address review comments

compiler/rustc_ast/src/ast.rs

@@ -1884,6 +1884,16 @@ impl Clone for Ty {
     }
 }
 
+impl Ty {
+    pub fn peel_refs(&self) -> &Self {
+        let mut final_ty = self;
+        while let TyKind::Rptr(_, MutTy { ty, .. }) = &final_ty.kind {
+            final_ty = &ty;
+        }
+        final_ty
+    }
+}
+
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct BareFnTy {
     pub unsafety: Unsafe,

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -442,7 +442,11 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
 
         if !self.type_ascription_suggestion(&mut err, base_span) {
             let mut fallback = false;
-            if let PathSource::Trait(AliasPossibility::Maybe) = source {
+            if let (
+                PathSource::Trait(AliasPossibility::Maybe),
+                Some(Res::Def(DefKind::Struct | DefKind::Enum | DefKind::Union, _)),
+            ) = (source, res)
+            {
                 if let Some(bounds @ [_, .., _]) = self.diagnostic_metadata.current_trait_object {
                     fallback = true;
                     let spans: Vec<Span> = bounds
@@ -580,7 +584,7 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
             return false;
         };
 
-        if let ast::TyKind::Path(None, type_param_path) = &ty.kind {
+        if let ast::TyKind::Path(None, type_param_path) = &ty.peel_refs().kind {
             // Confirm that the `SelfTy` is a type parameter.
             let partial_res = if let Ok(Some(partial_res)) = self.resolve_qpath_anywhere(
                 bounded_ty.id,
@@ -603,20 +607,24 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
                 return false;
             }
             if let (
-                [ast::PathSegment { ident, args: None, .. }],
+                [ast::PathSegment { ident: constrain_ident, args: None, .. }],
                 [ast::GenericBound::Trait(poly_trait_ref, ast::TraitBoundModifier::None)],
             ) = (&type_param_path.segments[..], &bounds[..])
             {
-                if let [ast::PathSegment { ident: bound_ident, args: None, .. }] =
+                if let [ast::PathSegment { ident, args: None, .. }] =
                     &poly_trait_ref.trait_ref.path.segments[..]
                 {
-                    if bound_ident.span == span {
+                    if ident.span == span {
                         err.span_suggestion_verbose(
                             *where_span,
-                            &format!("constrain the associated type to `{}`", bound_ident),
+                            &format!("constrain the associated type to `{}`", ident),
                             format!(
                                 "{}: {}<{} = {}>",
-                                ident,
+                                self.r
+                                    .session
+                                    .source_map()
+                                    .span_to_snippet(ty.span) // Account for `<&'a T as Foo>::Bar`.
+                                    .unwrap_or_else(|_| constrain_ident.to_string()),
                                 path.segments[..*position]
                                     .iter()
                                     .map(|segment| path_segment_to_string(segment))
@@ -627,7 +635,7 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
                                     .map(|segment| path_segment_to_string(segment))
                                     .collect::<Vec<_>>()
                                     .join("::"),
-                                bound_ident,
+                                ident,
                             ),
                             Applicability::MaybeIncorrect,
                         );

src/test/ui/traits/assoc_type_bound_with_struct.rs

@@ -6,4 +6,14 @@ struct Foo<T> where T: Bar, <T as Bar>::Baz: String { //~ ERROR expected trait,
     t: T,
 }
 
+struct Qux<'a, T> where T: Bar, <&'a T as Bar>::Baz: String { //~ ERROR expected trait, found struct
+    t: &'a T,
+}
+
+fn foo<T: Bar>(_: T) where <T as Bar>::Baz: String { //~ ERROR expected trait, found struct
+}
+
+fn qux<'a, T: Bar>(_: &'a T) where <&'a T as Bar>::Baz: String { //~ ERROR expected trait, found
+}
+
 fn main() {}

src/test/ui/traits/assoc_type_bound_with_struct.stderr

@@ -18,6 +18,66 @@ help: a trait with a similar name exists
 LL | struct Foo<T> where T: Bar, <T as Bar>::Baz: ToString {
    |                                              ^^^^^^^^
 
-error: aborting due to previous error
+error[E0404]: expected trait, found struct `String`
+  --> $DIR/assoc_type_bound_with_struct.rs:9:54
+   |
+LL | struct Qux<'a, T> where T: Bar, <&'a T as Bar>::Baz: String {
+   |                                                      ^^^^^^ not a trait
+   | 
+  ::: $SRC_DIR/alloc/src/string.rs:LL:COL
+   |
+LL | pub trait ToString {
+   | ------------------ similarly named trait `ToString` defined here
+   |
+help: constrain the associated type to `String`
+   |
+LL | struct Qux<'a, T> where T: Bar, &'a T: Bar<Baz = String> {
+   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^
+help: a trait with a similar name exists
+   |
+LL | struct Qux<'a, T> where T: Bar, <&'a T as Bar>::Baz: ToString {
+   |                                                      ^^^^^^^^
+
+error[E0404]: expected trait, found struct `String`
+  --> $DIR/assoc_type_bound_with_struct.rs:13:45
+   |
+LL | fn foo<T: Bar>(_: T) where <T as Bar>::Baz: String {
+   |                                             ^^^^^^ not a trait
+   | 
+  ::: $SRC_DIR/alloc/src/string.rs:LL:COL
+   |
+LL | pub trait ToString {
+   | ------------------ similarly named trait `ToString` defined here
+   |
+help: constrain the associated type to `String`
+   |
+LL | fn foo<T: Bar>(_: T) where T: Bar<Baz = String> {
+   |                            ^^^^^^^^^^^^^^^^^^^^
+help: a trait with a similar name exists
+   |
+LL | fn foo<T: Bar>(_: T) where <T as Bar>::Baz: ToString {
+   |                                             ^^^^^^^^
+
+error[E0404]: expected trait, found struct `String`
+  --> $DIR/assoc_type_bound_with_struct.rs:16:57
+   |
+LL | fn qux<'a, T: Bar>(_: &'a T) where <&'a T as Bar>::Baz: String {
+   |                                                         ^^^^^^ not a trait
+   | 
+  ::: $SRC_DIR/alloc/src/string.rs:LL:COL
+   |
+LL | pub trait ToString {
+   | ------------------ similarly named trait `ToString` defined here
+   |
+help: constrain the associated type to `String`
+   |
+LL | fn qux<'a, T: Bar>(_: &'a T) where &'a T: Bar<Baz = String> {
+   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^
+help: a trait with a similar name exists
+   |
+LL | fn qux<'a, T: Bar>(_: &'a T) where <&'a T as Bar>::Baz: ToString {
+   |                                                         ^^^^^^^^
+
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0404`.