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
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
//! Stream utilities for Tokio.
//!
//! A `Stream` is an asynchronous sequence of values. It can be thought of as an asynchronous version of the standard library's `Iterator` trait.
//!
//! This module provides helpers to work with them.

mod all;
use all::AllFuture;

mod any;
use any::AnyFuture;

mod chain;
use chain::Chain;

mod collect;
use collect::Collect;
pub use collect::FromStream;

mod empty;
pub use empty::{empty, Empty};

mod filter;
use filter::Filter;

mod filter_map;
use filter_map::FilterMap;

mod fold;
use fold::FoldFuture;

mod fuse;
use fuse::Fuse;

mod iter;
pub use iter::{iter, Iter};

mod map;
use map::Map;

mod merge;
use merge::Merge;

mod next;
use next::Next;

mod once;
pub use once::{once, Once};

mod pending;
pub use pending::{pending, Pending};

mod stream_map;
pub use stream_map::StreamMap;

mod skip;
use skip::Skip;

mod skip_while;
use skip_while::SkipWhile;

mod try_next;
use try_next::TryNext;

mod take;
use take::Take;

mod take_while;
use take_while::TakeWhile;

cfg_time! {
    mod timeout;
    use timeout::Timeout;
    use std::time::Duration;
}

pub use futures_core::Stream;

/// An extension trait for `Stream`s that provides a variety of convenient
/// combinator functions.
pub trait StreamExt: Stream {
    /// Consumes and returns the next value in the stream or `None` if the
    /// stream is finished.
    ///
    /// Equivalent to:
    ///
    /// ```ignore
    /// async fn next(&mut self) -> Option<Self::Item>;
    /// ```
    ///
    /// Note that because `next` doesn't take ownership over the stream,
    /// the [`Stream`] type must be [`Unpin`]. If you want to use `next` with a
    /// [`!Unpin`](Unpin) stream, you'll first have to pin the stream. This can
    /// be done by boxing the stream using [`Box::pin`] or
    /// pinning it to the stack using the `pin_mut!` macro from the `pin_utils`
    /// crate.
    ///
    /// # Examples
    ///
    /// ```
    /// # #[tokio::main]
    /// # async fn main() {
    /// use tokio::stream::{self, StreamExt};
    ///
    /// let mut stream = stream::iter(1..=3);
    ///
    /// assert_eq!(stream.next().await, Some(1));
    /// assert_eq!(stream.next().await, Some(2));
    /// assert_eq!(stream.next().await, Some(3));
    /// assert_eq!(stream.next().await, None);
    /// # }
    /// ```
    fn next(&mut self) -> Next<'_, Self>
    where
        Self: Unpin,
    {
        Next::new(self)
    }

    /// Consumes and returns the next item in the stream. If an error is
    /// encountered before the next item, the error is returned instead.
    ///
    /// Equivalent to:
    ///
    /// ```ignore
    /// async fn try_next(&mut self) -> Result<Option<T>, E>;
    /// ```
    ///
    /// This is similar to the [`next`](StreamExt::next) combinator,
    /// but returns a [`Result<Option<T>, E>`](Result) rather than
    /// an [`Option<Result<T, E>>`](Option), making for easy use
    /// with the [`?`](std::ops::Try) operator.
    ///
    /// # Examples
    ///
    /// ```
    /// # #[tokio::main]
    /// # async fn main() {
    /// use tokio::stream::{self, StreamExt};
    ///
    /// let mut stream = stream::iter(vec![Ok(1), Ok(2), Err("nope")]);
    ///
    /// assert_eq!(stream.try_next().await, Ok(Some(1)));
    /// assert_eq!(stream.try_next().await, Ok(Some(2)));
    /// assert_eq!(stream.try_next().await, Err("nope"));
    /// # }
    /// ```
    fn try_next<T, E>(&mut self) -> TryNext<'_, Self>
    where
        Self: Stream<Item = Result<T, E>> + Unpin,
    {
        TryNext::new(self)
    }

