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    #![allow(clippy::needless_doctest_main)]
    #![warn(
        missing_debug_implementations,
        missing_docs,
        rust_2018_idioms,
        unreachable_pub
    )]
    #![cfg_attr(docsrs, deny(broken_intra_doc_links))]
    #![doc(test(
        no_crate_inject,
        attr(deny(warnings, rust_2018_idioms), allow(dead_code, unused_variables))
    ))]
    #![cfg_attr(docsrs, feature(doc_cfg))]

    //! Utilities for working with Tokio.
    //!
    //! This crate is not versioned in lockstep with the core
    //! [`tokio`] crate. However, `tokio-util` _will_ respect Rust's
    //! semantic versioning policy, especially with regard to breaking changes.
    //!
    //! [`tokio`]: https://docs.rs/tokio

    #[macro_use]
    mod cfg;

    mod loom;

    cfg_codec! {
        pub mod codec;
    }

    cfg_net! {
        pub mod udp;
    }

    cfg_compat! {
        pub mod compat;
    }

    cfg_io! {
        pub mod io;
    }

    cfg_rt! {
        pub mod context;
    }

    pub mod sync;

    pub mod either;

    #[cfg(feature = "time")]
    pub mod time;

    #[cfg(any(feature = "io", feature = "codec"))]
    mod util {
        use tokio::io::{AsyncRead, AsyncWrite, ReadBuf};

        use bytes::{Buf, BufMut};
        use futures_core::ready;
        use std::io::{self, IoSlice};
        use std::mem::MaybeUninit;
        use std::pin::Pin;
        use std::task::{Context, Poll};

        /// Try to read data from an `AsyncRead` into an implementer of the [`BufMut`] trait.
        ///
        /// [`BufMut`]: bytes::Buf
        ///
        /// # Example
        ///
        /// ```
        /// use bytes::{Bytes, BytesMut};
        /// use tokio_stream as stream;
        /// use tokio::io::Result;
        /// use tokio_util::io::{StreamReader, poll_read_buf};
        /// use futures::future::poll_fn;
        /// use std::pin::Pin;
        /// # #[tokio::main]
        /// # async fn main() -> std::io::Result<()> {
        ///
        /// // Create a reader from an iterator. This particular reader will always be
        /// // ready.
        /// let mut read = StreamReader::new(stream::iter(vec![Result::Ok(Bytes::from_static(&[0, 1, 2, 3]))]));
        ///
        /// let mut buf = BytesMut::new();
        /// let mut reads = 0;
        ///
        /// loop {
        ///     reads += 1;
        ///     let n = poll_fn(|cx| poll_read_buf(Pin::new(&mut read), cx, &mut buf)).await?;
        ///
        ///     if n == 0 {
        ///         break;
        ///     }
        /// }
        ///
        /// // one or more reads might be necessary.
        /// assert!(reads >= 1);
        /// assert_eq!(&buf[..], &[0, 1, 2, 3]);
        /// # Ok(())
        /// # }
        /// ```
        #[cfg_attr(not(feature = "io"), allow(unreachable_pub))]
        pub fn poll_read_buf<T: AsyncRead, B: BufMut>(
            io: Pin<&mut T>,
            cx: &mut Context<'_>,
            buf: &mut B,
        ) -> Poll<io::Result<usize>> {
            if !buf.has_remaining_mut() {
                return Poll::Ready(Ok(0));
            }

            let n = {
                let dst = buf.chunk_mut();
                let dst = unsafe { &mut *(dst as *mut _ as *mut [MaybeUninit<u8>]) };
                let mut buf = ReadBuf::uninit(dst);
                let ptr = buf.filled().as_ptr();
                ready!(io.poll_read(cx, &mut buf)?);

                // Ensure the pointer does not change from under us
                assert_eq!(ptr, buf.filled().as_ptr());
                buf.filled().len()
            };

            // Safety: This is guaranteed to be the number of initialized (and read)
            // bytes due to the invariants provided by `ReadBuf::filled`.
            unsafe {
                buf.advance_mut(n);
            }

            Poll::Ready(Ok(n))
        }

        /// Try to write data from an implementer of the [`Buf`] trait to an
        /// [`AsyncWrite`], advancing the buffer's internal cursor.
        ///
        /// This function will use [vectored writes] when the [`AsyncWrite`] supports
        /// vectored writes.
        ///
        /// # Examples
        ///
        /// [`File`] implements [`AsyncWrite`] and [`Cursor<&[u8]>`] implements
        /// [`Buf`]:
        ///
        /// ```no_run
        /// use tokio_util::io::poll_write_buf;
        /// use tokio::io;
        /// use tokio::fs::File;
        ///
        /// use bytes::Buf;
        /// use std::io::Cursor;
        /// use std::pin::Pin;
        /// use futures::future::poll_fn;
        ///
        /// #[tokio::main]
        /// async fn main() -> io::Result<()> {
        ///     let mut file = File::create("foo.txt").await?;
        ///     let mut buf = Cursor::new(b"data to write");
        ///
        ///     // Loop until the entire contents of the buffer are written to
        ///     // the file.
        ///     while buf.has_remaining() {
        ///         poll_fn(|cx| poll_write_buf(Pin::new(&mut file), cx, &mut buf)).await?;
        ///     }
        ///
        ///     Ok(())
        /// }
        /// ```
        ///
        /// [`Buf`]: bytes::Buf
        /// [`AsyncWrite`]: tokio::io::AsyncWrite
        /// [`File`]: tokio::fs::File
        /// [vectored writes]: tokio::io::AsyncWrite::poll_write_vectored
        #[cfg_attr(not(feature = "io"), allow(unreachable_pub))]
        pub fn poll_write_buf<T: AsyncWrite, B: Buf>(
            io: Pin<&mut T>,
            cx: &mut Context<'_>,
            buf: &mut B,
        ) -> Poll<io::Result<usize>> {
            const MAX_BUFS: usize = 64;

            if !buf.has_remaining() {
                return Poll::Ready(Ok(0));
            }

            let n = if io.is_write_vectored() {
                let mut slices = [IoSlice::new(&[]); MAX_BUFS];
                let cnt = buf.chunks_vectored(&mut slices);
                ready!(io.poll_write_vectored(cx, &slices[..cnt]))?
            } else {
                ready!(io.poll_write(cx, buf.chunk()))?
            };

            buf.advance(n);

            Poll::Ready(Ok(n))
        }
    }