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    //! Asynchronous sinks
    //!
    //! This crate contains the `Sink` trait which allows values to be sent
    //! asynchronously.

    #![cfg_attr(not(feature = "std"), no_std)]
    #![warn(missing_docs, missing_debug_implementations, rust_2018_idioms, unreachable_pub)]
    // It cannot be included in the published code because this lints have false positives in the minimum required version.
    #![cfg_attr(test, warn(single_use_lifetimes))]
    #![warn(clippy::all)]
    #![doc(test(attr(deny(warnings), allow(dead_code, unused_assignments, unused_variables))))]

    #[cfg(feature = "alloc")]
    extern crate alloc;

    use core::ops::DerefMut;
    use core::pin::Pin;
    use core::task::{Context, Poll};

    /// 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 stream.
    #[must_use = "sinks do nothing unless polled"]
    pub trait Sink<Item> {
        /// The type of value produced by the sink when an error occurs.
        type Error;

        /// Attempts to prepare the `Sink` to receive a value.
        ///
        /// This method must be called and return `Poll::Ready(Ok(()))` prior to
        /// each call to `start_send`.
        ///
        /// This method returns `Poll::Ready` once the underlying sink is ready to
        /// receive data. If this method returns `Poll::Pending`, the current task
        /// is registered to be notified (via `cx.waker().wake_by_ref()`) when `poll_ready`
        /// should be called again.
        ///
        /// In most cases, if the sink encounters an error, the sink will
        /// permanently be unable to receive items.
        fn poll_ready(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>>;

        /// Begin the process of sending a value to the sink.
        /// Each call to this function must be preceded by a successful call to
        /// `poll_ready` which returned `Poll::Ready(Ok(()))`.
        ///
        /// 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_flush` or `poll_close` in order to guarantee completion of a
        /// send**.
        ///
        /// Implementations of `poll_ready` and `start_send` will usually involve
        /// flushing behind the scenes in order to make room for new messages.
        /// It is only necessary to call `poll_flush` if you need to guarantee that
        /// *all* of the items placed into the `Sink` have been sent.
        ///
        /// In most cases, if the sink encounters an error, the sink will
        /// permanently be unable to receive items.
        fn start_send(self: Pin<&mut Self>, item: Item) -> Result<(), Self::Error>;

        /// Flush any remaining output from this sink.
        ///
        /// Returns `Poll::Ready(Ok(()))` when no buffered items remain. If this
        /// value is returned then it is guaranteed that all previous values sent
        /// via `start_send` have been flushed.
        ///
        /// Returns `Poll::Pending` if there is more work left to do, in which
        /// case the current task is scheduled (via `cx.waker().wake_by_ref()`) to wake up when
        /// `poll_flush` should be called again.
        ///
        /// In most cases, if the sink encounters an error, the sink will
        /// permanently be unable to receive items.
        fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>>;

        /// Flush any remaining output and close this sink, if necessary.
        ///
        /// Returns `Poll::Ready(Ok(()))` when no buffered items remain and the sink
        /// has been successfully closed.
        ///
        /// Returns `Poll::Pending` if there is more work left to do, in which
        /// case the current task is scheduled (via `cx.waker().wake_by_ref()`) to wake up when
        /// `poll_close` should be called again.
        ///
        /// If this function encounters an error, the sink should be considered to
        /// have failed permanently, and no more `Sink` methods should be called.
        fn poll_close(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>>;
    }

    impl<S: ?Sized + Sink<Item> + Unpin, Item> Sink<Item> for &mut S {
        type Error = S::Error;

        fn poll_ready(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
            Pin::new(&mut **self).poll_ready(cx)
        }

        fn start_send(mut self: Pin<&mut Self>, item: Item) -> Result<(), Self::Error> {
            Pin::new(&mut **self).start_send(item)
        }

        fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
            Pin::new(&mut **self).poll_flush(cx)
        }

        fn poll_close(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
            Pin::new(&mut **self).poll_close(cx)
        }
    }

    impl<P, Item> Sink<Item> for Pin<P>
    where
        P: DerefMut + Unpin,
        P::Target: Sink<Item>,
    {
        type Error = <P::Target as Sink<Item>>::Error;

        fn poll_ready(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
            self.get_mut().as_mut().poll_ready(cx)
        }

        fn start_send(self: Pin<&mut Self>, item: Item) -> Result<(), Self::Error> {
            self.get_mut().as_mut().start_send(item)
        }

        fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
            self.get_mut().as_mut().poll_flush(cx)
        }

        fn poll_close(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
            self.get_mut().as_mut().poll_close(cx)
        }
    }

    #[cfg(feature = "alloc")]
    mod if_alloc {
        use super::*;
        use core::convert::Infallible as Never;

        impl<T> Sink<T> for alloc::vec::Vec<T> {
            type Error = Never;

            fn poll_ready(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
                Poll::Ready(Ok(()))
            }

            fn start_send(self: Pin<&mut Self>, item: T) -> Result<(), Self::Error> {
                // TODO: impl<T> Unpin for Vec<T> {}
                unsafe { self.get_unchecked_mut() }.push(item);
                Ok(())
            }

            fn poll_flush(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
                Poll::Ready(Ok(()))
            }

            fn poll_close(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
                Poll::Ready(Ok(()))
            }
        }

        impl<T> Sink<T> for alloc::collections::VecDeque<T> {
            type Error = Never;

            fn poll_ready(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
                Poll::Ready(Ok(()))
            }

            fn start_send(self: Pin<&mut Self>, item: T) -> Result<(), Self::Error> {
                // TODO: impl<T> Unpin for Vec<T> {}
                unsafe { self.get_unchecked_mut() }.push_back(item);
                Ok(())
            }

            fn poll_flush(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
                Poll::Ready(Ok(()))
            }

            fn poll_close(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
                Poll::Ready(Ok(()))
            }
        }

        impl<S: ?Sized + Sink<Item> + Unpin, Item> Sink<Item> for alloc::boxed::Box<S> {
            type Error = S::Error;

            fn poll_ready(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
                Pin::new(&mut **self).poll_ready(cx)
            }

            fn start_send(mut self: Pin<&mut Self>, item: Item) -> Result<(), Self::Error> {
                Pin::new(&mut **self).start_send(item)
            }

            fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
                Pin::new(&mut **self).poll_flush(cx)
            }

            fn poll_close(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
                Pin::new(&mut **self).poll_close(cx)
            }
        }
    }