/rust/registry/src/index.crates.io-1949cf8c6b5b557f/futures-util-0.3.28/src/future/either.rs

Source
use core::pin::Pin;
use core::task::{Context, Poll};
use futures_core::future::{FusedFuture, Future};
use futures_core::stream::{FusedStream, Stream};
#[cfg(feature = "sink")]
use futures_sink::Sink;

/// Combines two different futures, streams, or sinks having the same associated types into a single type.
///
/// This is useful when conditionally choosing between two distinct future types:
///
/// ```rust
/// use futures::future::Either;
///
/// # futures::executor::block_on(async {
/// let cond = true;
///
/// let fut = if cond {
///     Either::Left(async move { 12 })
/// } else {
///     Either::Right(async move { 44 })
/// };
///
/// assert_eq!(fut.await, 12);
/// # })
/// ```
#[derive(Debug, Clone)]
pub enum Either<A, B> {
    /// First branch of the type
    Left(/* #[pin] */ A),
    /// Second branch of the type
    Right(/* #[pin] */ B),
}

impl<A, B> Either<A, B> {
    /// Convert `Pin<&Either<A, B>>` to `Either<Pin<&A>, Pin<&B>>`,
    /// pinned projections of the inner variants.
    pub fn as_pin_ref(self: Pin<&Self>) -> Either<Pin<&A>, Pin<&B>> {
        // SAFETY: We can use `new_unchecked` because the `inner` parts are
        // guaranteed to be pinned, as they come from `self` which is pinned.
        unsafe {
            match *Pin::get_ref(self) {
                Either::Left(ref inner) => Either::Left(Pin::new_unchecked(inner)),
                Either::Right(ref inner) => Either::Right(Pin::new_unchecked(inner)),
            }
        }
    }

    /// Convert `Pin<&mut Either<A, B>>` to `Either<Pin<&mut A>, Pin<&mut B>>`,
    /// pinned projections of the inner variants.
    pub fn as_pin_mut(self: Pin<&mut Self>) -> Either<Pin<&mut A>, Pin<&mut B>> {
        // SAFETY: `get_unchecked_mut` is fine because we don't move anything.
        // We can use `new_unchecked` because the `inner` parts are guaranteed
        // to be pinned, as they come from `self` which is pinned, and we never
        // offer an unpinned `&mut A` or `&mut B` through `Pin<&mut Self>`. We
        // also don't have an implementation of `Drop`, nor manual `Unpin`.
        unsafe {
            match *Pin::get_unchecked_mut(self) {
                Either::Left(ref mut inner) => Either::Left(Pin::new_unchecked(inner)),
                Either::Right(ref mut inner) => Either::Right(Pin::new_unchecked(inner)),
            }
        }
    }
}

impl<A, B, T> Either<(T, A), (T, B)> {
    /// Factor out a homogeneous type from an either of pairs.
    ///
    /// Here, the homogeneous type is the first element of the pairs.
    pub fn factor_first(self) -> (T, Either<A, B>) {
        match self {
            Either::Left((x, a)) => (x, Either::Left(a)),
            Either::Right((x, b)) => (x, Either::Right(b)),
        }
    }
}

impl<A, B, T> Either<(A, T), (B, T)> {
    /// Factor out a homogeneous type from an either of pairs.
    ///
    /// Here, the homogeneous type is the second element of the pairs.
    pub fn factor_second(self) -> (Either<A, B>, T) {
        match self {
            Either::Left((a, x)) => (Either::Left(a), x),
            Either::Right((b, x)) => (Either::Right(b), x),
        }
    }
}

impl<T> Either<T, T> {
    /// Extract the value of an either over two equivalent types.
    pub fn into_inner(self) -> T {
        match self {
            Either::Left(x) => x,
            Either::Right(x) => x,
        }
    }
}

impl<A, B> Future for Either<A, B>
where
    A: Future,
    B: Future<Output = A::Output>,
{
    type Output = A::Output;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        match self.as_pin_mut() {
            Either::Left(x) => x.poll(cx),
            Either::Right(x) => x.poll(cx),
        }
    }
}

impl<A, B> FusedFuture for Either<A, B>
where
    A: FusedFuture,
    B: FusedFuture<Output = A::Output>,
{
    fn is_terminated(&self) -> bool {
        match self {
            Either::Left(x) => x.is_terminated(),
            Either::Right(x) => x.is_terminated(),
        }
    }
}

impl<A, B> Stream for Either<A, B>
where
    A: Stream,
    B: Stream<Item = A::Item>,
{
    type Item = A::Item;

