1 | use futures_core::task::{Context, Poll}; |
2 | use futures_io::{AsyncBufRead, AsyncRead}; |
3 | use std::fmt; |
4 | use std::io; |
5 | use std::pin::Pin; |
6 | |
7 | /// Reader for the [`empty()`] function. |
8 | #[must_use = "readers do nothing unless polled" ] |
9 | pub 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 | /// ``` |
31 | pub fn empty() -> Empty { |
32 | Empty { _priv: () } |
33 | } |
34 | |
35 | impl 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 | |
46 | impl 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 | |
55 | impl fmt::Debug for Empty { |
56 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
57 | f.pad("Empty { .. }" ) |
58 | } |
59 | } |
60 | |