);
+            builder.msg(Kind::Test, compiler.stage, format!("book {}", self.name), host, host);
 
         // Do a breadth-first traversal of the `src/doc` directory and just run
         // tests for all files that end in `*.md`
         let mut stack = vec![builder.src.join(self.path)];
-        let _time = helpers::timeit(&builder);
+        let _time = helpers::timeit(builder);
         let mut files = Vec::new();
         while let Some(p) = stack.pop() {
             if p.is_dir() {
@@ -2227,7 +2227,7 @@ impl Step for ErrorIndex {
 
         let guard =
             builder.msg(Kind::Test, compiler.stage, "error-index", compiler.host, compiler.host);
-        let _time = helpers::timeit(&builder);
+        let _time = helpers::timeit(builder);
         builder.run_quiet(&mut tool);
         drop(guard);
         // The tests themselves need to link to std, so make sure it is
@@ -2315,11 +2315,8 @@ impl Step for CrateLibrustc {
         let builder = run.builder;
         let host = run.build_triple();
         let compiler = builder.compiler_for(builder.top_stage, host, host);
-        let crates = run
-            .paths
-            .iter()
-            .map(|p| builder.crate_paths[&p.assert_single_path().path].clone())
-            .collect();
+        let crates =
+            run.paths.iter().map(|p| builder.crate_paths[&p.assert_single_path().path]).collect();
 
         builder.ensure(CrateLibrustc { compiler, target: run.target, crates });
     }
@@ -2351,7 +2348,7 @@ fn run_cargo_test<'a>(
 ) -> bool {
     let mut cargo =
         prepare_cargo_test(cargo, libtest_args, crates, primary_crate, compiler, target, builder);
-    let _time = helpers::timeit(&builder);
+    let _time = helpers::timeit(builder);
     let _group = description.into().and_then(|what| {
         builder.msg_sysroot_tool(Kind::Test, compiler.stage, what, compiler.host, target)
     });
@@ -2406,7 +2403,7 @@ fn prepare_cargo_test(
             if krate.has_lib {
                 cargo.arg("--lib");
             }
-            cargo.args(&["--bins", "--examples", "--tests", "--benches"]);
+            cargo.args(["--bins", "--examples", "--tests", "--benches"]);
         }
         DocTests::Yes => {}
     }
@@ -2468,11 +2465,8 @@ impl Step for Crate {
         let builder = run.builder;
         let host = run.build_triple();
         let compiler = builder.compiler_for(builder.top_stage, host, host);
-        let crates = run
-            .paths
-            .iter()
-            .map(|p| builder.crate_paths[&p.assert_single_path().path].clone())
-            .collect();
+        let crates =
+            run.paths.iter().map(|p| builder.crate_paths[&p.assert_single_path().path]).collect();
 
         builder.ensure(Crate { compiler, target: run.target, mode: Mode::Std, crates });
     }
@@ -2844,11 +2838,11 @@ impl Step for Bootstrap {
         let compiler = builder.compiler(0, host);
         let _guard = builder.msg(Kind::Test, 0, "bootstrap", host, host);
 
-        let mut check_bootstrap = Command::new(&builder.python());
+        let mut check_bootstrap = Command::new(builder.python());
         check_bootstrap
             .args(["-m", "unittest", "bootstrap_test.py"])
             .env("BUILD_DIR", &builder.out)
-            .env("BUILD_PLATFORM", &builder.build.build.triple)
+            .env("BUILD_PLATFORM", builder.build.build.triple)
             .current_dir(builder.src.join("src/bootstrap/"));
         // NOTE: we intentionally don't pass test_args here because the args for unittest and cargo test are mutually incompatible.
         // Use `python -m unittest` manually if you want to pass arguments.
@@ -3171,7 +3165,7 @@ impl Step for CodegenCranelift {
             &compiler.host,
             target
         ));
-        let _time = helpers::timeit(&builder);
+        let _time = helpers::timeit(builder);
 
         // FIXME handle vendoring for source tarballs before removing the --skip-test below
         let download_dir = builder.out.join("cg_clif_download");
@@ -3300,7 +3294,7 @@ impl Step for CodegenGCC {
             &compiler.host,
             target
         ));
-        let _time = helpers::timeit(&builder);
+        let _time = helpers::timeit(builder);
 
         // FIXME: Uncomment the `prepare` command below once vendoring is implemented.
         /*
diff --git a/src/bootstrap/src/core/build_steps/tool.rs b/src/bootstrap/src/core/build_steps/tool.rs
index a3daf22c9a9..ba867a04ec5 100644
--- a/src/bootstrap/src/core/build_steps/tool.rs
+++ b/src/bootstrap/src/core/build_steps/tool.rs
@@ -819,7 +819,7 @@ impl<'a> Builder<'a> {
         if compiler.host.is_msvc() {
             let curpaths = env::var_os("PATH").unwrap_or_default();
             let curpaths = env::split_paths(&curpaths).collect::<Vec<_>>();
-            for &(ref k, ref v) in self.cc.borrow()[&compiler.host].env() {
+            for (k, v) in self.cc.borrow()[&compiler.host].env() {
                 if k != "PATH" {
                     continue;
                 }
diff --git a/src/bootstrap/src/core/build_steps/toolstate.rs b/src/bootstrap/src/core/build_steps/toolstate.rs
index a451f92b6bc..deb782cad0c 100644
--- a/src/bootstrap/src/core/build_steps/toolstate.rs
+++ b/src/bootstrap/src/core/build_steps/toolstate.rs
@@ -346,7 +346,7 @@ fn prepare_toolstate_config(token: &str) {
 
     let credential = format!("https://{token}:x-oauth-basic@github.com\n",);
     let git_credential_path = PathBuf::from(t!(env::var("HOME"))).join(".git-credentials");
-    t!(fs::write(&git_credential_path, credential));
+    t!(fs::write(git_credential_path, credential));
 }
 
 /// Reads the latest toolstate from the toolstate repo.
@@ -389,7 +389,7 @@ fn commit_toolstate_change(current_toolstate: &ToolstateData) {
         // Upload the test results (the new commit-to-toolstate mapping) to the toolstate repo.
         // This does *not* change the "current toolstate"; that only happens post-landing
         // via `src/ci/docker/publish_toolstate.sh`.
-        publish_test_results(&current_toolstate);
+        publish_test_results(current_toolstate);
 
         // `git commit` failing means nothing to commit.
         let status = t!(Command::new("git")
diff --git a/src/bootstrap/src/core/builder.rs b/src/bootstrap/src/core/builder.rs
index 50264ca3b3a..97819403ab7 100644
--- a/src/bootstrap/src/core/builder.rs
+++ b/src/bootstrap/src/core/builder.rs
@@ -290,7 +290,7 @@ impl PathSet {
 
 const PATH_REMAP: &[(&str, &str)] = &[("rust-analyzer-proc-macro-srv", "proc-macro-srv-cli")];
 
-fn remap_paths(paths: &mut Vec<&Path>) {
+fn remap_paths(paths: &mut [&Path]) {
     for path in paths.iter_mut() {
         for &(search, replace) in PATH_REMAP {
             if path.to_str() == Some(search) {
@@ -329,7 +329,7 @@ impl StepDescription {
     }
 
     fn is_excluded(&self, builder: &Builder<'_>, pathset: &PathSet) -> bool {
-        if builder.config.skip.iter().any(|e| pathset.has(&e, builder.kind)) {
+        if builder.config.skip.iter().any(|e| pathset.has(e, builder.kind)) {
             if !matches!(builder.config.dry_run, DryRun::SelfCheck) {
                 println!("Skipping {pathset:?} because it is excluded");
             }
@@ -369,8 +369,7 @@ impl StepDescription {
         }
 
         // strip CurDir prefix if present
-        let mut paths: Vec<_> =
-            paths.into_iter().map(|p| p.strip_prefix(".").unwrap_or(p)).collect();
+        let mut paths: Vec<_> = paths.iter().map(|p| p.strip_prefix(".").unwrap_or(p)).collect();
 
         remap_paths(&mut paths);
 
@@ -378,7 +377,7 @@ impl StepDescription {
         // (This is separate from the loop below to avoid having to handle multiple paths in `is_suite_path` somehow.)
         paths.retain(|path| {
             for (desc, should_run) in v.iter().zip(&should_runs) {
-                if let Some(suite) = should_run.is_suite_path(&path) {
+                if let Some(suite) = should_run.is_suite_path(path) {
                     desc.maybe_run(builder, vec![suite.clone()]);
                     return false;
                 }
@@ -537,7 +536,7 @@ impl<'a> ShouldRun<'a> {
                 .iter()
                 .map(|p| {
                     // assert only if `p` isn't submodule
-                    if submodules_paths.iter().find(|sm_p| p.contains(*sm_p)).is_none() {
+                    if !submodules_paths.iter().any(|sm_p| p.contains(sm_p)) {
                         assert!(
                             self.builder.src.join(p).exists(),
                             "`should_run.paths` should correspond to real on-disk paths - use `alias` if there is no relevant path: {}",
@@ -1208,7 +1207,7 @@ impl<'a> Builder<'a> {
     }
 
     pub fn rustdoc_cmd(&self, compiler: Compiler) -> Command {
-        let mut cmd = Command::new(&self.bootstrap_out.join("rustdoc"));
+        let mut cmd = Command::new(self.bootstrap_out.join("rustdoc"));
         cmd.env("RUSTC_STAGE", compiler.stage.to_string())
             .env("RUSTC_SYSROOT", self.sysroot(compiler))
             // Note that this is *not* the sysroot_libdir because rustdoc must be linked
@@ -1351,7 +1350,7 @@ impl<'a> Builder<'a> {
 
