Tracking Issue for `poll_ready`

[djc]

This is a tracking issue for Poll::ready() (PR #72994).
The feature gate for the issue is #![feature(poll_ready)].

Steps

• Implement the API
• Adjust documentation (see instructions on rustc-dev-guide) [not applicable?]
• Stabilization PR (see instructions on rustc-dev-guide)

Rationale: I have this code in Quinn:

.filter_map(|(i, x)| match Pin::new(x.as_mut().unwrap()).poll(cx) {
    Poll::Ready(y) => {
        *x = None;
        Some((i, y))
    }
    Poll::Pending => None,
})

And this would seem quite a bit nicer:

.filter_map(|(i, x)| {
    Pin::new(x.as_mut().unwrap()).poll(cx).ready().map(|y| {
        *x = None;
        (i, y)
    })
})

One potential downside is that this adds one more ready() after futures::ready!() and futures::future::ready(). As it is a method (compared to a free function or a macro), it seems it's still clearly distinct, but could be confusing. It otherwise seems to be the idiomatic naming choice though.

[KodrAus]

The PR that implemented this feature has been closed (just from inactivity). Since there isn't a feature to track here I'll go ahead and close this issue, but if there's a follow-up to #72994 that comes back then let's re-open this!