Auto merge of #121796 - oli-obk:eager_opaque_checks3, r=lcnr

Make `DefiningAnchor::Bind` only store the opaque types that may be constrained, instead of the current infcx root item.

This makes `Bind` almost always be empty, so we can start forwarding it to queries, allowing us to remove `Bubble` entirely (not done in this PR)

The only behaviour change is in diagnostics.

r? `@lcnr` `@compiler-errors`

16 files changed, 105 insertions, 111 deletions

diff --git a/compiler/rustc_borrowck/src/consumers.rs b/compiler/rustc_borrowck/src/consumers.rs
index a58fe2b7447..31307ef1410 100644
--- a/compiler/rustc_borrowck/src/consumers.rs
+++ b/compiler/rustc_borrowck/src/consumers.rs
@@ -106,7 +106,7 @@ pub fn get_body_with_borrowck_facts(
     options: ConsumerOptions,
 ) -> BodyWithBorrowckFacts<'_> {
     let (input_body, promoted) = tcx.mir_promoted(def);
-    let infcx = tcx.infer_ctxt().with_opaque_type_inference(DefiningAnchor::Bind(def)).build();
+    let infcx = tcx.infer_ctxt().with_opaque_type_inference(DefiningAnchor::bind(tcx, def)).build();
     let input_body: &Body<'_> = &input_body.borrow();
     let promoted: &IndexSlice<_, _> = &promoted.borrow();
     *super::do_mir_borrowck(&infcx, input_body, promoted, Some(options)).1.unwrap()
diff --git a/compiler/rustc_borrowck/src/lib.rs b/compiler/rustc_borrowck/src/lib.rs
index 8dcfe014b65..415ba095a1b 100644
--- a/compiler/rustc_borrowck/src/lib.rs
+++ b/compiler/rustc_borrowck/src/lib.rs
@@ -126,10 +126,7 @@ fn mir_borrowck(tcx: TyCtxt<'_>, def: LocalDefId) -> &BorrowCheckResult<'_> {
         return tcx.arena.alloc(result);
     }
 
-    let hir_owner = tcx.local_def_id_to_hir_id(def).owner;
-
-    let infcx =
-        tcx.infer_ctxt().with_opaque_type_inference(DefiningAnchor::Bind(hir_owner.def_id)).build();
+    let infcx = tcx.infer_ctxt().with_opaque_type_inference(DefiningAnchor::bind(tcx, def)).build();
     let promoted: &IndexSlice<_, _> = &promoted.borrow();
     let opt_closure_req = do_mir_borrowck(&infcx, input_body, promoted, None).0;
     debug!("mir_borrowck done");
diff --git a/compiler/rustc_borrowck/src/region_infer/opaque_types.rs b/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
index 12b02c7fcfa..8a172233037 100644
--- a/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
+++ b/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
@@ -311,13 +311,13 @@ fn check_opaque_type_well_formed<'tcx>(
         parent_def_id = tcx.local_parent(parent_def_id);
     }
 
-    // FIXME(-Znext-solver): We probably should use `DefiningAnchor::Error`
+    // FIXME(-Znext-solver): We probably should use `DefiningAnchor::Bind(&[])`
     // and prepopulate this `InferCtxt` with known opaque values, rather than
     // using the `Bind` anchor here. For now it's fine.
     let infcx = tcx
         .infer_ctxt()
         .with_next_trait_solver(next_trait_solver)
-        .with_opaque_type_inference(DefiningAnchor::Bind(parent_def_id))
+        .with_opaque_type_inference(DefiningAnchor::bind(tcx, parent_def_id))
         .build();
     let ocx = ObligationCtxt::new(&infcx);
     let identity_args = GenericArgs::identity_for_item(tcx, def_id);
diff --git a/compiler/rustc_hir_analysis/src/check/check.rs b/compiler/rustc_hir_analysis/src/check/check.rs
index 1b24c2b61fd..cb739ac48a8 100644
--- a/compiler/rustc_hir_analysis/src/check/check.rs
+++ b/compiler/rustc_hir_analysis/src/check/check.rs
@@ -347,7 +347,7 @@ fn check_opaque_meets_bounds<'tcx>(
 
     let infcx = tcx
         .infer_ctxt()
-        .with_opaque_type_inference(DefiningAnchor::Bind(defining_use_anchor))
+        .with_opaque_type_inference(DefiningAnchor::bind(tcx, defining_use_anchor))
         .build();
     let ocx = ObligationCtxt::new(&infcx);
 
