```1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
```
``````//! Definition of the MaybeDone combinator

use super::assert_future;
use core::mem;
use core::pin::Pin;
use futures_core::future::{FusedFuture, Future};

/// A future that may have completed.
///
/// This is created by the [`maybe_done()`] function.
#[derive(Debug)]
pub enum MaybeDone<Fut: Future> {
/// A not-yet-completed future
Future(/* #[pin] */ Fut),
/// The output of the completed future
Done(Fut::Output),
/// The empty variant after the result of a [`MaybeDone`] has been
/// taken using the [`take_output`](MaybeDone::take_output) method.
Gone,
}

impl<Fut: Future + Unpin> Unpin for MaybeDone<Fut> {}

/// Wraps a future into a `MaybeDone`
///
/// # Examples
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future;
/// use futures::pin_mut;
///
/// let future = future::maybe_done(async { 5 });
/// pin_mut!(future);
/// assert_eq!(future.as_mut().take_output(), None);
/// let () = future.as_mut().await;
/// assert_eq!(future.as_mut().take_output(), Some(5));
/// assert_eq!(future.as_mut().take_output(), None);
/// # });
/// ```
pub fn maybe_done<Fut: Future>(future: Fut) -> MaybeDone<Fut> {
assert_future::<(), _>(MaybeDone::Future(future))
}

impl<Fut: Future> MaybeDone<Fut> {
/// Returns an [`Option`] containing a mutable reference to the output of the future.
/// The output of this method will be [`Some`] if and only if the inner
/// future has been completed and [`take_output`](MaybeDone::take_output)
/// has not yet been called.
#[inline]
pub fn output_mut(self: Pin<&mut Self>) -> Option<&mut Fut::Output> {
unsafe {
match self.get_unchecked_mut() {
MaybeDone::Done(res) => Some(res),
_ => None,
}
}
}

/// Attempt to take the output of a `MaybeDone` without driving it
/// towards completion.
#[inline]
pub fn take_output(self: Pin<&mut Self>) -> Option<Fut::Output> {
match &*self {
Self::Done(_) => {}
Self::Future(_) | Self::Gone => return None,
}
unsafe {
match mem::replace(self.get_unchecked_mut(), Self::Gone) {
MaybeDone::Done(output) => Some(output),
_ => unreachable!(),
}
}
}
}

impl<Fut: Future> FusedFuture for MaybeDone<Fut> {
fn is_terminated(&self) -> bool {
match self {
Self::Future(_) => false,
Self::Done(_) | Self::Gone => true,
}
}
}

impl<Fut: Future> Future for MaybeDone<Fut> {
type Output = ();

fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
unsafe {
match self.as_mut().get_unchecked_mut() {
MaybeDone::Future(f) => {