         // See comment in rustc_llvm/build.rs for why this is necessary, largely llvm-config
         // needs to not accidentally link to libLLVM in stage0/lib.
-        cargo.env("REAL_LIBRARY_PATH_VAR", &helpers::dylib_path_var());
+        cargo.env("REAL_LIBRARY_PATH_VAR", helpers::dylib_path_var());
         if let Some(e) = env::var_os(helpers::dylib_path_var()) {
             cargo.env("REAL_LIBRARY_PATH", e);
         }
@@ -1620,8 +1619,8 @@ impl<'a> Builder<'a> {
             .env("RUSTBUILD_NATIVE_DIR", self.native_dir(target))
             .env("RUSTC_REAL", self.rustc(compiler))
             .env("RUSTC_STAGE", stage.to_string())
-            .env("RUSTC_SYSROOT", &sysroot)
-            .env("RUSTC_LIBDIR", &libdir)
+            .env("RUSTC_SYSROOT", sysroot)
+            .env("RUSTC_LIBDIR", libdir)
             .env("RUSTDOC", self.bootstrap_out.join("rustdoc"))
             .env(
                 "RUSTDOC_REAL",
@@ -1754,7 +1753,7 @@ impl<'a> Builder<'a> {
         cargo.env("RUSTC_BOOTSTRAP", "1");
 
         if self.config.dump_bootstrap_shims {
-            prepare_behaviour_dump_dir(&self.build);
+            prepare_behaviour_dump_dir(self.build);
 
             cargo
                 .env("DUMP_BOOTSTRAP_SHIMS", self.build.out.join("bootstrap-shims-dump"))
@@ -1793,7 +1792,7 @@ impl<'a> Builder<'a> {
         // platform-specific environment variable as a workaround.
         if mode == Mode::ToolRustc || mode == Mode::Codegen {
             if let Some(llvm_config) = self.llvm_config(target) {
-                let llvm_libdir = output(Command::new(&llvm_config).arg("--libdir"));
+                let llvm_libdir = output(Command::new(llvm_config).arg("--libdir"));
                 add_link_lib_path(vec![llvm_libdir.trim().into()], &mut cargo);
             }
         }
@@ -2080,7 +2079,7 @@ impl<'a> Builder<'a> {
 
         if self.config.print_step_timings && !self.config.dry_run() {
             let step_string = format!("{step:?}");
-            let brace_index = step_string.find("{").unwrap_or(0);
+            let brace_index = step_string.find('{').unwrap_or(0);
             let type_string = type_name::<S>();
             println!(
                 "[TIMING] {} {} -- {}.{:03}",
@@ -2429,7 +2428,7 @@ impl Cargo {
                     _ => s.display().to_string(),
                 }
             };
-            let triple_underscored = target.triple.replace("-", "_");
+            let triple_underscored = target.triple.replace('-', "_");
             let cc = ccacheify(&builder.cc(target));
             self.command.env(format!("CC_{triple_underscored}"), &cc);
 
diff --git a/src/bootstrap/src/core/config/config.rs b/src/bootstrap/src/core/config/config.rs
index 1605776f772..927d46c67a9 100644
--- a/src/bootstrap/src/core/config/config.rs
+++ b/src/bootstrap/src/core/config/config.rs
@@ -468,7 +468,7 @@ pub struct TargetSelectionList(Vec<TargetSelection>);
 
 pub fn target_selection_list(s: &str) -> Result<TargetSelectionList, String> {
     Ok(TargetSelectionList(
-        s.split(",").filter(|s| !s.is_empty()).map(TargetSelection::from_user).collect(),
+        s.split(',').filter(|s| !s.is_empty()).map(TargetSelection::from_user).collect(),
     ))
 }
 
@@ -963,10 +963,10 @@ impl<'de> serde::de::Visitor<'de> for OptimizeVisitor {
     where
         E: serde::de::Error,
     {
-        if ["s", "z"].iter().find(|x| **x == value).is_some() {
+        if matches!(value, "s" | "z") {
             Ok(RustOptimize::String(value.to_string()))
         } else {
-            Err(format_optimize_error_msg(value)).map_err(serde::de::Error::custom)
+            Err(serde::de::Error::custom(format_optimize_error_msg(value)))
         }
     }
 
@@ -977,7 +977,7 @@ impl<'de> serde::de::Visitor<'de> for OptimizeVisitor {
         if matches!(value, 0..=3) {
             Ok(RustOptimize::Int(value as u8))
         } else {
-            Err(format_optimize_error_msg(value)).map_err(serde::de::Error::custom)
+            Err(serde::de::Error::custom(format_optimize_error_msg(value)))
         }
     }
 
@@ -1144,41 +1144,44 @@ define_config! {
 
 impl Config {
     pub fn default_opts() -> Config {
-        let mut config = Config::default();
-        config.bypass_bootstrap_lock = false;
-        config.llvm_optimize = true;
-        config.ninja_in_file = true;
-        config.llvm_static_stdcpp = false;
-        config.backtrace = true;
-        config.rust_optimize = RustOptimize::Bool(true);
-        config.rust_optimize_tests = true;
-        config.submodules = None;
-        config.docs = true;
-        config.docs_minification = true;
-        config.rust_rpath = true;
-        config.rust_strip = false;
-        config.channel = "dev".to_string();
-        config.codegen_tests = true;
-        config.rust_dist_src = true;
-        config.rust_codegen_backends = vec![INTERNER.intern_str("llvm")];
-        config.deny_warnings = true;
-        config.bindir = "bin".into();
-        config.dist_include_mingw_linker = true;
-        config.dist_compression_profile = "fast".into();
-        config.rustc_parallel = true;
-
-        config.stdout_is_tty = std::io::stdout().is_terminal();
-        config.stderr_is_tty = std::io::stderr().is_terminal();
-
-        // set by build.rs
-        config.build = TargetSelection::from_user(&env!("BUILD_TRIPLE"));
-
-        let manifest_dir = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
-        // Undo `src/bootstrap`
-        config.src = manifest_dir.parent().unwrap().parent().unwrap().to_owned();
-        config.out = PathBuf::from("build");
-
-        config
+        Config {
+            bypass_bootstrap_lock: false,
+            llvm_optimize: true,
+            ninja_in_file: true,
+            llvm_static_stdcpp: false,
+            backtrace: true,
+            rust_optimize: RustOptimize::Bool(true),
+            rust_optimize_tests: true,
+            submodules: None,
+            docs: true,
+            docs_minification: true,
+            rust_rpath: true,
+            rust_strip: false,
+            channel: "dev".to_string(),
+            codegen_tests: true,
+            rust_dist_src: true,
+            rust_codegen_backends: vec![INTERNER.intern_str("llvm")],
+            deny_warnings: true,
+            bindir: "bin".into(),
+            dist_include_mingw_linker: true,
+            dist_compression_profile: "fast".into(),
+            rustc_parallel: true,
+
+            stdout_is_tty: std::io::stdout().is_terminal(),
+            stderr_is_tty: std::io::stderr().is_terminal(),
+
+            // set by build.rs
+            build: TargetSelection::from_user(env!("BUILD_TRIPLE")),
+
+            src: {
+                let manifest_dir = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
+                // Undo `src/bootstrap`
+                manifest_dir.parent().unwrap().parent().unwrap().to_owned()
+            },
+            out: PathBuf::from("build"),
+
+            ..Default::default()
+        }
     }
 
     pub fn parse(args: &[String]) -> Config {
@@ -1204,7 +1207,7 @@ impl Config {
     }
 
     pub(crate) fn parse_inner(args: &[String], get_toml: impl Fn(&Path) -> TomlConfig) -> Config {
-        let mut flags = Flags::parse(&args);
+        let mut flags = Flags::parse(args);
         let mut config = Config::default_opts();
 
         // Set flags.
@@ -1252,7 +1255,7 @@ impl Config {
             // Bootstrap is quite bad at handling /? in front of paths
             let src = match s.strip_prefix("\\\\?\\") {
                 Some(p) => PathBuf::from(p),
-                None => PathBuf::from(git_root),
+                None => git_root,
             };
             // If this doesn't have at least `stage0.json`, we guessed wrong. This can happen when,
             // for example, the build directory is inside of another unrelated git directory.
@@ -1278,7 +1281,7 @@ impl Config {
             .to_path_buf();
         }
 
-        let stage0_json = t!(std::fs::read(&config.src.join("src").join("stage0.json")));
+        let stage0_json = t!(std::fs::read(config.src.join("src").join("stage0.json")));
 
         config.stage0_metadata = t!(serde_json::from_slice::<Stage0Metadata>(&stage0_json));
 
@@ -1324,8 +1327,7 @@ impl Config {
         let mut override_toml = TomlConfig::default();
         for option in flags.set.iter() {
             fn get_table(option: &str) -> Result<TomlConfig, toml::de::Error> {
-                toml::from_str(&option)
-                    .and_then(|table: toml::Value| TomlConfig::deserialize(table))
+                toml::from_str(option).and_then(|table: toml::Value| TomlConfig::deserialize(table))
             }
 
             let mut err = match get_table(option) {
@@ -1337,7 +1339,7 @@ impl Config {
             };
             // We want to be able to set string values without quotes,
             // like in `configure.py`. Try adding quotes around the right hand side
-            if let Some((key, value)) = option.split_once("=") {
+            if let Some((key, value)) = option.split_once('=') {
                 if !value.contains('"') {
                     match get_table(&format!(r#"{key}="{value}""#)) {
                         Ok(v) => {
@@ -1660,7 +1662,7 @@ impl Config {
                 llvm_libunwind.map(|v| v.parse().expect("failed to parse rust.llvm-libunwind"));
 
             if let Some(ref backends) = codegen_backends {
-                let available_backends = vec!["llvm", "cranelift", "gcc"];
+                let available_backends = ["llvm", "cranelift", "gcc"];
 
                 config.rust_codegen_backends = backends.iter().map(|s| {
                     if let Some(backend) = s.strip_prefix(CODEGEN_BACKEND_PREFIX) {
@@ -1808,7 +1810,7 @@ impl Config {
                 let mut target = Target::from_triple(&triple);
 
                 if let Some(ref s) = cfg.llvm_config {
-                    if config.download_rustc_commit.is_some() && triple == &*config.build.triple {
+                    if config.download_rustc_commit.is_some() && triple == *config.build.triple {
                         panic!(
                             "setting llvm_config for the host is incompatible with download-rustc"
                         );
@@ -1847,7 +1849,7 @@ impl Config {
                 target.rpath = cfg.rpath;
 
                 if let Some(ref backends) = cfg.codegen_backends {
-                    let available_backends = vec!["llvm", "cranelift", "gcc"];
+                    let available_backends = ["llvm", "cranelift", "gcc"];
 
