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
 838
 839
 840
 841
 842
 843
 844
 845
 846
 847
 848
 849
 850
 851
 852
 853
 854
 855
 856
 857
 858
 859
 860
 861
 862
 863
 864
 865
 866
 867
 868
 869
 870
 871
 872
 873
 874
 875
 876
 877
 878
 879
 880
 881
 882
 883
 884
 885
 886
 887
 888
 889
 890
 891
 892
 893
 894
 895
 896
 897
 898
 899
 900
 901
 902
 903
 904
 905
 906
 907
 908
 909
 910
 911
 912
 913
 914
 915
 916
 917
 918
 919
 920
 921
 922
 923
 924
 925
 926
 927
 928
 929
 930
 931
 932
 933
 934
 935
 936
 937
 938
 939
 940
 941
 942
 943
 944
 945
 946
 947
 948
 949
 950
 951
 952
 953
 954
 955
 956
 957
 958
 959
 960
 961
 962
 963
 964
 965
 966
 967
 968
 969
 970
 971
 972
 973
 974
 975
 976
 977
 978
 979
 980
 981
 982
 983
 984
 985
 986
 987
 988
 989
 990
 991
 992
 993
 994
 995
 996
 997
 998
 999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
//! Streams
//!
//! This module contains a number of functions for working with `Stream`s,
//! including the `StreamExt` trait which adds methods to `Stream` types.

use core::marker::Unpin;
use core::pin::Pin;
use either::Either;
use futures_core::future::Future;
use futures_core::stream::Stream;
use futures_core::task::{LocalWaker, Poll};
use futures_sink::Sink;

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

mod repeat;
pub use self::repeat::{repeat, Repeat};

mod chain;
pub use self::chain::Chain;

mod concat;
pub use self::concat::Concat;

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

mod filter;
pub use self::filter::Filter;

mod filter_map;
pub use self::filter_map::FilterMap;

mod flatten;
pub use self::flatten::Flatten;

mod fold;
pub use self::fold::Fold;

mod forward;
pub use self::forward::Forward;

mod for_each;
pub use self::for_each::ForEach;

mod fuse;
pub use self::fuse::Fuse;

mod into_future;
pub use self::into_future::StreamFuture;

mod inspect;
pub use self::inspect::Inspect;

mod map;
pub use self::map::Map;

mod next;
pub use self::next::Next;

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

mod peek;
pub use self::peek::Peekable;

mod poll_fn;
pub use self::poll_fn::{poll_fn, PollFn};

mod select;
pub use self::select::Select;

mod skip;
pub use self::skip::Skip;

mod skip_while;
pub use self::skip_while::SkipWhile;

mod take;
pub use self::take::Take;

mod take_while;
pub use self::take_while::TakeWhile;

mod then;
pub use self::then::Then;

mod unfold;
pub use self::unfold::{unfold, Unfold};

mod zip;
pub use self::zip::Zip;

if_std! {
    use std;
    use std::iter::Extend;

    mod buffer_unordered;
    pub use self::buffer_unordered::BufferUnordered;

    mod buffered;
    pub use self::buffered::Buffered;

    mod catch_unwind;
    pub use self::catch_unwind::CatchUnwind;

    mod chunks;
    pub use self::chunks::Chunks;

    mod collect;
    pub use self::collect::Collect;

    mod for_each_concurrent;
    pub use self::for_each_concurrent::ForEachConcurrent;

    mod futures_ordered;
    pub use self::futures_ordered::{futures_ordered, FuturesOrdered};

    mod futures_unordered;
    pub use self::futures_unordered::{futures_unordered, FuturesUnordered};

    mod split;
    pub use self::split::{SplitStream, SplitSink, ReuniteError};

    // ToDo
    // mod select_all;
    // pub use self::select_all::{select_all, SelectAll};
}

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

/// An extension trait for `Stream`s that provides a variety of convenient
/// combinator functions.
pub trait StreamExt: Stream {
    /// Creates a future that resolves to the next item in the stream.
    ///
    /// 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::pinned`] or
    /// pinning it to the stack using the `pin_mut!` macro from the `pin_utils`
    /// crate.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(pin)]
    /// use futures::executor::block_on;
    /// use futures::stream::{self, StreamExt};
    ///
    /// let mut stream = stream::iter(1..=3);
    ///
    /// assert_eq!(block_on(stream.next()), Some(1));
    /// assert_eq!(block_on(stream.next()), Some(2));
    /// assert_eq!(block_on(stream.next()), Some(3));
    /// assert_eq!(block_on(stream.next()), None);
    /// ```
    fn next(&mut self) -> Next<'_, Self>
        where Self: Sized + Unpin,
    {
        Next::new(self)
    }

    /// Converts this stream into a future of `(next_item, tail_of_stream)`.
    /// If the stream terminates, then the next item is [`None`].
    ///
    /// The returned future can be used to compose streams and futures together
    /// by placing everything into the "world of futures".
    ///
    /// Note that because `into_future` moves the stream, the [`Stream`] type
    /// must be [`Unpin`]. If you want to use `into_future` with a
    /// [`!Unpin`](Unpin) stream, you'll first have to pin the stream. This can
    /// be done by boxing the stream using [`Box::pinned`] or
    /// pinning it to the stack using the `pin_mut!` macro from the `pin_utils`
    /// crate.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(pin)]
    /// use futures::executor::block_on;
    /// use futures::stream::{self, StreamExt};
    ///
    /// let stream = stream::iter(1..=3);
    ///
    /// let (item, stream) = block_on(stream.into_future());
    /// assert_eq!(Some(1), item);
    ///
    /// let (item, stream) = block_on(stream.into_future());
    /// assert_eq!(Some(2), item);
    /// ```
    fn into_future(self) -> StreamFuture<Self>
        where Self: Sized + Unpin,
    {
        StreamFuture::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
    ///
    /// ```
    /// use futures::executor::block_on;
    /// use futures::stream::{self, StreamExt};
    ///
    /// let stream = stream::iter(1..=3);
    /// let stream = stream.map(|x| x + 3);
    ///
    /// assert_eq!(vec![4, 5, 6], block_on(stream.collect::<Vec<_>>()));
    /// ```
    fn map<T, F>(self, f: F) -> Map<Self, F>
        where F: FnMut(Self::Item) -> T,
              Self: Sized
    {
        Map::new(self, f)
    }

