For a rigid projection, recursively look at the self type's item bounds to fix the associated_type_bounds feature

compiler/rustc_ast_lowering/src/lib.rs

@@ -1094,24 +1094,22 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
                 // Piggy-back on the `impl Trait` context to figure out the correct behavior.
                 let desugar_kind = match itctx {
-                    // We are in the return position:
-                    //
-                    //     fn foo() -> impl Iterator<Item: Debug>
-                    //
-                    // so desugar to
-                    //
-                    //     fn foo() -> impl Iterator<Item = impl Debug>
-                    ImplTraitContext::ReturnPositionOpaqueTy { .. }
-                    | ImplTraitContext::TypeAliasesOpaqueTy { .. } => DesugarKind::ImplTrait,
-
-                    // We are in the argument position, but within a dyn type:
+                    // in an argument, RPIT, or TAIT, if we are within a dyn type:
                     //
                     //     fn foo(x: dyn Iterator<Item: Debug>)
                     //
-                    // so desugar to
+                    // then desugar to:
                     //
                     //     fn foo(x: dyn Iterator<Item = impl Debug>)
-                    ImplTraitContext::Universal if self.is_in_dyn_type => DesugarKind::ImplTrait,
+                    //
+                    // This is because dyn traits must have all of their associated types specified.
+                    ImplTraitContext::ReturnPositionOpaqueTy { .. }
+                    | ImplTraitContext::TypeAliasesOpaqueTy { .. }
+                    | ImplTraitContext::Universal
+                        if self.is_in_dyn_type =>
+                    {
+                        DesugarKind::ImplTrait
+                    }
 
                     ImplTraitContext::Disallowed(position) if self.is_in_dyn_type => {
                         DesugarKind::Error(position)

compiler/rustc_trait_selection/src/solve/assembly/mod.rs

@@ -542,7 +542,27 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         goal: Goal<'tcx, G>,
         candidates: &mut Vec<Candidate<'tcx>>,
     ) {
-        let alias_ty = match goal.predicate.self_ty().kind() {
+        let () = self.probe(|_| ProbeKind::NormalizedSelfTyAssembly).enter(|ecx| {
+            ecx.assemble_alias_bound_candidates_recur(goal.predicate.self_ty(), goal, candidates);
+        });
+    }
+
+    /// For some deeply nested `<T>::A::B::C::D` rigid associated type,
+    /// we should explore the item bounds for all levels, since the
+    /// `associated_type_bounds` feature means that a parent associated
+    /// type may carry bounds for a nested associated type.
+    ///
+    /// If we have a projection, check that its self type is a rigid projection.
+    /// If so, continue searching by recursively calling after normalization.
+    // FIXME: This may recurse infinitely, but I can't seem to trigger it without
+    // hitting another overflow error something. Add a depth parameter needed later.
+    fn assemble_alias_bound_candidates_recur<G: GoalKind<'tcx>>(
+        &mut self,
+        self_ty: Ty<'tcx>,
+        goal: Goal<'tcx, G>,
+        candidates: &mut Vec<Candidate<'tcx>>,
+    ) {
+        let (kind, alias_ty) = match *self_ty.kind() {
             ty::Bool
             | ty::Char
             | ty::Int(_)
@@ -567,23 +587,56 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             | ty::Param(_)
             | ty::Placeholder(..)
             | ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
-            | ty::Alias(ty::Inherent, _)
-            | ty::Alias(ty::Weak, _)
             | ty::Error(_) => return,
-            ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_))
-            | ty::Bound(..) => bug!("unexpected self type for `{goal:?}`"),
-            // Excluding IATs and type aliases here as they don't have meaningful item bounds.
-            ty::Alias(ty::Projection | ty::Opaque, alias_ty) => alias_ty,
+            ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) | ty::Bound(..) => {
+                bug!("unexpected self type for `{goal:?}`")
+            }
+
+            ty::Infer(ty::TyVar(_)) => {
+                // If we hit infer when normalizing the self type of an alias,
+                // then bail with ambiguity. We should never encounter this on
+                // the *first* iteration of this recursive function.
+                if let Ok(result) =
+                    self.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
+                {
+                    candidates.push(Candidate { source: CandidateSource::AliasBound, result });
+                }
+                return;
+            }
+
+            ty::Alias(kind @ (ty::Projection | ty::Opaque), alias_ty) => (kind, alias_ty),
+            ty::Alias(ty::Inherent | ty::Weak, _) => {
+                unreachable!("Weak and Inherent aliases should have been normalized away already")
+            }
         };
 