    /// Maps this stream's items to a different type, returning a new stream of
    /// the resulting type.
    ///
    /// The provided closure is executed over all elements of this stream as
    /// they are made available. It is executed inline with calls to
    /// [`poll_next`](Stream::poll_next).
    ///
    /// Note that this function consumes the stream passed into it and returns a
    /// wrapped version of it, similar to the existing `map` methods in the
    /// standard library.
    ///
    /// # Examples
    ///
    /// ```
    /// # #[tokio::main]
    /// # async fn main() {
    /// use tokio::stream::{self, StreamExt};
    ///
    /// let stream = stream::iter(1..=3);
    /// let mut stream = stream.map(|x| x + 3);
    ///
    /// assert_eq!(stream.next().await, Some(4));
    /// assert_eq!(stream.next().await, Some(5));
    /// assert_eq!(stream.next().await, Some(6));
    /// # }
    /// ```
    fn map<T, F>(self, f: F) -> Map<Self, F>
    where
        F: FnMut(Self::Item) -> T,
        Self: Sized,
    {
        Map::new(self, f)
    }

    /// Combine two streams into one by interleaving the output of both as it
    /// is produced.
    ///
    /// Values are produced from the merged stream in the order they arrive from
    /// the two source streams. If both source streams provide values
    /// simultaneously, the merge stream alternates between them. This provides
    /// some level of fairness. You should not chain calls to `merge`, as this
    /// will break the fairness of the merging.
    ///
    /// The merged stream completes once **both** source streams complete. When
    /// one source stream completes before the other, the merge stream
    /// exclusively polls the remaining stream.
    ///
    /// For merging multiple streams, consider using [`StreamMap`] instead.
    ///
    /// [`StreamMap`]: crate::stream::StreamMap
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::stream::StreamExt;
    /// use tokio::sync::mpsc;
    /// use tokio::time;
    ///
    /// use std::time::Duration;
    ///
    /// # /*
    /// #[tokio::main]
    /// # */
    /// # #[tokio::main(basic_scheduler)]
    /// async fn main() {
    /// # time::pause();
    ///     let (mut tx1, rx1) = mpsc::channel(10);
    ///     let (mut tx2, rx2) = mpsc::channel(10);
    ///
    ///     let mut rx = rx1.merge(rx2);
    ///
    ///     tokio::spawn(async move {
    ///         // Send some values immediately
    ///         tx1.send(1).await.unwrap();
    ///         tx1.send(2).await.unwrap();
    ///
    ///         // Let the other task send values
    ///         time::delay_for(Duration::from_millis(20)).await;
    ///
    ///         tx1.send(4).await.unwrap();
    ///     });
    ///
    ///     tokio::spawn(async move {
    ///         // Wait for the first task to send values
    ///         time::delay_for(Duration::from_millis(5)).await;
    ///
    ///         tx2.send(3).await.unwrap();
    ///
    ///         time::delay_for(Duration::from_millis(25)).await;
    ///
    ///         // Send the final value
    ///         tx2.send(5).await.unwrap();
    ///     });
    ///
    ///    assert_eq!(1, rx.next().await.unwrap());
    ///    assert_eq!(2, rx.next().await.unwrap());
    ///    assert_eq!(3, rx.next().await.unwrap());
    ///    assert_eq!(4, rx.next().await.unwrap());
    ///    assert_eq!(5, rx.next().await.unwrap());
    ///
    ///    // The merged stream is consumed
    ///    assert!(rx.next().await.is_none());
    /// }
    /// ```
    fn merge<U>(self, other: U) -> Merge<Self, U>
    where
        U: Stream<Item = Self::Item>,
        Self: Sized,
    {
        Merge::new(self, other)
    }

    /// Filters the values produced by this stream according to the provided
    /// predicate.
    ///
    /// As values of this stream are made available, the provided predicate `f`
    /// will be run against them. If the predicate
    /// resolves to `true`, then the stream will yield the value, but if the
    /// predicate resolves to `false`, then the value
    /// will be discarded and the next value will be produced.
    ///
    /// Note that this function consumes the stream passed into it and returns a
    /// wrapped version of it, similar to [`Iterator::filter`] method in the
    /// standard library.
    ///
    /// # Examples
    ///
    /// ```
    /// # #[tokio::main]
    /// # async fn main() {
    /// use tokio::stream::{self, StreamExt};
    ///
    /// let stream = stream::iter(1..=8);
    /// let mut evens = stream.filter(|x| x % 2 == 0);
    ///
    /// assert_eq!(Some(2), evens.next().await);
    /// assert_eq!(Some(4), evens.next().await);
    /// assert_eq!(Some(6), evens.next().await);
    /// assert_eq!(Some(8), evens.next().await);
    /// assert_eq!(None, evens.next().await);
    /// # }
    /// ```
    fn filter<F>(self, f: F) -> Filter<Self, F>
    where
        F: FnMut(&Self::Item) -> bool,
        Self: Sized,
    {
        Filter::new(self, f)
    }