@@ -1558,7 +1558,7 @@ pub(super) fn check_coroutine_obligations(
         .ignoring_regions()
         // Bind opaque types to type checking root, as they should have been checked by borrowck,
         // but may show up in some cases, like when (root) obligations are stalled in the new solver.
-        .with_opaque_type_inference(DefiningAnchor::Bind(typeck.hir_owner.def_id))
+        .with_opaque_type_inference(DefiningAnchor::bind(tcx, typeck.hir_owner.def_id))
         .build();
 
     let mut fulfillment_cx = <dyn TraitEngine<'_>>::new(&infcx);
diff --git a/compiler/rustc_hir_typeck/src/inherited.rs b/compiler/rustc_hir_typeck/src/inherited.rs
index 4ad46845f0b..b0950ed2800 100644
--- a/compiler/rustc_hir_typeck/src/inherited.rs
+++ b/compiler/rustc_hir_typeck/src/inherited.rs
@@ -79,7 +79,7 @@ impl<'tcx> Inherited<'tcx> {
         let infcx = tcx
             .infer_ctxt()
             .ignoring_regions()
-            .with_opaque_type_inference(DefiningAnchor::Bind(def_id))
+            .with_opaque_type_inference(DefiningAnchor::bind(tcx, def_id))
             .build();
         let typeck_results = RefCell::new(ty::TypeckResults::new(hir_owner));
 
diff --git a/compiler/rustc_infer/src/infer/mod.rs b/compiler/rustc_infer/src/infer/mod.rs
index 15cdd6a910e..89e4e88b3df 100644
--- a/compiler/rustc_infer/src/infer/mod.rs
+++ b/compiler/rustc_infer/src/infer/mod.rs
@@ -242,9 +242,10 @@ pub struct InferCtxt<'tcx> {
     /// short lived InferCtxt within queries. The opaque type obligations are forwarded
     /// to the outside until the end up in an `InferCtxt` for typeck or borrowck.
     ///
-    /// Its default value is `DefiningAnchor::Error`, this way it is easier to catch errors that
+    /// Its default value is `DefiningAnchor::Bind(&[])`, which means no opaque types may be defined.
+    /// This way it is easier to catch errors that
     /// might come up during inference or typeck.
-    pub defining_use_anchor: DefiningAnchor,
+    pub defining_use_anchor: DefiningAnchor<'tcx>,
 
     /// Whether this inference context should care about region obligations in
     /// the root universe. Most notably, this is used during hir typeck as region
@@ -605,7 +606,7 @@ impl fmt::Display for FixupError {
 /// Used to configure inference contexts before their creation.
 pub struct InferCtxtBuilder<'tcx> {
     tcx: TyCtxt<'tcx>,
-    defining_use_anchor: DefiningAnchor,
+    defining_use_anchor: DefiningAnchor<'tcx>,
     considering_regions: bool,
     skip_leak_check: bool,
     /// Whether we are in coherence mode.
@@ -620,7 +621,7 @@ impl<'tcx> TyCtxt<'tcx> {
     fn infer_ctxt(self) -> InferCtxtBuilder<'tcx> {
         InferCtxtBuilder {
             tcx: self,
-            defining_use_anchor: DefiningAnchor::Error,
+            defining_use_anchor: DefiningAnchor::Bind(ty::List::empty()),
             considering_regions: true,
             skip_leak_check: false,
             intercrate: false,
@@ -636,7 +637,7 @@ impl<'tcx> InferCtxtBuilder<'tcx> {
     /// It is only meant to be called in two places, for typeck
     /// (via `Inherited::build`) and for the inference context used
     /// in mir borrowck.
-    pub fn with_opaque_type_inference(mut self, defining_use_anchor: DefiningAnchor) -> Self {
+    pub fn with_opaque_type_inference(mut self, defining_use_anchor: DefiningAnchor<'tcx>) -> Self {
         self.defining_use_anchor = defining_use_anchor;
         self
     }
@@ -1208,13 +1209,11 @@ impl<'tcx> InferCtxt<'tcx> {
 
