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+//! Asynchronous sinks.
+//!
+//! This module contains:
+//!
+//! - The [`Sink`] trait, which allows you to asynchronously write data.
+//! - The [`SinkExt`] trait, which provides adapters for chaining and composing
+//!   sinks.
+
+use crate::future::{assert_future, Either};
+use core::pin::Pin;
+use futures_core::future::Future;
+use futures_core::stream::{Stream, TryStream};
+use futures_core::task::{Context, Poll};
+
+#[cfg(feature = "compat")]
+use crate::compat::CompatSink;
+
+pub use futures_sink::Sink;
+
+mod close;
+pub use self::close::Close;
+
+mod drain;
+pub use self::drain::{drain, Drain};
+
+mod fanout;
+pub use self::fanout::Fanout;
+
+mod feed;
+pub use self::feed::Feed;
+
+mod flush;
+pub use self::flush::Flush;
+
+mod err_into;
+pub use self::err_into::SinkErrInto;
+
+mod map_err;
+pub use self::map_err::SinkMapErr;
+
+mod send;
+pub use self::send::Send;
+
+mod send_all;
+pub use self::send_all::SendAll;
+
+mod unfold;
+pub use self::unfold::{unfold, Unfold};
+
+mod with;
+pub use self::with::With;
+
+mod with_flat_map;
+pub use self::with_flat_map::WithFlatMap;
+
+#[cfg(feature = "alloc")]
+mod buffer;
+#[cfg(feature = "alloc")]
+pub use self::buffer::Buffer;
+
+impl<T: ?Sized, Item> SinkExt<Item> for T where T: Sink<Item> {}
+
+/// An extension trait for `Sink`s that provides a variety of convenient
+/// combinator functions.
+pub trait SinkExt<Item>: Sink<Item> {
+    /// 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, Fut, F, E>(self, f: F) -> With<Self, Item, U, Fut, F>
+    where
+        F: FnMut(U) -> Fut,
+        Fut: Future<Output = Result<Item, E>>,
+        E: From<Self::Error>,
+        Self: Sized,
+    {
+        assert_sink::<U, E, _>(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
+    ///
+    /// ```
+    /// # futures::executor::block_on(async {
+    /// use futures::channel::mpsc;
+    /// use futures::sink::SinkExt;
+    /// use futures::stream::{self, StreamExt};
+    ///
+    /// let (tx, rx) = mpsc::channel(5);
+    ///
+    /// let mut tx = tx.with_flat_map(|x| {
+    ///     stream::iter(vec![Ok(42); x])
+    /// });
+    ///
+    /// tx.send(5).await.unwrap();
+    /// drop(tx);
+    /// let received: Vec<i32> = rx.collect().await;
+    /// assert_eq!(received, vec![42, 42, 42, 42, 42]);
+    /// # });
+    /// ```
+    fn with_flat_map<U, St, F>(self, f: F) -> WithFlatMap<Self, Item, U, St, F>
+    where
+        F: FnMut(U) -> St,
+        St: Stream<Item = Result<Item, Self::Error>>,
+        Self: Sized,
+    {
+        assert_sink::<U, Self::Error, _>(WithFlatMap::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<E, F>(self, f: F) -> SinkMapErr<Self, F>
+    where
+        F: FnOnce(Self::Error) -> E,
+        Self: Sized,
+    {
+        assert_sink::<Item, E, _>(SinkMapErr::new(self, f))
+    }
+
+    /// Map this sink's error to a different error type using the `Into` trait.
+    ///
+    /// If wanting to map errors of a `Sink + Stream`, use `.sink_err_into().err_into()`.
+    fn sink_err_into<E>(self) -> err_into::SinkErrInto<Self, Item, E>
+    where
+        Self: Sized,
+        Self::Error: Into<E>,
+    {
+        assert_sink::<Item, E, _>(SinkErrInto::new(self))
+    }
+
+    /// Adds a fixed-size buffer to the current sink.
+    ///
+    /// The resulting sink will buffer up to `capacity` items when the
+    /// underlying sink is unwilling to accept additional items. Calling `flush`
+    /// 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 `std` or `alloc` feature of this
+    /// library is activated, and it is activated by default.
+    #[cfg(feature = "alloc")]
+    fn buffer(self, capacity: usize) -> Buffer<Self, Item>
+    where
+        Self: Sized,
+    {
+        assert_sink::<Item, Self::Error, _>(Buffer::new(self, capacity))
+    }
+
+    /// Close the sink.
+    fn close(&mut self) -> Close<'_, Self, Item>
+    where
+        Self: Unpin,
+    {
+        assert_future::<Result<(), Self::Error>, _>(Close::new(self))
+    }
+
+    /// 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<Si>(self, other: Si) -> Fanout<Self, Si>
+    where
+        Self: Sized,
+        Item: Clone,
+        Si: Sink<Item, Error = Self::Error>,
+    {
+        assert_sink::<Item, Self::Error, _>(Fanout::new(self, other))
+    }
+
+    /// Flush the sink, processing all pending items.
