New format for E0221

Issue #35386

Author: tvladyslav

src/librustc_typeck/astconv.rs

@@ -1285,6 +1285,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> AstConv<'gcx, 'tcx>+'o {
                                            "ambiguous associated type `{}` in bounds of `{}`",
                                            assoc_name,
                                            ty_param_name);
+            err.span_label(span, &format!("ambiguous associated type `{}`", assoc_name));
 
             for bound in &bounds {
                 span_note!(&mut err, span,

src/test/compile-fail/E0221.rs

@@ -18,7 +18,11 @@ trait Foo {
 trait Bar : Foo {
     type A: T2;
     fn do_something() {
-        let _: Self::A; //~ ERROR E0221
+        let _: Self::A;
+        //~^ ERROR E0221
+        //~| NOTE ambiguous associated type `A`
+        //~| NOTE associated type `Self` could derive from `Foo`
+        //~| NOTE associated type `Self` could derive from `Bar`
     }
 }