                     target.codegen_backends = Some(backends.iter().map(|s| {
                         if let Some(backend) = s.strip_prefix(CODEGEN_BACKEND_PREFIX) {
@@ -1874,7 +1876,7 @@ impl Config {
             let build_target = config
                 .target_config
                 .entry(config.build)
-                .or_insert_with(|| Target::from_triple(&triple));
+                .or_insert_with(|| Target::from_triple(triple));
 
             check_ci_llvm!(build_target.llvm_config);
             check_ci_llvm!(build_target.llvm_filecheck);
@@ -2208,7 +2210,7 @@ impl Config {
     }
 
     pub fn sanitizers_enabled(&self, target: TargetSelection) -> bool {
-        self.target_config.get(&target).map(|t| t.sanitizers).flatten().unwrap_or(self.sanitizers)
+        self.target_config.get(&target).and_then(|t| t.sanitizers).unwrap_or(self.sanitizers)
     }
 
     pub fn needs_sanitizer_runtime_built(&self, target: TargetSelection) -> bool {
@@ -2243,7 +2245,7 @@ impl Config {
     }
 
     pub fn rpath_enabled(&self, target: TargetSelection) -> bool {
-        self.target_config.get(&target).map(|t| t.rpath).flatten().unwrap_or(self.rust_rpath)
+        self.target_config.get(&target).and_then(|t| t.rpath).unwrap_or(self.rust_rpath)
     }
 
     pub fn llvm_enabled(&self, target: TargetSelection) -> bool {
@@ -2274,7 +2276,7 @@ impl Config {
     }
 
     pub fn default_codegen_backend(&self, target: TargetSelection) -> Option<Interned<String>> {
-        self.codegen_backends(target).get(0).cloned()
+        self.codegen_backends(target).first().cloned()
     }
 
     pub fn git_config(&self) -> GitConfig<'_> {
@@ -2303,9 +2305,9 @@ impl Config {
             .next()
             .unwrap()
             .to_owned();
-        let rustc_version = Version::parse(&rustc_output.trim()).unwrap();
+        let rustc_version = Version::parse(rustc_output.trim()).unwrap();
         let source_version =
-            Version::parse(&fs::read_to_string(self.src.join("src/version")).unwrap().trim())
+            Version::parse(fs::read_to_string(self.src.join("src/version")).unwrap().trim())
                 .unwrap();
         if !(source_version == rustc_version
             || (source_version.major == rustc_version.major
@@ -2333,7 +2335,7 @@ impl Config {
         };
 
         // Handle running from a directory other than the top level
-        let top_level = output(self.git().args(&["rev-parse", "--show-toplevel"]));
+        let top_level = output(self.git().args(["rev-parse", "--show-toplevel"]));
         let top_level = top_level.trim_end();
         let compiler = format!("{top_level}/compiler/");
         let library = format!("{top_level}/library/");
@@ -2344,7 +2346,7 @@ impl Config {
             self.git()
                 .arg("rev-list")
                 .arg(format!("--author={}", self.stage0_metadata.config.git_merge_commit_email))
-                .args(&["-n1", "--first-parent", "HEAD"]),
+                .args(["-n1", "--first-parent", "HEAD"]),
         );
         let commit = merge_base.trim_end();
         if commit.is_empty() {
@@ -2358,7 +2360,7 @@ impl Config {
         // Warn if there were changes to the compiler or standard library since the ancestor commit.
         let has_changes = !t!(self
             .git()
-            .args(&["diff-index", "--quiet", &commit, "--", &compiler, &library])
+            .args(["diff-index", "--quiet", commit, "--", &compiler, &library])
             .status())
         .success();
         if has_changes {
@@ -2397,7 +2399,7 @@ impl Config {
                 // there are some untracked changes in the the given paths.
                 false
             } else {
-                llvm::is_ci_llvm_available(&self, asserts)
+                llvm::is_ci_llvm_available(self, asserts)
             }
         };
         match download_ci_llvm {
@@ -2406,7 +2408,7 @@ impl Config {
             // FIXME: "if-available" is deprecated. Remove this block later (around mid 2024)
             // to not break builds between the recent-to-old checkouts.
             Some(StringOrBool::String(s)) if s == "if-available" => {
-                llvm::is_ci_llvm_available(&self, asserts)
+                llvm::is_ci_llvm_available(self, asserts)
             }
             Some(StringOrBool::String(s)) if s == "if-unchanged" => if_unchanged(),
             Some(StringOrBool::String(other)) => {
@@ -2424,7 +2426,7 @@ impl Config {
         if_unchanged: bool,
     ) -> Option<String> {
         // Handle running from a directory other than the top level
-        let top_level = output(self.git().args(&["rev-parse", "--show-toplevel"]));
+        let top_level = output(self.git().args(["rev-parse", "--show-toplevel"]));
         let top_level = top_level.trim_end();
 
         // Look for a version to compare to based on the current commit.
@@ -2433,7 +2435,7 @@ impl Config {
             self.git()
                 .arg("rev-list")
                 .arg(format!("--author={}", self.stage0_metadata.config.git_merge_commit_email))
-                .args(&["-n1", "--first-parent", "HEAD"]),
+                .args(["-n1", "--first-parent", "HEAD"]),
         );
         let commit = merge_base.trim_end();
         if commit.is_empty() {
@@ -2446,7 +2448,7 @@ impl Config {
 
         // Warn if there were changes to the compiler or standard library since the ancestor commit.
         let mut git = self.git();
-        git.args(&["diff-index", "--quiet", &commit, "--"]);
+        git.args(["diff-index", "--quiet", commit, "--"]);
 
         for path in modified_paths {
             git.arg(format!("{top_level}/{path}"));
diff --git a/src/bootstrap/src/core/download.rs b/src/bootstrap/src/core/download.rs
index b4ae3578ce3..185089a646b 100644
--- a/src/bootstrap/src/core/download.rs
+++ b/src/bootstrap/src/core/download.rs
@@ -159,7 +159,7 @@ impl Config {
             ";
             nix_build_succeeded = try_run(
                 self,
-                Command::new("nix-build").args(&[
+                Command::new("nix-build").args([
                     Path::new("-E"),
                     Path::new(NIX_EXPR),
                     Path::new("-o"),
@@ -188,7 +188,7 @@ impl Config {
             let dynamic_linker_path = nix_deps_dir.join("nix-support/dynamic-linker");
             // FIXME: can we support utf8 here? `args` doesn't accept Vec<u8>, only OsString ...
             let dynamic_linker = t!(String::from_utf8(t!(fs::read(dynamic_linker_path))));
-            patchelf.args(&["--set-interpreter", dynamic_linker.trim_end()]);
+            patchelf.args(["--set-interpreter", dynamic_linker.trim_end()]);
         }
 
         let _ = try_run(self, patchelf.arg(fname));
@@ -218,7 +218,7 @@ impl Config {
         println!("downloading {url}");
         // Try curl. If that fails and we are on windows, fallback to PowerShell.
         let mut curl = Command::new("curl");
-        curl.args(&[
+        curl.args([
             "-y",
             "30",
             "-Y",
@@ -242,7 +242,7 @@ impl Config {
             if self.build.contains("windows-msvc") {
                 eprintln!("Fallback to PowerShell");
                 for _ in 0..3 {
-                    if try_run(self, Command::new("PowerShell.exe").args(&[
+                    if try_run(self, Command::new("PowerShell.exe").args([
                         "/nologo",
                         "-Command",
                         "[Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12;",
@@ -388,7 +388,7 @@ impl Config {
         let bin_root = self.out.join(host.triple).join("stage0");
         let clippy_stamp = bin_root.join(".clippy-stamp");
         let cargo_clippy = bin_root.join("bin").join(exe("cargo-clippy", host));
-        if cargo_clippy.exists() && !program_out_of_date(&clippy_stamp, &date) {
+        if cargo_clippy.exists() && !program_out_of_date(&clippy_stamp, date) {
             return cargo_clippy;
         }
 
@@ -421,14 +421,14 @@ impl Config {
             DownloadSource::Dist,
             format!("rustfmt-{version}-{build}.tar.xz", build = host.triple),
             "rustfmt-preview",
-            &date,
+            date,
             "rustfmt",
         );
         self.download_component(
             DownloadSource::Dist,
             format!("rustc-{version}-{build}.tar.xz", build = host.triple),
             "rustc",
-            &date,
+            date,
             "rustfmt",
         );
 
@@ -665,7 +665,7 @@ download-rustc = false
         }
         let llvm_root = self.ci_llvm_root();
         let llvm_stamp = llvm_root.join(".llvm-stamp");
-        let llvm_sha = detect_llvm_sha(&self, self.rust_info.is_managed_git_subrepository());
+        let llvm_sha = detect_llvm_sha(self, self.rust_info.is_managed_git_subrepository());
         let key = format!("{}{}", llvm_sha, self.llvm_assertions);
         if program_out_of_date(&llvm_stamp, &key) && !self.dry_run() {
             self.download_ci_llvm(&llvm_sha);
@@ -685,11 +685,11 @@ download-rustc = false
             // rebuild.
             let now = filetime::FileTime::from_system_time(std::time::SystemTime::now());
             let llvm_config = llvm_root.join("bin").join(exe("llvm-config", self.build));
-            t!(filetime::set_file_times(&llvm_config, now, now));
+            t!(filetime::set_file_times(llvm_config, now, now));
 
             if self.should_fix_bins_and_dylibs() {
                 let llvm_lib = llvm_root.join("lib");
-                for entry in t!(fs::read_dir(&llvm_lib)) {
+                for entry in t!(fs::read_dir(llvm_lib)) {
                     let lib = t!(entry).path();
                     if lib.extension().map_or(false, |ext| ext == "so") {
                         self.fix_bin_or_dylib(&lib);
diff --git a/src/bootstrap/src/lib.rs b/src/bootstrap/src/lib.rs
index 121ed88c92f..965d788bb83 100644
--- a/src/bootstrap/src/lib.rs
+++ b/src/bootstrap/src/lib.rs
@@ -467,7 +467,7 @@ impl Build {
         }
 
