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    //! Definition of the MaybeDone combinator

    use std::future::Future;
    use std::mem;
    use std::pin::Pin;
    use std::task::{Context, Poll};

    /// A future that may have completed.
    #[derive(Debug)]
    pub enum MaybeDone<Fut: Future> {
        /// A not-yet-completed future
        Future(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,
    }

    // Safe because we never generate `Pin<&mut Fut::Output>`
    impl<Fut: Future + Unpin> Unpin for MaybeDone<Fut> {}

    /// Wraps a future into a `MaybeDone`
    pub fn maybe_done<Fut: Future>(future: Fut) -> MaybeDone<Fut> {
        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.
        pub fn output_mut(self: Pin<&mut Self>) -> Option<&mut Fut::Output> {
            unsafe {
                let this = self.get_unchecked_mut();
                match this {
                    MaybeDone::Done(res) => Some(res),
                    _ => None,
                }
            }
        }

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

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

        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
            let res = unsafe {
                match self.as_mut().get_unchecked_mut() {
                    MaybeDone::Future(a) => ready!(Pin::new_unchecked(a).poll(cx)),
                    MaybeDone::Done(_) => return Poll::Ready(()),
                    MaybeDone::Gone => panic!("MaybeDone polled after value taken"),
                }
            };
            self.set(MaybeDone::Done(res));
            Poll::Ready(())
        }
    }