     #[instrument(level = "debug", skip(self), ret)]
     pub fn take_opaque_types(&self) -> opaque_types::OpaqueTypeMap<'tcx> {
-        debug_assert_ne!(self.defining_use_anchor, DefiningAnchor::Error);
         std::mem::take(&mut self.inner.borrow_mut().opaque_type_storage.opaque_types)
     }
 
     #[instrument(level = "debug", skip(self), ret)]
     pub fn clone_opaque_types(&self) -> opaque_types::OpaqueTypeMap<'tcx> {
-        debug_assert_ne!(self.defining_use_anchor, DefiningAnchor::Error);
         self.inner.borrow().opaque_type_storage.opaque_types.clone()
     }
 
diff --git a/compiler/rustc_infer/src/infer/opaque_types/mod.rs b/compiler/rustc_infer/src/infer/opaque_types/mod.rs
index 3d6829ba657..a6f8115c27e 100644
--- a/compiler/rustc_infer/src/infer/opaque_types/mod.rs
+++ b/compiler/rustc_infer/src/infer/opaque_types/mod.rs
@@ -150,9 +150,6 @@ impl<'tcx> InferCtxt<'tcx> {
                         }
                     }
                     DefiningAnchor::Bubble => {}
-                    DefiningAnchor::Error => {
-                        return None;
-                    }
                 }
                 if let ty::Alias(ty::Opaque, ty::AliasTy { def_id: b_def_id, .. }) = *b.kind() {
                     // We could accept this, but there are various ways to handle this situation, and we don't
@@ -378,28 +375,14 @@ impl<'tcx> InferCtxt<'tcx> {
     /// in its defining scope.
     #[instrument(skip(self), level = "trace", ret)]
     pub fn opaque_type_origin(&self, def_id: LocalDefId) -> Option<OpaqueTyOrigin> {
-        let opaque_hir_id = self.tcx.local_def_id_to_hir_id(def_id);
-        let parent_def_id = match self.defining_use_anchor {
-            DefiningAnchor::Bubble | DefiningAnchor::Error => return None,
+        let defined_opaque_types = match self.defining_use_anchor {
+            DefiningAnchor::Bubble => return None,
             DefiningAnchor::Bind(bind) => bind,
         };
 
         let origin = self.tcx.opaque_type_origin(def_id);
-        let in_definition_scope = match origin {
-            // Async `impl Trait`
-            hir::OpaqueTyOrigin::AsyncFn(parent) => parent == parent_def_id,
-            // Anonymous `impl Trait`
-            hir::OpaqueTyOrigin::FnReturn(parent) => parent == parent_def_id,
-            // Named `type Foo = impl Bar;`
-            hir::OpaqueTyOrigin::TyAlias { in_assoc_ty, .. } => {
-                if in_assoc_ty {
-                    self.tcx.opaque_types_defined_by(parent_def_id).contains(&def_id)
-                } else {
-                    may_define_opaque_type(self.tcx, parent_def_id, opaque_hir_id)
-                }
-            }
-        };
-        in_definition_scope.then_some(origin)
+
+        defined_opaque_types.contains(&def_id).then_some(origin)
     }
 }
 
