Note that type param is chosen by caller when suggesting return impl …

…Trait

Revives rust-lang#112088 and rust-lang#104755.

Co-authored-by: sladynnunes <snunes@usc.edu>
Co-authored-by: Thalia Nero <chrisvn00@gmail.com>

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -889,7 +889,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             self.dcx(),
                             errors::ExpectedReturnTypeLabel::Other { span: hir_ty.span, expected },
                         );
-                        self.try_suggest_return_impl_trait(err, expected, ty, fn_id);
+                        self.try_suggest_return_impl_trait(err, expected, found, fn_id);
                         return true;
                     }
                 }
@@ -1010,6 +1010,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             format!("impl {all_bounds_str}"),
             Applicability::MaybeIncorrect,
         );
+
+        err.note(format!(
+            "the caller chooses the type of {} which can be different from {}",
+            expected_ty_as_param.name,
+            found
+        ));
     }
 
     pub(in super::super) fn suggest_missing_break_or_return_expr(

tests/ui/return/return-impl-trait.stderr

@@ -12,6 +12,7 @@ LL |     ()
    |
    = note: expected type parameter `T`
                    found unit type `()`
+   = note: the caller chooses the type of T which can be different from ()
 
 error[E0308]: mismatched types
   --> $DIR/return-impl-trait.rs:23:5
@@ -28,6 +29,7 @@ LL |     ()
    |
    = note: expected type parameter `T`
                    found unit type `()`
+   = note: the caller chooses the type of T which can be different from ()
 
 error: aborting due to 2 previous errors