    /// Filters the values produced by this stream while simultaneously mapping
    /// them to a different type according to the provided closure.
    ///
    /// As values of this stream are made available, the provided function will
    /// be run on them. If the predicate `f` resolves to
    /// [`Some(item)`](Some) then the stream will yield the value `item`, but if
    /// it resolves to [`None`], then the value will be skipped.
    ///
    /// Note that this function consumes the stream passed into it and returns a
    /// wrapped version of it, similar to [`Iterator::filter_map`] method in the
    /// standard library.
    ///
    /// # Examples
    /// ```
    /// # #[tokio::main]
    /// # async fn main() {
    /// use tokio::stream::{self, StreamExt};
    ///
    /// let stream = stream::iter(1..=8);
    /// let mut evens = stream.filter_map(|x| {
    ///     if x % 2 == 0 { Some(x + 1) } else { None }
    /// });
    ///
    /// assert_eq!(Some(3), evens.next().await);
    /// assert_eq!(Some(5), evens.next().await);
    /// assert_eq!(Some(7), evens.next().await);
    /// assert_eq!(Some(9), evens.next().await);
    /// assert_eq!(None, evens.next().await);
    /// # }
    /// ```
    fn filter_map<T, F>(self, f: F) -> FilterMap<Self, F>
    where
        F: FnMut(Self::Item) -> Option<T>,
        Self: Sized,
    {
        FilterMap::new(self, f)
    }

    /// Creates a stream which ends after the first `None`.
    ///
    /// After a stream returns `None`, behavior is undefined. Future calls to
    /// `poll_next` may or may not return `Some(T)` again or they may panic.
    /// `fuse()` adapts a stream, ensuring that after `None` is given, it will
    /// return `None` forever.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::stream::{Stream, StreamExt};
    ///
    /// use std::pin::Pin;
    /// use std::task::{Context, Poll};
    ///
    /// // a stream which alternates between Some and None
    /// struct Alternate {
    ///     state: i32,
    /// }
    ///
    /// impl Stream for Alternate {
    ///     type Item = i32;
    ///
    ///     fn poll_next(mut self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Option<i32>> {
    ///         let val = self.state;
    ///         self.state = self.state + 1;
    ///
    ///         // if it's even, Some(i32), else None
    ///         if val % 2 == 0 {
    ///             Poll::Ready(Some(val))
    ///         } else {
    ///             Poll::Ready(None)
    ///         }
    ///     }
    /// }
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let mut stream = Alternate { state: 0 };
    ///
    ///     // the stream goes back and forth
    ///     assert_eq!(stream.next().await, Some(0));
    ///     assert_eq!(stream.next().await, None);
    ///     assert_eq!(stream.next().await, Some(2));
    ///     assert_eq!(stream.next().await, None);
    ///
    ///     // however, once it is fused
    ///     let mut stream = stream.fuse();
    ///
    ///     assert_eq!(stream.next().await, Some(4));
    ///     assert_eq!(stream.next().await, None);
    ///
    ///     // it will always return `None` after the first time.
    ///     assert_eq!(stream.next().await, None);
    ///     assert_eq!(stream.next().await, None);
    ///     assert_eq!(stream.next().await, None);
    /// }
    /// ```
    fn fuse(self) -> Fuse<Self>
    where
        Self: Sized,
    {
        Fuse::new(self)
    }

    /// Creates a new stream of at most `n` items of the underlying stream.
    ///
    /// Once `n` items have been yielded from this stream then it will always
    /// return that the stream is done.
    ///
    /// # Examples
    ///
    /// ```
    /// # #[tokio::main]
    /// # async fn main() {
    /// use tokio::stream::{self, StreamExt};
    ///
    /// let mut stream = stream::iter(1..=10).take(3);
    ///
    /// assert_eq!(Some(1), stream.next().await);
    /// assert_eq!(Some(2), stream.next().await);
    /// assert_eq!(Some(3), stream.next().await);
    /// assert_eq!(None, stream.next().await);
    /// # }
    /// ```
    fn take(self, n: usize) -> Take<Self>
    where
        Self: Sized,
    {
        Take::new(self, n)
    }