         for assumption in
             self.tcx().item_bounds(alias_ty.def_id).instantiate(self.tcx(), alias_ty.args)
         {
             match G::consider_alias_bound_candidate(self, goal, assumption) {
                 Ok(result) => {
-                    candidates.push(Candidate { source: CandidateSource::AliasBound, result })
+                    candidates.push(Candidate { source: CandidateSource::AliasBound, result });
+                }
+                Err(NoSolution) => {}
+            }
+        }
+
+        if kind != ty::Projection {
+            return;
+        }
+
+        match self.try_normalize_ty(goal.param_env, alias_ty.self_ty()) {
+            // Recurse on the self type of the projection.
+            Some(next_self_ty) => {
+                self.assemble_alias_bound_candidates_recur(next_self_ty, goal, candidates);
+            }
+            // Bail if we overflow when normalizing, adding an ambiguous candidate.
+            None => {
+                if let Ok(result) =
+                    self.evaluate_added_goals_and_make_canonical_response(Certainty::OVERFLOW)
+                {
+                    candidates.push(Candidate { source: CandidateSource::AliasBound, result });
                 }
-                Err(NoSolution) => (),
             }
         }
     }

compiler/rustc_trait_selection/src/traits/project.rs

@@ -40,6 +40,7 @@ use rustc_middle::ty::{self, Term, ToPredicate, Ty, TyCtxt};
 use rustc_span::symbol::sym;
 
 use std::collections::BTreeMap;
+use std::ops::ControlFlow;
 
 pub use rustc_middle::traits::Reveal;
 
@@ -1614,32 +1615,44 @@ fn assemble_candidates_from_trait_def<'cx, 'tcx>(
     candidate_set: &mut ProjectionCandidateSet<'tcx>,
 ) {
     debug!("assemble_candidates_from_trait_def(..)");
+    let mut ambiguous = false;
+    selcx.for_each_item_bound(
+        obligation.predicate.self_ty(),
+        |selcx, clause, _| {
+            let Some(clause) = clause.as_projection_clause() else {
+                return ControlFlow::Continue(());
+            };
 
-    let tcx = selcx.tcx();
-    // Check whether the self-type is itself a projection.
-    // If so, extract what we know from the trait and try to come up with a good answer.
-    let bounds = match *obligation.predicate.self_ty().kind() {
-        // Excluding IATs and type aliases here as they don't have meaningful item bounds.
-        ty::Alias(ty::Projection | ty::Opaque, ref data) => {
-            tcx.item_bounds(data.def_id).instantiate(tcx, data.args)
-        }
-        ty::Infer(ty::TyVar(_)) => {
-            // If the self-type is an inference variable, then it MAY wind up
-            // being a projected type, so induce an ambiguity.
-            candidate_set.mark_ambiguous();
-            return;
-        }
-        _ => return,
-    };
+            let is_match =
+                selcx.infcx.probe(|_| selcx.match_projection_projections(obligation, clause, true));
 
-    assemble_candidates_from_predicates(
-        selcx,
-        obligation,
-        candidate_set,
-        ProjectionCandidate::TraitDef,
-        bounds.iter(),
-        true,
+            match is_match {
+                ProjectionMatchesProjection::Yes => {
+                    candidate_set.push_candidate(ProjectionCandidate::TraitDef(clause));
+
+                    if !obligation.predicate.has_non_region_infer() {
+                        // HACK: Pick the first trait def candidate for a fully
+                        // inferred predicate. This is to allow duplicates that
+                        // differ only in normalization.
+                        return ControlFlow::Break(());
+                    }
+                }
+                ProjectionMatchesProjection::Ambiguous => {
+                    candidate_set.mark_ambiguous();
+                }
+                ProjectionMatchesProjection::No => {}
+            }
+
+            ControlFlow::Continue(())
+        },
+        // `ProjectionCandidateSet` is borrowed in the above closure,
+        // so just mark ambiguous outside of the closure.
+        || ambiguous = true,
     );
+
+    if ambiguous {
+        candidate_set.mark_ambiguous();
+    }
 }
 
 /// In the case of a trait object like

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -6,13 +6,16 @@
 //!
 //! [rustc dev guide]:https://rustc-dev-guide.rust-lang.org/traits/resolution.html#candidate-assembly
 
