Add a note when suggesting to use Impl Trait

Add tests

Update compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

Co-authored-by: Lukas <[email protected]>

Fixed tests

Author: sladyn98

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -856,6 +856,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             format!("impl {}", all_bounds_str),
             Applicability::MaybeIncorrect,
         );
+        err.note(format!("the type of `{}` is chosen by the caller", expected_ty_as_param.name));
     }
 
     pub(in super::super) fn suggest_missing_break_or_return_expr(

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

@@ -53,6 +53,7 @@ LL |     "don't suggest this, because the generic param is used in the bound."
    |
    = note: expected type parameter `T`
                    found reference `&'static str`
+   = note: the type of `T` is chosen by the caller
 
 error: aborting due to 4 previous errors
 

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

@@ -12,6 +12,7 @@ LL |     ()
    |
    = note: expected type parameter `T`
                    found unit type `()`
+   = note: the type of `T` is chosen by the caller
 
 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 type of `T` is chosen by the caller
 
 error: aborting due to 2 previous errors