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
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
//! Asynchronous streams.

use core::ops::DerefMut;
use core::pin::Pin;
use core::task::{Context, Poll};

/// An owned dynamically typed [`Stream`] for use in cases where you can't
/// statically type your result or need to add some indirection.
///
/// This type is often created by the [`boxed`] method on [`StreamExt`]. See its documentation for more.
///
/// [`boxed`]: https://docs.rs/futures/latest/futures/stream/trait.StreamExt.html#method.boxed
/// [`StreamExt`]: https://docs.rs/futures/latest/futures/stream/trait.StreamExt.html
#[cfg(feature = "alloc")]
pub type BoxStream<'a, T> = Pin<alloc::boxed::Box<dyn Stream<Item = T> + Send + 'a>>;

/// `BoxStream`, but without the `Send` requirement.
///
/// This type is often created by the [`boxed_local`] method on [`StreamExt`]. See its documentation for more.
///
/// [`boxed_local`]: https://docs.rs/futures/latest/futures/stream/trait.StreamExt.html#method.boxed_local
/// [`StreamExt`]: https://docs.rs/futures/latest/futures/stream/trait.StreamExt.html
#[cfg(feature = "alloc")]
pub type LocalBoxStream<'a, T> = Pin<alloc::boxed::Box<dyn Stream<Item = T> + 'a>>;

/// A stream of values produced asynchronously.
///
/// If `Future<Output = T>` is an asynchronous version of `T`, then `Stream<Item
/// = T>` is an asynchronous version of `Iterator<Item = T>`. A stream
/// represents a sequence of value-producing events that occur asynchronously to
/// the caller.
///
/// The trait is modeled after `Future`, but allows `poll_next` to be called
/// even after a value has been produced, yielding `None` once the stream has
/// been fully exhausted.
#[must_use = "streams do nothing unless polled"]
pub trait Stream {
    /// Values yielded by the stream.
    type Item;

    /// Attempt to pull out the next value of this stream, registering the
    /// current task for wakeup if the value is not yet available, and returning
    /// `None` if the stream is exhausted.
    ///
    /// # Return value
    ///
    /// There are several possible return values, each indicating a distinct
    /// stream state:
    ///
    /// - `Poll::Pending` means that this stream's next value is not ready
    ///   yet. Implementations will ensure that the current task will be notified
    ///   when the next value may be ready.
    ///
    /// - `Poll::Ready(Some(val))` means that the stream has successfully
    ///   produced a value, `val`, and may produce further values on subsequent
    ///   `poll_next` calls.
    ///
    /// - `Poll::Ready(None)` means that the stream has terminated, and
    ///   `poll_next` should not be invoked again.
    ///
    /// # Panics
    ///
    /// Once a stream has finished (returned `Ready(None)` from `poll_next`), calling its
    /// `poll_next` method again may panic, block forever, or cause other kinds of
    /// problems; the `Stream` trait places no requirements on the effects of
    /// such a call. However, as the `poll_next` method is not marked `unsafe`,
    /// Rust's usual rules apply: calls must never cause undefined behavior
    /// (memory corruption, incorrect use of `unsafe` functions, or the like),
    /// regardless of the stream's state.
    ///
    /// If this is difficult to guard against then the [`fuse`] adapter can be used
    /// to ensure that `poll_next` always returns `Ready(None)` in subsequent
    /// calls.
    ///
    /// [`fuse`]: https://docs.rs/futures/0.3/futures/stream/trait.StreamExt.html#method.fuse
    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>>;