@@ -656,43 +639,3 @@ impl<'tcx> InferCtxt<'tcx> {
         }
     }
 }
-
-/// Returns `true` if `opaque_hir_id` is a sibling or a child of a sibling of `def_id`.
-///
-/// Example:
-/// ```ignore UNSOLVED (is this a bug?)
-/// # #![feature(type_alias_impl_trait)]
-/// pub mod foo {
-///     pub mod bar {
-///         pub trait Bar { /* ... */ }
-///         pub type Baz = impl Bar;
-///
-///         # impl Bar for () {}
-///         fn f1() -> Baz { /* ... */ }
-///     }
-///     fn f2() -> bar::Baz { /* ... */ }
-/// }
-/// ```
-///
-/// Here, `def_id` is the `LocalDefId` of the defining use of the opaque type (e.g., `f1` or `f2`),
-/// and `opaque_hir_id` is the `HirId` of the definition of the opaque type `Baz`.
-/// For the above example, this function returns `true` for `f1` and `false` for `f2`.
-fn may_define_opaque_type(tcx: TyCtxt<'_>, def_id: LocalDefId, opaque_hir_id: hir::HirId) -> bool {
-    let mut hir_id = tcx.local_def_id_to_hir_id(def_id);
-
-    // Named opaque types can be defined by any siblings or children of siblings.
-    let scope = tcx.hir().get_defining_scope(opaque_hir_id);
-    // We walk up the node tree until we hit the root or the scope of the opaque type.
-    while hir_id != scope && hir_id != hir::CRATE_HIR_ID {
-        hir_id = tcx.hir().get_parent_item(hir_id).into();
-    }
-    // Syntactically, we are allowed to define the concrete type if:
-    let res = hir_id == scope;
-    trace!(
-        "may_define_opaque_type(def={:?}, opaque_node={:?}) = {}",
-        tcx.hir_node(hir_id),
-        tcx.hir_node(opaque_hir_id),
-        res
-    );
-    res
-}
diff --git a/compiler/rustc_middle/src/mir/type_foldable.rs b/compiler/rustc_middle/src/mir/type_foldable.rs
index 93a9bbf64c9..b798f078800 100644
--- a/compiler/rustc_middle/src/mir/type_foldable.rs
+++ b/compiler/rustc_middle/src/mir/type_foldable.rs
@@ -1,6 +1,7 @@
 //! `TypeFoldable` implementations for MIR types
 
 use rustc_ast::InlineAsmTemplatePiece;
+use rustc_hir::def_id::LocalDefId;
 
 use super::*;
 
@@ -44,6 +45,15 @@ impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for &'tcx [Span] {
     }
 }
 
+impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for &'tcx ty::List<LocalDefId> {
+    fn try_fold_with<F: FallibleTypeFolder<TyCtxt<'tcx>>>(
+        self,
+        _folder: &mut F,
+    ) -> Result<Self, F::Error> {
+        Ok(self)
+    }
+}
+
 impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for &'tcx ty::List<PlaceElem<'tcx>> {
     fn try_fold_with<F: FallibleTypeFolder<TyCtxt<'tcx>>>(
         self,
diff --git a/compiler/rustc_middle/src/traits/mod.rs b/compiler/rustc_middle/src/traits/mod.rs
index a6e4702d819..aea58653351 100644
--- a/compiler/rustc_middle/src/traits/mod.rs
+++ b/compiler/rustc_middle/src/traits/mod.rs
@@ -12,8 +12,8 @@ pub mod util;
 use crate::infer::canonical::Canonical;
 use crate::mir::ConstraintCategory;
 use crate::ty::abstract_const::NotConstEvaluatable;
-use crate::ty::GenericArgsRef;
 use crate::ty::{self, AdtKind, Ty};
+use crate::ty::{GenericArgsRef, TyCtxt};
 