    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        match self.as_pin_mut() {
            Either::Left(x) => x.poll_next(cx),
            Either::Right(x) => x.poll_next(cx),
        }
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        match self {
            Either::Left(x) => x.size_hint(),
            Either::Right(x) => x.size_hint(),
        }
    }
}

impl<A, B> FusedStream for Either<A, B>
where
    A: FusedStream,
    B: FusedStream<Item = A::Item>,
{
    fn is_terminated(&self) -> bool {
        match self {
            Either::Left(x) => x.is_terminated(),
            Either::Right(x) => x.is_terminated(),
        }
    }
}

#[cfg(feature = "sink")]
impl<A, B, Item> Sink<Item> for Either<A, B>
where
    A: Sink<Item>,
    B: Sink<Item, Error = A::Error>,
{
    type Error = A::Error;

    fn poll_ready(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
        match self.as_pin_mut() {
            Either::Left(x) => x.poll_ready(cx),
            Either::Right(x) => x.poll_ready(cx),
        }
    }

    fn start_send(self: Pin<&mut Self>, item: Item) -> Result<(), Self::Error> {
        match self.as_pin_mut() {
            Either::Left(x) => x.start_send(item),
            Either::Right(x) => x.start_send(item),
        }
    }

    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
        match self.as_pin_mut() {
            Either::Left(x) => x.poll_flush(cx),
            Either::Right(x) => x.poll_flush(cx),
        }
    }

    fn poll_close(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
        match self.as_pin_mut() {
            Either::Left(x) => x.poll_close(cx),
            Either::Right(x) => x.poll_close(cx),
        }
    }
}

#[cfg(feature = "io")]
#[cfg(feature = "std")]
mod if_std {
    use super::*;

    use core::pin::Pin;
    use core::task::{Context, Poll};
    use futures_io::{
        AsyncBufRead, AsyncRead, AsyncSeek, AsyncWrite, IoSlice, IoSliceMut, Result, SeekFrom,
    };

    impl<A, B> AsyncRead for Either<A, B>
    where
        A: AsyncRead,
        B: AsyncRead,
    {
        fn poll_read(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            buf: &mut [u8],
        ) -> Poll<Result<usize>> {
            match self.as_pin_mut() {
                Either::Left(x) => x.poll_read(cx, buf),
                Either::Right(x) => x.poll_read(cx, buf),
            }
        }

        fn poll_read_vectored(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            bufs: &mut [IoSliceMut<'_>],
        ) -> Poll<Result<usize>> {
            match self.as_pin_mut() {
                Either::Left(x) => x.poll_read_vectored(cx, bufs),
                Either::Right(x) => x.poll_read_vectored(cx, bufs),
            }
        }
    }

    impl<A, B> AsyncWrite for Either<A, B>
    where
        A: AsyncWrite,
        B: AsyncWrite,
    {
        fn poll_write(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            buf: &[u8],
        ) -> Poll<Result<usize>> {
            match self.as_pin_mut() {
                Either::Left(x) => x.poll_write(cx, buf),
                Either::Right(x) => x.poll_write(cx, buf),
            }
        }

        fn poll_write_vectored(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            bufs: &[IoSlice<'_>],
        ) -> Poll<Result<usize>> {
            match self.as_pin_mut() {
                Either::Left(x) => x.poll_write_vectored(cx, bufs),
                Either::Right(x) => x.poll_write_vectored(cx, bufs),
            }
        }

        fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<()>> {
            match self.as_pin_mut() {
                Either::Left(x) => x.poll_flush(cx),
                Either::Right(x) => x.poll_flush(cx),
            }
        }

        fn poll_close(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<()>> {
            match self.as_pin_mut() {
                Either::Left(x) => x.poll_close(cx),
                Either::Right(x) => x.poll_close(cx),
            }
        }
    }

