Revision control
Copy as Markdown
Other Tools
//! Definition of the [`MaybeDone`] combinator.
use pin_project_lite::pin_project;
use std::future::{Future, IntoFuture};
use std::pin::Pin;
use std::task::{Context, Poll};
pin_project! {
/// A future that may have completed.
#[derive(Debug)]
#[project = MaybeDoneProj]
#[project_replace = MaybeDoneProjReplace]
pub enum MaybeDone<Fut: Future> {
/// A not-yet-completed future.
Future { #[pin] future: Fut },
/// The output of the completed future.
Done { output: Fut::Output },
/// The empty variant after the result of a [`MaybeDone`] has been
/// taken using the [`take_output`](MaybeDone::take_output) method.
Gone,
}
}
/// Wraps a future into a `MaybeDone`.
pub fn maybe_done<F: IntoFuture>(future: F) -> MaybeDone<F::IntoFuture> {
MaybeDone::Future {
future: future.into_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> {
match self.project() {
MaybeDoneProj::Done { output } => Some(output),
_ => 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> {
match *self {
MaybeDone::Done { .. } => {}
MaybeDone::Future { .. } | MaybeDone::Gone => return None,
};
if let MaybeDoneProjReplace::Done { output } = self.project_replace(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 output = match self.as_mut().project() {
MaybeDoneProj::Future { future } => ready!(future.poll(cx)),
MaybeDoneProj::Done { .. } => return Poll::Ready(()),
MaybeDoneProj::Gone => panic!("MaybeDone polled after value taken"),
};
self.set(MaybeDone::Done { output });
Poll::Ready(())
}
}