Make nightly_error_messages feature compatible with latest nightly

axum-macros/tests/debug_handler/fail/not_send.stderr

@@ -4,7 +4,7 @@ error: future cannot be sent between threads safely
 3 | #[debug_handler]
   | ^^^^^^^^^^^^^^^^ future returned by `handler` is not `Send`
   |
-  = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
+  = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`, which is required by `impl Future<Output = ()>: Send`
 note: future is not `Send` as this value is used across an await
  --> tests/debug_handler/fail/not_send.rs:6:14
   |
@@ -12,8 +12,6 @@ note: future is not `Send` as this value is used across an await
   |         --- has type `Rc<()>` which is not `Send`
 6 |     async {}.await;
   |              ^^^^^ await occurs here, with `_rc` maybe used later
-7 | }
-  | - `_rc` is later dropped here
 note: required by a bound in `check`
  --> tests/debug_handler/fail/not_send.rs:3:1
   |