Function write_all_buf 

pub(crate) fn write_all_buf<'a, W, B>(
    writer: &'a mut W,
    buf: &'a mut B,
) -> WriteAllBuf<'a, W, B> 
where
    W: AsyncWrite + Unpin,
    B: Buf,

Tries to write some bytes from the given buf to the writer in an asynchronous manner, returning a future.