tokio/future/
maybe_done.rs

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 = match self.as_mut().project() {
65            MaybeDoneProj::Future { future } => 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}
tokio/future/maybe_done.rs

tokio/future/
maybe_done.rs
