1use futures_core::task::{Context, Poll};
2use futures_io::{AsyncBufRead, AsyncRead};
3use std::fmt;
4use std::io;
5use std::pin::Pin;
6
7/// Reader for the [`empty()`] function.
8#[must_use = "readers do nothing unless polled"]
9pub struct Empty {
10 _priv: (),
11}
12
13/// Constructs a new handle to an empty reader.
14///
15/// All reads from the returned reader will return `Poll::Ready(Ok(0))`.
16///
17/// # Examples
18///
19/// A slightly sad example of not reading anything into a buffer:
20///
21/// ```
22/// # futures::executor::block_on(async {
23/// use futures::io::{self, AsyncReadExt};
24///
25/// let mut buffer = String::new();
26/// let mut reader = io::empty();
27/// reader.read_to_string(&mut buffer).await?;
28/// assert!(buffer.is_empty());
29/// # Ok::<(), Box<dyn std::error::Error>>(()) }).unwrap();
30/// ```
31pub fn empty() -> Empty {
32 Empty { _priv: () }
33}
34
35impl AsyncRead for Empty {
36 #[inline]
37 fn poll_read(
38 self: Pin<&mut Self>,
39 _: &mut Context<'_>,
40 _: &mut [u8],
41 ) -> Poll<io::Result<usize>> {
42 Poll::Ready(Ok(0))
43 }
44}
45
46impl AsyncBufRead for Empty {
47 #[inline]
48 fn poll_fill_buf(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
49 Poll::Ready(Ok(&[]))
50 }
51 #[inline]
52 fn consume(self: Pin<&mut Self>, _: usize) {}
53}
54
55impl fmt::Debug for Empty {
56 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
57 f.pad("Empty { .. }")
58 }
59}
60