Auto merge of #122012 - matthiaskrgr:rollup-bzqjj2n, r=matthiaskrgr

Rollup of 10 pull requests

Successful merges:

 - #121213 (Add an example to demonstrate how Rc::into_inner works)
 - #121262 (Add vector time complexity)
 - #121287 (Clarify/add `must_use` message for Rc/Arc/Weak::into_raw.)
 - #121664 (Adjust error `yield`/`await` lowering)
 - #121826 (Use root obligation on E0277 for some cases)
 - #121838 (Use the correct logic for nested impl trait in assoc types)
 - #121913 (Don't panic when waiting on poisoned queries)
 - #121987 (pattern analysis: abort on arity mismatch)
 - #121993 (Avoid using unnecessary queries when printing the query stack in panics)
 - #121997 (interpret/cast: make more matches on FloatTy properly exhaustive)

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

8 files changed, 176 insertions, 132 deletions

diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
index 0b1cb124080..85b599902b9 100644
--- a/compiler/rustc_ast_lowering/src/expr.rs
+++ b/compiler/rustc_ast_lowering/src/expr.rs
@@ -760,10 +760,28 @@ impl<'hir> LoweringContext<'_, 'hir> {
             Some(hir::CoroutineKind::Coroutine(_))
             | Some(hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::Gen, _))
             | None => {
-                return hir::ExprKind::Err(self.dcx().emit_err(AwaitOnlyInAsyncFnAndBlocks {
-                    await_kw_span,
-                    item_span: self.current_item,
-                }));
+                // Lower to a block `{ EXPR; <error> }` so that the awaited expr
+                // is not accidentally orphaned.
+                let stmt_id = self.next_id();
+                let expr_err = self.expr(
+                    expr.span,
+                    hir::ExprKind::Err(self.dcx().emit_err(AwaitOnlyInAsyncFnAndBlocks {
+                        await_kw_span,
+                        item_span: self.current_item,
+                    })),
+                );
+                return hir::ExprKind::Block(
+                    self.block_all(
+                        expr.span,
+                        arena_vec![self; hir::Stmt {
+                            hir_id: stmt_id,
+                            kind: hir::StmtKind::Semi(expr),
+                            span: expr.span,
+                        }],
+                        Some(self.arena.alloc(expr_err)),
+                    ),
+                    None,
+                );
             }
         };
 
@@ -1496,12 +1514,31 @@ impl<'hir> LoweringContext<'_, 'hir> {
     }
 
     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
+        let yielded =
+            opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));
+
         let is_async_gen = match self.coroutine_kind {
             Some(hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::Gen, _)) => false,
             Some(hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::AsyncGen, _)) => true,
             Some(hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::Async, _)) => {
-                return hir::ExprKind::Err(
-                    self.dcx().emit_err(AsyncCoroutinesNotSupported { span }),
+                // Lower to a block `{ EXPR; <error> }` so that the awaited expr
+                // is not accidentally orphaned.
+                let stmt_id = self.next_id();
+                let expr_err = self.expr(
+                    yielded.span,
+                    hir::ExprKind::Err(self.dcx().emit_err(AsyncCoroutinesNotSupported { span })),
+                );
+                return hir::ExprKind::Block(
+                    self.block_all(
+                        yielded.span,
+                        arena_vec![self; hir::Stmt {
+                            hir_id: stmt_id,
+                            kind: hir::StmtKind::Semi(yielded),
+                            span: yielded.span,
+                        }],
+                        Some(self.arena.alloc(expr_err)),
+                    ),
+                    None,
                 );
             }
             Some(hir::CoroutineKind::Coroutine(_)) => {
@@ -1531,9 +1568,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
             }
         };
 
-        let yielded =
-            opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));
-
         if is_async_gen {
             // `yield $expr` is transformed into `task_context = yield async_gen_ready($expr)`.
             // This ensures that we store our resumed `ResumeContext` correctly, and also that
diff --git a/compiler/rustc_const_eval/src/interpret/cast.rs b/compiler/rustc_const_eval/src/interpret/cast.rs
index a88e130cd4b..2cebea9d145 100644
--- a/compiler/rustc_const_eval/src/interpret/cast.rs
+++ b/compiler/rustc_const_eval/src/interpret/cast.rs
@@ -182,13 +182,14 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     ) -> InterpResult<'tcx, ImmTy<'tcx, M::Provenance>> {
         use rustc_type_ir::TyKind::*;
 
