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    //! Asynchronous sinks
    //!
    //! This module contains the `Sink` trait, along with a number of adapter types
    //! for it. An overview is available in the documentation for the trait itself.
    //!
    //! You can find more information/tutorials about streams [online at
    //! https://tokio.rs][online]
    //!
    //! [online]: https://tokio.rs/docs/getting-started/streams-and-sinks/

    use {IntoFuture, Poll, StartSend};
    use stream::Stream;

    mod with;
    mod with_flat_map;
    // mod with_map;
    // mod with_filter;
    // mod with_filter_map;
    mod flush;
    mod from_err;
    mod send;
    mod send_all;
    mod map_err;
    mod fanout;

    if_std! {
        mod buffer;
        mod wait;

        pub use self::buffer::Buffer;
        pub use self::wait::Wait;

        // TODO: consider expanding this via e.g. FromIterator
        impl<T> Sink for ::std::vec::Vec<T> {
            type SinkItem = T;
            type SinkError = (); // Change this to ! once it stabilizes

            fn start_send(&mut self, item: Self::SinkItem)
                          -> StartSend<Self::SinkItem, Self::SinkError>
            {
                self.push(item);
                Ok(::AsyncSink::Ready)
            }

            fn poll_complete(&mut self) -> Poll<(), Self::SinkError> {
                Ok(::Async::Ready(()))
            }

            fn close(&mut self) -> Poll<(), Self::SinkError> {
                Ok(::Async::Ready(()))
            }
        }

        /// A type alias for `Box<Sink + Send>`
        pub type BoxSink<T, E> = ::std::boxed::Box<Sink<SinkItem = T, SinkError = E> +
                                                   ::core::marker::Send>;

        impl<S: ?Sized + Sink> Sink for ::std::boxed::Box<S> {
            type SinkItem = S::SinkItem;
            type SinkError = S::SinkError;

            fn start_send(&mut self, item: Self::SinkItem)
                          -> StartSend<Self::SinkItem, Self::SinkError> {
                (**self).start_send(item)
            }

            fn poll_complete(&mut self) -> Poll<(), Self::SinkError> {
                (**self).poll_complete()
            }

            fn close(&mut self) -> Poll<(), Self::SinkError> {
                (**self).close()
            }
        }
    }

    pub use self::with::With;
    pub use self::with_flat_map::WithFlatMap;
    pub use self::flush::Flush;
    pub use self::send::Send;
    pub use self::send_all::SendAll;
    pub use self::map_err::SinkMapErr;
    pub use self::from_err::SinkFromErr;
    pub use self::fanout::Fanout;

    /// A `Sink` is a value into which other values can be sent, asynchronously.
    ///
    /// Basic examples of sinks include the sending side of:
    ///
    /// - Channels
    /// - Sockets
    /// - Pipes
    ///
    /// In addition to such "primitive" sinks, it's typical to layer additional
    /// functionality, such as buffering, on top of an existing sink.
    ///
    /// Sending to a sink is "asynchronous" in the sense that the value may not be
    /// sent in its entirety immediately. Instead, values are sent in a two-phase
    /// way: first by initiating a send, and then by polling for completion. This
    /// two-phase setup is analogous to buffered writing in synchronous code, where
    /// writes often succeed immediately, but internally are buffered and are
    /// *actually* written only upon flushing.
    ///
    /// In addition, the `Sink` may be *full*, in which case it is not even possible
    /// to start the sending process.
    ///
    /// As with `Future` and `Stream`, the `Sink` trait is built from a few core
    /// required methods, and a host of default methods for working in a
    /// higher-level way. The `Sink::send_all` combinator is of particular
    /// importance: you can use it to send an entire stream to a sink, which is
    /// the simplest way to ultimately consume a sink.
    ///
    /// You can find more information/tutorials about streams [online at
    /// https://tokio.rs][online]
    ///
    /// [online]: https://tokio.rs/docs/getting-started/streams-and-sinks/
    pub trait Sink {
        /// The type of value that the sink accepts.
        type SinkItem;

        /// The type of value produced by the sink when an error occurs.
        type SinkError;

