tokio/io/util/
empty.rs

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
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>();
    }
}