-        let val = match src.layout.ty.kind() {
-            // Floating point
-            Float(FloatTy::F32) => self.cast_from_float(src.to_scalar().to_f32()?, cast_to.ty),
-            Float(FloatTy::F64) => self.cast_from_float(src.to_scalar().to_f64()?, cast_to.ty),
-            _ => {
-                bug!("Can't cast 'Float' type into {}", cast_to.ty);
-            }
+        let Float(fty) = src.layout.ty.kind() else {
+            bug!("FloatToFloat/FloatToInt cast: source type {} is not a float type", src.layout.ty)
+        };
+        let val = match fty {
+            FloatTy::F16 => unimplemented!("f16_f128"),
+            FloatTy::F32 => self.cast_from_float(src.to_scalar().to_f32()?, cast_to.ty),
+            FloatTy::F64 => self.cast_from_float(src.to_scalar().to_f64()?, cast_to.ty),
+            FloatTy::F128 => unimplemented!("f16_f128"),
         };
         Ok(ImmTy::from_scalar(val, cast_to))
     }
@@ -275,6 +276,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         trace!("cast_from_scalar: {}, {} -> {}", v, src_layout.ty, cast_ty);
 
         Ok(match *cast_ty.kind() {
+            // int -> int
             Int(_) | Uint(_) => {
                 let size = match *cast_ty.kind() {
                     Int(t) => Integer::from_int_ty(self, t).size(),
@@ -285,15 +287,26 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 Scalar::from_uint(v, size)
             }
 
-            Float(FloatTy::F32) if signed => Scalar::from_f32(Single::from_i128(v as i128).value),
-            Float(FloatTy::F64) if signed => Scalar::from_f64(Double::from_i128(v as i128).value),
-            Float(FloatTy::F32) => Scalar::from_f32(Single::from_u128(v).value),
-            Float(FloatTy::F64) => Scalar::from_f64(Double::from_u128(v).value),
-
-            Char => {
-                // `u8` to `char` cast
-                Scalar::from_u32(u8::try_from(v).unwrap().into())
+            // signed int -> float
+            Float(fty) if signed => {
+                let v = v as i128;
+                match fty {
+                    FloatTy::F16 => unimplemented!("f16_f128"),
+                    FloatTy::F32 => Scalar::from_f32(Single::from_i128(v).value),
+                    FloatTy::F64 => Scalar::from_f64(Double::from_i128(v).value),
+                    FloatTy::F128 => unimplemented!("f16_f128"),
+                }
             }
+            // unsigned int -> float
+            Float(fty) => match fty {
+                FloatTy::F16 => unimplemented!("f16_f128"),
+                FloatTy::F32 => Scalar::from_f32(Single::from_u128(v).value),
+                FloatTy::F64 => Scalar::from_f64(Double::from_u128(v).value),
+                FloatTy::F128 => unimplemented!("f16_f128"),
+            },
+
+            // u8 -> char
+            Char => Scalar::from_u32(u8::try_from(v).unwrap().into()),
 
             // Casts to bool are not permitted by rustc, no need to handle them here.
             _ => span_bug!(self.cur_span(), "invalid int to {} cast", cast_ty),
@@ -339,14 +352,13 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 let v = f.to_i128(size.bits_usize()).value;
                 Scalar::from_int(v, size)
             }
-            // float -> f32
-            Float(FloatTy::F32) => {
-                Scalar::from_f32(adjust_nan(self, f, f.convert(&mut false).value))
-            }
-            // float -> f64
-            Float(FloatTy::F64) => {
-                Scalar::from_f64(adjust_nan(self, f, f.convert(&mut false).value))
-            }
+            // float -> float
+            Float(fty) => match fty {
+                FloatTy::F16 => unimplemented!("f16_f128"),
+                FloatTy::F32 => Scalar::from_f32(adjust_nan(self, f, f.convert(&mut false).value)),
+                FloatTy::F64 => Scalar::from_f64(adjust_nan(self, f, f.convert(&mut false).value)),
+                FloatTy::F128 => unimplemented!("f16_f128"),
+            },
             // That's it.
             _ => span_bug!(self.cur_span(), "invalid float to {} cast", dest_ty),
         }
diff --git a/compiler/rustc_hir_typeck/src/method/suggest.rs b/compiler/rustc_hir_typeck/src/method/suggest.rs
index 440d0ad1140..893b3f9534d 100644
--- a/compiler/rustc_hir_typeck/src/method/suggest.rs
+++ b/compiler/rustc_hir_typeck/src/method/suggest.rs
@@ -166,7 +166,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return false;
         }
 
