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    //! Contains utilities for stdout and stderr.
    use crate::io::AsyncWrite;
    use std::pin::Pin;
    use std::task::{Context, Poll};
    /// # Windows
    /// AsyncWrite adapter that finds last char boundary in given buffer and does not write the rest,
    /// if buffer contents seems to be utf8. Otherwise it only trims buffer down to MAX_BUF.
    /// That's why, wrapped writer will always receive well-formed utf-8 bytes.
    /// # Other platforms
    /// passes data to `inner` as is
    #[derive(Debug)]
    pub(crate) struct SplitByUtf8BoundaryIfWindows<W> {
        inner: W,
    }

    impl<W> SplitByUtf8BoundaryIfWindows<W> {
        pub(crate) fn new(inner: W) -> Self {
            Self { inner }
        }
    }

    // this constant is defined by Unicode standard.
    const MAX_BYTES_PER_CHAR: usize = 4;

    // Subject for tweaking here
    const MAGIC_CONST: usize = 8;

    impl<W> crate::io::AsyncWrite for SplitByUtf8BoundaryIfWindows<W>
    where
        W: AsyncWrite + Unpin,
    {
        fn poll_write(
            mut self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            mut buf: &[u8],
        ) -> Poll<Result<usize, std::io::Error>> {
            // just a closure to avoid repetitive code
            let mut call_inner = move |buf| Pin::new(&mut self.inner).poll_write(cx, buf);

            // 1. Only windows stdio can suffer from non-utf8.
            // We also check for `test` so that we can write some tests
            // for further code. Since `AsyncWrite` can always shrink
            // buffer at its discretion, excessive (i.e. in tests) shrinking
            // does not break correctness.
            // 2. If buffer is small, it will not be shrinked.
            // That's why, it's "textness" will not change, so we don't have
            // to fixup it.
            if cfg!(not(any(target_os = "windows", test))) || buf.len() <= crate::io::blocking::MAX_BUF
            {
                return call_inner(buf);
            }

            buf = &buf[..crate::io::blocking::MAX_BUF];

            // Now there are two possibilites.
            // If caller gave is binary buffer, we **should not** shrink it
            // anymore, because excessive shrinking hits performance.
            // If caller gave as binary buffer, we  **must** additionaly
            // shrink it to strip incomplete char at the end of buffer.
            // that's why check we will perform now is allowed to have
            // false-positive.

            // Now let's look at the first MAX_BYTES_PER_CHAR * MAGIC_CONST bytes.
            // if they are (possibly incomplete) utf8, then we can be quite sure
            // that input buffer was utf8.

            let have_to_fix_up = match std::str::from_utf8(&buf[..MAX_BYTES_PER_CHAR * MAGIC_CONST]) {
                Ok(_) => true,
                Err(err) => {
                    let incomplete_bytes = MAX_BYTES_PER_CHAR * MAGIC_CONST - err.valid_up_to();
                    incomplete_bytes < MAX_BYTES_PER_CHAR
                }
            };

            if have_to_fix_up {
                // We must pop several bytes at the end which form incomplete
                // character. To achieve it, we exploit UTF8 encoding:
                // for any code point, all bytes except first start with 0b10 prefix.
                // see https://en.wikipedia.org/wiki/UTF-8#Encoding for details
                let trailing_incomplete_char_size = buf
                    .iter()
                    .rev()
                    .take(MAX_BYTES_PER_CHAR)
                    .position(|byte| *byte < 0b1000_0000 || *byte >= 0b1100_0000)
                    .unwrap_or(0)
                    + 1;
                buf = &buf[..buf.len() - trailing_incomplete_char_size];
            }

            call_inner(buf)
        }

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

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

    #[cfg(test)]
    #[cfg(not(loom))]
    mod tests {
        use crate::io::AsyncWriteExt;
        use std::io;
        use std::pin::Pin;
        use std::task::Context;
        use std::task::Poll;

        const MAX_BUF: usize = 16 * 1024;

        struct TextMockWriter;

        impl crate::io::AsyncWrite for TextMockWriter {
            fn poll_write(
                self: Pin<&mut Self>,
                _cx: &mut Context<'_>,
                buf: &[u8],
            ) -> Poll<Result<usize, io::Error>> {
                assert!(buf.len() <= MAX_BUF);
                assert!(std::str::from_utf8(buf).is_ok());
                Poll::Ready(Ok(buf.len()))
            }

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

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

        struct LoggingMockWriter {
            write_history: Vec<usize>,
        }

        impl LoggingMockWriter {
            fn new() -> Self {
                LoggingMockWriter {
                    write_history: Vec::new(),
                }
            }
        }

        impl crate::io::AsyncWrite for LoggingMockWriter {
            fn poll_write(
                mut self: Pin<&mut Self>,
                _cx: &mut Context<'_>,
                buf: &[u8],
            ) -> Poll<Result<usize, io::Error>> {
                assert!(buf.len() <= MAX_BUF);
                self.write_history.push(buf.len());
                Poll::Ready(Ok(buf.len()))
            }

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

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

        #[test]
        fn test_splitter() {
            let data = str::repeat("█", MAX_BUF);
            let mut wr = super::SplitByUtf8BoundaryIfWindows::new(TextMockWriter);
            let fut = async move {
                wr.write_all(data.as_bytes()).await.unwrap();
            };
            crate::runtime::Builder::new_current_thread()
                .build()
                .unwrap()
                .block_on(fut);
        }

        #[test]
        fn test_pseudo_text() {
            // In this test we write a piece of binary data, whose beginning is
            // text though. We then validate that even in this corner case buffer
            // was not shrinked too much.
            let checked_count = super::MAGIC_CONST * super::MAX_BYTES_PER_CHAR;
            let mut data: Vec<u8> = str::repeat("a", checked_count).into();
            data.extend(std::iter::repeat(0b1010_1010).take(MAX_BUF - checked_count + 1));
            let mut writer = LoggingMockWriter::new();
            let mut splitter = super::SplitByUtf8BoundaryIfWindows::new(&mut writer);
            crate::runtime::Builder::new_current_thread()
                .build()
                .unwrap()
                .block_on(async {
                    splitter.write_all(&data).await.unwrap();
                });
            // Check that at most two writes were performed
            assert!(writer.write_history.len() <= 2);
            // Check that all has been written
            assert_eq!(
                writer.write_history.iter().copied().sum::<usize>(),
                data.len()
            );
            // Check that at most MAX_BYTES_PER_CHAR + 1 (i.e. 5) bytes were shrinked
            // from the buffer: one because it was outside of MAX_BUF boundary, and
            // up to one "utf8 code point".
            assert!(data.len() - writer.write_history[0] <= super::MAX_BYTES_PER_CHAR + 1);
        }
    }