fn flush_if_completed_line(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        let this = self.project();
        match this.buf_writer.buffer().last().copied() {
            Some(b'\n') => this.buf_writer.flush_buf(cx),
            _ => Poll::Ready(Ok(())),
        }
    }
Flush `buf_writer` if last char is "new line"
futures-util/src/io/line_writer.rs