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    #![allow(non_snake_case)]

    use core::fmt;
    use core::mem;

    use {Future, Poll, IntoFuture, Async};

    macro_rules! generate {
        ($(
            $(#[$doc:meta])*
            ($Join:ident, $new:ident, <A, $($B:ident),*>),
        )*) => ($(
            $(#[$doc])*
            #[must_use = "futures do nothing unless polled"]
            pub struct $Join<A, $($B),*>
                where A: Future,
                      $($B: Future<Error=A::Error>),*
            {
                a: MaybeDone<A>,
                $($B: MaybeDone<$B>,)*
            }

            impl<A, $($B),*> fmt::Debug for $Join<A, $($B),*>
                where A: Future + fmt::Debug,
                      A::Item: fmt::Debug,
                      $(
                          $B: Future<Error=A::Error> + fmt::Debug,
                          $B::Item: fmt::Debug
                      ),*
            {
                fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
                    fmt.debug_struct(stringify!($Join))
                        .field("a", &self.a)
                        $(.field(stringify!($B), &self.$B))*
                        .finish()
                }
            }

            pub fn $new<A, $($B),*>(a: A, $($B: $B),*) -> $Join<A, $($B),*>
                where A: Future,
                      $($B: Future<Error=A::Error>),*
            {
                $Join {
                    a: MaybeDone::NotYet(a),
                    $($B: MaybeDone::NotYet($B)),*
                }
            }

            impl<A, $($B),*> $Join<A, $($B),*>
                where A: Future,
                      $($B: Future<Error=A::Error>),*
            {
                fn erase(&mut self) {
                    self.a = MaybeDone::Gone;
                    $(self.$B = MaybeDone::Gone;)*
                }
            }

            impl<A, $($B),*> Future for $Join<A, $($B),*>
                where A: Future,
                      $($B: Future<Error=A::Error>),*
            {
                type Item = (A::Item, $($B::Item),*);
                type Error = A::Error;

                fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
                    let mut all_done = match self.a.poll() {
                        Ok(done) => done,
                        Err(e) => {
                            self.erase();
                            return Err(e)
                        }
                    };
                    $(
                        all_done = match self.$B.poll() {
                            Ok(done) => all_done && done,
                            Err(e) => {
                                self.erase();
                                return Err(e)
                            }
                        };
                    )*

                    if all_done {
                        Ok(Async::Ready((self.a.take(), $(self.$B.take()),*)))
                    } else {
                        Ok(Async::NotReady)
                    }
                }
            }

            impl<A, $($B),*> IntoFuture for (A, $($B),*)
                where A: IntoFuture,
            $(
                $B: IntoFuture<Error=A::Error>
            ),*
            {
                type Future = $Join<A::Future, $($B::Future),*>;
                type Item = (A::Item, $($B::Item),*);
                type Error = A::Error;

                fn into_future(self) -> Self::Future {
                    match self {
                        (a, $($B),+) => {
                            $new(
                                IntoFuture::into_future(a),
                                $(IntoFuture::into_future($B)),+
                            )
                        }
                    }
                }
            }

        )*)
    }

    generate! {
        /// Future for the `join` combinator, waiting for two futures to
        /// complete.
        ///
        /// This is created by the `Future::join` method.
        (Join, new, <A, B>),

        /// Future for the `join3` combinator, waiting for three futures to
        /// complete.
        ///
        /// This is created by the `Future::join3` method.
        (Join3, new3, <A, B, C>),

        /// Future for the `join4` combinator, waiting for four futures to
        /// complete.
        ///
        /// This is created by the `Future::join4` method.
        (Join4, new4, <A, B, C, D>),

        /// Future for the `join5` combinator, waiting for five futures to
        /// complete.
        ///
        /// This is created by the `Future::join5` method.
        (Join5, new5, <A, B, C, D, E>),
    }

    #[derive(Debug)]
    enum MaybeDone<A: Future> {
        NotYet(A),
        Done(A::Item),
        Gone,
    }

    impl<A: Future> MaybeDone<A> {
        fn poll(&mut self) -> Result<bool, A::Error> {
            let res = match *self {
                MaybeDone::NotYet(ref mut a) => a.poll()?,
                MaybeDone::Done(_) => return Ok(true),
                MaybeDone::Gone => panic!("cannot poll Join twice"),
            };
            match res {
                Async::Ready(res) => {
                    *self = MaybeDone::Done(res);
                    Ok(true)
                }
                Async::NotReady => Ok(false),
            }
        }

        fn take(&mut self) -> A::Item {
            match mem::replace(self, MaybeDone::Gone) {
                MaybeDone::Done(a) => a,
                _ => panic!(),
            }
        }
    }

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