-        match ty.kind() {
+        match ty.peel_refs().kind() {
             ty::Param(param) => {
                 let generics = self.tcx.generics_of(self.body_id);
                 let generic_param = generics.type_param(&param, self.tcx);
@@ -184,7 +184,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     }
                 }
             }
-            ty::Alias(ty::AliasKind::Opaque, _) => {
+            ty::Slice(..) | ty::Adt(..) | ty::Alias(ty::AliasKind::Opaque, _) => {
                 for unsatisfied in unsatisfied_predicates.iter() {
                     if is_iterator_predicate(unsatisfied.0, self.tcx) {
                         return true;
diff --git a/compiler/rustc_pattern_analysis/src/usefulness.rs b/compiler/rustc_pattern_analysis/src/usefulness.rs
index 0261768d916..c518844cc5e 100644
--- a/compiler/rustc_pattern_analysis/src/usefulness.rs
+++ b/compiler/rustc_pattern_analysis/src/usefulness.rs
@@ -1001,19 +1001,26 @@ impl<'p, Cx: TypeCx> PatStack<'p, Cx> {
     /// Only call if `ctor.is_covered_by(self.head().ctor())` is true.
     fn pop_head_constructor(
         &self,
+        cx: &Cx,
         ctor: &Constructor<Cx>,
         ctor_arity: usize,
         ctor_is_relevant: bool,
-    ) -> PatStack<'p, Cx> {
+    ) -> Result<PatStack<'p, Cx>, Cx::Error> {
         // We pop the head pattern and push the new fields extracted from the arguments of
         // `self.head()`.
         let mut new_pats = self.head().specialize(ctor, ctor_arity);
+        if new_pats.len() != ctor_arity {
+            return Err(cx.bug(format_args!(
+                "uncaught type error: pattern {:?} has inconsistent arity (expected arity {ctor_arity})",
+                self.head().as_pat().unwrap()
+            )));
+        }
         new_pats.extend_from_slice(&self.pats[1..]);
         // `ctor` is relevant for this row if it is the actual constructor of this row, or if the
         // row has a wildcard and `ctor` is relevant for wildcards.
         let ctor_is_relevant =
             !matches!(self.head().ctor(), Constructor::Wildcard) || ctor_is_relevant;
-        PatStack { pats: new_pats, relevant: self.relevant && ctor_is_relevant }
+        Ok(PatStack { pats: new_pats, relevant: self.relevant && ctor_is_relevant })
     }
 }
 
@@ -1083,18 +1090,19 @@ impl<'p, Cx: TypeCx> MatrixRow<'p, Cx> {
     /// Only call if `ctor.is_covered_by(self.head().ctor())` is true.
     fn pop_head_constructor(
         &self,
+        cx: &Cx,
         ctor: &Constructor<Cx>,
         ctor_arity: usize,
         ctor_is_relevant: bool,
         parent_row: usize,
-    ) -> MatrixRow<'p, Cx> {
-        MatrixRow {
-            pats: self.pats.pop_head_constructor(ctor, ctor_arity, ctor_is_relevant),
+    ) -> Result<MatrixRow<'p, Cx>, Cx::Error> {
+        Ok(MatrixRow {
+            pats: self.pats.pop_head_constructor(cx, ctor, ctor_arity, ctor_is_relevant)?,
             parent_row,
             is_under_guard: self.is_under_guard,
             useful: false,
             intersects: BitSet::new_empty(0), // Initialized in `Matrix::expand_and_push`.
-        }
+        })
     }
 }
 