    /// Returns the bounds on the remaining length of the stream.
    ///
    /// Specifically, `size_hint()` returns a tuple where the first element
    /// is the lower bound, and the second element is the upper bound.
    ///
    /// The second half of the tuple that is returned is an [`Option`]`<`[`usize`]`>`.
    /// A [`None`] here means that either there is no known upper bound, or the
    /// upper bound is larger than [`usize`].
    ///
    /// # Implementation notes
    ///
    /// It is not enforced that a stream implementation yields the declared
    /// number of elements. A buggy stream may yield less than the lower bound
    /// or more than the upper bound of elements.
    ///
    /// `size_hint()` is primarily intended to be used for optimizations such as
    /// reserving space for the elements of the stream, but must not be
    /// trusted to e.g., omit bounds checks in unsafe code. An incorrect
    /// implementation of `size_hint()` should not lead to memory safety
    /// violations.
    ///
    /// That said, the implementation should provide a correct estimation,
    /// because otherwise it would be a violation of the trait's protocol.
    ///
    /// The default implementation returns `(0, `[`None`]`)` which is correct for any
    /// stream.
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        (0, None)
    }
}

impl<S: ?Sized + Stream + Unpin> Stream for &mut S {
    type Item = S::Item;

    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        S::poll_next(Pin::new(&mut **self), cx)
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        (**self).size_hint()
    }
}

impl<P> Stream for Pin<P>
where
    P: DerefMut + Unpin,
    P::Target: Stream,
{
    type Item = <P::Target as Stream>::Item;

    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        self.get_mut().as_mut().poll_next(cx)
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        (**self).size_hint()
    }
}

/// A stream which tracks whether or not the underlying stream
/// should no longer be polled.
///
/// `is_terminated` will return `true` if a future should no longer be polled.
/// Usually, this state occurs after `poll_next` (or `try_poll_next`) returned
/// `Poll::Ready(None)`. However, `is_terminated` may also return `true` if a
/// stream has become inactive and can no longer make progress and should be
/// ignored or dropped rather than being polled again.
pub trait FusedStream: Stream {
    /// Returns `true` if the stream should no longer be polled.
    fn is_terminated(&self) -> bool;
}

impl<F: ?Sized + FusedStream + Unpin> FusedStream for &mut F {
    fn is_terminated(&self) -> bool {
        <F as FusedStream>::is_terminated(&**self)
    }
}

impl<P> FusedStream for Pin<P>
where
    P: DerefMut + Unpin,
    P::Target: FusedStream,
{
    fn is_terminated(&self) -> bool {
        <P::Target as FusedStream>::is_terminated(&**self)
    }
}

mod private_try_stream {
    use super::Stream;

    pub trait Sealed {}

    impl<S, T, E> Sealed for S where S: ?Sized + Stream<Item = Result<T, E>> {}
}

/// A convenience for streams that return `Result` values that includes
/// a variety of adapters tailored to such futures.
pub trait TryStream: Stream + private_try_stream::Sealed {
    /// The type of successful values yielded by this future
    type Ok;

    /// The type of failures yielded by this future
    type Error;

    /// Poll this `TryStream` as if it were a `Stream`.
    ///
    /// This method is a stopgap for a compiler limitation that prevents us from
    /// directly inheriting from the `Stream` trait; in the future it won't be
    /// needed.
    fn try_poll_next(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Option<Result<Self::Ok, Self::Error>>>;
}

impl<S, T, E> TryStream for S
where
    S: ?Sized + Stream<Item = Result<T, E>>,
{
    type Ok = T;
    type Error = E;

    fn try_poll_next(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Option<Result<Self::Ok, Self::Error>>> {
        self.poll_next(cx)
    }
}

#[cfg(feature = "alloc")]
mod if_alloc {
    use super::*;
    use alloc::boxed::Box;

    impl<S: ?Sized + Stream + Unpin> Stream for Box<S> {
        type Item = S::Item;

        fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
            Pin::new(&mut **self).poll_next(cx)
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            (**self).size_hint()
        }
    }

    #[cfg(feature = "std")]
    impl<S: Stream> Stream for std::panic::AssertUnwindSafe<S> {
        type Item = S::Item;

        fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<S::Item>> {
            unsafe { self.map_unchecked_mut(|x| &mut x.0) }.poll_next(cx)
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            self.0.size_hint()
        }
    }

    impl<S: ?Sized + FusedStream + Unpin> FusedStream for Box<S> {
        fn is_terminated(&self) -> bool {
            <S as FusedStream>::is_terminated(&**self)
        }
    }
}