105 lines
3.1 KiB
Rust
105 lines
3.1 KiB
Rust
//! Definition of the MaybeDone combinator
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use super::assert_future;
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use core::mem;
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use core::pin::Pin;
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use futures_core::future::{FusedFuture, Future};
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use futures_core::ready;
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use futures_core::task::{Context, Poll};
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/// A future that may have completed.
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///
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/// This is created by the [`maybe_done()`] function.
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#[derive(Debug)]
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pub enum MaybeDone<Fut: Future> {
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/// A not-yet-completed future
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Future(/* #[pin] */ Fut),
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/// The output of the completed future
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Done(Fut::Output),
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/// The empty variant after the result of a [`MaybeDone`] has been
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/// taken using the [`take_output`](MaybeDone::take_output) method.
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Gone,
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}
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impl<Fut: Future + Unpin> Unpin for MaybeDone<Fut> {}
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/// Wraps a future into a `MaybeDone`
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///
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/// # Examples
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///
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/// ```
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/// # futures::executor::block_on(async {
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/// use futures::future;
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/// use futures::pin_mut;
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///
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/// let future = future::maybe_done(async { 5 });
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/// pin_mut!(future);
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/// assert_eq!(future.as_mut().take_output(), None);
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/// let () = future.as_mut().await;
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/// assert_eq!(future.as_mut().take_output(), Some(5));
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/// assert_eq!(future.as_mut().take_output(), None);
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/// # });
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/// ```
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pub fn maybe_done<Fut: Future>(future: Fut) -> MaybeDone<Fut> {
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assert_future::<(), _>(MaybeDone::Future(future))
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}
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impl<Fut: Future> MaybeDone<Fut> {
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/// Returns an [`Option`] containing a mutable reference to the output of the future.
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/// The output of this method will be [`Some`] if and only if the inner
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/// future has been completed and [`take_output`](MaybeDone::take_output)
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/// has not yet been called.
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#[inline]
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pub fn output_mut(self: Pin<&mut Self>) -> Option<&mut Fut::Output> {
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unsafe {
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match self.get_unchecked_mut() {
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MaybeDone::Done(res) => Some(res),
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_ => None,
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}
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}
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}
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/// Attempt to take the output of a `MaybeDone` without driving it
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/// towards completion.
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#[inline]
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pub fn take_output(self: Pin<&mut Self>) -> Option<Fut::Output> {
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match &*self {
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Self::Done(_) => {}
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Self::Future(_) | Self::Gone => return None,
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}
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unsafe {
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match mem::replace(self.get_unchecked_mut(), Self::Gone) {
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MaybeDone::Done(output) => Some(output),
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_ => unreachable!(),
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}
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}
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}
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}
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impl<Fut: Future> FusedFuture for MaybeDone<Fut> {
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fn is_terminated(&self) -> bool {
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match self {
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Self::Future(_) => false,
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Self::Done(_) | Self::Gone => true,
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}
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}
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}
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impl<Fut: Future> Future for MaybeDone<Fut> {
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type Output = ();
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fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
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unsafe {
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match self.as_mut().get_unchecked_mut() {
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MaybeDone::Future(f) => {
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let res = ready!(Pin::new_unchecked(f).poll(cx));
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self.set(Self::Done(res));
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}
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MaybeDone::Done(_) => {}
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MaybeDone::Gone => panic!("MaybeDone polled after value taken"),
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}
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}
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Poll::Ready(())
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}
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}
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