error: future cannot be sent between threads safely --> $DIR/issue-71137.rs:20:3 | LL | fn fake_spawn(f: F) { } | ---- required by this bound in `fake_spawn` ... LL | fake_spawn(wrong_mutex()); | ^^^^^^^^^^ future returned by `wrong_mutex` is not `Send` | = help: within `impl Future`, the trait `Send` is not implemented for `MutexGuard<'_, i32>` note: future is not `Send` as this value is used across an await --> $DIR/issue-71137.rs:12:5 | LL | let mut guard = m.lock().unwrap(); | --------- has type `MutexGuard<'_, i32>` which is not `Send` LL | (async { "right"; }).await; | ^^^^^^^^^^^^^^^^^^^^^^^^^^ await occurs here, with `mut guard` maybe used later LL | *guard += 1; LL | } | - `mut guard` is later dropped here error: aborting due to previous error