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//! Definition of the MaybeDone combinator
use std::future::Future;
use std::mem;
use std::pin::Pin;
use std::task::{Context, Poll};
/// A future that may have completed.
#[derive(Debug)]
pub enum MaybeDone<Fut: Future> {
/// A not-yet-completed future
Future(Fut),
/// The output of the completed future
Done(Fut::Output),
/// The empty variant after the result of a [`MaybeDone`] has been
/// taken using the [`take_output`](MaybeDone::take_output) method.
Gone,
}
// Safe because we never generate `Pin<&mut Fut::Output>`
impl<Fut: Future + Unpin> Unpin for MaybeDone<Fut> {}
/// Wraps a future into a `MaybeDone`
pub fn maybe_done<Fut: Future>(future: Fut) -> MaybeDone<Fut> {
MaybeDone::Future(future)
}
impl<Fut: Future> MaybeDone<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 been completed and [`take_output`](MaybeDone::take_output)
/// has not yet been called.
pub fn output_mut(self: Pin<&mut Self>) -> Option<&mut Fut::Output> {
unsafe {
let this = self.get_unchecked_mut();
match this {
MaybeDone::Done(res) => Some(res),
_ => None,
}
}
}
/// Attempts to take the output of a `MaybeDone` without driving it
/// towards completion.
#[inline]
pub fn take_output(self: Pin<&mut Self>) -> Option<Fut::Output> {
unsafe {
let this = self.get_unchecked_mut();
match this {
MaybeDone::Done(_) => {}
MaybeDone::Future(_) | MaybeDone::Gone => return None,
};
if let MaybeDone::Done(output) = mem::replace(this, MaybeDone::Gone) {
Some(output)
} else {
unreachable!()
}
}
}
}
impl<Fut: Future> Future for MaybeDone<Fut> {
type Output = ();
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let res = unsafe {
match self.as_mut().get_unchecked_mut() {
MaybeDone::Future(a) => ready!(Pin::new_unchecked(a).poll(cx)),
MaybeDone::Done(_) => return Poll::Ready(()),
MaybeDone::Gone => panic!("MaybeDone polled after value taken"),
}
};
self.set(MaybeDone::Done(res));
Poll::Ready(())
}
}
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