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::{Applicability, Diag, EmissionGuarantee};
@@ -1001,10 +1001,14 @@ pub enum CodegenObligationError {
 /// opaques are replaced with inference vars eagerly in the old solver (e.g.
 /// in projection, and in the signature during function type-checking).
 #[derive(Debug, PartialEq, Eq, Clone, Copy, Hash, HashStable, TypeFoldable, TypeVisitable)]
-pub enum DefiningAnchor {
-    /// Define opaques which are in-scope of the `LocalDefId`. Also, eagerly
-    /// replace opaque types in `replace_opaque_types_with_inference_vars`.
-    Bind(LocalDefId),
+pub enum DefiningAnchor<'tcx> {
+    /// Define opaques which are in-scope of the current item being analyzed.
+    /// Also, eagerly replace these opaque types in `replace_opaque_types_with_inference_vars`.
+    ///
+    /// If the list is empty, do not allow any opaques to be defined. This is used to catch type mismatch
+    /// errors when handling opaque types, and also should be used when we would
+    /// otherwise reveal opaques (such as [`Reveal::All`] reveal mode).
+    Bind(&'tcx ty::List<LocalDefId>),
     /// In contexts where we don't currently know what opaques are allowed to be
     /// defined, such as (old solver) canonical queries, we will simply allow
     /// opaques to be defined, but "bubble" them up in the canonical response or
@@ -1013,8 +1017,10 @@ pub enum DefiningAnchor {
     /// We do not eagerly replace opaque types in `replace_opaque_types_with_inference_vars`,
     /// which may affect what predicates pass and fail in the old trait solver.
     Bubble,
-    /// Do not allow any opaques to be defined. This is used to catch type mismatch
-    /// errors when handling opaque types, and also should be used when we would
-    /// otherwise reveal opaques (such as [`Reveal::All`] reveal mode).
-    Error,
+}
+
+impl<'tcx> DefiningAnchor<'tcx> {
+    pub fn bind(tcx: TyCtxt<'tcx>, item: LocalDefId) -> Self {
+        Self::Bind(tcx.opaque_types_defined_by(item))
+    }
 }
diff --git a/compiler/rustc_middle/src/traits/solve.rs b/compiler/rustc_middle/src/traits/solve.rs
index 95139b50cb8..dc4cd203415 100644
--- a/compiler/rustc_middle/src/traits/solve.rs
+++ b/compiler/rustc_middle/src/traits/solve.rs
@@ -114,7 +114,7 @@ impl MaybeCause {
 #[derive(Debug, PartialEq, Eq, Clone, Copy, Hash, HashStable, TypeFoldable, TypeVisitable)]
 pub struct QueryInput<'tcx, T> {
     pub goal: Goal<'tcx, T>,
-    pub anchor: DefiningAnchor,
+    pub anchor: DefiningAnchor<'tcx>,
     pub predefined_opaques_in_body: PredefinedOpaques<'tcx>,
 }
 
diff --git a/compiler/rustc_middle/src/ty/codec.rs b/compiler/rustc_middle/src/ty/codec.rs
index 69ae05ca820..ddbc0bffaed 100644
--- a/compiler/rustc_middle/src/ty/codec.rs
+++ b/compiler/rustc_middle/src/ty/codec.rs
@@ -17,6 +17,7 @@ use crate::traits;
 use crate::ty::GenericArgsRef;
 use crate::ty::{self, AdtDef, Ty};
 use rustc_data_structures::fx::FxHashMap;
+use rustc_hir::def_id::LocalDefId;
 use rustc_middle::ty::TyCtxt;
 use rustc_serialize::{Decodable, Encodable};
 use rustc_span::Span;
@@ -431,6 +432,15 @@ impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D> for ty::List<Fi
     }
 }
 
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D> for ty::List<LocalDefId> {
+    fn decode(decoder: &mut D) -> &'tcx Self {
+        let len = decoder.read_usize();
+        decoder.interner().mk_local_def_ids_from_iter(
+            (0..len).map::<LocalDefId, _>(|_| Decodable::decode(decoder)),
+        )
+    }
+}
+
 impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D>
     for ty::List<(VariantIdx, FieldIdx)>
 {
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index b08e5df30ef..c415c06c21b 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -2014,6 +2014,14 @@ impl<'tcx> TyCtxt<'tcx> {
         self.intern_local_def_ids(clauses)
     }
 
+    pub fn mk_local_def_ids_from_iter<I, T>(self, iter: I) -> T::Output
+    where
+        I: Iterator<Item = T>,
+        T: CollectAndApply<LocalDefId, &'tcx List<LocalDefId>>,
+    {
+        T::collect_and_apply(iter, |xs| self.mk_local_def_ids(xs))
+    }
+
     pub fn mk_const_list_from_iter<I, T>(self, iter: I) -> T::Output
     where
         I: Iterator<Item = T>,
diff --git a/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs b/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs
index 22f52c27362..3b858cb449f 100644
--- a/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs
+++ b/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs
@@ -16,7 +16,7 @@ use rustc_middle::traits::solve::{
     CanonicalInput, CanonicalResponse, Certainty, IsNormalizesToHack, PredefinedOpaques,
     PredefinedOpaquesData, QueryResult,
 };
-use rustc_middle::traits::{specialization_graph, DefiningAnchor};
+use rustc_middle::traits::specialization_graph;
 use rustc_middle::ty::{
     self, InferCtxtLike, OpaqueTypeKey, Ty, TyCtxt, TypeFoldable, TypeSuperVisitable,
     TypeVisitable, TypeVisitableExt, TypeVisitor,
@@ -258,10 +258,7 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
         // instead of taking them. This would cause an ICE here, since we have
         // assertions against dropping an `InferCtxt` without taking opaques.
         // FIXME: Once we remove support for the old impl we can remove this.
-        if input.anchor != DefiningAnchor::Error {
-            // This seems ok, but fragile.
-            let _ = infcx.take_opaque_types();
-        }
+        let _ = infcx.take_opaque_types();
 