         // Make a symbolic link so we can use a consistent directory in the documentation.
-        let build_triple = build.out.join(&build.build.triple);
+        let build_triple = build.out.join(build.build.triple);
         t!(fs::create_dir_all(&build_triple));
         let host = build.out.join("host");
         if host.is_symlink() {
@@ -491,7 +491,7 @@ impl Build {
     ///
     /// `relative_path` should be relative to the root of the git repository, not an absolute path.
     pub(crate) fn update_submodule(&self, relative_path: &Path) {
-        if !self.config.submodules(&self.rust_info()) {
+        if !self.config.submodules(self.rust_info()) {
             return;
         }
 
@@ -507,11 +507,11 @@ impl Build {
 
         // check_submodule
         let checked_out_hash =
-            output(Command::new("git").args(&["rev-parse", "HEAD"]).current_dir(&absolute_path));
+            output(Command::new("git").args(["rev-parse", "HEAD"]).current_dir(&absolute_path));
         // update_submodules
         let recorded = output(
             Command::new("git")
-                .args(&["ls-tree", "HEAD"])
+                .args(["ls-tree", "HEAD"])
                 .arg(relative_path)
                 .current_dir(&self.config.src),
         );
@@ -529,7 +529,7 @@ impl Build {
         println!("Updating submodule {}", relative_path.display());
         self.run(
             Command::new("git")
-                .args(&["submodule", "-q", "sync"])
+                .args(["submodule", "-q", "sync"])
                 .arg(relative_path)
                 .current_dir(&self.config.src),
         );
@@ -560,7 +560,7 @@ impl Build {
                 let branch = branch.strip_prefix("heads/").unwrap_or(&branch);
                 git.arg("-c").arg(format!("branch.{branch}.remote=origin"));
             }
-            git.args(&["submodule", "update", "--init", "--recursive", "--depth=1"]);
+            git.args(["submodule", "update", "--init", "--recursive", "--depth=1"]);
             if progress {
                 git.arg("--progress");
             }
@@ -577,7 +577,7 @@ impl Build {
         let has_local_modifications = !self.run_cmd(
             BootstrapCommand::from(
                 Command::new("git")
-                    .args(&["diff-index", "--quiet", "HEAD"])
+                    .args(["diff-index", "--quiet", "HEAD"])
                     .current_dir(&absolute_path),
             )
             .allow_failure()
@@ -587,14 +587,14 @@ impl Build {
             }),
         );
         if has_local_modifications {
-            self.run(Command::new("git").args(&["stash", "push"]).current_dir(&absolute_path));
+            self.run(Command::new("git").args(["stash", "push"]).current_dir(&absolute_path));
         }
 
-        self.run(Command::new("git").args(&["reset", "-q", "--hard"]).current_dir(&absolute_path));
-        self.run(Command::new("git").args(&["clean", "-qdfx"]).current_dir(&absolute_path));
+        self.run(Command::new("git").args(["reset", "-q", "--hard"]).current_dir(&absolute_path));
+        self.run(Command::new("git").args(["clean", "-qdfx"]).current_dir(&absolute_path));
 
         if has_local_modifications {
-            self.run(Command::new("git").args(&["stash", "pop"]).current_dir(absolute_path));
+            self.run(Command::new("git").args(["stash", "pop"]).current_dir(absolute_path));
         }
     }
 
@@ -602,20 +602,20 @@ impl Build {
     /// This avoids contributors checking in a submodule change by accident.
     pub fn update_existing_submodules(&self) {
         // Avoid running git when there isn't a git checkout.
-        if !self.config.submodules(&self.rust_info()) {
+        if !self.config.submodules(self.rust_info()) {
             return;
         }
         let output = output(
             self.config
                 .git()
-                .args(&["config", "--file"])
+                .args(["config", "--file"])
                 .arg(&self.config.src.join(".gitmodules"))
-                .args(&["--get-regexp", "path"]),
+                .args(["--get-regexp", "path"]),
         );
         for line in output.lines() {
             // Look for `submodule.$name.path = $path`
             // Sample output: `submodule.src/rust-installer.path src/tools/rust-installer`
-            let submodule = Path::new(line.splitn(2, ' ').nth(1).unwrap());
+            let submodule = Path::new(line.split_once(' ').unwrap().1);
             // Don't update the submodule unless it's already been cloned.
             if GitInfo::new(false, submodule).is_managed_git_subrepository() {
                 self.update_submodule(submodule);
@@ -630,26 +630,26 @@ impl Build {
         }
 
         // Download rustfmt early so that it can be used in rust-analyzer configs.
-        let _ = &builder::Builder::new(&self).initial_rustfmt();
+        let _ = &builder::Builder::new(self).initial_rustfmt();
 
         // hardcoded subcommands
         match &self.config.cmd {
             Subcommand::Format { check } => {
                 return core::build_steps::format::format(
-                    &builder::Builder::new(&self),
+                    &builder::Builder::new(self),
                     *check,
                     &self.config.paths,
                 );
             }
             Subcommand::Suggest { run } => {
-                return core::build_steps::suggest::suggest(&builder::Builder::new(&self), *run);
+                return core::build_steps::suggest::suggest(&builder::Builder::new(self), *run);
             }
             _ => (),
         }
 
         {
-            let builder = builder::Builder::new(&self);
-            if let Some(path) = builder.paths.get(0) {
+            let builder = builder::Builder::new(self);
+            if let Some(path) = builder.paths.first() {
                 if path == Path::new("nonexistent/path/to/trigger/cargo/metadata") {
                     return;
                 }
@@ -659,14 +659,14 @@ impl Build {
         if !self.config.dry_run() {
             {
                 self.config.dry_run = DryRun::SelfCheck;
-                let builder = builder::Builder::new(&self);
+                let builder = builder::Builder::new(self);
                 builder.execute_cli();
             }
             self.config.dry_run = DryRun::Disabled;
-            let builder = builder::Builder::new(&self);
+            let builder = builder::Builder::new(self);
             builder.execute_cli();
         } else {
-            let builder = builder::Builder::new(&self);
+            let builder = builder::Builder::new(self);
             builder.execute_cli();
         }
 
@@ -936,7 +936,7 @@ impl Build {
         static SYSROOT_CACHE: OnceLock<PathBuf> = OnceLock::new();
         SYSROOT_CACHE.get_or_init(|| {
             let mut rustc = Command::new(&self.initial_rustc);
-            rustc.args(&["--print", "sysroot"]);
+            rustc.args(["--print", "sysroot"]);
             output(&mut rustc).trim().into()
         })
     }
@@ -1162,7 +1162,7 @@ impl Build {
     fn group(&self, msg: &str) -> Option<gha::Group> {
         match self.config.dry_run {
             DryRun::SelfCheck => None,
-            DryRun::Disabled | DryRun::UserSelected => Some(gha::group(&msg)),
+            DryRun::Disabled | DryRun::UserSelected => Some(gha::group(msg)),
         }
     }
 
@@ -1322,7 +1322,7 @@ impl Build {
             .target_config
             .get(&target)
             .and_then(|t| t.musl_root.as_ref())
-            .or_else(|| self.config.musl_root.as_ref())
+            .or(self.config.musl_root.as_ref())
             .map(|p| &**p)
     }
 
@@ -1511,11 +1511,11 @@ impl Build {
 
     /// Returns the `a.b.c` version that the given package is at.
     fn release_num(&self, package: &str) -> String {
-        let toml_file_name = self.src.join(&format!("src/tools/{package}/Cargo.toml"));
-        let toml = t!(fs::read_to_string(&toml_file_name));
+        let toml_file_name = self.src.join(format!("src/tools/{package}/Cargo.toml"));
+        let toml = t!(fs::read_to_string(toml_file_name));
         for line in toml.lines() {
             if let Some(stripped) =
-                line.strip_prefix("version = \"").and_then(|s| s.strip_suffix("\""))
+                line.strip_prefix("version = \"").and_then(|s| s.strip_suffix('"'))
             {
                 return stripped.to_owned();
             }
@@ -1618,7 +1618,7 @@ impl Build {
         if src == dst {
             return;
         }
-        let _ = fs::remove_file(&dst);
+        let _ = fs::remove_file(dst);
         let metadata = t!(src.symlink_metadata());
         let mut src = src.to_path_buf();
         if metadata.file_type().is_symlink() {
@@ -1908,7 +1908,7 @@ pub fn prepare_behaviour_dump_dir(build: &Build) {
 
     let dump_path = build.out.join("bootstrap-shims-dump");
 
-    let initialized = INITIALIZED.get().unwrap_or_else(|| &false);
+    let initialized = INITIALIZED.get().unwrap_or(&false);
     if !initialized {
         // clear old dumps
         if dump_path.exists() {
diff --git a/src/bootstrap/src/utils/bin_helpers.rs b/src/bootstrap/src/utils/bin_helpers.rs
index 9c4e039ea69..5fbbe0bde0e 100644
--- a/src/bootstrap/src/utils/bin_helpers.rs
+++ b/src/bootstrap/src/utils/bin_helpers.rs
@@ -39,8 +39,7 @@ pub(crate) fn maybe_dump(dump_name: String, cmd: &Command) {
     if let Ok(dump_dir) = env::var("DUMP_BOOTSTRAP_SHIMS") {
         let dump_file = format!("{dump_dir}/{dump_name}");
 
