Add FutureExt::also_poll

[coolreader18]

trait FutureExt {
    fn also_poll<Also>(self, also: Also) -> AlsoPoll<Self, Also> // Output = Self::Output
    where
        Self: Sized,
        Also: Future<Output = ()>,
    { // AlsoPoll pseudocode:
        let mut also = also.fuse();
        poll_fn(|cx| {
            let _ = also.poll(cx);
            self.poll(cx)
        })
    }
}

I've found this sort of function helpful when trying to avoid a Barbara Battles Buffered Streams type situation; specifically in my case, when you have a select-loop and one of the branches needs to await, but you don't want to starve something from another branch (pseudocode):

let mut fut = foo();
let mut stream = bar();
loop {
    select! {
        x = &mut fut => {
            do_thing_with(x);
            fut = foo();
        }
        y = stream.next() => {
            handle_stream_item(y).also_poll(&mut fut).await;
        }
    }
}

Without that .also_poll(&mut fut), fut would be starved of polls for the duration of handle_stream_item, potentially causing deadlocks or very long delays through the rest of the system if fut was queued to receive a semaphore permit (e.g. a mutex, a mpsc send() permit) because now it can't release it until handle_stream_item is done. That was my specific use case, at least, but I feel that this is a valuable primitive to have in general as folks start to look at stuff like Stream::poll_progress for combating BBBS (.also_poll(poll_fn(|cx| foo.poll_progress(cx))) is I feel a quintessential potential use case for this function).

Definitely open to naming suggestions, I feel like as it is it might not be super intuitive, but I do like how foo.also_poll(bar).await reads.

[coolreader18]

Not sure whether the base should be master or 0.3 - it's compatible with either but maybe 0.3 is just for bugfixes now?

[taiki-e]

Not sure whether the base should be master or 0.3

The base should bemaster. #2813