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    use {Future, Poll, Async};

    /// A future which "fuses" a future once it's been resolved.
    ///
    /// Normally futures can behave unpredictable once they're used after a future
    /// has been resolved, but `Fuse` is always defined to return `Async::NotReady`
    /// from `poll` after it has resolved successfully or returned an error.
    ///
    /// This is created by the `Future::fuse` method.
    #[derive(Debug)]
    #[must_use = "futures do nothing unless polled"]
    pub struct Fuse<A: Future> {
        future: Option<A>,
    }

    pub fn new<A: Future>(f: A) -> Fuse<A> {
        Fuse {
            future: Some(f),
        }
    }

    impl<A: Future> Fuse<A> {
        /// Returns whether the underlying future has finished or not.
        ///
        /// If this method returns `true`, then all future calls to `poll`
        /// are guaranteed to return `Ok(Async::NotReady)`. If this returns
        /// false, then the underlying future has not been driven to
        /// completion.
        pub fn is_done(&self) -> bool {
            self.future.is_none()
        }
    }

    impl<A: Future> Future for Fuse<A> {
        type Item = A::Item;
        type Error = A::Error;

        fn poll(&mut self) -> Poll<A::Item, A::Error> {
            let res = self.future.as_mut().map(|f| f.poll());
            match res.unwrap_or(Ok(Async::NotReady)) {
                res @ Ok(Async::Ready(_)) |
                res @ Err(_) => {
                    self.future = None;
                    res
                }
                Ok(Async::NotReady) => Ok(Async::NotReady)
            }
        }
    }

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