@@ -1217,7 +1225,7 @@ impl<'p, Cx: TypeCx> Matrix<'p, Cx> {
         };
         for (i, row) in self.rows().enumerate() {
             if ctor.is_covered_by(pcx.cx, row.head().ctor())? {
-                let new_row = row.pop_head_constructor(ctor, arity, ctor_is_relevant, i);
+                let new_row = row.pop_head_constructor(pcx.cx, ctor, arity, ctor_is_relevant, i)?;
                 matrix.expand_and_push(new_row);
             }
         }
diff --git a/compiler/rustc_query_impl/src/plumbing.rs b/compiler/rustc_query_impl/src/plumbing.rs
index 1b546bf9103..b06d75be390 100644
--- a/compiler/rustc_query_impl/src/plumbing.rs
+++ b/compiler/rustc_query_impl/src/plumbing.rs
@@ -17,6 +17,7 @@ use rustc_middle::dep_graph::{
 use rustc_middle::query::on_disk_cache::AbsoluteBytePos;
 use rustc_middle::query::on_disk_cache::{CacheDecoder, CacheEncoder, EncodedDepNodeIndex};
 use rustc_middle::query::Key;
+use rustc_middle::ty::print::with_reduced_queries;
 use rustc_middle::ty::tls::{self, ImplicitCtxt};
 use rustc_middle::ty::{self, TyCtxt};
 use rustc_query_system::dep_graph::{DepNodeParams, HasDepContext};
@@ -304,6 +305,10 @@ pub(crate) fn create_query_frame<
     kind: DepKind,
     name: &'static str,
 ) -> QueryStackFrame {
+    // If reduced queries are requested, we may be printing a query stack due
+    // to a panic. Avoid using `default_span` and `def_kind` in that case.
+    let reduce_queries = with_reduced_queries();
+
     // Avoid calling queries while formatting the description
     let description = ty::print::with_no_queries!(do_describe(tcx, key));
     let description = if tcx.sess.verbose_internals() {
@@ -311,7 +316,7 @@ pub(crate) fn create_query_frame<
     } else {
         description
     };
-    let span = if kind == dep_graph::dep_kinds::def_span {
+    let span = if kind == dep_graph::dep_kinds::def_span || reduce_queries {
         // The `def_span` query is used to calculate `default_span`,
         // so exit to avoid infinite recursion.
         None
@@ -319,7 +324,7 @@ pub(crate) fn create_query_frame<
         Some(key.default_span(tcx))
     };
     let def_id = key.key_as_def_id();
-    let def_kind = if kind == dep_graph::dep_kinds::def_kind {
+    let def_kind = if kind == dep_graph::dep_kinds::def_kind || reduce_queries {
         // Try to avoid infinite recursion.
         None
     } else {
diff --git a/compiler/rustc_query_system/src/query/plumbing.rs b/compiler/rustc_query_system/src/query/plumbing.rs
index 40517407ee6..3d9848395a2 100644
--- a/compiler/rustc_query_system/src/query/plumbing.rs
+++ b/compiler/rustc_query_system/src/query/plumbing.rs
@@ -285,9 +285,8 @@ where
                     let lock = query.query_state(qcx).active.get_shard_by_value(&key).lock();
 
                     match lock.get(&key) {
-                        Some(QueryResult::Poisoned) => {
-                            panic!("query '{}' not cached due to poisoning", query.name())
-                        }
+                        // The query we waited on panicked. Continue unwinding here.
+                        Some(QueryResult::Poisoned) => FatalError.raise(),
                         _ => panic!(
                             "query '{}' result must be in the cache or the query must be poisoned after a wait",
                             query.name()
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
index 62933bce76b..7a930937255 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
@@ -416,9 +416,59 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                     ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_predicate)) => {
                         let trait_predicate = bound_predicate.rebind(trait_predicate);
                         let trait_predicate = self.resolve_vars_if_possible(trait_predicate);
-                        let trait_ref = trait_predicate.to_poly_trait_ref();
 
