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    use core::pin::Pin;
    use futures_core::future::{FusedFuture, Future};
    use futures_core::ready;
    use futures_core::task::{Context, Poll};
    use pin_project_lite::pin_project;

    use crate::fns::FnOnce1;

    pin_project! {
        /// Internal Map future
        #[project = MapProj]
        #[project_replace = MapProjReplace]
        #[derive(Debug)]
        #[must_use = "futures do nothing unless you `.await` or poll them"]
        pub enum Map<Fut, F> {
            Incomplete {
                #[pin]
                future: Fut,
                f: F,
            },
            Complete,
        }
    }

    impl<Fut, F> Map<Fut, F> {
        /// Creates a new Map.
        pub(crate) fn new(future: Fut, f: F) -> Self {
            Self::Incomplete { future, f }
        }
    }

    impl<Fut, F, T> FusedFuture for Map<Fut, F>
    where
        Fut: Future,
        F: FnOnce1<Fut::Output, Output = T>,
    {
        fn is_terminated(&self) -> bool {
            match self {
                Self::Incomplete { .. } => false,
                Self::Complete => true,
            }
        }
    }

    impl<Fut, F, T> Future for Map<Fut, F>
    where
        Fut: Future,
        F: FnOnce1<Fut::Output, Output = T>,
    {
        type Output = T;

        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
            match self.as_mut().project() {
                MapProj::Incomplete { future, .. } => {
                    let output = ready!(future.poll(cx));
                    match self.project_replace(Map::Complete) {
                        MapProjReplace::Incomplete { f, .. } => Poll::Ready(f.call_once(output)),
                        MapProjReplace::Complete => unreachable!(),
                    }
                }
                MapProj::Complete => {
                    panic!("Map must not be polled after it returned `Poll::Ready`")
                }
            }
        }
    }