1//! Definition of the [`MaybeDone`] combinator.
2
3use pin_project_lite::pin_project;
4use std::future::{Future, IntoFuture};
5use std::pin::Pin;
6use std::task::{ready, Context, Poll};
7
8pin_project! {
9 /// A future that may have completed.
10 #[derive(Debug)]
11 #[project = MaybeDoneProj]
12 #[project_replace = MaybeDoneProjReplace]
13 #[repr(C)] // https://github.com/rust-lang/miri/issues/3780
14 pub enum MaybeDone<Fut: Future> {
15 /// A not-yet-completed future.
16 Future { #[pin] future: Fut },
17 /// The output of the completed future.
18 Done { output: Fut::Output },
19 /// The empty variant after the result of a [`MaybeDone`] has been
20 /// taken using the [`take_output`](MaybeDone::take_output) method.
21 Gone,
22 }
23}
24
25/// Wraps a future into a `MaybeDone`.
26pub fn maybe_done<F: IntoFuture>(future: F) -> MaybeDone<F::IntoFuture> {
27 MaybeDone::Future {
28 future: future.into_future(),
29 }
30}
31
32impl<Fut: Future> MaybeDone<Fut> {
33 /// Returns an [`Option`] containing a mutable reference to the output of the future.
34 /// The output of this method will be [`Some`] if and only if the inner
35 /// future has been completed and [`take_output`](MaybeDone::take_output)
36 /// has not yet been called.
37 pub fn output_mut(self: Pin<&mut Self>) -> Option<&mut Fut::Output> {
38 match self.project() {
39 MaybeDoneProj::Done { output } => Some(output),
40 _ => None,
41 }
42 }
43
44 /// Attempts to take the output of a `MaybeDone` without driving it
45 /// towards completion.
46 #[inline]
47 pub fn take_output(self: Pin<&mut Self>) -> Option<Fut::Output> {
48 match *self {
49 MaybeDone::Done { .. } => {}
50 MaybeDone::Future { .. } | MaybeDone::Gone => return None,
51 };
52 if let MaybeDoneProjReplace::Done { output } = self.project_replace(MaybeDone::Gone) {
53 Some(output)
54 } else {
55 unreachable!()
56 }
57 }
58}
59
60impl<Fut: Future> Future for MaybeDone<Fut> {
61 type Output = ();
62
63 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
64 let output: ::Output = match self.as_mut().project() {
65 MaybeDoneProj::Future { future: Pin<&mut Fut> } => ready!(future.poll(cx)),
66 MaybeDoneProj::Done { .. } => return Poll::Ready(()),
67 MaybeDoneProj::Gone => panic!("MaybeDone polled after value taken"),
68 };
69 self.set(MaybeDone::Done { output });
70 Poll::Ready(())
71 }
72}
73
74// Test for https://github.com/tokio-rs/tokio/issues/6729
75#[cfg(test)]
76mod miri_tests {
77 use super::maybe_done;
78
79 use std::{
80 future::Future,
81 pin::Pin,
82 sync::Arc,
83 task::{Context, Poll, Wake},
84 };
85
86 struct ThingAdder<'a> {
87 thing: &'a mut String,
88 }
89
90 impl Future for ThingAdder<'_> {
91 type Output = ();
92
93 fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Self::Output> {
94 unsafe {
95 *self.get_unchecked_mut().thing += ", world";
96 }
97 Poll::Pending
98 }
99 }
100
101 #[test]
102 fn maybe_done_miri() {
103 let mut thing = "hello".to_owned();
104
105 // The async block is necessary to trigger the miri failure.
106 #[allow(clippy::redundant_async_block)]
107 let fut = async move { ThingAdder { thing: &mut thing }.await };
108
109 let mut fut = maybe_done(fut);
110 let mut fut = unsafe { Pin::new_unchecked(&mut fut) };
111
112 let waker = Arc::new(DummyWaker).into();
113 let mut ctx = Context::from_waker(&waker);
114 assert_eq!(fut.as_mut().poll(&mut ctx), Poll::Pending);
115 assert_eq!(fut.as_mut().poll(&mut ctx), Poll::Pending);
116 }
117
118 struct DummyWaker;
119
120 impl Wake for DummyWaker {
121 fn wake(self: Arc<Self>) {}
122 }
123}
124