Like an Atomic<Option<Box<dyn FnOnce + Send + 'static>>>.

This is a barebones concurrency utility that is useful for building larger abstractions on top of. For example, the seg_queue example shows how this can be used to elevant crossbeam's SegQueue (a concurrent queue which does not support blocking, only polling) into an mpsc queue which supports both blocking and async/await:

pub struct Sender<T>(Arc<State<T>>);

pub struct Receiver<T>(Arc<State<T>>);

struct State<T> {
    queue: SegQueue<T>,
    callback_cell: CallbackCell,
}

fn new_queue<T>() -> (Sender<T>, Receiver<T>) {
    let state_1 = Arc::new(State {
        queue: SegQueue::new(),
        callback_cell: CallbackCell::new(),
    });
    let state_2 = Arc::clone(&state_1);
    (Sender(state_1), Receiver(state_2))
}

impl<T> Sender<T> {
    fn send(&self, item: T) {
        self.0.queue.push(item);
        self.0.callback_cell.take_call();
    }
}

impl<T> Receiver<T> {
    fn recv_blocking(&mut self) -> T {
        if let Some(item) = self.0.queue.pop() {
            return item;
        }
        let parker = Parker::new();
        loop {
            let unparker = parker.unparker().clone();
            self.0.callback_cell.put(move || unparker.unpark());
            if let Some(item) = self.0.queue.pop() {
                return item;
            }
            parker.park();
        }
    }

    async fn recv_async(&mut self) -> T {
        if let Some(item) = self.0.queue.pop() {
            return item;
        }
        let notify_1 = Arc::new(Notify::new());
        loop {
            let notify_2 = Arc::clone(&notify_1);
            self.0.callback_cell.put(move || notify_2.notify_one());
            if let Some(item) = self.0.queue.pop() {
                return item;
            }
            notify_1.notified().await;
        }
    }
}

A naive way of implementing this would involve two layers of indirection:

• First, the FnOnce could be boxed into a Box<dyn FnOnce>, achieving dynamic dispatch.
• Then, that could be boxed into a Box<Box<dyn FnOnce>>, making it a normal pointer rather than a fat pointer.
• That outer Box could be converted into a raw pointer and then into a usize and stored in an AtomicUsize.

This utility, however, does this in only one heap allocation rather than two, through slightly clever usage of monomorphization and the std::alloc API.