    /// Take elements from this stream while the provided predicate
    /// resolves to `true`.
    ///
    /// This function, like `Iterator::take_while`, will take elements from the
    /// stream until the predicate `f` resolves to `false`. Once one element
    /// returns false it will always return that the stream is done.
    ///
    /// # Examples
    ///
    /// ```
    /// # #[tokio::main]
    /// # async fn main() {
    /// use tokio::stream::{self, StreamExt};
    ///
    /// let mut stream = stream::iter(1..=10).take_while(|x| *x <= 3);
    ///
    /// assert_eq!(Some(1), stream.next().await);
    /// assert_eq!(Some(2), stream.next().await);
    /// assert_eq!(Some(3), stream.next().await);
    /// assert_eq!(None, stream.next().await);
    /// # }
    /// ```
    fn take_while<F>(self, f: F) -> TakeWhile<Self, F>
    where
        F: FnMut(&Self::Item) -> bool,
        Self: Sized,
    {
        TakeWhile::new(self, f)
    }

    /// Creates a new stream that will skip the `n` first items of the
    /// underlying stream.
    ///
    /// # Examples
    ///
    /// ```
    /// # #[tokio::main]
    /// # async fn main() {
    /// use tokio::stream::{self, StreamExt};
    ///
    /// let mut stream = stream::iter(1..=10).skip(7);
    ///
    /// assert_eq!(Some(8), stream.next().await);
    /// assert_eq!(Some(9), stream.next().await);
    /// assert_eq!(Some(10), stream.next().await);
    /// assert_eq!(None, stream.next().await);
    /// # }
    /// ```
    fn skip(self, n: usize) -> Skip<Self>
    where
        Self: Sized,
    {
        Skip::new(self, n)
    }

    /// Skip elements from the underlying stream while the provided predicate
    /// resolves to `true`.
    ///
    /// This function, like [`Iterator::skip_while`], will ignore elemets from the
    /// stream until the predicate `f` resolves to `false`. Once one element
    /// returns false, the rest of the elements will be yielded.
    ///
    /// [`Iterator::skip_while`]: std::iter::Iterator::skip_while()
    ///
    /// # Examples
    ///
    /// ```
    /// # #[tokio::main]
    /// # async fn main() {
    /// use tokio::stream::{self, StreamExt};
    /// let mut stream = stream::iter(vec![1,2,3,4,1]).skip_while(|x| *x < 3);
    ///
    /// assert_eq!(Some(3), stream.next().await);
    /// assert_eq!(Some(4), stream.next().await);
    /// assert_eq!(Some(1), stream.next().await);
    /// assert_eq!(None, stream.next().await);
    /// # }
    /// ```
    fn skip_while<F>(self, f: F) -> SkipWhile<Self, F>
    where
        F: FnMut(&Self::Item) -> bool,
        Self: Sized,
    {
        SkipWhile::new(self, f)
    }

    /// Tests if every element of the stream matches a predicate.
    ///
    /// `all()` takes a closure that returns `true` or `false`. It applies
    /// this closure to each element of the stream, and if they all return
    /// `true`, then so does `all`. If any of them return `false`, it
    /// returns `false`. An empty stream returns `true`.
    ///
    /// `all()` is short-circuiting; in other words, it will stop processing
    /// as soon as it finds a `false`, given that no matter what else happens,
    /// the result will also be `false`.
    ///
    /// An empty stream returns `true`.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// # #[tokio::main]
    /// # async fn main() {
    /// use tokio::stream::{self, StreamExt};
    ///
    /// let a = [1, 2, 3];
    ///
    /// assert!(stream::iter(&a).all(|&x| x > 0).await);
    ///
    /// assert!(!stream::iter(&a).all(|&x| x > 2).await);
    /// # }
    /// ```
    ///
    /// Stopping at the first `false`:
    ///
    /// ```
    /// # #[tokio::main]
    /// # async fn main() {
    /// use tokio::stream::{self, StreamExt};
    ///
    /// let a = [1, 2, 3];
    ///
    /// let mut iter = stream::iter(&a);
    ///
    /// assert!(!iter.all(|&x| x != 2).await);
    ///
    /// // we can still use `iter`, as there are more elements.
    /// assert_eq!(iter.next().await, Some(&3));
    /// # }
    /// ```
    fn all<F>(&mut self, f: F) -> AllFuture<'_, Self, F>
    where
        Self: Unpin,
        F: FnMut(Self::Item) -> bool,
    {
        AllFuture::new(self, f)
    }

