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