+    ///
+    /// This adapter is intended to be used when you want to stop sending to the sink
+    /// until all current requests are processed.
+    fn flush(&mut self) -> Flush<'_, Self, Item>
+    where
+        Self: Unpin,
+    {
+        assert_future::<Result<(), Self::Error>, _>(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 `feed` or `send_all`,
+    /// rather than flushing between each item.**
+    fn send(&mut self, item: Item) -> Send<'_, Self, Item>
+    where
+        Self: Unpin,
+    {
+        assert_future::<Result<(), Self::Error>, _>(Send::new(self, item))
+    }
+
+    /// A future that completes after the given item has been received
+    /// by the sink.
+    ///
+    /// Unlike `send`, the returned future does not flush the sink.
+    /// It is the caller's responsibility to ensure all pending items
+    /// are processed, which can be done via `flush` or `close`.
+    fn feed(&mut self, item: Item) -> Feed<'_, Self, Item>
+    where
+        Self: Unpin,
+    {
+        assert_future::<Result<(), Self::Error>, _>(Feed::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, and the sink has
+    /// been flushed. Note that the sink is **not** closed. If the stream produces
+    /// an error, that error will be returned by this future without flushing the sink.
+    ///
+    /// 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`.
+    fn send_all<'a, St>(&'a mut self, stream: &'a mut St) -> SendAll<'a, Self, St>
+    where
+        St: TryStream<Ok = Item, Error = Self::Error> + Stream + Unpin + ?Sized,
+        // St: Stream<Item = Result<Item, Self::Error>> + Unpin + ?Sized,
+        Self: Unpin,
+    {
+        // TODO: type mismatch resolving `<St as Stream>::Item == std::result::Result<Item, <Self as futures_sink::Sink<Item>>::Error>`
+        // assert_future::<Result<(), Self::Error>, _>(SendAll::new(self, stream))
+        SendAll::new(self, stream)
+    }
+
+    /// Wrap this sink in an `Either` sink, making it the left-hand variant
+    /// of that `Either`.
+    ///
+    /// This can be used in combination with the `right_sink` method to write `if`
+    /// statements that evaluate to different streams in different branches.
+    fn left_sink<Si2>(self) -> Either<Self, Si2>
+    where
+        Si2: Sink<Item, Error = Self::Error>,
+        Self: Sized,
+    {
+        assert_sink::<Item, Self::Error, _>(Either::Left(self))
+    }
+
+    /// Wrap this stream in an `Either` stream, making it the right-hand variant
+    /// of that `Either`.
+    ///
+    /// This can be used in combination with the `left_sink` method to write `if`
+    /// statements that evaluate to different streams in different branches.
+    fn right_sink<Si1>(self) -> Either<Si1, Self>
+    where
+        Si1: Sink<Item, Error = Self::Error>,
+        Self: Sized,
+    {
+        assert_sink::<Item, Self::Error, _>(Either::Right(self))
+    }
+
+    /// Wraps a [`Sink`] into a sink compatible with libraries using
+    /// futures 0.1 `Sink`. Requires the `compat` feature to be enabled.
+    #[cfg(feature = "compat")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "compat")))]
+    fn compat(self) -> CompatSink<Self, Item>
+    where
+        Self: Sized + Unpin,
+    {
+        CompatSink::new(self)
+    }
+
+    /// A convenience method for calling [`Sink::poll_ready`] on [`Unpin`]
+    /// sink types.
+    fn poll_ready_unpin(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>>
+    where
+        Self: Unpin,
+    {
+        Pin::new(self).poll_ready(cx)
+    }
+
+    /// A convenience method for calling [`Sink::start_send`] on [`Unpin`]
+    /// sink types.
+    fn start_send_unpin(&mut self, item: Item) -> Result<(), Self::Error>
+    where
+        Self: Unpin,
+    {
+        Pin::new(self).start_send(item)
+    }
+
+    /// A convenience method for calling [`Sink::poll_flush`] on [`Unpin`]
+    /// sink types.
+    fn poll_flush_unpin(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>>
+    where
+        Self: Unpin,
+    {
+        Pin::new(self).poll_flush(cx)
+    }
+
+    /// A convenience method for calling [`Sink::poll_close`] on [`Unpin`]
+    /// sink types.
+    fn poll_close_unpin(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>>
+    where
+        Self: Unpin,
+    {
+        Pin::new(self).poll_close(cx)
+    }
+}
+
+// Just a helper function to ensure the sinks we're returning all have the
+// right implementations.
+pub(crate) fn assert_sink<T, E, S>(sink: S) -> S
+where
+    S: Sink<T, Error = E>,
+{
+    sink
+}