Don't suggest replacing closure parameter with type name

When a closure has an inferred parameter type like `|ch|` and the
expected type differs in borrowing (e.g., `char` vs `&char`), the
suggestion code would incorrectly suggest `|char|` instead of the
valid `|ch: char|`.

This happened because the code couldn't walk explicit `&` references
in the HIR when the type is inferred, and fell back to replacing the
entire parameter span with the expected type name.

Fix by only emitting the suggestion when we can properly identify the
`&` syntax to remove.

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -5323,12 +5323,9 @@ fn hint_missing_borrow<'tcx>(
                     ty = mut_ty.ty;
                     left -= 1;
                 }
-                let sugg = if left == 0 {
-                    (span, String::new())
-                } else {
-                    (arg.span, expected_arg.to_string())
-                };
-                remove_borrow.push(sugg);
+                if left == 0 {
+                    remove_borrow.push((span, String::new()));
+                }
             }
         }
     }

tests/ui/closures/multiple-fn-bounds.stderr

@@ -19,11 +19,6 @@ note: required by a bound in `foo`
    |
 LL | fn foo<F: Fn(&char) -> bool + Fn(char) -> bool>(f: F) {
    |                               ^^^^^^^^^^^^^^^^ required by this bound in `foo`
-help: consider adjusting the signature so it does not borrow its argument
-   |
-LL -     foo(move |x| v);
-LL +     foo(move |char| v);
-   |
 
 error: aborting due to 1 previous error