         result
     }
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 71fcc47dba3..ad5b7debad7 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
@@ -32,7 +32,7 @@ use rustc_hir::{GenericParam, Item, Node};
 use rustc_infer::infer::error_reporting::TypeErrCtxt;
 use rustc_infer::infer::{InferOk, TypeTrace};
 use rustc_middle::traits::select::OverflowError;
-use rustc_middle::traits::{DefiningAnchor, SignatureMismatchData};
+use rustc_middle::traits::SignatureMismatchData;
 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::fold::{BottomUpFolder, TypeFolder, TypeSuperFoldable};
@@ -3390,19 +3390,12 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
             obligation.cause.span,
             format!("cannot check whether the hidden type of {name} satisfies auto traits"),
         );
+
+        err.note(
+            "fetching the hidden types of an opaque inside of the defining scope is not supported. \
+            You can try moving the opaque type and the item that actually registers a hidden type into a new submodule",
+        );
         err.span_note(self.tcx.def_span(def_id), "opaque type is declared here");
-        match self.defining_use_anchor {
-            DefiningAnchor::Bubble | DefiningAnchor::Error => {}
-            DefiningAnchor::Bind(bind) => {
-                err.span_note(
-                    self.tcx.def_ident_span(bind).unwrap_or_else(|| self.tcx.def_span(bind)),
-                    "this item depends on auto traits of the hidden type, \
-                    but may also be registering the hidden type. \
-                    This is not supported right now. \
-                    You can try moving the opaque type and the item that actually registers a hidden type into a new submodule".to_string(),
-                );
-            }
-        };
 
         self.note_obligation_cause(&mut err, &obligation);
         self.point_at_returns_when_relevant(&mut err, &obligation);
diff --git a/compiler/rustc_ty_utils/src/opaque_types.rs b/compiler/rustc_ty_utils/src/opaque_types.rs
index 3dcc8382289..4a1064b29f6 100644
--- a/compiler/rustc_ty_utils/src/opaque_types.rs
+++ b/compiler/rustc_ty_utils/src/opaque_types.rs
@@ -54,7 +54,7 @@ impl<'tcx> OpaqueTypeCollector<'tcx> {
         self.span = old;
     }
 
-    fn parent_trait_ref(&self) -> Option<ty::TraitRef<'tcx>> {
+    fn parent_impl_trait_ref(&self) -> Option<ty::TraitRef<'tcx>> {
         let parent = self.parent()?;
         if matches!(self.tcx.def_kind(parent), DefKind::Impl { .. }) {
             Some(self.tcx.impl_trait_ref(parent)?.instantiate_identity())
@@ -210,6 +210,7 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for OpaqueTypeCollector<'tcx> {
             ty::Alias(ty::Opaque, alias_ty) if alias_ty.def_id.is_local() => {
                 self.visit_opaque_ty(alias_ty);
             }
+            // Skips type aliases, as they are meant to be transparent.
             ty::Alias(ty::Weak, alias_ty) if alias_ty.def_id.is_local() => {
                 self.tcx
                     .type_of(alias_ty.def_id)
@@ -220,11 +221,11 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for OpaqueTypeCollector<'tcx> {
                 // 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.
-                if let Some(parent_trait_ref) = self.parent_trait_ref() {
+                if let Some(impl_trait_ref) = self.parent_impl_trait_ref() {
                     // 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.tcx) == parent_trait_ref {
+                    if alias_ty.trait_ref(self.tcx) == impl_trait_ref {
                         let parent = self.parent().expect("we should have a parent here");
 
