pub fn take(&self) -> Option<Waker> {
        // AcqRel ordering is used in order to acquire the value of the `task`
        // cell as well as to establish a `release` ordering with whatever
        // memory the `AtomicWaker` is associated with.
        match self.state.fetch_or(WAKING, AcqRel) {
            WAITING => {
                // The waking lock has been acquired.
                let waker = unsafe { (*self.waker.get()).take() };

                // Release the lock
                self.state.fetch_and(!WAKING, Release);

                waker
            }
            state => {
                // There is a concurrent thread currently updating the
                // associated task.
                //
                // Nothing more to do as the `WAKING` bit has been set. It
                // doesn't matter if there are concurrent registering threads or
                // not.
                //
                debug_assert!(
                    state == REGISTERING || state == REGISTERING | WAKING || state == WAKING
                );
                None
            }
        }
    }
Returns the last `Waker` passed to `register`, so that the user can wake it.


Sometimes, just waking the AtomicWaker is not fine grained enough. This allows the user
to take the waker and then wake it separately, rather than performing both steps in one
atomic action.

If a waker has not been registered, this returns `None`.
futures-core/src/task/__internal/atomic_waker.rs