-        let mut file =
-            OpenOptions::new().create(true).write(true).append(true).open(&dump_file).unwrap();
+        let mut file = OpenOptions::new().create(true).append(true).open(dump_file).unwrap();
 
         let cmd_dump = format!("{:?}\n", cmd);
         let cmd_dump = cmd_dump.replace(&env::var("BUILD_OUT").unwrap(), "${BUILD_OUT}");
diff --git a/src/bootstrap/src/utils/cache.rs b/src/bootstrap/src/utils/cache.rs
index 1b2aa9c234b..2b86585a9d3 100644
--- a/src/bootstrap/src/utils/cache.rs
+++ b/src/bootstrap/src/utils/cache.rs
@@ -64,7 +64,7 @@ unsafe impl<T> Sync for Interned<T> {}
 
 impl fmt::Display for Interned<String> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let s: &str = &*self;
+        let s: &str = self;
         f.write_str(s)
     }
 }
@@ -74,7 +74,7 @@ where
     Self: Deref<Target = U>,
 {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let s: &U = &*self;
+        let s: &U = self;
         f.write_fmt(format_args!("{s:?}"))
     }
 }
@@ -132,7 +132,7 @@ impl<T: Hash + Clone + Eq> TyIntern<T> {
         B: Eq + Hash + ToOwned<Owned = T> + ?Sized,
         T: Borrow<B>,
     {
-        if let Some(i) = self.set.get(&item) {
+        if let Some(i) = self.set.get(item) {
             return *i;
         }
         let item = item.to_owned();
@@ -233,7 +233,7 @@ impl Cache {
         let type_id = TypeId::of::<S>();
         let stepcache = cache
             .entry(type_id)
-            .or_insert_with(|| Box::new(HashMap::<S, S::Output>::new()))
+            .or_insert_with(|| Box::<HashMap<S, S::Output>>::default())
             .downcast_mut::<HashMap<S, S::Output>>()
             .expect("invalid type mapped");
         assert!(!stepcache.contains_key(&step), "processing {step:?} a second time");
@@ -245,7 +245,7 @@ impl Cache {
         let type_id = TypeId::of::<S>();
         let stepcache = cache
             .entry(type_id)
-            .or_insert_with(|| Box::new(HashMap::<S, S::Output>::new()))
+            .or_insert_with(|| Box::<HashMap<S, S::Output>>::default())
             .downcast_mut::<HashMap<S, S::Output>>()
             .expect("invalid type mapped");
         stepcache.get(step).cloned()
diff --git a/src/bootstrap/src/utils/cc_detect.rs b/src/bootstrap/src/utils/cc_detect.rs
index fb5b9d8c88f..ff2992bc896 100644
--- a/src/bootstrap/src/utils/cc_detect.rs
+++ b/src/bootstrap/src/utils/cc_detect.rs
@@ -35,7 +35,7 @@ use crate::{Build, CLang, GitRepo};
 // try to infer the archiver path from the C compiler path.
 // In the future this logic should be replaced by calling into the `cc` crate.
 fn cc2ar(cc: &Path, target: TargetSelection) -> Option<PathBuf> {
-    if let Some(ar) = env::var_os(format!("AR_{}", target.triple.replace("-", "_"))) {
+    if let Some(ar) = env::var_os(format!("AR_{}", target.triple.replace('-', "_"))) {
         Some(PathBuf::from(ar))
     } else if let Some(ar) = env::var_os("AR") {
         Some(PathBuf::from(ar))
@@ -172,11 +172,9 @@ fn default_compiler(
         // When compiling for android we may have the NDK configured in the
         // config.toml in which case we look there. Otherwise the default
         // compiler already takes into account the triple in question.
-        t if t.contains("android") => build
-            .config
-            .android_ndk
-            .as_ref()
-            .map(|ndk| ndk_compiler(compiler, &*target.triple, ndk)),
+        t if t.contains("android") => {
+            build.config.android_ndk.as_ref().map(|ndk| ndk_compiler(compiler, &target.triple, ndk))
+        }
 
         // The default gcc version from OpenBSD may be too old, try using egcc,
         // which is a gcc version from ports, if this is the case.
@@ -230,7 +228,7 @@ fn default_compiler(
 }
 
 pub(crate) fn ndk_compiler(compiler: Language, triple: &str, ndk: &Path) -> PathBuf {
-    let mut triple_iter = triple.split("-");
+    let mut triple_iter = triple.split('-');
     let triple_translated = if let Some(arch) = triple_iter.next() {
         let arch_new = match arch {
             "arm" | "armv7" | "armv7neon" | "thumbv7" | "thumbv7neon" => "armv7a",
diff --git a/src/bootstrap/src/utils/change_tracker.rs b/src/bootstrap/src/utils/change_tracker.rs
index fe5b5fe3175..b813d82ca6f 100644
--- a/src/bootstrap/src/utils/change_tracker.rs
+++ b/src/bootstrap/src/utils/change_tracker.rs
@@ -2,6 +2,8 @@
 //! with the goal of keeping developers synchronized with important modifications in
 //! the bootstrap.
 
+use std::fmt::Display;
+
 #[cfg(test)]
 mod tests;
 
@@ -24,11 +26,11 @@ pub enum ChangeSeverity {
     Warning,
 }
 
-impl ToString for ChangeSeverity {
-    fn to_string(&self) -> String {
+impl Display for ChangeSeverity {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
-            ChangeSeverity::Info => "INFO".to_string(),
-            ChangeSeverity::Warning => "WARNING".to_string(),
+            ChangeSeverity::Info => write!(f, "INFO"),
+            ChangeSeverity::Warning => write!(f, "WARNING"),
         }
     }
 }
@@ -40,7 +42,7 @@ pub fn find_recent_config_change_ids(current_id: usize) -> Vec<ChangeInfo> {
         // older one); otherwise, return the full list (assuming the user provided
         // the incorrect change-id by accident).
         if let Some(config) = CONFIG_CHANGE_HISTORY.iter().max_by_key(|config| config.change_id) {
-            if &current_id > &config.change_id {
+            if current_id > config.change_id {
                 return Vec::new();
             }
         }
diff --git a/src/bootstrap/src/utils/channel.rs b/src/bootstrap/src/utils/channel.rs
index e59d7f22aaa..88988c33916 100644
--- a/src/bootstrap/src/utils/channel.rs
+++ b/src/bootstrap/src/utils/channel.rs
@@ -97,7 +97,7 @@ impl GitInfo {
 
     pub fn version(&self, build: &Build, num: &str) -> String {
         let mut version = build.release(num);
-        if let Some(ref inner) = self.info() {
+        if let Some(inner) = self.info() {
             version.push_str(" (");
             version.push_str(&inner.short_sha);
             version.push(' ');
@@ -150,7 +150,7 @@ pub fn read_commit_info_file(root: &Path) -> Option<Info> {
 /// root.
 pub fn write_commit_info_file(root: &Path, info: &Info) {
     let commit_info = format!("{}\n{}\n{}\n", info.sha, info.short_sha, info.commit_date);
-    t!(fs::write(root.join("git-commit-info"), &commit_info));
+    t!(fs::write(root.join("git-commit-info"), commit_info));
 }
 
 /// Write the commit hash to the `git-commit-hash` file given the project root.
diff --git a/src/bootstrap/src/utils/helpers.rs b/src/bootstrap/src/utils/helpers.rs
index d1f713af917..a40ee189001 100644
--- a/src/bootstrap/src/utils/helpers.rs
+++ b/src/bootstrap/src/utils/helpers.rs
@@ -425,7 +425,7 @@ pub fn get_clang_cl_resource_dir(clang_cl_path: &str) -> PathBuf {
     // Similar to how LLVM does it, to find clang's library runtime directory:
     // - we ask `clang-cl` to locate the `clang_rt.builtins` lib.
     let mut builtins_locator = Command::new(clang_cl_path);
-    builtins_locator.args(&["/clang:-print-libgcc-file-name", "/clang:--rtlib=compiler-rt"]);
+    builtins_locator.args(["/clang:-print-libgcc-file-name", "/clang:--rtlib=compiler-rt"]);
 
     let clang_rt_builtins = output(&mut builtins_locator);
     let clang_rt_builtins = Path::new(clang_rt_builtins.trim());
@@ -475,7 +475,7 @@ pub fn dir_is_empty(dir: &Path) -> bool {
 /// the "y" part from the string.
 pub fn extract_beta_rev(version: &str) -> Option<String> {
     let parts = version.splitn(2, "-beta.").collect::<Vec<_>>();
-    let count = parts.get(1).and_then(|s| s.find(' ').map(|p| (&s[..p]).to_string()));
+    let count = parts.get(1).and_then(|s| s.find(' ').map(|p| s[..p].to_string()));
 
     count
 }
@@ -559,11 +559,10 @@ pub fn check_cfg_arg(name: &str, values: Option<&[&str]>) -> String {
     // ',values("tvos","watchos")' or '' (nothing) when there are no values.
     let next = match values {
         Some(values) => {
-            let mut tmp =
-                values.iter().map(|val| [",", "\"", val, "\""]).flatten().collect::<String>();
+            let mut tmp = values.iter().flat_map(|val| [",", "\"", val, "\""]).collect::<String>();
 
             tmp.insert_str(1, "values(");
-            tmp.push_str(")");
+            tmp.push(')');
             tmp
         }
         None => "".to_string(),
diff --git a/src/bootstrap/src/utils/render_tests.rs b/src/bootstrap/src/utils/render_tests.rs
index bff47f65c51..bfbb53f8c81 100644
--- a/src/bootstrap/src/utils/render_tests.rs
+++ b/src/bootstrap/src/utils/render_tests.rs
@@ -15,10 +15,10 @@ use termcolor::{Color, ColorSpec, WriteColor};
 const TERSE_TESTS_PER_LINE: usize = 88;
 
 pub(crate) fn add_flags_and_try_run_tests(builder: &Builder<'_>, cmd: &mut Command) -> bool {
-    if cmd.get_args().position(|arg| arg == "--").is_none() {
+    if !cmd.get_args().any(|arg| arg == "--") {
         cmd.arg("--");
     }
-    cmd.args(&["-Z", "unstable-options", "--format", "json"]);
+    cmd.args(["-Z", "unstable-options", "--format", "json"]);
 