                         for &assoc in self.tcx.associated_items(parent).in_definition_order() {
@@ -241,7 +242,7 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for OpaqueTypeCollector<'tcx> {
 
                             let impl_args = alias_ty.args.rebase_onto(
                                 self.tcx,
-                                parent_trait_ref.def_id,
+                                impl_trait_ref.def_id,
                                 ty::GenericArgs::identity_for_item(self.tcx, parent),
                             );
 
@@ -259,6 +260,24 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for OpaqueTypeCollector<'tcx> {
                             }
                         }
                     }
+                } else if let Some(ty::ImplTraitInTraitData::Trait { fn_def_id, .. }) =
+                    self.tcx.opt_rpitit_info(alias_ty.def_id)
+                    && fn_def_id == self.item.into()
+                {
+                    // RPITIT in trait definitions get desugared to an associated type. For
+                    // default methods we also create an opaque type this associated type
+                    // normalizes to. The associated type is only known to normalize to the
+                    // opaque if it is fully concrete. There could otherwise be an impl
+                    // overwriting the default method.
+                    //
+                    // However, we have to be able to normalize the associated type while inside
+                    // of the default method. This is normally handled by adding an unchecked
+                    // `Projection(<Self as Trait>::synthetic_assoc_ty, trait_def::opaque)`
+                    // assumption to the `param_env` of the default method. We also separately
+                    // rely on that assumption here.
+                    let ty = self.tcx.type_of(alias_ty.def_id).instantiate(self.tcx, alias_ty.args);
+                    let ty::Alias(ty::Opaque, alias_ty) = ty.kind() else { bug!("{ty:?}") };
+                    self.visit_opaque_ty(alias_ty);
                 }
             }
             ty::Adt(def, _) if def.did().is_local() => {
diff --git a/compiler/rustc_ty_utils/src/sig_types.rs b/compiler/rustc_ty_utils/src/sig_types.rs
index 5527d853e30..72fcc95c3b3 100644
--- a/compiler/rustc_ty_utils/src/sig_types.rs
+++ b/compiler/rustc_ty_utils/src/sig_types.rs
@@ -23,8 +23,14 @@ pub fn walk_types<'tcx, V: SpannedTypeVisitor<'tcx>>(
     match kind {
         // Walk over the signature of the function
         DefKind::AssocFn | DefKind::Fn => {
-            let ty_sig = tcx.fn_sig(item).instantiate_identity();
             let hir_sig = tcx.hir_node_by_def_id(item).fn_decl().unwrap();
+            // If the type of the item uses `_`, we're gonna error out anyway, but
+            // typeck (which type_of invokes below), will call back into opaque_types_defined_by
+            // causing a cycle. So we just bail out in this case.
+            if hir_sig.output.get_infer_ret_ty().is_some() {
+                return V::Result::output();
+            }
+            let ty_sig = tcx.fn_sig(item).instantiate_identity();
             // Walk over the inputs and outputs manually in order to get good spans for them.
             try_visit!(visitor.visit(hir_sig.output.span(), ty_sig.output()));
             for (hir, ty) in hir_sig.inputs.iter().zip(ty_sig.inputs().iter()) {
@@ -39,6 +45,12 @@ pub fn walk_types<'tcx, V: SpannedTypeVisitor<'tcx>>(
         // Walk over the type of the item
         DefKind::Static(_) | DefKind::Const | DefKind::AssocConst | DefKind::AnonConst => {
             if let Some(ty) = tcx.hir_node_by_def_id(item).ty() {
+                // If the type of the item uses `_`, we're gonna error out anyway, but
+                // typeck (which type_of invokes below), will call back into opaque_types_defined_by
+                // causing a cycle. So we just bail out in this case.
+                if ty.is_suggestable_infer_ty() {
+                    return V::Result::output();
+                }
                 // Associated types in traits don't necessarily have a type that we can visit
                 try_visit!(visitor.visit(ty.span, tcx.type_of(item).instantiate_identity()));
             }