pub(crate) unsafe fn complete(&self) {
        self.status.store(COMPLETE, SeqCst);
    }
Alert the mutex that the task has completed execution and should not be
notified again.

# Safety

Callable only from the `POLLING`/`REPOLL` states, i.e. between
successful calls to `notify` and `wait`/`complete`.
futures-executor/src/unpark_mutex.rs