        /// Begin the process of sending a value to the sink.
        ///
        /// As the name suggests, this method only *begins* the process of sending
        /// the item. If the sink employs buffering, the item isn't fully processed
        /// until the buffer is fully flushed. Since sinks are designed to work with
        /// asynchronous I/O, the process of actually writing out the data to an
        /// underlying object takes place asynchronously. **You *must* use
        /// `poll_complete` in order to drive completion of a send**. In particular,
        /// `start_send` does not begin the flushing process
        ///
        /// # Return value
        ///
        /// This method returns `AsyncSink::Ready` if the sink was able to start
        /// sending `item`. In that case, you *must* ensure that you call
        /// `poll_complete` to process the sent item to completion. Note, however,
        /// that several calls to `start_send` can be made prior to calling
        /// `poll_complete`, which will work on completing all pending items.
        ///
        /// The method returns `AsyncSink::NotReady` if the sink was unable to begin
        /// sending, usually due to being full. The sink must have attempted to
        /// complete processing any outstanding requests (equivalent to
        /// `poll_complete`) before yielding this result. The current task will be
        /// automatically scheduled for notification when the sink may be ready to
        /// receive new values.
        ///
        /// # Errors
        ///
        /// If the sink encounters an error other than being temporarily full, it
        /// uses the `Err` variant to signal that error. In most cases, such errors
        /// mean that the sink will permanently be unable to receive items.
        ///
        /// # Panics
        ///
        /// This method may panic in a few situations, depending on the specific
        /// sink:
        ///
        /// - It is called outside of the context of a task.
        /// - A previous call to `start_send` or `poll_complete` yielded an error.
        fn start_send(&mut self, item: Self::SinkItem)
                      -> StartSend<Self::SinkItem, Self::SinkError>;

        /// Flush all output from this sink, if necessary.
        ///
        /// Some sinks may buffer intermediate data as an optimization to improve
        /// throughput. In other words, if a sink has a corresponding receiver then
        /// a successful `start_send` above may not guarantee that the value is
        /// actually ready to be received by the receiver. This function is intended
        /// to be used to ensure that values do indeed make their way to the
        /// receiver.
        ///
        /// This function will attempt to process any pending requests on behalf of
        /// the sink and drive it to completion.
        ///
        /// # Return value
        ///
        /// Returns `Ok(Async::Ready(()))` when no buffered items remain. If this
        /// value is returned then it is guaranteed that all previous values sent
        /// via `start_send` will be guaranteed to be available to a listening
        /// receiver.
        ///
        /// Returns `Ok(Async::NotReady)` if there is more work left to do, in which
        /// case the current task is scheduled to wake up when more progress may be
        /// possible.
        ///
        /// # Errors
        ///
        /// Returns `Err` if the sink encounters an error while processing one of
        /// its pending requests. Due to the buffered nature of requests, it is not
        /// generally possible to correlate the error with a particular request. As
        /// with `start_send`, these errors are generally "fatal" for continued use
        /// of the sink.
        ///
        /// # Panics
        ///
        /// This method may panic in a few situations, depending on the specific sink:
        ///
        /// - It is called outside of the context of a task.
        /// - A previous call to `start_send` or `poll_complete` yielded an error.
        ///
        /// # Compatibility nodes
        ///
        /// The name of this method may be slightly misleading as the original
        /// intention was to have this method be more general than just flushing
        /// requests. Over time though it was decided to trim back the ambitions of
        /// this method to what it's always done, just flushing.
        ///
        /// In the 0.2 release series of futures this method will be renamed to
        /// `poll_flush`. For 0.1, however, the breaking change is not happening
        /// yet.
        fn poll_complete(&mut self) -> Poll<(), Self::SinkError>;

