error: future cannot be sent between threads safely
  --> tests/ui/send-not-implemented.rs:8:5
   |
 8 |     async fn test(&self) {
   |     ^^^^^ future created by async block is not `Send`
   |
   = help: within `{async block@$DIR/tests/ui/send-not-implemented.rs:8:5: 8:10}`, the trait `Send` is not implemented for `std::sync::MutexGuard<'_, ()>`
note: future is not `Send` as this value is used across an await
  --> tests/ui/send-not-implemented.rs:11:13
   |
10 |         let _guard = mutex.lock().unwrap();
   |             ------ has type `std::sync::MutexGuard<'_, ()>` which is not `Send`
11 |         f().await;
   |             ^^^^^ await occurs here, with `_guard` maybe used later
   = note: required for the cast from `Pin<Box<{async block@$DIR/tests/ui/send-not-implemented.rs:8:5: 8:10}>>` to `Pin<Box<dyn Future<Output = ()> + Send>>`

error: future cannot be sent between threads safely
  --> tests/ui/send-not-implemented.rs:14:5
   |
14 |     async fn test_ret(&self) -> bool {
   |     ^^^^^ future created by async block is not `Send`
   |
   = help: within `{async block@$DIR/tests/ui/send-not-implemented.rs:14:5: 14:10}`, the trait `Send` is not implemented for `std::sync::MutexGuard<'_, ()>`
note: future is not `Send` as this value is used across an await
  --> tests/ui/send-not-implemented.rs:17:13
   |
16 |         let _guard = mutex.lock().unwrap();
   |             ------ has type `std::sync::MutexGuard<'_, ()>` which is not `Send`
17 |         f().await;
   |             ^^^^^ await occurs here, with `_guard` maybe used later
   = note: required for the cast from `Pin<Box<{async block@$DIR/tests/ui/send-not-implemented.rs:14:5: 14:10}>>` to `Pin<Box<dyn Future<Output = bool> + Send>>`