     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

    use std::mem;

    use super::{task, Future, Pin, Poll};

    pub(crate) trait Started: Future {
        fn started(&self) -> bool;
    }

    pub(crate) fn lazy<F, R>(func: F) -> Lazy<F, R>
    where
        F: FnOnce() -> R,
        R: Future + Unpin,
    {
        Lazy {
            inner: Inner::Init(func),
        }
    }

    // FIXME: allow() required due to `impl Trait` leaking types to this lint
    #[allow(missing_debug_implementations)]
    pub(crate) struct Lazy<F, R> {
        inner: Inner<F, R>,
    }

    enum Inner<F, R> {
        Init(F),
        Fut(R),
        Empty,
    }

    impl<F, R> Started for Lazy<F, R>
    where
        F: FnOnce() -> R,
        R: Future + Unpin,
    {
        fn started(&self) -> bool {
            match self.inner {
                Inner::Init(_) => false,
                Inner::Fut(_) | Inner::Empty => true,
            }
        }
    }

    impl<F, R> Future for Lazy<F, R>
    where
        F: FnOnce() -> R,
        R: Future + Unpin,
    {
        type Output = R::Output;

        fn poll(mut self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
            if let Inner::Fut(ref mut f) = self.inner {
                return Pin::new(f).poll(cx);
            }

            match mem::replace(&mut self.inner, Inner::Empty) {
                Inner::Init(func) => {
                    let mut fut = func();
                    let ret = Pin::new(&mut fut).poll(cx);
                    self.inner = Inner::Fut(fut);
                    ret
                }
                _ => unreachable!("lazy state wrong"),
            }
        }
    }

    // The closure `F` is never pinned
    impl<F, R: Unpin> Unpin for Lazy<F, R> {}