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use crate::io::util::poll_proceed_and_make_progress;
use crate::io::{AsyncBufRead, AsyncRead, AsyncSeek, AsyncWrite, ReadBuf};
use std::fmt;
use std::io::{self, SeekFrom};
use std::pin::Pin;
use std::task::{ready, Context, Poll};
cfg_io_util! {
/// `Empty` ignores any data written via [`AsyncWrite`], and will always be empty
/// (returning zero bytes) when read via [`AsyncRead`].
///
/// This struct is generally created by calling [`empty`]. Please see
/// the documentation of [`empty()`][`empty`] for more details.
///
/// This is an asynchronous version of [`std::io::empty`][std].
///
/// [`empty`]: fn@empty
/// [std]: std::io::empty
pub struct Empty {
_p: (),
}
/// Creates a value that is always at EOF for reads, and ignores all data written.
///
/// All writes on the returned instance will return `Poll::Ready(Ok(buf.len()))`
/// and the contents of the buffer will not be inspected.
///
/// All reads from the returned instance will return `Poll::Ready(Ok(0))`.
///
/// This is an asynchronous version of [`std::io::empty`][std].
///
/// [std]: std::io::empty
///
/// # Examples
///
/// A slightly sad example of not reading anything into a buffer:
///
/// ```
/// use tokio::io::{self, AsyncReadExt};
///
/// #[tokio::main]
/// async fn main() {
/// let mut buffer = String::new();
/// io::empty().read_to_string(&mut buffer).await.unwrap();
/// assert!(buffer.is_empty());
/// }
/// ```
///
/// A convoluted way of getting the length of a buffer:
///
/// ```
/// use tokio::io::{self, AsyncWriteExt};
///
/// #[tokio::main]
/// async fn main() {
/// let buffer = vec![1, 2, 3, 5, 8];
/// let num_bytes = io::empty().write(&buffer).await.unwrap();
/// assert_eq!(num_bytes, 5);
/// }
/// ```
pub fn empty() -> Empty {
Empty { _p: () }
}
}
impl AsyncRead for Empty {
#[inline]
fn poll_read(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
_: &mut ReadBuf<'_>,
) -> Poll<io::Result<()>> {
ready!(crate::trace::trace_leaf(cx));
ready!(poll_proceed_and_make_progress(cx));
Poll::Ready(Ok(()))
}
}
impl AsyncBufRead for Empty {
#[inline]
fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
ready!(crate::trace::trace_leaf(cx));
ready!(poll_proceed_and_make_progress(cx));
Poll::Ready(Ok(&[]))
}
#[inline]
fn consume(self: Pin<&mut Self>, _: usize) {}
}
impl AsyncWrite for Empty {
#[inline]
fn poll_write(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
buf: &[u8],
) -> Poll<io::Result<usize>> {
ready!(crate::trace::trace_leaf(cx));
ready!(poll_proceed_and_make_progress(cx));
Poll::Ready(Ok(buf.len()))
}
#[inline]
fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
ready!(crate::trace::trace_leaf(cx));
ready!(poll_proceed_and_make_progress(cx));
Poll::Ready(Ok(()))
}
#[inline]
fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
ready!(crate::trace::trace_leaf(cx));
ready!(poll_proceed_and_make_progress(cx));
Poll::Ready(Ok(()))
}
#[inline]
fn is_write_vectored(&self) -> bool {
true
}
#[inline]
fn poll_write_vectored(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
bufs: &[io::IoSlice<'_>],
) -> Poll<Result<usize, io::Error>> {
ready!(crate::trace::trace_leaf(cx));
ready!(poll_proceed_and_make_progress(cx));
let num_bytes = bufs.iter().map(|b| b.len()).sum();
Poll::Ready(Ok(num_bytes))
}
}
impl AsyncSeek for Empty {
#[inline]
fn start_seek(self: Pin<&mut Self>, _position: SeekFrom) -> io::Result<()> {
Ok(())
}
#[inline]
fn poll_complete(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<u64>> {
ready!(crate::trace::trace_leaf(cx));
ready!(poll_proceed_and_make_progress(cx));
Poll::Ready(Ok(0))
}
}
impl fmt::Debug for Empty {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.pad("Empty { .. }")
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn assert_unpin() {
crate::is_unpin::<Empty>();
}
}