error: future cannot be sent between threads safely
  --> $DIR/send-not-implemented.rs:8:26
   |
8  |       async fn test(&self) {
   |  __________________________^
9  | |         let mutex = Mutex::new(());
10 | |         let _guard = mutex.lock().unwrap();
11 | |         f().await;
12 | |     }
   | |_____^ future created by async block is not `Send`
   |
   = help: within `impl Future`, the trait `Send` is not implemented for `MutexGuard<'_, ()>`
note: future is not `Send` as this value is used across an await
  --> $DIR/send-not-implemented.rs:11:9
   |
10 |         let _guard = mutex.lock().unwrap();
   |             ------ has type `MutexGuard<'_, ()>` which is not `Send`
11 |         f().await;
   |         ^^^^^^^^^ await occurs here, with `_guard` maybe used later
12 |     }
   |     - `_guard` is later dropped here
   = note: required for the cast to the object type `dyn Future<Output = ()> + Send`