-                        if let Some(guar) = self.emit_specialized_closure_kind_error(&obligation, trait_ref) {
+                        // Let's use the root obligation as the main message, when we care about the
+                        // most general case ("X doesn't implement Pattern<'_>") over the case that
+                        // happened to fail ("char doesn't implement Fn(&mut char)").
+                        //
+                        // We rely on a few heuristics to identify cases where this root
+                        // obligation is more important than the leaf obligation:
+                        let (main_trait_predicate, o) = if let ty::PredicateKind::Clause(
+                            ty::ClauseKind::Trait(root_pred)
+                        ) = root_obligation.predicate.kind().skip_binder()
+                            && !trait_predicate.self_ty().skip_binder().has_escaping_bound_vars()
+                            && !root_pred.self_ty().has_escaping_bound_vars()
+                            // The type of the leaf predicate is (roughly) the same as the type
+                            // from the root predicate, as a proxy for "we care about the root"
+                            // FIXME: this doesn't account for trivial derefs, but works as a first
+                            // approximation.
+                            && (
+                                // `T: Trait` && `&&T: OtherTrait`, we want `OtherTrait`
+                                self.can_eq(
+                                    obligation.param_env,
+                                    trait_predicate.self_ty().skip_binder(),
+                                    root_pred.self_ty().peel_refs(),
+                                )
+                                // `&str: Iterator` && `&str: IntoIterator`, we want `IntoIterator`
+                                || self.can_eq(
+                                    obligation.param_env,
+                                    trait_predicate.self_ty().skip_binder(),
+                                    root_pred.self_ty(),
+                                )
+                            )
+                            // The leaf trait and the root trait are different, so as to avoid
+                            // talking about `&mut T: Trait` and instead remain talking about
+                            // `T: Trait` instead
+                            && trait_predicate.def_id() != root_pred.def_id()
+                            // The root trait is not `Unsize`, as to avoid talking about it in
+                            // `tests/ui/coercion/coerce-issue-49593-box-never.rs`.
+                            && Some(root_pred.def_id()) != self.tcx.lang_items().unsize_trait()
+                        {
+                            (
+                                self.resolve_vars_if_possible(
+                                    root_obligation.predicate.kind().rebind(root_pred),
+                                ),
+                                root_obligation,
+                            )
+                        } else {
+                            (trait_predicate, &obligation)
+                        };
+                        let trait_ref = main_trait_predicate.to_poly_trait_ref();
+
+                        if let Some(guar) = self.emit_specialized_closure_kind_error(
+                            &obligation,
+                            trait_ref,
+                        ) {
                             return guar;
                         }
 
@@ -459,8 +509,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                             notes,
                             parent_label,
                             append_const_msg,
-                        } = self.on_unimplemented_note(trait_ref, &obligation, &mut long_ty_file);
-
+                        } = self.on_unimplemented_note(trait_ref, o, &mut long_ty_file);
                         let have_alt_message = message.is_some() || label.is_some();
                         let is_try_conversion = self.is_try_conversion(span, trait_ref.def_id());
                         let is_unsize =
@@ -483,7 +532,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                         };
 
                         let err_msg = self.get_standard_error_message(
-                            &trait_predicate,
+                            &main_trait_predicate,
                             message,
                             predicate_is_const,
                             append_const_msg,
diff --git a/compiler/rustc_ty_utils/src/opaque_types.rs b/compiler/rustc_ty_utils/src/opaque_types.rs
index 86c7551882a..28259516973 100644
--- a/compiler/rustc_ty_utils/src/opaque_types.rs
+++ b/compiler/rustc_ty_utils/src/opaque_types.rs
@@ -22,11 +22,20 @@ struct OpaqueTypeCollector<'tcx> {
     seen: FxHashSet<LocalDefId>,
 
     span: Option<Span>,
+
+    mode: CollectionMode,
+}
+
+enum CollectionMode {
+    /// For impl trait in assoc types we only permit collecting them from
+    /// associated types of the same impl block.
+    ImplTraitInAssocTypes,
+    TypeAliasImplTraitTransition,
 }
 
 impl<'tcx> OpaqueTypeCollector<'tcx> {
-    fn new(tcx: TyCtxt<'tcx>, item: LocalDefId) -> Self {
-        Self { tcx, opaques: Vec::new(), item, seen: Default::default(), span: None }
+    fn new(tcx: TyCtxt<'tcx>, item: LocalDefId, mode: CollectionMode) -> Self {
+        Self { tcx, opaques: Vec::new(), item, seen: Default::default(), span: None, mode }
     }
 
     fn span(&self) -> Span {
@@ -251,6 +260,9 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for OpaqueTypeCollector<'tcx> {
                 }
             }
             ty::Adt(def, _) if def.did().is_local() => {
+                if let CollectionMode::ImplTraitInAssocTypes = self.mode {
+                    return ControlFlow::Continue(());
+                }
                 if !self.seen.insert(def.did().expect_local()) {
                     return ControlFlow::Continue(());
                 }
@@ -275,89 +287,13 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for OpaqueTypeCollector<'tcx> {
     }
 }
 
