Fix span when suggesting to add an associated type bound

compiler/rustc_middle/src/ty/error.rs

@@ -628,6 +628,7 @@ impl<T> Trait<T> for X {
                             assoc_substs,
                             ty,
                             msg,
+                            false,
                         ) {
                             return true;
                         }
@@ -646,6 +647,7 @@ impl<T> Trait<T> for X {
                             assoc_substs,
                             ty,
                             msg,
+                            false,
                         );
                     }
                 }
@@ -771,13 +773,24 @@ fn foo(&self) -> Self::T { String::new() }
     ) -> bool {
         let assoc = self.associated_item(proj_ty.item_def_id);
         if let ty::Opaque(def_id, _) = *proj_ty.self_ty().kind() {
-            self.constrain_associated_type_structured_suggestion(
+            let opaque_local_def_id = def_id.expect_local();
+            let opaque_hir_id = self.hir().local_def_id_to_hir_id(opaque_local_def_id);
+            let opaque_hir_ty = match &self.hir().expect_item(opaque_hir_id).kind {
+                hir::ItemKind::OpaqueTy(opaque_hir_ty) => opaque_hir_ty,
+                _ => bug!("The HirId comes from a `ty::Opaque`"),
+            };
+
+            let (trait_ref, assoc_substs) = proj_ty.trait_ref_and_own_substs(self);
+
+            self.constrain_generic_bound_associated_type_structured_suggestion(
                 db,
-                self.def_span(def_id),
-                &assoc,
-                proj_ty.trait_ref_and_own_substs(self).1,
+                &trait_ref,
+                opaque_hir_ty.bounds,
+                assoc,
+                assoc_substs,
                 ty,
-                &msg,
+                msg,
+                true,
             )
         } else {
             false
@@ -899,6 +912,11 @@ fn foo(&self) -> Self::T { String::new() }
 
     /// Given a slice of `hir::GenericBound`s, if any of them corresponds to the `trait_ref`
     /// requirement, provide a structured suggestion to constrain it to a given type `ty`.
+    ///
+    /// `is_bound_surely_present` indicates whether we know the bound we're looking for is
+    /// inside `bounds`. If that's the case then we can consider `bounds` containing only one
+    /// trait bound as the one we're looking for. This can help in cases where the associated
+    /// type is defined on a supertrait of the one present in the bounds.
     fn constrain_generic_bound_associated_type_structured_suggestion(
         self,
         db: &mut DiagnosticBuilder<'_>,
@@ -908,23 +926,30 @@ fn foo(&self) -> Self::T { String::new() }
         assoc_substs: &[ty::GenericArg<'tcx>],
         ty: Ty<'tcx>,
         msg: &str,
+        is_bound_surely_present: bool,
     ) -> bool {
         // FIXME: we would want to call `resolve_vars_if_possible` on `ty` before suggesting.
-        bounds.iter().any(|bound| match bound {
-            hir::GenericBound::Trait(ptr, hir::TraitBoundModifier::None) => {
-                // Relate the type param against `T` in `<A as T>::Foo`.
-                ptr.trait_ref.trait_def_id() == Some(trait_ref.def_id)
-                    && self.constrain_associated_type_structured_suggestion(
-                        db,
-                        ptr.span,
-                        assoc,
-                        assoc_substs,
-                        ty,
-                        msg,
-                    )
-            }
-            _ => false,
-        })
+
+        let trait_bounds = bounds.iter().filter_map(|bound| match bound {
+            hir::GenericBound::Trait(ptr, hir::TraitBoundModifier::None) => Some(ptr),
+            _ => None,
+        });
+
+        let matching_trait_bounds = trait_bounds
+            .clone()
+            .filter(|ptr| ptr.trait_ref.trait_def_id() == Some(trait_ref.def_id))
+            .collect::<Vec<_>>();
+
+        let span = match &matching_trait_bounds[..] {
+            &[ptr] => ptr.span,
+            &[] if is_bound_surely_present => match &trait_bounds.collect::<Vec<_>>()[..] {
+                &[ptr] => ptr.span,
+                _ => return false,
+            },
+            _ => return false,
+        };
+
+        self.constrain_associated_type_structured_suggestion(db, span, assoc, assoc_substs, ty, msg)
     }
 
     /// Given a span corresponding to a bound, provide a structured suggestion to set an