`async`: `send_timeout` & `send_deadline`

[NathanSWard]

Currently send_timeout/deadline are implemented via blocking. This means if I want to make an async variant of these two, I need to use another library such as tokio::time::timeout - which notably is OK.

However, the problem comes in when the timeout happens before the value is successfully sent on the channel. There is no way to retrieve the queued message from the SendFut.

let (tx, rx) = bounded(n);

let mut fut = tx.send_async(42);
let not_sent = match tokio::time::timeout(DURATION, &mut fut).await {
    Ok(Ok(())) => {  None /* value was sent */ },
    Ok(Err(flume::SendError(value))) => { Some(value) /* channel was closed/disconnected */ },
    Err(_) => { 
        // timeout happened but there is no way to get the value back out of SendFut
        fut.into_inner().unwrap() // maybe something like `fut.into_inner()` ??
    }
};

Ideally SendFut would have an into_inner(self) -> Option<T> that would return Some(T) if the value was not yet send successfully.

[zesterer]

Yes, this is definitely missing functionality. I think this would be relatively easy to implement: internally we could just use a regular old async timeout (tokio or otherwise) and then use self.hook.take() to pull the value out of the future. Would you be interested in implementing this?

[NathanSWard]

Sure, I'll take a stab at the implementation and post a PR for it.

[NathanSWard]

See here for the PR :)