1 //! Definition of the MaybeDone combinator
2
3 use super::assert_future;
4 use core::mem;
5 use core::pin::Pin;
6 use futures_core::future::{FusedFuture, Future};
7 use futures_core::ready;
8 use 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)]
14 pub 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
24 impl<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 /// ```
maybe_done<Fut: Future>(future: Fut) -> MaybeDone<Fut>43 pub fn maybe_done<Fut: Future>(future: Fut) -> MaybeDone<Fut> {
44 assert_future::<(), _>(MaybeDone::Future(future))
45 }
46
47 impl<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]
output_mut(self: Pin<&mut Self>) -> Option<&mut Fut::Output>53 pub fn output_mut(self: Pin<&mut Self>) -> Option<&mut Fut::Output> {
54 unsafe {
55 match self.get_unchecked_mut() {
56 MaybeDone::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]
take_output(self: Pin<&mut Self>) -> Option<Fut::Output>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 MaybeDone::Done(output) => Some(output),
73 _ => unreachable!(),
74 }
75 }
76 }
77 }
78
79 impl<Fut: Future> FusedFuture for MaybeDone<Fut> {
is_terminated(&self) -> bool80 fn is_terminated(&self) -> bool {
81 match self {
82 Self::Future(_) => false,
83 Self::Done(_) | Self::Gone => true,
84 }
85 }
86 }
87
88 impl<Fut: Future> Future for MaybeDone<Fut> {
89 type Output = ();
90
poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>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 MaybeDone::Future(f) => {
95 let res = ready!(Pin::new_unchecked(f).poll(cx));
96 self.set(Self::Done(res));
97 }
98 MaybeDone::Done(_) => {}
99 MaybeDone::Gone => panic!("MaybeDone polled after value taken"),
100 }
101 }
102 Poll::Ready(())
103 }
104 }
105