    impl<A, B> AsyncSeek for Either<A, B>
    where
        A: AsyncSeek,
        B: AsyncSeek,
    {
        fn poll_seek(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            pos: SeekFrom,
        ) -> Poll<Result<u64>> {
            match self.as_pin_mut() {
                Either::Left(x) => x.poll_seek(cx, pos),
                Either::Right(x) => x.poll_seek(cx, pos),
            }
        }
    }

    impl<A, B> AsyncBufRead for Either<A, B>
    where
        A: AsyncBufRead,
        B: AsyncBufRead,
    {
        fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<&[u8]>> {
            match self.as_pin_mut() {
                Either::Left(x) => x.poll_fill_buf(cx),
                Either::Right(x) => x.poll_fill_buf(cx),
            }
        }

        fn consume(self: Pin<&mut Self>, amt: usize) {
            match self.as_pin_mut() {
                Either::Left(x) => x.consume(amt),
                Either::Right(x) => x.consume(amt),
            }
        }
    }
}

Coverage Report

Created: 2026-08-14 06:55

Line	Count	Source
1		use core::pin::Pin;
2		use core::task::{Context, Poll};
3		use futures_core::future::{FusedFuture, Future};
4		use futures_core::stream::{FusedStream, Stream};
5		#[cfg(feature = "sink")]
6		use futures_sink::Sink;
7
8		/// Combines two different futures, streams, or sinks having the same associated types into a single type.
9		///
10		/// This is useful when conditionally choosing between two distinct future types:
11		///
12		/// ```rust
13		/// use futures::future::Either;
14		///
15		/// # futures::executor::block_on(async {
16		/// let cond = true;
17		///
18		/// let fut = if cond {
19		/// Either::Left(async move { 12 })
20		/// } else {
21		/// Either::Right(async move { 44 })
22		/// };
23		///
24		/// assert_eq!(fut.await, 12);
25		/// # })
26		/// ```
27		#[derive(Debug, Clone)]
28		pub enum Either<A, B> {
29		/// First branch of the type
30		Left(/* #[pin] */ A),
31		/// Second branch of the type
32		Right(/* #[pin] */ B),
33		}
34
35		impl<A, B> Either<A, B> {
36		/// Convert `Pin<&Either<A, B>>` to `Either<Pin<&A>, Pin<&B>>`,
37		/// pinned projections of the inner variants.
38	0	pub fn as_pin_ref(self: Pin<&Self>) -> Either<Pin<&A>, Pin<&B>> {
39		// SAFETY: We can use `new_unchecked` because the `inner` parts are
40		// guaranteed to be pinned, as they come from `self` which is pinned.
41		unsafe {
42	0	match *Pin::get_ref(self) {
43	0	Either::Left(ref inner) => Either::Left(Pin::new_unchecked(inner)),
44	0	Either::Right(ref inner) => Either::Right(Pin::new_unchecked(inner)),
45		}
46		}
47	0	}
48
49		/// Convert `Pin<&mut Either<A, B>>` to `Either<Pin<&mut A>, Pin<&mut B>>`,
50		/// pinned projections of the inner variants.
51	0	pub fn as_pin_mut(self: Pin<&mut Self>) -> Either<Pin<&mut A>, Pin<&mut B>> {
52		// SAFETY: `get_unchecked_mut` is fine because we don't move anything.
53		// We can use `new_unchecked` because the `inner` parts are guaranteed
54		// to be pinned, as they come from `self` which is pinned, and we never
55		// offer an unpinned `&mut A` or `&mut B` through `Pin<&mut Self>`. We
56		// also don't have an implementation of `Drop`, nor manual `Unpin`.
57		unsafe {
58	0	match *Pin::get_unchecked_mut(self) {
59	0	Either::Left(ref mut inner) => Either::Left(Pin::new_unchecked(inner)),
