Implement `poll_read` and `poll_write` for `UdpSocket`

[Demi-Marie]

Currently, it is difficult to use a UdpSocket in an AsyncWrite implementation, as it does not expose any low-level methods that return Poll.

[Demi-Marie]

Would a high-quality PR implementing this be accepted?

We don't really expose any poll_* methods. But note that you can poll futures returned by read() and write() -- would something like this work for you?

Pin::new(&mut socket.read(buf)).poll(cx);

[yoshuawuyts]

Related PR: #636

[Demi-Marie]

We don't really expose any poll_* methods. But note that you can poll futures returned by read() and write() -- would something like this work for you?

Pin::new(&mut socket.read(buf)).poll(cx);

It does not compile because the return value of socket.read is not Unpin.

[tomaka]

Something like that should work:

let mut fut = socket.read(buf);
futures::pin_mut!(fut);
fut.poll(cx)

[Demi-Marie]

@tomaka thanks!