1//! Definition of the MaybeDone combinator
2
3use super::assert_future;
4use core::mem;
5use core::pin::Pin;
6use futures_core::future::{FusedFuture, Future};
7use futures_core::ready;
8use futures_core::task::{Context, Poll};
9
10/// A future that may have completed.
11///
12/// This is created by the [`maybe_done()`] function.
13#[derive(Debug)]
14pub enum MaybeDone<Fut: Future> {
15 /// A not-yet-completed future
16 Future(/* #[pin] */ Fut),
17 /// The output of the completed future
18 Done(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
24impl<Fut: Future + Unpin> Unpin for MaybeDone<Fut> {}
25
26/// Wraps a future into a `MaybeDone`
27///
28/// # Examples
29///
30/// ```
31/// # futures::executor::block_on(async {
32/// use futures::future;
33/// use futures::pin_mut;
34///
35/// let future = future::maybe_done(async { 5 });
36/// pin_mut!(future);
37/// assert_eq!(future.as_mut().take_output(), None);
38/// let () = future.as_mut().await;
39/// assert_eq!(future.as_mut().take_output(), Some(5));
40/// assert_eq!(future.as_mut().take_output(), None);
41/// # });
42/// ```
43pub fn maybe_done<Fut: Future>(future: Fut) -> MaybeDone<Fut> {
44 assert_future::<(), _>(MaybeDone::Future(future))
45}
46
47impl<Fut: Future> MaybeDone<Fut> {
48 /// Returns an [`Option`] containing a mutable reference to the output of the future.
49 /// The output of this method will be [`Some`] if and only if the inner
50 /// future has been completed and [`take_output`](MaybeDone::take_output)
51 /// has not yet been called.
52 #[inline]
53 pub fn output_mut(self: Pin<&mut Self>) -> Option<&mut Fut::Output> {
54 unsafe {
55 match self.get_unchecked_mut() {
56 Self::Done(res) => Some(res),
57 _ => None,
58 }
59 }
60 }
61
62 /// Attempt to take the output of a `MaybeDone` without driving it
63 /// towards completion.
64 #[inline]
65 pub fn take_output(self: Pin<&mut Self>) -> Option<Fut::Output> {
66 match &*self {
67 Self::Done(_) => {}
68 Self::Future(_) | Self::Gone => return None,
69 }
70 unsafe {
71 match mem::replace(self.get_unchecked_mut(), Self::Gone) {
72 Self::Done(output) => Some(output),
73 _ => unreachable!(),
74 }
75 }
76 }
77}
78
79impl<Fut: Future> FusedFuture for MaybeDone<Fut> {
80 fn is_terminated(&self) -> bool {
81 match self {
82 Self::Future(_) => false,
83 Self::Done(_) | Self::Gone => true,
84 }
85 }
86}
87
88impl<Fut: Future> Future for MaybeDone<Fut> {
89 type Output = ();
90
91 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
92 unsafe {
93 match self.as_mut().get_unchecked_mut() {
94 Self::Future(f: &mut Fut) => {
95 let res: ::Output = ready!(Pin::new_unchecked(f).poll(cx));
96 self.set(Self::Done(res));
97 }
98 Self::Done(_) => {}
99 Self::Gone => panic!("MaybeDone polled after value taken"),
100 }
101 }
102 Poll::Ready(())
103 }
104}
105