60	0	Either::Right(ref mut inner) => Either::Right(Pin::new_unchecked(inner)),
61		}
62		}
63	0	}
64		}
65
66		impl<A, B, T> Either<(T, A), (T, B)> {
67		/// Factor out a homogeneous type from an either of pairs.
68		///
69		/// Here, the homogeneous type is the first element of the pairs.
70	0	pub fn factor_first(self) -> (T, Either<A, B>) {
71	0	match self {
72	0	Either::Left((x, a)) => (x, Either::Left(a)),
73	0	Either::Right((x, b)) => (x, Either::Right(b)),
74		}
75	0	}
76		}
77
78		impl<A, B, T> Either<(A, T), (B, T)> {
79		/// Factor out a homogeneous type from an either of pairs.
80		///
81		/// Here, the homogeneous type is the second element of the pairs.
82	0	pub fn factor_second(self) -> (Either<A, B>, T) {
83	0	match self {
84	0	Either::Left((a, x)) => (Either::Left(a), x),
85	0	Either::Right((b, x)) => (Either::Right(b), x),
86		}
87	0	}
88		}
89
90		impl<T> Either<T, T> {
91		/// Extract the value of an either over two equivalent types.
92	0	pub fn into_inner(self) -> T {
93	0	match self {
94	0	Either::Left(x) => x,
95	0	Either::Right(x) => x,
96		}
97	0	}
98		}
99
100		impl<A, B> Future for Either<A, B>
101		where
102		A: Future,
103		B: Future<Output = A::Output>,
104		{
105		type Output = A::Output;
106
107	0	fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
108	0	match self.as_pin_mut() {
109	0	Either::Left(x) => x.poll(cx),
110	0	Either::Right(x) => x.poll(cx),
111		}
112	0	}
113		}
114
115		impl<A, B> FusedFuture for Either<A, B>
116		where
117		A: FusedFuture,
118		B: FusedFuture<Output = A::Output>,
119		{
120	0	fn is_terminated(&self) -> bool {
121	0	match self {
122	0	Either::Left(x) => x.is_terminated(),
123	0	Either::Right(x) => x.is_terminated(),
124		}
125	0	}
126		}
127
128		impl<A, B> Stream for Either<A, B>
129		where
130		A: Stream,
131		B: Stream<Item = A::Item>,
132		{
133		type Item = A::Item;
134
135	0	fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
136	0	match self.as_pin_mut() {
137	0	Either::Left(x) => x.poll_next(cx),
138	0	Either::Right(x) => x.poll_next(cx),
139		}
140	0	}
141
142	0	fn size_hint(&self) -> (usize, Option<usize>) {
143	0	match self {
144	0	Either::Left(x) => x.size_hint(),
145	0	Either::Right(x) => x.size_hint(),
146		}
147	0	}
148		}
149
150		impl<A, B> FusedStream for Either<A, B>
151		where
152		A: FusedStream,
153		B: FusedStream<Item = A::Item>,
154		{
155	0	fn is_terminated(&self) -> bool {
156	0	match self {
157	0	Either::Left(x) => x.is_terminated(),
158	0	Either::Right(x) => x.is_terminated(),
159		}
160	0	}
161		}
162
163		#[cfg(feature = "sink")]
164		impl<A, B, Item> Sink<Item> for Either<A, B>
165		where
166		A: Sink<Item>,
167		B: Sink<Item, Error = A::Error>,
168		{
169		type Error = A::Error;
170
171	0	fn poll_ready(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
172	0	match self.as_pin_mut() {
173	0	Either::Left(x) => x.poll_ready(cx),
174	0	Either::Right(x) => x.poll_ready(cx),
175		}
176	0	}
177
178	0	fn start_send(self: Pin<&mut Self>, item: Item) -> Result<(), Self::Error> {
179	0	match self.as_pin_mut() {
180	0	Either::Left(x) => x.start_send(item),
181	0	Either::Right(x) => x.start_send(item),
182		}
183	0	}
184
185	0	fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
186	0	match self.as_pin_mut() {
187	0	Either::Left(x) => x.poll_flush(cx),
