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Add Future::poll_once #92116

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Add Future::poll_once #92116

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27 changes: 27 additions & 0 deletions library/core/src/future/future.rs
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Original file line number Diff line number Diff line change
@@ -1,5 +1,6 @@
#![stable(feature = "futures_api", since = "1.36.0")]

use crate::future::PollOnce;
use crate::marker::Unpin;
use crate::ops;
use crate::pin::Pin;
Expand Down Expand Up @@ -101,6 +102,32 @@ pub trait Future {
#[lang = "poll"]
#[stable(feature = "futures_api", since = "1.36.0")]
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>;

/// Poll a future once.
///
/// # Examples
///
/// ```rust
/// #![feature(future_poll_once)]
///
/// use std::future::{self, Future};
/// use std::task::Poll;
///
/// # let _ = async {
/// let f = future::ready(1);
/// assert_eq!(f.poll_once().await, Poll::Ready(1));
///
/// let mut f = future::pending::<()>();
/// assert_eq!(f.poll_once().await, Poll::Pending);
/// # };
/// ```
#[unstable(feature = "future_poll_once", issue = "92115")]
fn poll_once(self) -> PollOnce<Self>
where
Self: Sized,
{
PollOnce { future: self }
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@danielhenrymantilla danielhenrymantilla Jan 31, 2022 •
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Suggested change
PollOnce { future: self }
PollOnce { future: Some(self) }

}
}

#[stable(feature = "futures_api", since = "1.36.0")]
Expand Down
4 changes: 4 additions & 0 deletions library/core/src/future/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,7 @@ mod future;
mod into_future;
mod pending;
mod poll_fn;
mod poll_once;
mod ready;

#[stable(feature = "futures_api", since = "1.36.0")]
Expand All @@ -29,6 +30,9 @@ pub use ready::{ready, Ready};
#[unstable(feature = "future_poll_fn", issue = "72302")]
pub use poll_fn::{poll_fn, PollFn};

#[unstable(feature = "future_poll_once", issue = "92115")]
pub use poll_once::PollOnce;

/// This type is needed because:
///
/// a) Generators cannot implement `for<'a, 'b> Generator<&'a mut Context<'b>>`, so we need to pass
Expand Down
32 changes: 32 additions & 0 deletions library/core/src/future/poll_once.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,32 @@
use crate::fmt;
use crate::future::Future;
use crate::pin::Pin;
use crate::task::{Context, Poll};

/// Resolves to the output of polling a future once.
///
/// This `struct` is created by [`poll_once()`]. See its
/// documentation for more.
#[unstable(feature = "future_poll_once", issue = "92115")]
pub struct PollOnce<F> {
pub(crate) future: F,
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@danielhenrymantilla danielhenrymantilla Jan 31, 2022 •
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Idea to support ?Unpin:

Suggested change
pub(crate) future: F,
pub(crate) future: Option<F>,

(and a impl<F> Unpin for PollOnce<F>)

}

#[unstable(feature = "future_poll_once", issue = "92115")]
impl<F> fmt::Debug for PollOnce<F> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_tuple("PollOnce").finish()
}
}

#[unstable(feature = "future_poll_once", issue = "92115")]
impl<F> Future for PollOnce<F>
where
F: Future + Unpin,
{
type Output = Poll<F::Output>;

fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
Poll::Ready(Pin::new(&mut self.future).poll(cx))
}
Comment on lines +25 to +31
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@danielhenrymantilla danielhenrymantilla Jan 31, 2022 •
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Suggested change
F: Future + Unpin,
{
type Output = Poll<F::Output>;
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
Poll::Ready(Pin::new(&mut self.future).poll(cx))
}
F: Future,
{
type Output = Poll<F::Output>;
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let future = pin!(self.future.take().expect("Polled after completion"));
Poll::Ready(future.poll(cx))
}

If the dependency on a macro is deemed unsatisfactory to begin with, we could always hand-roll a Pin::new_unchecked(). This is also a nice instance to showcase the safe .pinned() (🚲 🏠) closure-based combinator:

self.future
    .take()
    .expect("Polled after completion")
    .pinned(|future| Poll::Ready(future.poll(cx)))

}