+use std::ops::ControlFlow;
+
 use hir::def_id::DefId;
 use hir::LangItem;
+use rustc_data_structures::fx::FxHashSet;
 use rustc_hir as hir;
 use rustc_infer::traits::ObligationCause;
 use rustc_infer::traits::{Obligation, PolyTraitObligation, SelectionError};
 use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
-use rustc_middle::ty::{self, Ty, TypeVisitableExt};
+use rustc_middle::ty::{self, ToPolyTraitRef, Ty, TypeVisitableExt};
 
 use crate::traits;
 use crate::traits::query::evaluate_obligation::InferCtxtExt;
@@ -158,11 +161,50 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             _ => return,
         }
 
-        let result = self
-            .infcx
-            .probe(|_| self.match_projection_obligation_against_definition_bounds(obligation));
+        self.infcx.probe(|_| {
+            let poly_trait_predicate = self.infcx.resolve_vars_if_possible(obligation.predicate);
+            let placeholder_trait_predicate =
+                self.infcx.enter_forall_and_leak_universe(poly_trait_predicate);
+            debug!(?placeholder_trait_predicate);
+
+            // The bounds returned by `item_bounds` may contain duplicates after
+            // normalization, so try to deduplicate when possible to avoid
+            // unnecessary ambiguity.
+            let mut distinct_normalized_bounds = FxHashSet::default();
+            self.for_each_item_bound::<!>(
+                placeholder_trait_predicate.self_ty(),
+                |selcx, bound, idx| {
+                    let Some(bound) = bound.as_trait_clause() else {
+                        return ControlFlow::Continue(());
+                    };
+                    if bound.polarity() != placeholder_trait_predicate.polarity {
+                        return ControlFlow::Continue(());
+                    }
 
-        candidates.vec.extend(result.into_iter().map(|idx| ProjectionCandidate(idx)));
+                    selcx.infcx.probe(|_| {
+                        match selcx.match_normalize_trait_ref(
+                            obligation,
+                            bound.to_poly_trait_ref(),
+                            placeholder_trait_predicate.trait_ref,
+                        ) {
+                            Ok(None) => {
+                                candidates.vec.push(ProjectionCandidate(idx));
+                            }
+                            Ok(Some(normalized_trait))
+                                if distinct_normalized_bounds.insert(normalized_trait) =>
+                            {
+                                candidates.vec.push(ProjectionCandidate(idx));
+                            }
+                            _ => {}
+                        }
+                    });
+
+                    ControlFlow::Continue(())
+                },
+                // On ambiguity.
+                || candidates.ambiguous = true,
+            );
+        });
     }
 
     /// Given an obligation like `<SomeTrait for T>`, searches the obligations that the caller

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -162,20 +162,26 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             self.infcx.enter_forall_and_leak_universe(trait_predicate).trait_ref;
         let placeholder_self_ty = placeholder_trait_predicate.self_ty();
         let placeholder_trait_predicate = ty::Binder::dummy(placeholder_trait_predicate);
-        let (def_id, args) = match *placeholder_self_ty.kind() {
-            // Excluding IATs and type aliases here as they don't have meaningful item bounds.
-            ty::Alias(ty::Projection | ty::Opaque, ty::AliasTy { def_id, args, .. }) => {
-                (def_id, args)
-            }
-            _ => bug!("projection candidate for unexpected type: {:?}", placeholder_self_ty),
-        };
 
-        let candidate_predicate =
-            tcx.item_bounds(def_id).map_bound(|i| i[idx]).instantiate(tcx, args);
+        let candidate_predicate = self
+            .for_each_item_bound(
+                placeholder_self_ty,
+                |_, clause, clause_idx| {
+                    if clause_idx == idx {
+                        ControlFlow::Break(clause)
+                    } else {
+                        ControlFlow::Continue(())
+                    }
+                },
+                || unreachable!(),
+            )
+            .break_value()
+            .expect("expected to index into clause that exists");
         let candidate = candidate_predicate
             .as_trait_clause()
             .expect("projection candidate is not a trait predicate")
             .map_bound(|t| t.trait_ref);
+
         let mut obligations = Vec::new();
         let candidate = normalize_with_depth_to(
             self,
@@ -194,8 +200,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 .map_err(|_| Unimplemented)
         })?);
 
-        if let ty::Alias(ty::Projection, ..) = placeholder_self_ty.kind() {
-            let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, args);
+        // FIXME(compiler-errors): I don't think this is needed.
+        if let ty::Alias(ty::Projection, alias_ty) = placeholder_self_ty.kind() {
+            let predicates = tcx.predicates_of(alias_ty.def_id).instantiate_own(tcx, alias_ty.args);
             for (predicate, _) in predicates {
                 let normalized = normalize_with_depth_to(
                     self,