     try_run_tests(builder, cmd, false)
 }
@@ -303,19 +303,19 @@ impl Outcome<'_> {
     fn write_short(&self, writer: &mut dyn WriteColor) -> Result<(), std::io::Error> {
         match self {
             Outcome::Ok => {
-                writer.set_color(&ColorSpec::new().set_fg(Some(Color::Green)))?;
+                writer.set_color(ColorSpec::new().set_fg(Some(Color::Green)))?;
                 write!(writer, ".")?;
             }
             Outcome::BenchOk => {
-                writer.set_color(&ColorSpec::new().set_fg(Some(Color::Cyan)))?;
+                writer.set_color(ColorSpec::new().set_fg(Some(Color::Cyan)))?;
                 write!(writer, "b")?;
             }
             Outcome::Failed => {
-                writer.set_color(&ColorSpec::new().set_fg(Some(Color::Red)))?;
+                writer.set_color(ColorSpec::new().set_fg(Some(Color::Red)))?;
                 write!(writer, "F")?;
             }
             Outcome::Ignored { .. } => {
-                writer.set_color(&ColorSpec::new().set_fg(Some(Color::Yellow)))?;
+                writer.set_color(ColorSpec::new().set_fg(Some(Color::Yellow)))?;
                 write!(writer, "i")?;
             }
         }
@@ -325,19 +325,19 @@ impl Outcome<'_> {
     fn write_long(&self, writer: &mut dyn WriteColor) -> Result<(), std::io::Error> {
         match self {
             Outcome::Ok => {
-                writer.set_color(&ColorSpec::new().set_fg(Some(Color::Green)))?;
+                writer.set_color(ColorSpec::new().set_fg(Some(Color::Green)))?;
                 write!(writer, "ok")?;
             }
             Outcome::BenchOk => {
-                writer.set_color(&ColorSpec::new().set_fg(Some(Color::Cyan)))?;
+                writer.set_color(ColorSpec::new().set_fg(Some(Color::Cyan)))?;
                 write!(writer, "benchmarked")?;
             }
             Outcome::Failed => {
-                writer.set_color(&ColorSpec::new().set_fg(Some(Color::Red)))?;
+                writer.set_color(ColorSpec::new().set_fg(Some(Color::Red)))?;
                 write!(writer, "FAILED")?;
             }
             Outcome::Ignored { reason } => {
-                writer.set_color(&ColorSpec::new().set_fg(Some(Color::Yellow)))?;
+                writer.set_color(ColorSpec::new().set_fg(Some(Color::Yellow)))?;
                 write!(writer, "ignored")?;
                 if let Some(reason) = reason {
                     write!(writer, ", {reason}")?;
diff --git a/src/bootstrap/src/utils/tarball.rs b/src/bootstrap/src/utils/tarball.rs
index a8393f88f8a..573d923ed8f 100644
--- a/src/bootstrap/src/utils/tarball.rs
+++ b/src/bootstrap/src/utils/tarball.rs
@@ -226,8 +226,7 @@ impl<'a> Tarball<'a> {
         if self.include_target_in_component_name {
             component_name.push('-');
             component_name.push_str(
-                &self
-                    .target
+                self.target
                     .as_ref()
                     .expect("include_target_in_component_name used in a targetless tarball"),
             );
@@ -326,7 +325,7 @@ impl<'a> Tarball<'a> {
             assert!(!formats.is_empty(), "dist.compression-formats can't be empty");
             cmd.arg("--compression-formats").arg(formats.join(","));
         }
-        cmd.args(&["--compression-profile", &self.builder.config.dist_compression_profile]);
+        cmd.args(["--compression-profile", &self.builder.config.dist_compression_profile]);
         self.builder.run(&mut cmd);
 
         // Ensure there are no symbolic links in the tarball. In particular,
@@ -347,7 +346,7 @@ impl<'a> Tarball<'a> {
             .config
             .dist_compression_formats
             .as_ref()
-            .and_then(|formats| formats.get(0))
+            .and_then(|formats| formats.first())
             .map(|s| s.as_str())
             .unwrap_or("gz");
 
diff --git a/src/tools/compiletest/src/runtest.rs b/src/tools/compiletest/src/runtest.rs
index ed120ee877a..f3e2a940f36 100644
--- a/src/tools/compiletest/src/runtest.rs
+++ b/src/tools/compiletest/src/runtest.rs
@@ -1936,7 +1936,7 @@ impl<'test> TestCx<'test> {
     fn document(&self, out_dir: &Path) -> ProcRes {
         if self.props.build_aux_docs {
             for rel_ab in &self.props.aux_builds {
-                let aux_testpaths = self.compute_aux_test_paths(rel_ab);
+                let aux_testpaths = self.compute_aux_test_paths(&self.testpaths, rel_ab);
                 let aux_props =
                     self.props.from_aux_file(&aux_testpaths.file, self.revision, self.config);
                 let aux_cx = TestCx {
@@ -2092,24 +2092,18 @@ impl<'test> TestCx<'test> {
         proc_res
     }
 
-    /// For each `aux-build: foo/bar` annotation, we check to find the
-    /// file in an `auxiliary` directory relative to the test itself.
-    fn compute_aux_test_paths(&self, rel_ab: &str) -> TestPaths {
-        let test_ab = self
-            .testpaths
-            .file
-            .parent()
-            .expect("test file path has no parent")
-            .join("auxiliary")
-            .join(rel_ab);
+    /// For each `aux-build: foo/bar` annotation, we check to find the file in an `auxiliary`
+    /// directory relative to the test itself (not any intermediate auxiliaries).
+    fn compute_aux_test_paths(&self, of: &TestPaths, rel_ab: &str) -> TestPaths {
+        let test_ab =
+            of.file.parent().expect("test file path has no parent").join("auxiliary").join(rel_ab);
         if !test_ab.exists() {
             self.fatal(&format!("aux-build `{}` source not found", test_ab.display()))
         }
 
         TestPaths {
             file: test_ab,
-            relative_dir: self
-                .testpaths
+            relative_dir: of
                 .relative_dir
                 .join(self.output_testname_unique())
                 .join("auxiliary")
@@ -2135,7 +2129,7 @@ impl<'test> TestCx<'test> {
         self.config.target.contains("vxworks") && !self.is_vxworks_pure_static()
     }
 
-    fn build_all_auxiliary(&self, rustc: &mut Command) -> PathBuf {
+    fn aux_output_dir(&self) -> PathBuf {
         let aux_dir = self.aux_output_dir_name();
 
         if !self.props.aux_builds.is_empty() {
@@ -2143,22 +2137,26 @@ impl<'test> TestCx<'test> {
             create_dir_all(&aux_dir).unwrap();
         }
 
+        aux_dir
+    }
+
+    fn build_all_auxiliary(&self, of: &TestPaths, aux_dir: &Path, rustc: &mut Command) {
         for rel_ab in &self.props.aux_builds {
-            self.build_auxiliary(rel_ab, &aux_dir);
+            self.build_auxiliary(of, rel_ab, &aux_dir);
         }
 
         for (aux_name, aux_path) in &self.props.aux_crates {
-            let is_dylib = self.build_auxiliary(&aux_path, &aux_dir);
+            let is_dylib = self.build_auxiliary(of, &aux_path, &aux_dir);
             let lib_name =
                 get_lib_name(&aux_path.trim_end_matches(".rs").replace('-', "_"), is_dylib);
             rustc.arg("--extern").arg(format!("{}={}/{}", aux_name, aux_dir.display(), lib_name));
         }
-
-        aux_dir
     }
 
     fn compose_and_run_compiler(&self, mut rustc: Command, input: Option<String>) -> ProcRes {
-        let aux_dir = self.build_all_auxiliary(&mut rustc);
+        let aux_dir = self.aux_output_dir();
+        self.build_all_auxiliary(&self.testpaths, &aux_dir, &mut rustc);
+
         self.props.unset_rustc_env.iter().fold(&mut rustc, Command::env_remove);
         rustc.envs(self.props.rustc_env.clone());
         self.compose_and_run(
@@ -2172,10 +2170,10 @@ impl<'test> TestCx<'test> {
     /// Builds an aux dependency.
     ///
     /// Returns whether or not it is a dylib.
-    fn build_auxiliary(&self, source_path: &str, aux_dir: &Path) -> bool {
-        let aux_testpaths = self.compute_aux_test_paths(source_path);
+    fn build_auxiliary(&self, of: &TestPaths, source_path: &str, aux_dir: &Path) -> bool {
+        let aux_testpaths = self.compute_aux_test_paths(of, source_path);
         let aux_props = self.props.from_aux_file(&aux_testpaths.file, self.revision, self.config);
-        let aux_output = TargetLocation::ThisDirectory(self.aux_output_dir_name());
+        let aux_output = TargetLocation::ThisDirectory(aux_dir.to_path_buf());
         let aux_cx = TestCx {
             config: self.config,
             props: &aux_props,
@@ -2193,6 +2191,7 @@ impl<'test> TestCx<'test> {
             LinkToAux::No,
             Vec::new(),
         );
+        aux_cx.build_all_auxiliary(of, aux_dir, &mut aux_rustc);
 
         for key in &aux_props.unset_rustc_env {
             aux_rustc.env_remove(key);
@@ -3034,7 +3033,8 @@ impl<'test> TestCx<'test> {
             LinkToAux::Yes,
             Vec::new(),
         );
-        new_rustdoc.build_all_auxiliary(&mut rustc);
+        let aux_dir = new_rustdoc.aux_output_dir();
+        new_rustdoc.build_all_auxiliary(&new_rustdoc.testpaths, &aux_dir, &mut rustc);
 
         let proc_res = new_rustdoc.document(&compare_dir);
         if !proc_res.status.success() {
diff --git a/tests/ui-fulldeps/session-diagnostic/diagnostic-derive-doc-comment-field.stderr b/tests/ui-fulldeps/session-diagnostic/diagnostic-derive-doc-comment-field.stderr
index 0d61e15b0f1..7844e60c654 100644
--- a/tests/ui-fulldeps/session-diagnostic/diagnostic-derive-doc-comment-field.stderr
+++ b/tests/ui-fulldeps/session-diagnostic/diagnostic-derive-doc-comment-field.stderr
@@ -8,7 +8,7 @@ LL |     arg: NotIntoDiagnosticArg,
    |          ^^^^^^^^^^^^^^^^^^^^ the trait `IntoDiagnosticArg` is not implemented for `NotIntoDiagnosticArg`
    |
    = help: normalized in stderr
-note: required by a bound in `rustc_errors::diagnostic::<impl DiagnosticBuilder<'a, G>>::arg`
+note: required by a bound in `DiagnosticBuilder::<'a, G>::arg`
   --> $COMPILER_DIR/rustc_errors/src/diagnostic.rs:LL:CC
    = note: this error originates in the macro `with_fn` (in Nightly builds, run with -Z macro-backtrace for more info)
 