    /// Tests if any element of the stream matches a predicate.
    ///
    /// `any()` takes a closure that returns `true` or `false`. It applies
    /// this closure to each element of the stream, and if any of them return
    /// `true`, then so does `any()`. If they all return `false`, it
    /// returns `false`.
    ///
    /// `any()` is short-circuiting; in other words, it will stop processing
    /// as soon as it finds a `true`, given that no matter what else happens,
    /// the result will also be `true`.
    ///
    /// An empty stream returns `false`.
    ///
    /// Basic usage:
    ///
    /// ```
    /// # #[tokio::main]
    /// # async fn main() {
    /// use tokio::stream::{self, StreamExt};
    ///
    /// let a = [1, 2, 3];
    ///
    /// assert!(stream::iter(&a).any(|&x| x > 0).await);
    ///
    /// assert!(!stream::iter(&a).any(|&x| x > 5).await);
    /// # }
    /// ```
    ///
    /// Stopping at the first `true`:
    ///
    /// ```
    /// # #[tokio::main]
    /// # async fn main() {
    /// use tokio::stream::{self, StreamExt};
    ///
    /// let a = [1, 2, 3];
    ///
    /// let mut iter = stream::iter(&a);
    ///
    /// assert!(iter.any(|&x| x != 2).await);
    ///
    /// // we can still use `iter`, as there are more elements.
    /// assert_eq!(iter.next().await, Some(&2));
    /// # }
    /// ```
    fn any<F>(&mut self, f: F) -> AnyFuture<'_, Self, F>
    where
        Self: Unpin,
        F: FnMut(Self::Item) -> bool,
    {
        AnyFuture::new(self, f)
    }

    /// Combine two streams into one by first returning all values from the
    /// first stream then all values from the second stream.
    ///
    /// As long as `self` still has values to emit, no values from `other` are
    /// emitted, even if some are ready.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::stream::{self, StreamExt};
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let one = stream::iter(vec![1, 2, 3]);
    ///     let two = stream::iter(vec![4, 5, 6]);
    ///
    ///     let mut stream = one.chain(two);
    ///
    ///     assert_eq!(stream.next().await, Some(1));
    ///     assert_eq!(stream.next().await, Some(2));
    ///     assert_eq!(stream.next().await, Some(3));
    ///     assert_eq!(stream.next().await, Some(4));
    ///     assert_eq!(stream.next().await, Some(5));
    ///     assert_eq!(stream.next().await, Some(6));
    ///     assert_eq!(stream.next().await, None);
    /// }
    /// ```
    fn chain<U>(self, other: U) -> Chain<Self, U>
    where
        U: Stream<Item = Self::Item>,
        Self: Sized,
    {
        Chain::new(self, other)
    }

    /// A combinator that applies a function to every element in a stream
    /// producing a single, final value.
    ///
    /// # Examples
    /// Basic usage:
    /// ```
    /// # #[tokio::main]
    /// # async fn main() {
    /// use tokio::stream::{self, *};
    ///
    /// let s = stream::iter(vec![1u8, 2, 3]);
    /// let sum = s.fold(0, |acc, x| acc + x).await;
    ///
    /// assert_eq!(sum, 6);
    /// # }
    /// ```
    fn fold<B, F>(self, init: B, f: F) -> FoldFuture<Self, B, F>
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B,
    {
        FoldFuture::new(self, init, f)
    }

