async_std/future/
maybe_done.rs

1//! A type that wraps a future to keep track of its completion status.
2//!
3//! This implementation was taken from the original `macro_rules` `join/try_join`
4//! macros in the `futures-preview` crate.
5
6use std::future::Future;
7use std::mem;
8use std::pin::Pin;
9use std::task::{Context, Poll};
10
11use futures_core::ready;
12
13/// A future that may have completed.
14#[derive(Debug)]
15pub(crate) enum MaybeDone<Fut: Future> {
16    /// A not-yet-completed future
17    Future(Fut),
18
19    /// The output of the completed future
20    Done(Fut::Output),
21
22    /// The empty variant after the result of a [`MaybeDone`] has been
23    /// taken using the [`take`](MaybeDone::take) method.
24    Gone,
25}
26
27impl<Fut: Future> MaybeDone<Fut> {
28    /// Create a new instance of `MaybeDone`.
29    pub(crate) fn new(future: Fut) -> MaybeDone<Fut> {
30        Self::Future(future)
31    }
32
33    /// Returns an [`Option`] containing a 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`](MaybeDone::take)
36    /// has not yet been called.
37    #[inline]
38    pub(crate) fn output(self: Pin<&Self>) -> Option<&Fut::Output> {
39        let this = self.get_ref();
40        match this {
41            MaybeDone::Done(res) => Some(res),
42            _ => None,
43        }
44    }
45
46    /// Attempt to take the output of a `MaybeDone` without driving it
47    /// towards completion.
48    #[inline]
49    pub(crate) fn take(self: Pin<&mut Self>) -> Option<Fut::Output> {
50        unsafe {
51            let this = self.get_unchecked_mut();
52            match this {
53                MaybeDone::Done(_) => {}
54                MaybeDone::Future(_) | MaybeDone::Gone => return None,
55            };
56            if let MaybeDone::Done(output) = mem::replace(this, MaybeDone::Gone) {
57                Some(output)
58            } else {
59                unreachable!()
60            }
61        }
62    }
63}
64
65impl<Fut: Future> Future for MaybeDone<Fut> {
66    type Output = ();
67
68    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
69        let res = unsafe {
70            match Pin::as_mut(&mut self).get_unchecked_mut() {
71                MaybeDone::Future(a) => ready!(Pin::new_unchecked(a).poll(cx)),
72                MaybeDone::Done(_) => return Poll::Ready(()),
73                MaybeDone::Gone => panic!("MaybeDone polled after value taken"),
74            }
75        };
76        self.set(MaybeDone::Done(res));
77        Poll::Ready(())
78    }
79}
async_std/future/maybe_done.rs

async_std/future/
maybe_done.rs