@@ -22,7 +22,7 @@ LL |     arg: NotIntoDiagnosticArg,
    |          ^^^^^^^^^^^^^^^^^^^^ the trait `IntoDiagnosticArg` is not implemented for `NotIntoDiagnosticArg`
    |
    = help: normalized in stderr
-note: required by a bound in `rustc_errors::diagnostic::<impl DiagnosticBuilder<'a, G>>::arg`
+note: required by a bound in `DiagnosticBuilder::<'a, G>::arg`
   --> $COMPILER_DIR/rustc_errors/src/diagnostic.rs:LL:CC
    = note: this error originates in the macro `with_fn` (in Nightly builds, run with -Z macro-backtrace for more info)
 
diff --git a/tests/ui-fulldeps/session-diagnostic/diagnostic-derive.stderr b/tests/ui-fulldeps/session-diagnostic/diagnostic-derive.stderr
index ddbb3c6df26..8732629db47 100644
--- a/tests/ui-fulldeps/session-diagnostic/diagnostic-derive.stderr
+++ b/tests/ui-fulldeps/session-diagnostic/diagnostic-derive.stderr
@@ -628,7 +628,7 @@ LL |     other: Hello,
    |            ^^^^^ the trait `IntoDiagnosticArg` is not implemented for `Hello`
    |
    = help: normalized in stderr
-note: required by a bound in `rustc_errors::diagnostic::<impl DiagnosticBuilder<'a, G>>::arg`
+note: required by a bound in `DiagnosticBuilder::<'a, G>::arg`
   --> $COMPILER_DIR/rustc_errors/src/diagnostic.rs:LL:CC
    = note: this error originates in the macro `with_fn` (in Nightly builds, run with -Z macro-backtrace for more info)
 
diff --git a/tests/ui/async-await/async-fn/impl-header.rs b/tests/ui/async-await/async-fn/impl-header.rs
index b9ae90292bb..9af5f1f42a9 100644
--- a/tests/ui/async-await/async-fn/impl-header.rs
+++ b/tests/ui/async-await/async-fn/impl-header.rs
@@ -3,6 +3,10 @@
 struct F;
 
 impl async Fn<()> for F {}
-//~^ ERROR expected type, found keyword `async`
+//~^ ERROR `async` trait implementations are unsupported
+//~| ERROR the precise format of `Fn`-family traits' type parameters is subject to change
+//~| ERROR manual implementations of `Fn` are experimental
+//~| ERROR expected a `FnMut()` closure, found `F`
+//~| ERROR not all trait items implemented, missing: `call`
 
 fn main() {}
diff --git a/tests/ui/async-await/async-fn/impl-header.stderr b/tests/ui/async-await/async-fn/impl-header.stderr
index 02cb4326242..2fb862af04e 100644
--- a/tests/ui/async-await/async-fn/impl-header.stderr
+++ b/tests/ui/async-await/async-fn/impl-header.stderr
@@ -1,8 +1,47 @@
-error: expected type, found keyword `async`
+error: `async` trait implementations are unsupported
   --> $DIR/impl-header.rs:5:6
    |
 LL | impl async Fn<()> for F {}
-   |      ^^^^^ expected type
+   |      ^^^^^
 
-error: aborting due to 1 previous error
+error[E0658]: the precise format of `Fn`-family traits' type parameters is subject to change
+  --> $DIR/impl-header.rs:5:12
+   |
+LL | impl async Fn<()> for F {}
+   |            ^^^^^^
+   |
+   = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
+   = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0183]: manual implementations of `Fn` are experimental
+  --> $DIR/impl-header.rs:5:12
+   |
+LL | impl async Fn<()> for F {}
+   |            ^^^^^^ manual implementations of `Fn` are experimental
+   |
+   = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
+
+error[E0277]: expected a `FnMut()` closure, found `F`
+  --> $DIR/impl-header.rs:5:23
+   |
+LL | impl async Fn<()> for F {}
+   |                       ^ expected an `FnMut()` closure, found `F`
+   |
+   = help: the trait `FnMut<()>` is not implemented for `F`
+   = note: wrap the `F` in a closure with no arguments: `|| { /* code */ }`
+note: required by a bound in `Fn`
+  --> $SRC_DIR/core/src/ops/function.rs:LL:COL
+
+error[E0046]: not all trait items implemented, missing: `call`
+  --> $DIR/impl-header.rs:5:1
+   |
+LL | impl async Fn<()> for F {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^ missing `call` in implementation
+   |
+   = help: implement the missing item: `fn call(&self, _: ()) -> <Self as FnOnce<()>>::Output { todo!() }`
+
+error: aborting due to 5 previous errors
 
+Some errors have detailed explanations: E0046, E0183, E0277, E0658.
+For more information about an error, try `rustc --explain E0046`.
diff --git a/tests/ui/async-await/async-fn/mbe-async-trait-bound-theoretical-regression.rs b/tests/ui/async-await/async-fn/mbe-async-trait-bound-theoretical-regression.rs
new file mode 100644
index 00000000000..abc429772fd
--- /dev/null
+++ b/tests/ui/async-await/async-fn/mbe-async-trait-bound-theoretical-regression.rs
@@ -0,0 +1,21 @@
+// Demonstrates and records a theoretical regressions / breaking changes caused by the
+// introduction of async trait bounds.
+
+// Setting the edition to 2018 since we don't regress `demo! { dyn async }` in Rust <2018.
+//@ edition:2018
+
+macro_rules! demo {
+    ($ty:ty) => { compile_error!("ty"); };
+    //~^ ERROR ty
+    //~| ERROR ty
+    (impl $c:ident Trait) => {};
+    (dyn $c:ident Trait) => {};
+}
+
+demo! { impl async Trait }
+//~^ ERROR async closures are unstable
+
+demo! { dyn async Trait }
+//~^ ERROR async closures are unstable
+
+fn main() {}
diff --git a/tests/ui/async-await/async-fn/mbe-async-trait-bound-theoretical-regression.stderr b/tests/ui/async-await/async-fn/mbe-async-trait-bound-theoretical-regression.stderr
new file mode 100644
index 00000000000..13b8e72b49d
--- /dev/null
+++ b/tests/ui/async-await/async-fn/mbe-async-trait-bound-theoretical-regression.stderr
@@ -0,0 +1,47 @@
+error: ty
+  --> $DIR/mbe-async-trait-bound-theoretical-regression.rs:8:19
+   |
+LL |     ($ty:ty) => { compile_error!("ty"); };
+   |                   ^^^^^^^^^^^^^^^^^^^^
+...
+LL | demo! { impl async Trait }
+   | -------------------------- in this macro invocation
+   |
+   = note: this error originates in the macro `demo` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: ty
+  --> $DIR/mbe-async-trait-bound-theoretical-regression.rs:8:19
+   |
+LL |     ($ty:ty) => { compile_error!("ty"); };
+   |                   ^^^^^^^^^^^^^^^^^^^^
+...
+LL | demo! { dyn async Trait }
+   | ------------------------- in this macro invocation
+   |
+   = note: this error originates in the macro `demo` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0658]: async closures are unstable
+  --> $DIR/mbe-async-trait-bound-theoretical-regression.rs:15:14
+   |
+LL | demo! { impl async Trait }
+   |              ^^^^^
+   |
+   = note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information
+   = help: add `#![feature(async_closure)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = help: to use an async block, remove the `||`: `async {`
+
+error[E0658]: async closures are unstable
+  --> $DIR/mbe-async-trait-bound-theoretical-regression.rs:18:13
+   |
+LL | demo! { dyn async Trait }
+   |             ^^^^^
+   |
+   = note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information
+   = help: add `#![feature(async_closure)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = help: to use an async block, remove the `||`: `async {`
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0658`.
diff --git a/tests/ui/async-await/async-fn/trait-bounds-in-macro.rs b/tests/ui/async-await/async-fn/trait-bounds-in-macro.rs
new file mode 100644
index 00000000000..329a1528e8b
--- /dev/null
+++ b/tests/ui/async-await/async-fn/trait-bounds-in-macro.rs
@@ -0,0 +1,12 @@
+//@ edition: 2021
+
+macro_rules! x {
+    ($x:item) => {}
+}
+
+x! {
+    async fn foo() -> impl async Fn() { }
+    //~^ ERROR async closures are unstable
+}
+
+fn main() {}
diff --git a/tests/ui/async-await/async-fn/trait-bounds-in-macro.stderr b/tests/ui/async-await/async-fn/trait-bounds-in-macro.stderr
new file mode 100644
index 00000000000..f68c09737db
--- /dev/null
+++ b/tests/ui/async-await/async-fn/trait-bounds-in-macro.stderr
@@ -0,0 +1,14 @@
+error[E0658]: async closures are unstable
+  --> $DIR/trait-bounds-in-macro.rs:8:28
+   |
+LL |     async fn foo() -> impl async Fn() { }
+   |                            ^^^^^
+   |
+   = note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information
+   = help: add `#![feature(async_closure)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = help: to use an async block, remove the `||`: `async {`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.
diff --git a/tests/ui/coherence/occurs-check/associated-type.next.stderr b/tests/ui/coherence/occurs-check/associated-type.next.stderr
index e405f389f5e..6119e6149a7 100644
--- a/tests/ui/coherence/occurs-check/associated-type.next.stderr
+++ b/tests/ui/coherence/occurs-check/associated-type.next.stderr
@@ -1,11 +1,11 @@
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [*const ?1t, RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [*const ?1t, RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [*const ?1t, RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [*const ?1t, RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) }
 error[E0119]: conflicting implementations of trait `Overlap<for<'a> fn(&'a (), ())>` for type `for<'a> fn(&'a (), ())`
   --> $DIR/associated-type.rs:31:1
    |
diff --git a/tests/ui/coherence/occurs-check/associated-type.old.stderr b/tests/ui/coherence/occurs-check/associated-type.old.stderr
index 4a67a777f10..655809b827e 100644
--- a/tests/ui/coherence/occurs-check/associated-type.old.stderr
+++ b/tests/ui/coherence/occurs-check/associated-type.old.stderr
@@ -1,11 +1,11 @@
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!2_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!2_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!2_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!2_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [*const ?1t, RePlaceholder(!2_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [*const ?1t, RePlaceholder(!2_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [*const ?1t, RePlaceholder(!2_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [*const ?1t, RePlaceholder(!2_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) }
 error[E0119]: conflicting implementations of trait `Overlap<for<'a> fn(&'a (), _)>` for type `for<'a> fn(&'a (), _)`
   --> $DIR/associated-type.rs:31:1
    |
diff --git a/tests/ui/compiletest-self-test/aux-aux.rs b/tests/ui/compiletest-self-test/aux-aux.rs
new file mode 100644
index 00000000000..c87905ff775
--- /dev/null
+++ b/tests/ui/compiletest-self-test/aux-aux.rs
@@ -0,0 +1,14 @@
+//@ aux-crate: aux_aux_foo=aux_aux_foo.rs
+//@ aux-crate: aux_aux_bar=aux_aux_bar.rs
+//@ edition: 2021
+//@ compile-flags: --crate-type lib
+//@ check-pass
+
+use aux_aux_foo::Bar as IndirectBar;
+use aux_aux_bar::Bar as DirectBar;
+
+fn foo(x: IndirectBar) {}
+
+fn main() {
+    foo(DirectBar);
+}
diff --git a/tests/ui/compiletest-self-test/auxiliary/aux_aux_bar.rs b/tests/ui/compiletest-self-test/auxiliary/aux_aux_bar.rs
new file mode 100644
index 00000000000..eefcc270c38
--- /dev/null
+++ b/tests/ui/compiletest-self-test/auxiliary/aux_aux_bar.rs
@@ -0,0 +1,3 @@
+//@ edition: 2021
+
+pub struct Bar;
diff --git a/tests/ui/compiletest-self-test/auxiliary/aux_aux_foo.rs b/tests/ui/compiletest-self-test/auxiliary/aux_aux_foo.rs
new file mode 100644
index 00000000000..f96c6bb0b27
--- /dev/null
+++ b/tests/ui/compiletest-self-test/auxiliary/aux_aux_foo.rs
@@ -0,0 +1,4 @@
+//@ aux-crate: aux_aux_bar=aux_aux_bar.rs
+//@ edition: 2021
+
+pub use aux_aux_bar::Bar;
diff --git a/tests/ui/parser/bad-recover-kw-after-impl.rs b/tests/ui/parser/bad-recover-kw-after-impl.rs
index 23abceaf493..15c0b377c8a 100644
--- a/tests/ui/parser/bad-recover-kw-after-impl.rs
+++ b/tests/ui/parser/bad-recover-kw-after-impl.rs
@@ -1,4 +1,4 @@
-//@ check-pass
+// This is just `mbe-async-trait-bound-theoretical-regression.rs` in practice.
 