-struct ImplTraitInAssocTypeCollector<'tcx>(OpaqueTypeCollector<'tcx>);
-
-impl<'tcx> super::sig_types::SpannedTypeVisitor<'tcx> for ImplTraitInAssocTypeCollector<'tcx> {
-    #[instrument(skip(self), ret, level = "trace")]
-    fn visit(&mut self, span: Span, value: impl TypeVisitable<TyCtxt<'tcx>>) -> ControlFlow<!> {
-        let old = self.0.span;
-        self.0.span = Some(span);
-        value.visit_with(self);
-        self.0.span = old;
-
-        ControlFlow::Continue(())
-    }
-}
-
-impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for ImplTraitInAssocTypeCollector<'tcx> {
-    #[instrument(skip(self), ret, level = "trace")]
-    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<!> {
-        t.super_visit_with(self)?;
-        match t.kind() {
-            ty::Alias(ty::Opaque, alias_ty) if alias_ty.def_id.is_local() => {
-                self.0.visit_opaque_ty(alias_ty);
-            }
-            ty::Alias(ty::Projection, alias_ty) => {
-                // This avoids having to do normalization of `Self::AssocTy` by only
-                // supporting the case of a method defining opaque types from assoc types
-                // in the same impl block.
-                let parent_trait_ref = self
-                    .0
-                    .parent_trait_ref()
-                    .expect("impl trait in assoc type collector used on non-assoc item");
-                // If the trait ref of the associated item and the impl differs,
-                // then we can't use the impl's identity args below, so
-                // just skip.
-                if alias_ty.trait_ref(self.0.tcx) == parent_trait_ref {
-                    let parent = self.0.parent().expect("we should have a parent here");
-
-                    for &assoc in self.0.tcx.associated_items(parent).in_definition_order() {
-                        trace!(?assoc);
-                        if assoc.trait_item_def_id != Some(alias_ty.def_id) {
-                            continue;
-                        }
-
-                        // If the type is further specializable, then the type_of
-                        // is not actually correct below.
-                        if !assoc.defaultness(self.0.tcx).is_final() {
-                            continue;
-                        }
-
-                        let impl_args = alias_ty.args.rebase_onto(
-                            self.0.tcx,
-                            parent_trait_ref.def_id,
-                            ty::GenericArgs::identity_for_item(self.0.tcx, parent),
-                        );
-
-                        if check_args_compatible(self.0.tcx, assoc, impl_args) {
-                            return self
-                                .0
-                                .tcx
-                                .type_of(assoc.def_id)
-                                .instantiate(self.0.tcx, impl_args)
-                                .visit_with(self);
-                        } else {
-                            self.0.tcx.dcx().span_bug(
-                                self.0.tcx.def_span(assoc.def_id),
-                                "item had incorrect args",
-                            );
-                        }
-                    }
-                }
-            }
-            _ => trace!(kind=?t.kind()),
-        }
-        ControlFlow::Continue(())
-    }
-}
-
 fn impl_trait_in_assoc_types_defined_by<'tcx>(
     tcx: TyCtxt<'tcx>,
     item: LocalDefId,
 ) -> &'tcx ty::List<LocalDefId> {
-    let mut collector = ImplTraitInAssocTypeCollector(OpaqueTypeCollector::new(tcx, item));
+    let mut collector = OpaqueTypeCollector::new(tcx, item, CollectionMode::ImplTraitInAssocTypes);
     super::sig_types::walk_types(tcx, item, &mut collector);
-    tcx.mk_local_def_ids(&collector.0.opaques)
+    tcx.mk_local_def_ids(&collector.opaques)
 }
 
 fn opaque_types_defined_by<'tcx>(
@@ -366,7 +302,8 @@ fn opaque_types_defined_by<'tcx>(
 ) -> &'tcx ty::List<LocalDefId> {
     let kind = tcx.def_kind(item);
     trace!(?kind);
-    let mut collector = OpaqueTypeCollector::new(tcx, item);
+    let mut collector =
+        OpaqueTypeCollector::new(tcx, item, CollectionMode::TypeAliasImplTraitTransition);
     super::sig_types::walk_types(tcx, item, &mut collector);
     match kind {
         DefKind::AssocFn