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
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126

use super::Race as RaceTrait;

use core::fmt;
use core::future::{Future, IntoFuture};
use core::pin::Pin;
use core::task::{Context, Poll};

use pin_project::pin_project;

macro_rules! generate {
    ($(
        (<$($Fut:ident),*>),
    )*) => ($( #[allow(non_snake_case)] const _: () = {
        #[pin_project]
        #[must_use = "futures do nothing unless you `.await` or poll them"]
        #[allow(non_snake_case)]
        pub struct Race<T, $($Fut),*>
        where
            $($Fut: Future<Output = T>),*
        {
            done: bool,
            $(#[pin] $Fut: $Fut,)*
        }

        impl<T, $($Fut),*> fmt::Debug for Race<T, $($Fut),*>
        where
            $(
                $Fut: Future<Output = T> + fmt::Debug,
                T: fmt::Debug,
            )*
        {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                f.debug_struct(stringify!(Race))
                    $(.field(stringify!($Fut), &self.$Fut))*
                    .finish()
            }
        }

        impl<T, $($Fut),*> RaceTrait for ($($Fut),*)
        where
            $($Fut: IntoFuture<Output = T>),*
        {
            type Output = T;
            type Future = Race<T, $($Fut::IntoFuture),*>;

            fn race(self) -> Self::Future {
                let ($($Fut),*): ($($Fut),*) = self;
                Race {
                    done: false,
                    $($Fut: $Fut.into_future()),*
                }
            }
        }

        impl<T, $($Fut: Future),*> Future for Race<T, $($Fut),*>
        where
            $($Fut: Future<Output = T>),*
        {
            type Output = T;

            fn poll(
                self: Pin<&mut Self>, cx: &mut Context<'_>
            ) -> Poll<Self::Output> {
                let this = self.project();

                assert!(
                    !*this.done,
                    "Futures must not be polled after being completed"
                );

                $(
                    if let Poll::Ready(output) = Future::poll(this.$Fut, cx) {
                        *this.done = true;
                        return Poll::Ready(output);
                    }
                )*
                Poll::Pending
            }
        }
    }; )*)
}

generate! {
    (<A, B>),
    (<A, B, C>),
    (<A, B, C, D>),
    (<A, B, C, D, E>),
    (<A, B, C, D, E, F>),
    (<A, B, C, D, E, F, G>),
    (<A, B, C, D, E, F, G, H>),
    (<A, B, C, D, E, F, G, H, I>),
    (<A, B, C, D, E, F, G, H, I, J>),
    (<A, B, C, D, E, F, G, H, I, J, K>),
    (<A, B, C, D, E, F, G, H, I, J, K, L>),
}

#[cfg(test)]
mod test {
    use super::*;
    use std::future;

    // NOTE: we should probably poll in random order.
    #[test]
    fn no_fairness() {
        async_io::block_on(async {
            let res = (future::ready("hello"), future::ready("world"))
                .race()
                .await;
            assert_eq!(res, "hello");
        });
    }

    #[test]
    fn thruple() {
        async_io::block_on(async {
            let res = (
                future::pending(),
                future::ready("world"),
                future::ready("hello"),
            )
                .race()
                .await;
            assert_eq!(res, "world");
        });
    }
}