        /// A method to indicate that no more values will ever be pushed into this
        /// sink.
        ///
        /// This method is used to indicate that a sink will no longer even be given
        /// another value by the caller. That is, the `start_send` method above will
        /// be called no longer (nor `poll_complete`). This method is intended to
        /// model "graceful shutdown" in various protocols where the intent to shut
        /// down is followed by a little more blocking work.
        ///
        /// Callers of this function should work it it in a similar fashion to
        /// `poll_complete`. Once called it may return `NotReady` which indicates
        /// that more external work needs to happen to make progress. The current
        /// task will be scheduled to receive a notification in such an event,
        /// however.
        ///
        /// Note that this function will imply `poll_complete` above. That is, if a
        /// sink has buffered data, then it'll be flushed out during a `close`
        /// operation. It is not necessary to have `poll_complete` return `Ready`
        /// before a `close` is called. Once a `close` is called, though,
        /// `poll_complete` cannot be called.
        ///
        /// # Return value
        ///
        /// This function, like `poll_complete`, returns a `Poll`. The value is
        /// `Ready` once the close operation has completed. At that point it should
        /// be safe to drop the sink and deallocate associated resources.
        ///
        /// If the value returned is `NotReady` then the sink is not yet closed and
        /// work needs to be done to close it. The work has been scheduled and the
        /// current task will receive a notification when it's next ready to call
        /// this method again.
        ///
        /// Finally, this function may also return an error.
        ///
        /// # Errors
        ///
        /// This function will return an `Err` if any operation along the way during
        /// the close operation fails. An error typically is fatal for a sink and is
        /// unable to be recovered from, but in specific situations this may not
        /// always be true.
        ///
        /// Note that it's also typically an error to call `start_send` or
        /// `poll_complete` after the `close` function is called. This method will
        /// *initiate* a close, and continuing to send values after that (or attempt
        /// to flush) may result in strange behavior, panics, errors, etc. Once this
        /// method is called, it must be the only method called on this `Sink`.
        ///
        /// # Panics
        ///
        /// This method may panic or cause panics if:
        ///
        /// * It is called outside the context of a future's task
        /// * It is called and then `start_send` or `poll_complete` is called
        ///
        /// # Compatibility notes
        ///
        /// Note that this function is currently by default a provided function,
        /// defaulted to calling `poll_complete` above. This function was added
        /// in the 0.1 series of the crate as a backwards-compatible addition. It
        /// is intended that in the 0.2 series the method will no longer be a
        /// default method.
        ///
        /// It is highly recommended to consider this method a required method and
        /// to implement it whenever you implement `Sink` locally. It is especially
        /// crucial to be sure to close inner sinks, if applicable.
        #[cfg(feature = "with-deprecated")]
        fn close(&mut self) -> Poll<(), Self::SinkError> {
            self.poll_complete()
        }

        /// dox (you should see the above, not this)
        #[cfg(not(feature = "with-deprecated"))]
        fn close(&mut self) -> Poll<(), Self::SinkError>;

        /// Creates a new object which will produce a synchronous sink.
        ///
        /// The sink returned does **not** implement the `Sink` trait, and instead
        /// only has two methods: `send` and `flush`. These two methods correspond
        /// to `start_send` and `poll_complete` above except are executed in a
        /// blocking fashion.
        #[cfg(feature = "use_std")]
        fn wait(self) -> Wait<Self>
            where Self: Sized
        {
            wait::new(self)
        }

        /// Composes a function *in front of* the sink.
        ///
        /// This adapter produces a new sink that passes each value through the
        /// given function `f` before sending it to `self`.
        ///
        /// To process each value, `f` produces a *future*, which is then polled to
        /// completion before passing its result down to the underlying sink. If the
        /// future produces an error, that error is returned by the new sink.
        ///
        /// Note that this function consumes the given sink, returning a wrapped
        /// version, much like `Iterator::map`.
        fn with<U, F, Fut>(self, f: F) -> With<Self, U, F, Fut>
            where F: FnMut(U) -> Fut,
                  Fut: IntoFuture<Item = Self::SinkItem>,
                  Fut::Error: From<Self::SinkError>,
                  Self: Sized
        {
            with::new(self, f)
        }

        /// Composes a function *in front of* the sink.
        ///
        /// This adapter produces a new sink that passes each value through the
        /// given function `f` before sending it to `self`.
        ///
        /// To process each value, `f` produces a *stream*, of which each value
        /// is passed to the underlying sink. A new value will not be accepted until
        /// the stream has been drained
        ///
        /// Note that this function consumes the given sink, returning a wrapped
        /// version, much like `Iterator::flat_map`.
        ///
        /// # Examples
        /// ---
        /// Using this function with an iterator through use of the `stream::iter_ok()`
        /// function
        ///
        /// ```
        /// use futures::prelude::*;
        /// use futures::stream;
        /// use futures::sync::mpsc;
        ///
        /// let (tx, rx) = mpsc::channel::<i32>(5);
        ///
        /// let tx = tx.with_flat_map(|x| {
        ///     stream::iter_ok(vec![42; x].into_iter().map(|y| y))
        /// });
        /// tx.send(5).wait().unwrap();
        /// assert_eq!(rx.collect().wait(), Ok(vec![42, 42, 42, 42, 42]))
        /// ```
        fn with_flat_map<U, F, St>(self, f: F) -> WithFlatMap<Self, U, F, St>
            where F: FnMut(U) -> St,
                  St: Stream<Item = Self::SinkItem, Error=Self::SinkError>,
                  Self: Sized
            {
                with_flat_map::new(self, f)
            }

