Address review comments

3 files changed, 22 insertions, 7 deletions

diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs
index a8d75ba223a..a59520f16fe 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs
@@ -21,7 +21,8 @@ use tracing::{debug, instrument};
 use super::errors::GenericsArgsErrExtend;
 use crate::errors;
 use crate::hir_ty_lowering::{
-    AssocItemQSelf, FeedConstTy, HirTyLowerer, PredicateFilter, RegionInferReason,
+    AssocItemQSelf, FeedConstTy, HirTyLowerer, OverlappingAsssocItemConstraints, PredicateFilter,
+    RegionInferReason,
 };
 
 #[derive(Debug, Default)]
@@ -362,7 +363,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         param_ty,
                         bounds,
                         predicate_filter,
-                        false,
+                        OverlappingAsssocItemConstraints::Allowed,
                     );
                 }
                 hir::GenericBound::Outlives(lifetime) => {
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_trait.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_trait.rs
index a4179776572..c0b13773089 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_trait.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_trait.rs
@@ -23,7 +23,9 @@ use tracing::{debug, instrument};
 
 use super::HirTyLowerer;
 use crate::errors::SelfInTypeAlias;
-use crate::hir_ty_lowering::{GenericArgCountMismatch, PredicateFilter, RegionInferReason};
+use crate::hir_ty_lowering::{
+    GenericArgCountMismatch, OverlappingAsssocItemConstraints, PredicateFilter, RegionInferReason,
+};
 
 impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     /// Lower a trait object type from the HIR to our internal notion of a type.
@@ -60,7 +62,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 dummy_self,
                 &mut user_written_bounds,
                 PredicateFilter::SelfOnly,
-                true,
+                OverlappingAsssocItemConstraints::Forbidden,
             );
             if let Err(GenericArgCountMismatch { invalid_args, .. }) = result.correct {
                 potential_assoc_types.extend(invalid_args);
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
index 0ff1fabd7b3..cc5c0d0ad6c 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
@@ -332,6 +332,15 @@ pub(crate) enum GenericArgPosition {
     MethodCall,
 }
 
+/// Whether to allow duplicate associated iten constraints in a trait ref, e.g.
+/// `Trait<Assoc = Ty, Assoc = Ty>`. This is forbidden in `dyn Trait<...>`
+/// but allowed everywhere else.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub(crate) enum OverlappingAsssocItemConstraints {
+    Allowed,
+    Forbidden,
+}
+
 /// A marker denoting that the generic arguments that were
 /// provided did not match the respective generic parameters.
 #[derive(Clone, Debug)]
@@ -752,7 +761,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         self_ty: Ty<'tcx>,
         bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
         predicate_filter: PredicateFilter,
-        for_dyn: bool,
+        overlapping_assoc_item_constraints: OverlappingAsssocItemConstraints,
     ) -> GenericArgCountResult {
         let tcx = self.tcx();
 
@@ -909,7 +918,10 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             }
         }
 
-        let mut dup_constraints = FxIndexMap::default();
+        let mut dup_constraints = (overlapping_assoc_item_constraints
+            == OverlappingAsssocItemConstraints::Forbidden)
+            .then_some(FxIndexMap::default());
+
         for constraint in trait_segment.args().constraints {
             // Don't register any associated item constraints for negative bounds,
             // since we should have emitted an error for them earlier, and they
@@ -928,7 +940,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 poly_trait_ref,
                 constraint,
                 bounds,
-                for_dyn.then_some(&mut dup_constraints),
+                dup_constraints.as_mut(),
                 constraint.span,
                 predicate_filter,
             );