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use crate::io::{AsyncBufRead, AsyncRead, ReadBuf};
use std::fmt;
use std::io;
use std::pin::Pin;
use std::task::{Context, Poll};
cfg_io_util! {
/// An async reader which is always at EOF.
///
/// 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 new empty async reader.
///
/// All reads from the returned reader 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());
/// }
/// ```
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 fmt::Debug for Empty {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.pad("Empty { .. }")
}
}
cfg_coop! {
fn poll_proceed_and_make_progress(cx: &mut Context<'_>) -> Poll<()> {
let coop = ready!(crate::runtime::coop::poll_proceed(cx));
coop.made_progress();
Poll::Ready(())
}
}
cfg_not_coop! {
fn poll_proceed_and_make_progress(_: &mut Context<'_>) -> Poll<()> {
Poll::Ready(())
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn assert_unpin() {
crate::is_unpin::<Empty>();
}
}