    /// Drain stream pushing all emitted values into a collection.
    ///
    /// `collect` streams all values, awaiting as needed. Values are pushed into
    /// a collection. A number of different target collection types are
    /// supported, including [`Vec`](std::vec::Vec),
    /// [`String`](std::string::String), and [`Bytes`](bytes::Bytes).
    ///
    /// # `Result`
    ///
    /// `collect()` can also be used with streams of type `Result<T, E>` where
    /// `T: FromStream<_>`. In this case, `collect()` will stream as long as
    /// values yielded from the stream are `Ok(_)`. If `Err(_)` is encountered,
    /// streaming is terminated and `collect()` returns the `Err`.
    ///
    /// # Notes
    ///
    /// `FromStream` is currently a sealed trait. Stabilization is pending
    /// enhancements to the Rust language.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use tokio::stream::{self, StreamExt};
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let doubled: Vec<i32> =
    ///         stream::iter(vec![1, 2, 3])
    ///             .map(|x| x * 2)
    ///             .collect()
    ///             .await;
    ///
    ///     assert_eq!(vec![2, 4, 6], doubled);
    /// }
    /// ```
    ///
    /// Collecting a stream of `Result` values
    ///
    /// ```
    /// use tokio::stream::{self, StreamExt};
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     // A stream containing only `Ok` values will be collected
    ///     let values: Result<Vec<i32>, &str> =
    ///         stream::iter(vec![Ok(1), Ok(2), Ok(3)])
    ///             .collect()
    ///             .await;
    ///
    ///     assert_eq!(Ok(vec![1, 2, 3]), values);
    ///
    ///     // A stream containing `Err` values will return the first error.
    ///     let results = vec![Ok(1), Err("no"), Ok(2), Ok(3), Err("nein")];
    ///
    ///     let values: Result<Vec<i32>, &str> =
    ///         stream::iter(results)
    ///             .collect()
    ///             .await;
    ///
    ///     assert_eq!(Err("no"), values);
    /// }
    /// ```
    fn collect<T>(self) -> Collect<Self, T>
    where
        T: FromStream<Self::Item>,
        Self: Sized,
    {
        Collect::new(self)
    }

    /// Applies a per-item timeout to the passed stream.
    ///
    /// `timeout()` takes a `Duration` that represents the maximum amount of
    /// time each element of the stream has to complete before timing out.
    ///
    /// If the wrapped stream yields a value before the deadline is reached, the
    /// value is returned. Otherwise, an error is returned. The caller may decide
    /// to continue consuming the stream and will eventually get the next source
    /// stream value once it becomes available.
    ///
    /// # Notes
    ///
    /// This function consumes the stream passed into it and returns a
    /// wrapped version of it.
    ///
    /// Polling the returned stream will continue to poll the inner stream even
    /// if one or more items time out.
    ///
    /// # Examples
    ///
    /// Suppose we have a stream `int_stream` that yields 3 numbers (1, 2, 3):
    ///
    /// ```
    /// # #[tokio::main]
    /// # async fn main() {
    /// use tokio::stream::{self, StreamExt};
    /// use std::time::Duration;
    /// # let int_stream = stream::iter(1..=3);
    ///
    /// let mut int_stream = int_stream.timeout(Duration::from_secs(1));
    ///
    /// // When no items time out, we get the 3 elements in succession:
    /// assert_eq!(int_stream.try_next().await, Ok(Some(1)));
    /// assert_eq!(int_stream.try_next().await, Ok(Some(2)));
    /// assert_eq!(int_stream.try_next().await, Ok(Some(3)));
    /// assert_eq!(int_stream.try_next().await, Ok(None));
    ///
    /// // If the second item times out, we get an error and continue polling the stream:
    /// # let mut int_stream = stream::iter(vec![Ok(1), Err(()), Ok(2), Ok(3)]);
    /// assert_eq!(int_stream.try_next().await, Ok(Some(1)));
    /// assert!(int_stream.try_next().await.is_err());
    /// assert_eq!(int_stream.try_next().await, Ok(Some(2)));
    /// assert_eq!(int_stream.try_next().await, Ok(Some(3)));
    /// assert_eq!(int_stream.try_next().await, Ok(None));
    ///
    /// // If we want to stop consuming the source stream the first time an
    /// // element times out, we can use the `take_while` operator:
    /// # let int_stream = stream::iter(vec![Ok(1), Err(()), Ok(2), Ok(3)]);
    /// let mut int_stream = int_stream.take_while(Result::is_ok);
    ///
    /// assert_eq!(int_stream.try_next().await, Ok(Some(1)));
    /// assert_eq!(int_stream.try_next().await, Ok(None));
    /// # }
    /// ```
    #[cfg(all(feature = "time"))]
    #[cfg_attr(docsrs, doc(cfg(feature = "time")))]
    fn timeout(self, duration: Duration) -> Timeout<Self>
    where
        Self: Sized,
    {
        Timeout::new(self, duration)
    }
}

impl<St: ?Sized> StreamExt for St where St: Stream {}

/// Merge the size hints from two streams.
fn merge_size_hints(
    (left_low, left_high): (usize, Option<usize>),
    (right_low, right_hign): (usize, Option<usize>),
) -> (usize, Option<usize>) {
    let low = left_low.saturating_add(right_low);
    let high = match (left_high, right_hign) {
        (Some(h1), Some(h2)) => h1.checked_add(h2),
        _ => None,
    };
    (low, high)
}