      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
    127
    128
    129
    130
    131
    132
    133
    134
    135
    136
    137
    138
    139
    140
    141
    142
    143
    144
    145
    146
    147
    148
    149
    150
    151
    152
    153
    154
    155
    156

    use core::pin::Pin;
    use futures_core::future::{FusedFuture, Future};
    use futures_core::ready;
    use futures_core::stream::{FusedStream, Stream};
    #[cfg(feature = "sink")]
    use futures_sink::Sink;
    use futures_core::task::{Context, Poll};
    use pin_project_lite::pin_project;

    pin_project! {
        #[project = FlattenProj]
        #[derive(Debug)]
        pub enum Flatten<Fut1, Fut2> {
            First { #[pin] f: Fut1 },
            Second { #[pin] f: Fut2 },
            Empty,
        }
    }

    impl<Fut1, Fut2> Flatten<Fut1, Fut2> {
        pub(crate) fn new(future: Fut1) -> Self {
            Self::First { f: future }
        }
    }

    impl<Fut> FusedFuture for Flatten<Fut, Fut::Output>
        where Fut: Future,
              Fut::Output: Future,
    {
        fn is_terminated(&self) -> bool {
            match self {
                Self::Empty => true,
                _ => false,
            }
        }
    }

    impl<Fut> Future for Flatten<Fut, Fut::Output>
        where Fut: Future,
              Fut::Output: Future,
    {
        type Output = <Fut::Output as Future>::Output;

        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
            Poll::Ready(loop {
                match self.as_mut().project() {
                    FlattenProj::First { f } => {
                        let f = ready!(f.poll(cx));
                        self.set(Self::Second { f });
                    },
                    FlattenProj::Second { f } => {
                        let output = ready!(f.poll(cx));
                        self.set(Self::Empty);
                        break output;
                    },
                    FlattenProj::Empty => panic!("Flatten polled after completion"),
                }
            })
        }
    }

    impl<Fut> FusedStream for Flatten<Fut, Fut::Output>
        where Fut: Future,
              Fut::Output: Stream,
    {
        fn is_terminated(&self) -> bool {
            match self {
                Self::Empty => true,
                _ => false,
            }
        }
    }

    impl<Fut> Stream for Flatten<Fut, Fut::Output>
        where Fut: Future,
              Fut::Output: Stream,
    {
        type Item = <Fut::Output as Stream>::Item;

        fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
            Poll::Ready(loop {
                match self.as_mut().project() {
                    FlattenProj::First { f }  => {
                        let f = ready!(f.poll(cx));
                        self.set(Self::Second { f });
                    },
                    FlattenProj::Second { f } => {
                        let output = ready!(f.poll_next(cx));
                        if output.is_none() {
                            self.set(Self::Empty);
                        }
                        break output;
                    },
                    FlattenProj::Empty => break None,
                }
            })
        }
    }

    #[cfg(feature = "sink")]
    impl<Fut, Item> Sink<Item> for Flatten<Fut, Fut::Output>
    where
        Fut: Future,
        Fut::Output: Sink<Item>,
    {
        type Error = <Fut::Output as Sink<Item>>::Error;

        fn poll_ready(
            mut self: Pin<&mut Self>,
            cx: &mut Context<'_>,
        ) -> Poll<Result<(), Self::Error>> {
            Poll::Ready(loop {
                match self.as_mut().project() {
                    FlattenProj::First { f } => {
                        let f = ready!(f.poll(cx));
                        self.set(Self::Second { f });
                    },
                    FlattenProj::Second { f } => {
                        break ready!(f.poll_ready(cx));
                    },
                    FlattenProj::Empty => panic!("poll_ready called after eof"),
                }
            })
        }

        fn start_send(self: Pin<&mut Self>, item: Item) -> Result<(), Self::Error> {
            match self.project() {
                FlattenProj::First { .. } => panic!("poll_ready not called first"),
                FlattenProj::Second { f } => f.start_send(item),
                FlattenProj::Empty => panic!("start_send called after eof"),
            }
        }

        fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
            match self.project() {
                FlattenProj::First { .. } => Poll::Ready(Ok(())),
                FlattenProj::Second { f } => f.poll_flush(cx),
                FlattenProj::Empty => panic!("poll_flush called after eof"),
            }
        }

        fn poll_close(
            mut self: Pin<&mut Self>,
            cx: &mut Context<'_>,
        ) -> Poll<Result<(), Self::Error>> {
            let res = match self.as_mut().project() {
                FlattenProj::Second { f } => f.poll_close(cx),
                _ => Poll::Ready(Ok(())),
            };
            if res.is_ready() {
                self.set(Self::Empty);
            }
            res
        }
    }