sync: add notify method to Receiver from watch module

[govardhangdg]

Motivation

Attempt to solve #2404

Solution

Modified recv method to return Option<()> instead of Option< T >
I have added documentation test.

[Darksonn]

We should consider if Option<()> is the best return value here. Additionally, as I mentioned in #2404, I'm not convinced that notify is the best name for the function.

[Darksonn]

Huh, apparently it doesn't compile if you move the map outside due to the lifetime on the reference. I guess you should move it inside again.

I've thought a bit more about the Option<()> thing, and realized that we can totally do this:

pub async fn recv_ref<'a>(&'a mut self) -> Option<Ref<'a, T>> {
    let has_next = poll_fn(|cx| self.poll_recv_ref(cx).map(|opt| opt.is_some())).await;
    if has_next {
        Some(self.borrow())
    } else {
        None
    }
}

This seems like a better approach than simply waiting.

[carllerche]

We probably want to name this notified to match closed.

Then, it would be used as: rx.notified().await.

[Darksonn]

Are you interested in getting this PR across the finish line?

[carllerche]

This has been done in #2814. Thanks for the proposal and getting the work started 👍