188	0	Either::Right(x) => x.poll_flush(cx),
189		}
190	0	}
191
192	0	fn poll_close(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
193	0	match self.as_pin_mut() {
194	0	Either::Left(x) => x.poll_close(cx),
195	0	Either::Right(x) => x.poll_close(cx),
196		}
197	0	}
198		}
199
200		#[cfg(feature = "io")]
201		#[cfg(feature = "std")]
202		mod if_std {
203		use super::*;
204
205		use core::pin::Pin;
206		use core::task::{Context, Poll};
207		use futures_io::{
208		AsyncBufRead, AsyncRead, AsyncSeek, AsyncWrite, IoSlice, IoSliceMut, Result, SeekFrom,
209		};
210
211		impl<A, B> AsyncRead for Either<A, B>
212		where
213		A: AsyncRead,
214		B: AsyncRead,
215		{
216	0	fn poll_read(
217	0	self: Pin<&mut Self>,
218	0	cx: &mut Context<'_>,
219	0	buf: &mut [u8],
220	0	) -> Poll<Result<usize>> {
221	0	match self.as_pin_mut() {
222	0	Either::Left(x) => x.poll_read(cx, buf),
223	0	Either::Right(x) => x.poll_read(cx, buf),
224		}
225	0	}
226
227	0	fn poll_read_vectored(
228	0	self: Pin<&mut Self>,
229	0	cx: &mut Context<'_>,
230	0	bufs: &mut [IoSliceMut<'_>],
231	0	) -> Poll<Result<usize>> {
232	0	match self.as_pin_mut() {
233	0	Either::Left(x) => x.poll_read_vectored(cx, bufs),
234	0	Either::Right(x) => x.poll_read_vectored(cx, bufs),
235		}
236	0	}
237		}
238
239		impl<A, B> AsyncWrite for Either<A, B>
240		where
241		A: AsyncWrite,
242		B: AsyncWrite,
243		{
244	0	fn poll_write(
245	0	self: Pin<&mut Self>,
246	0	cx: &mut Context<'_>,
247	0	buf: &[u8],
248	0	) -> Poll<Result<usize>> {
249	0	match self.as_pin_mut() {
250	0	Either::Left(x) => x.poll_write(cx, buf),
251	0	Either::Right(x) => x.poll_write(cx, buf),
252		}
253	0	}
254
255	0	fn poll_write_vectored(
256	0	self: Pin<&mut Self>,
257	0	cx: &mut Context<'_>,
258	0	bufs: &[IoSlice<'_>],
259	0	) -> Poll<Result<usize>> {
260	0	match self.as_pin_mut() {
261	0	Either::Left(x) => x.poll_write_vectored(cx, bufs),
262	0	Either::Right(x) => x.poll_write_vectored(cx, bufs),
263		}
264	0	}
265
266	0	fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<()>> {
267	0	match self.as_pin_mut() {
268	0	Either::Left(x) => x.poll_flush(cx),
269	0	Either::Right(x) => x.poll_flush(cx),
270		}
271	0	}
272
273	0	fn poll_close(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<()>> {
274	0	match self.as_pin_mut() {
275	0	Either::Left(x) => x.poll_close(cx),
276	0	Either::Right(x) => x.poll_close(cx),
277		}
278	0	}
279		}
280
281		impl<A, B> AsyncSeek for Either<A, B>
282		where
283		A: AsyncSeek,
284		B: AsyncSeek,
285		{
286	0	fn poll_seek(
287	0	self: Pin<&mut Self>,
288	0	cx: &mut Context<'_>,
289	0	pos: SeekFrom,
290	0	) -> Poll<Result<u64>> {
291	0	match self.as_pin_mut() {
292	0	Either::Left(x) => x.poll_seek(cx, pos),
293	0	Either::Right(x) => x.poll_seek(cx, pos),
294		}
295	0	}
296		}
297
298		impl<A, B> AsyncBufRead for Either<A, B>
299		where
300		A: AsyncBufRead,
301		B: AsyncBufRead,
302		{
303	0	fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<&[u8]>> {
304	0	match self.as_pin_mut() {
305	0	Either::Left(x) => x.poll_fill_buf(cx),
306	0	Either::Right(x) => x.poll_fill_buf(cx),
307		}
308	0	}
309
310	0	fn consume(self: Pin<&mut Self>, amt: usize) {
311	0	match self.as_pin_mut() {
312	0	Either::Left(x) => x.consume(amt),
313	0	Either::Right(x) => x.consume(amt),
314		}
315	0	}
316		}
317		}