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use core::marker::Unpin;
use core::mem;
use core::pin::Pin;
use futures_core::future::Future;
use futures_core::task::{LocalWaker, Poll};
#[derive(Debug)]
pub enum MaybeDone<Fut: Future> {
Future(Fut),
Done(Fut::Output),
Gone,
}
impl<Fut: Future + Unpin> Unpin for MaybeDone<Fut> {}
pub fn maybe_done<Fut: Future>(future: Fut) -> MaybeDone<Fut> {
MaybeDone::Future(future)
}
impl<Fut: Future> MaybeDone<Fut> {
#[inline]
#[allow(clippy::needless_lifetimes)]
pub fn output_mut<'a>(self: Pin<&'a mut Self>) -> Option<&'a mut Fut::Output> {
unsafe {
let this = Pin::get_mut_unchecked(self);
match this {
MaybeDone::Done(res) => Some(res),
_ => None,
}
}
}
#[inline]
pub fn take_output(self: Pin<&mut Self>) -> Option<Fut::Output> {
unsafe {
let this = Pin::get_mut_unchecked(self);
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>, lw: &LocalWaker) -> Poll<Self::Output> {
let res = unsafe {
match Pin::get_mut_unchecked(self.as_mut()) {
MaybeDone::Future(a) => {
if let Poll::Ready(res) = Pin::new_unchecked(a).poll(lw) {
res
} else {
return Poll::Pending
}
}
MaybeDone::Done(_) => return Poll::Ready(()),
MaybeDone::Gone => panic!("MaybeDone polled after value taken"),
}
};
Pin::set(self, MaybeDone::Done(res));
Poll::Ready(())
}
}