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    use core::mem;
    use core::marker::PhantomData;

    use {IntoFuture, Future, Poll, Async, StartSend, AsyncSink};
    use sink::Sink;
    use stream::Stream;

    /// Sink for the `Sink::with` combinator, chaining a computation to run *prior*
    /// to pushing a value into the underlying sink.
    #[derive(Debug)]
    #[must_use = "sinks do nothing unless polled"]
    pub struct With<S, U, F, Fut>
        where S: Sink,
              F: FnMut(U) -> Fut,
              Fut: IntoFuture,
    {
        sink: S,
        f: F,
        state: State<Fut::Future, S::SinkItem>,
        _phantom: PhantomData<fn(U)>,
    }

    #[derive(Debug)]
    enum State<Fut, T> {
        Empty,
        Process(Fut),
        Buffered(T),
    }

    impl<Fut, T> State<Fut, T> {
        fn is_empty(&self) -> bool {
            if let State::Empty = *self {
                true
            } else {
                false
            }
        }
    }

    pub fn new<S, U, F, Fut>(sink: S, f: F) -> With<S, U, F, Fut>
        where S: Sink,
              F: FnMut(U) -> Fut,
              Fut: IntoFuture<Item = S::SinkItem>,
              Fut::Error: From<S::SinkError>,
    {
        With {
            state: State::Empty,
            sink: sink,
            f: f,
            _phantom: PhantomData,
        }
    }

    // Forwarding impl of Stream from the underlying sink
    impl<S, U, F, Fut> Stream for With<S, U, F, Fut>
        where S: Stream + Sink,
              F: FnMut(U) -> Fut,
              Fut: IntoFuture
    {
        type Item = S::Item;
        type Error = S::Error;

        fn poll(&mut self) -> Poll<Option<S::Item>, S::Error> {
            self.sink.poll()
        }
    }

    impl<S, U, F, Fut> With<S, U, F, Fut>
        where S: Sink,
              F: FnMut(U) -> Fut,
              Fut: IntoFuture<Item = S::SinkItem>,
              Fut::Error: From<S::SinkError>,
    {
        /// Get a shared reference to the inner sink.
        pub fn get_ref(&self) -> &S {
            &self.sink
        }

        /// Get a mutable reference to the inner sink.
        pub fn get_mut(&mut self) -> &mut S {
            &mut self.sink
        }

        /// Consumes this combinator, returning the underlying sink.
        ///
        /// Note that this may discard intermediate state of this combinator, so
        /// care should be taken to avoid losing resources when this is called.
        pub fn into_inner(self) -> S {
            self.sink
        }

        fn poll(&mut self) -> Poll<(), Fut::Error> {
            loop {
                match mem::replace(&mut self.state, State::Empty) {
                    State::Empty => break,
                    State::Process(mut fut) => {
                        match fut.poll()? {
                            Async::Ready(item) => {
                                self.state = State::Buffered(item);
                            }
                            Async::NotReady => {
                                self.state = State::Process(fut);
                                break
                            }
                        }
                    }
                    State::Buffered(item) => {
                        if let AsyncSink::NotReady(item) = self.sink.start_send(item)? {
                            self.state = State::Buffered(item);
                            break
                        }
                    }
                }
            }

            if self.state.is_empty() {
                Ok(Async::Ready(()))
            } else {
                Ok(Async::NotReady)
            }
        }
    }

    impl<S, U, F, Fut> Sink for With<S, U, F, Fut>
        where S: Sink,
              F: FnMut(U) -> Fut,
              Fut: IntoFuture<Item = S::SinkItem>,
              Fut::Error: From<S::SinkError>,
    {
        type SinkItem = U;
        type SinkError = Fut::Error;

        fn start_send(&mut self, item: Self::SinkItem) -> StartSend<Self::SinkItem, Fut::Error> {
            if self.poll()?.is_not_ready() {
                return Ok(AsyncSink::NotReady(item))
            }
            self.state = State::Process((self.f)(item).into_future());
            Ok(AsyncSink::Ready)
        }

        fn poll_complete(&mut self) -> Poll<(), Fut::Error> {
            // poll ourselves first, to push data downward
            let me_ready = self.poll()?;
            // always propagate `poll_complete` downward to attempt to make progress
            try_ready!(self.sink.poll_complete());
            Ok(me_ready)
        }

        fn close(&mut self) -> Poll<(), Fut::Error> {
            try_ready!(self.poll());
            Ok(self.sink.close()?)
        }
    }

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