 //@ edition:2021
 // for the `impl` + keyword test
@@ -11,5 +11,7 @@ macro_rules! impl_primitive {
 }
 
 impl_primitive!(impl async);
+//~^ ERROR expected identifier, found `<eof>`
+//~| ERROR async closures are unstable
 
 fn main() {}
diff --git a/tests/ui/parser/bad-recover-kw-after-impl.stderr b/tests/ui/parser/bad-recover-kw-after-impl.stderr
new file mode 100644
index 00000000000..f617cf65498
--- /dev/null
+++ b/tests/ui/parser/bad-recover-kw-after-impl.stderr
@@ -0,0 +1,23 @@
+error: expected identifier, found `<eof>`
+  --> $DIR/bad-recover-kw-after-impl.rs:13:22
+   |
+LL |     ($ty:ty) => {
+   |      ------ while parsing argument for this `ty` macro fragment
+...
+LL | impl_primitive!(impl async);
+   |                      ^^^^^ expected identifier
+
+error[E0658]: async closures are unstable
+  --> $DIR/bad-recover-kw-after-impl.rs:13:22
+   |
+LL | impl_primitive!(impl async);
+   |                      ^^^^^
+   |
+   = note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information
+   = help: add `#![feature(async_closure)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = help: to use an async block, remove the `||`: `async {`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.
diff --git a/tests/ui/parser/trait-object-delimiters.rs b/tests/ui/parser/trait-object-delimiters.rs
index 84cd16c2796..d6bc629aa11 100644
--- a/tests/ui/parser/trait-object-delimiters.rs
+++ b/tests/ui/parser/trait-object-delimiters.rs
@@ -8,7 +8,7 @@ fn foo2(_: &dyn (Drop + AsRef<str>)) {} //~ ERROR incorrect parentheses around t
 fn foo2_no_space(_: &dyn(Drop + AsRef<str>)) {} //~ ERROR incorrect parentheses around trait bounds
 
 fn foo3(_: &dyn {Drop + AsRef<str>}) {} //~ ERROR expected parameter name, found `{`
-//~^ ERROR expected one of `!`, `(`, `)`, `*`, `,`, `?`, `const`, `for`, `~`, lifetime, or path, found `{`
+//~^ ERROR expected one of `!`, `(`, `)`, `*`, `,`, `?`, `async`, `const`, `for`, `~`, lifetime, or path, found `{`
 //~| ERROR at least one trait is required for an object type
 
 fn foo4(_: &dyn <Drop + AsRef<str>>) {} //~ ERROR expected identifier, found `<`
diff --git a/tests/ui/parser/trait-object-delimiters.stderr b/tests/ui/parser/trait-object-delimiters.stderr
index 2ddb734cee0..2b1f8df991f 100644
--- a/tests/ui/parser/trait-object-delimiters.stderr
+++ b/tests/ui/parser/trait-object-delimiters.stderr
@@ -34,11 +34,11 @@ error: expected parameter name, found `{`
 LL | fn foo3(_: &dyn {Drop + AsRef<str>}) {}
    |                 ^ expected parameter name
 
-error: expected one of `!`, `(`, `)`, `*`, `,`, `?`, `const`, `for`, `~`, lifetime, or path, found `{`
+error: expected one of `!`, `(`, `)`, `*`, `,`, `?`, `async`, `const`, `for`, `~`, lifetime, or path, found `{`
   --> $DIR/trait-object-delimiters.rs:10:17
    |
 LL | fn foo3(_: &dyn {Drop + AsRef<str>}) {}
-   |                -^ expected one of 11 possible tokens
+   |                -^ expected one of 12 possible tokens
    |                |
    |                help: missing `,`
 
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/mbe-const-trait-bound-theoretical-regression.rs b/tests/ui/rfcs/rfc-2632-const-trait-impl/mbe-const-trait-bound-theoretical-regression.rs
index 99806922ba5..3dcdb0cad94 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/mbe-const-trait-bound-theoretical-regression.rs
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/mbe-const-trait-bound-theoretical-regression.rs
@@ -6,15 +6,16 @@
 
 macro_rules! demo {
     ($ty:ty) => { compile_error!("ty"); };
-    (impl $c:ident) => {};
-    (dyn $c:ident) => {};
+    //~^ ERROR ty
+    //~| ERROR ty
+    (impl $c:ident Trait) => {};
+    (dyn $c:ident Trait) => {};
 }
 
-demo! { impl const }
-//~^ ERROR expected identifier, found `<eof>`
+demo! { impl const Trait }
+//~^ ERROR const trait impls are experimental
 
-demo! { dyn const }
+demo! { dyn const Trait }
 //~^ ERROR const trait impls are experimental
-//~| ERROR expected identifier, found `<eof>`
 
 fn main() {}
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/mbe-const-trait-bound-theoretical-regression.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/mbe-const-trait-bound-theoretical-regression.stderr
index fd9184b9dff..f4b401b7386 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/mbe-const-trait-bound-theoretical-regression.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/mbe-const-trait-bound-theoretical-regression.stderr
@@ -1,31 +1,45 @@
-error: expected identifier, found `<eof>`
-  --> $DIR/mbe-const-trait-bound-theoretical-regression.rs:13:14
+error: ty
+  --> $DIR/mbe-const-trait-bound-theoretical-regression.rs:8:19
    |
 LL |     ($ty:ty) => { compile_error!("ty"); };
-   |      ------ while parsing argument for this `ty` macro fragment
+   |                   ^^^^^^^^^^^^^^^^^^^^
 ...
-LL | demo! { impl const }
-   |              ^^^^^ expected identifier
+LL | demo! { impl const Trait }
+   | -------------------------- in this macro invocation
+   |
+   = note: this error originates in the macro `demo` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: expected identifier, found `<eof>`
-  --> $DIR/mbe-const-trait-bound-theoretical-regression.rs:16:13
+error: ty
+  --> $DIR/mbe-const-trait-bound-theoretical-regression.rs:8:19
    |
 LL |     ($ty:ty) => { compile_error!("ty"); };
-   |      ------ while parsing argument for this `ty` macro fragment
+   |                   ^^^^^^^^^^^^^^^^^^^^
 ...
-LL | demo! { dyn const }
-   |             ^^^^^ expected identifier
+LL | demo! { dyn const Trait }
+   | ------------------------- in this macro invocation
+   |
+   = note: this error originates in the macro `demo` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0658]: const trait impls are experimental
+  --> $DIR/mbe-const-trait-bound-theoretical-regression.rs:15:14
+   |
+LL | demo! { impl const Trait }
+   |              ^^^^^
+   |
+   = note: see issue #67792 <https://github.com/rust-lang/rust/issues/67792> for more information
+   = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error[E0658]: const trait impls are experimental
-  --> $DIR/mbe-const-trait-bound-theoretical-regression.rs:16:13
+  --> $DIR/mbe-const-trait-bound-theoretical-regression.rs:18:13
    |
-LL | demo! { dyn const }
+LL | demo! { dyn const Trait }
    |             ^^^^^
    |
    = note: see issue #67792 <https://github.com/rust-lang/rust/issues/67792> for more information
    = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0658`.
diff --git a/tests/ui/traits/next-solver/issue-118950-root-region.stderr b/tests/ui/traits/next-solver/issue-118950-root-region.stderr
index f4638348358..e33320ed9e6 100644
--- a/tests/ui/traits/next-solver/issue-118950-root-region.stderr
+++ b/tests/ui/traits/next-solver/issue-118950-root-region.stderr
@@ -13,14 +13,14 @@ LL | #![feature(lazy_type_alias)]
    = note: see issue #112792 <https://github.com/rust-lang/rust/issues/112792> for more information
    = note: `#[warn(incomplete_features)]` on by default
 
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Weak, AliasTy { args: [ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Weak, AliasTy { args: [RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Weak, AliasTy { args: [ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Weak, AliasTy { args: [RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Weak, AliasTy { args: [ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Weak, AliasTy { args: [RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Weak, AliasTy { args: [ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Weak, AliasTy { args: [RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) })
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) }
 error[E0119]: conflicting implementations of trait `Overlap<fn(_)>` for type `fn(_)`
   --> $DIR/issue-118950-root-region.rs:19:1
    |