Handle RTN projections in assoc type restriction diagnostics

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -6074,7 +6074,16 @@ fn point_at_assoc_type_restriction<G: EmissionGuarantee>(
     let ty::ClauseKind::Projection(proj) = clause else {
         return;
     };
-    let name = tcx.item_name(proj.projection_term.def_id);
+    let Some(name) = tcx
+        .opt_rpitit_info(proj.projection_term.def_id)
+        .and_then(|data| match data {
+            ty::ImplTraitInTraitData::Trait { fn_def_id, .. } => Some(tcx.item_name(fn_def_id)),
+            ty::ImplTraitInTraitData::Impl { .. } => None,
+        })
+        .or_else(|| tcx.opt_item_name(proj.projection_term.def_id))
+    else {
+        return;
+    };
     let mut predicates = generics.predicates.iter().peekable();
     let mut prev: Option<(&hir::WhereBoundPredicate<'_>, Span)> = None;
     while let Some(pred) = predicates.next() {

tests/ui/associated-type-bounds/return-type-notation/return-type-notation-where-clause-doesnt-apply.rs

@@ -0,0 +1,18 @@
+// Regression test for https://github.com/rust-lang/rust/issues/152887
+//@ edition: 2024
+
+#![feature(return_type_notation)]
+
+pub trait Trait {
+    async fn func();
+}
+
+impl<T: Trait<func(..): Send>> Trait for T {}
+//~^ ERROR not all trait items implemented, missing: `func`
+
+fn check(_: impl Trait) {}
+
+fn main() {
+    check(());
+    //~^ ERROR overflow evaluating the requirement
+}

tests/ui/associated-type-bounds/return-type-notation/return-type-notation-where-clause-doesnt-apply.stderr

@@ -0,0 +1,34 @@
+error[E0046]: not all trait items implemented, missing: `func`
+  --> $DIR/return-type-notation-where-clause-doesnt-apply.rs:10:1
+   |
+LL |     async fn func();
+   |     ---------------- `func` from trait
+...
+LL | impl<T: Trait<func(..): Send>> Trait for T {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `func` in implementation
+
+error[E0275]: overflow evaluating the requirement `impl Future<Output = ()> { <() as Trait>::func(..) } == _`
+  --> $DIR/return-type-notation-where-clause-doesnt-apply.rs:16:5
+   |
+LL |     check(());
+   |     ^^^^^^^^^
+   |
+note: required for `()` to implement `Trait`
+  --> $DIR/return-type-notation-where-clause-doesnt-apply.rs:10:32
+   |
+LL | impl<T: Trait<func(..): Send>> Trait for T {}
+   |                         ----   ^^^^^     ^
+   |                         |
+   |                         unsatisfied trait bound introduced here
+   = note: 1 redundant requirement hidden
+   = note: required for `()` to implement `Trait`
+note: required by a bound in `check`
+  --> $DIR/return-type-notation-where-clause-doesnt-apply.rs:13:18
+   |
+LL | fn check(_: impl Trait) {}
+   |                  ^^^^^ required by this bound in `check`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0046, E0275.
+For more information about an error, try `rustc --explain E0046`.