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    //! Definition of the TryMaybeDone combinator

    use super::assert_future;
    use core::mem;
    use core::pin::Pin;
    use futures_core::future::{FusedFuture, Future, TryFuture};
    use futures_core::ready;
    use futures_core::task::{Context, Poll};

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

    impl<Fut: TryFuture + Unpin> Unpin for TryMaybeDone<Fut> {}

    /// Wraps a future into a `TryMaybeDone`
    pub fn try_maybe_done<Fut: TryFuture>(future: Fut) -> TryMaybeDone<Fut> {
        assert_future::<Result<(), Fut::Error>, _>(TryMaybeDone::Future(future))
    }

    impl<Fut: TryFuture> TryMaybeDone<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 completed successfully and [`take_output`](TryMaybeDone::take_output)
        /// has not yet been called.
        #[inline]
        pub fn output_mut(self: Pin<&mut Self>) -> Option<&mut Fut::Ok> {
            unsafe {
                match self.get_unchecked_mut() {
                    TryMaybeDone::Done(res) => Some(res),
                    _ => None,
                }
            }
        }

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

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

    impl<Fut: TryFuture> Future for TryMaybeDone<Fut> {
        type Output = Result<(), Fut::Error>;

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