        /*
        fn with_map<U, F>(self, f: F) -> WithMap<Self, U, F>
            where F: FnMut(U) -> Self::SinkItem,
                  Self: Sized;

        fn with_filter<F>(self, f: F) -> WithFilter<Self, F>
            where F: FnMut(Self::SinkItem) -> bool,
                  Self: Sized;

        fn with_filter_map<U, F>(self, f: F) -> WithFilterMap<Self, U, F>
            where F: FnMut(U) -> Option<Self::SinkItem>,
                  Self: Sized;
         */

        /// Transforms the error returned by the sink.
        fn sink_map_err<F, E>(self, f: F) -> SinkMapErr<Self, F>
            where F: FnOnce(Self::SinkError) -> E,
                  Self: Sized,
        {
            map_err::new(self, f)
        }

        /// Map this sink's error to any error implementing `From` for this sink's
        /// `Error`, returning a new sink.
        ///
        /// If wanting to map errors of a `Sink + Stream`, use `.sink_from_err().from_err()`.
        fn sink_from_err<E: From<Self::SinkError>>(self) -> from_err::SinkFromErr<Self, E>
            where Self: Sized,
        {
            from_err::new(self)
        }

        /// Adds a fixed-size buffer to the current sink.
        ///
        /// The resulting sink will buffer up to `amt` items when the underlying
        /// sink is unwilling to accept additional items. Calling `poll_complete` on
        /// the buffered sink will attempt to both empty the buffer and complete
        /// processing on the underlying sink.
        ///
        /// Note that this function consumes the given sink, returning a wrapped
        /// version, much like `Iterator::map`.
        ///
        /// This method is only available when the `use_std` feature of this
        /// library is activated, and it is activated by default.
        #[cfg(feature = "use_std")]
        fn buffer(self, amt: usize) -> Buffer<Self>
            where Self: Sized
        {
            buffer::new(self, amt)
        }

        /// Fanout items to multiple sinks.
        ///
        /// This adapter clones each incoming item and forwards it to both this as well as
        /// the other sink at the same time.
        fn fanout<S>(self, other: S) -> Fanout<Self, S>
            where Self: Sized,
                  Self::SinkItem: Clone,
                  S: Sink<SinkItem=Self::SinkItem, SinkError=Self::SinkError>
        {
            fanout::new(self, other)
        }

        /// A future that completes when the sink has finished processing all
        /// pending requests.
        ///
        /// The sink itself is returned after flushing is complete; this adapter is
        /// intended to be used when you want to stop sending to the sink until
        /// all current requests are processed.
        fn flush(self) -> Flush<Self>
            where Self: Sized
        {
            flush::new(self)
        }

        /// A future that completes after the given item has been fully processed
        /// into the sink, including flushing.
        ///
        /// Note that, **because of the flushing requirement, it is usually better
        /// to batch together items to send via `send_all`, rather than flushing
        /// between each item.**
        ///
        /// On completion, the sink is returned.
        fn send(self, item: Self::SinkItem) -> Send<Self>
            where Self: Sized
        {
            send::new(self, item)
        }

        /// A future that completes after the given stream has been fully processed
        /// into the sink, including flushing.
        ///
        /// This future will drive the stream to keep producing items until it is
        /// exhausted, sending each item to the sink. It will complete once both the
        /// stream is exhausted, the sink has received all items, the sink has been
        /// flushed, and the sink has been closed.
        ///
        /// Doing `sink.send_all(stream)` is roughly equivalent to
        /// `stream.forward(sink)`. The returned future will exhaust all items from
        /// `stream` and send them to `self`, closing `self` when all items have been
        /// received.
        ///
        /// On completion, the pair `(sink, source)` is returned.
        fn send_all<S>(self, stream: S) -> SendAll<Self, S>
            where S: Stream<Item = Self::SinkItem>,
                  Self::SinkError: From<S::Error>,
                  Self: Sized
        {
            send_all::new(self, stream)
        }
    }

    impl<'a, S: ?Sized + Sink> Sink for &'a mut S {
        type SinkItem = S::SinkItem;
        type SinkError = S::SinkError;

        fn start_send(&mut self, item: Self::SinkItem)
                      -> StartSend<Self::SinkItem, Self::SinkError> {
            (**self).start_send(item)
        }

        fn poll_complete(&mut self) -> Poll<(), Self::SinkError> {
            (**self).poll_complete()
        }

        fn close(&mut self) -> Poll<(), Self::SinkError> {
            (**self).close()
        }
    }

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