    /// Filters the values produced by this stream according to the provided
    /// asynchronous predicate.
    ///
    /// As values of this stream are made available, the provided predicate `f`
    /// will be run against them. If the predicate returns a `Future` which
    /// resolves to `true`, then the stream will yield the value, but if the
    /// predicate returns a `Future` which 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 the existing `filter` methods in the
    /// standard library.
    ///
    /// # Examples
    ///
    /// ```
    /// use futures::executor::block_on;
    /// use futures::future;
    /// use futures::stream::{self, StreamExt};
    ///
    /// let stream = stream::iter(1..=10);
    /// let evens = stream.filter(|x| future::ready(x % 2 == 0));
    ///
    /// assert_eq!(vec![2, 4, 6, 8, 10], block_on(evens.collect::<Vec<_>>()));
    /// ```
    fn filter<Fut, F>(self, f: F) -> Filter<Self, Fut, F>
        where F: FnMut(&Self::Item) -> Fut,
              Fut: Future<Output = 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 asynchronous closure.
    ///
    /// As values of this stream are made available, the provided function will
    /// be run on them. If the future returned by the predicate `f` resolves to
    /// [`Some(item)`](Some) then the stream will yield the value `item`, but if
    /// it resolves to [`None`] then 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 the existing `filter_map` methods in
    /// the standard library.
    ///
    /// # Examples
    /// ```
    /// use futures::executor::block_on;
    /// use futures::future;
    /// use futures::stream::{self, StreamExt};
    ///
    /// let stream = stream::iter(1..=10);
    /// let evens = stream.filter_map(|x| {
    ///     let ret = if x % 2 == 0 { Some(x + 1) } else { None };
    ///     future::ready(ret)
    /// });
    ///
    /// assert_eq!(vec![3, 5, 7, 9, 11], block_on(evens.collect::<Vec<_>>()));
    /// ```
    fn filter_map<Fut, T, F>(self, f: F) -> FilterMap<Self, Fut, F>
        where F: FnMut(Self::Item) -> Fut,
              Fut: Future<Output = Option<T>>,
              Self: Sized,
    {
        FilterMap::new(self, f)
    }

    /// Computes from this stream's items new items of a different type using
    /// an asynchronous closure.
    ///
    /// The provided closure `f` will be called with an `Item` once a value is
    /// ready, it returns a future which will then be run to completion
    /// to produce the next value on this stream.
    ///
    /// Note that this function consumes the stream passed into it and returns a
    /// wrapped version of it.
    ///
    /// # Examples
    ///
    /// ```
    /// use futures::executor::block_on;
    /// use futures::future;
    /// use futures::stream::{self, StreamExt};
    ///
    /// let stream = stream::iter(1..=3);
    /// let stream = stream.then(|x| future::ready(x + 3));
    ///
    /// assert_eq!(vec![4, 5, 6], block_on(stream.collect::<Vec<_>>()));
    /// ```
    fn then<Fut, F>(self, f: F) -> Then<Self, Fut, F>
        where F: FnMut(Self::Item) -> Fut,
              Fut: Future,
              Self: Sized
    {
        Then::new(self, f)
    }

    /// Collect all of the values of this stream into a vector, returning a
    /// future representing the result of that computation.
    ///
    /// The returned future will be resolved when the stream terminates.
    ///
    /// This method is only available when the `std` feature of this
    /// library is activated, and it is activated by default.
    ///
    /// # Examples
    ///
    /// ```
    /// use futures::channel::mpsc;
    /// use futures::executor::block_on;
    /// use futures::stream::StreamExt;
    /// use std::thread;
    ///
    /// let (mut tx, rx) = mpsc::unbounded();
    ///
    /// thread::spawn(move || {
    ///     for i in (1..=5) {
    ///         tx.unbounded_send(i).unwrap();
    ///     }
    /// });
    ///
    /// let output = block_on(rx.collect::<Vec<i32>>());
    /// assert_eq!(output, vec![1, 2, 3, 4, 5]);
    /// ```
    #[cfg(feature = "std")]
    fn collect<C: Default + Extend<Self::Item>>(self) -> Collect<Self, C>
        where Self: Sized
    {
        Collect::new(self)
    }

    /// Concatenate all items of a stream into a single extendable
    /// destination, returning a future representing the end result.
    ///
    /// This combinator will extend the first item with the contents
    /// of all the subsequent results of the stream. If the stream is
    /// empty, the default value will be returned.
    ///
    /// Works with all collections that implement the
    /// [`Extend`](std::iter::Extend) trait.
    ///
    /// # Examples
    ///
    /// ```
    /// use futures::channel::mpsc;
    /// use futures::executor::block_on;
    /// use futures::stream::StreamExt;
    /// use std::thread;
    ///
    /// let (mut tx, rx) = mpsc::unbounded();
    ///
    /// thread::spawn(move || {
    ///     for i in (0..3).rev() {
    ///         let n = i * 3;
    ///         tx.unbounded_send(vec![n + 1, n + 2, n + 3]).unwrap();
    ///     }
    /// });
    ///
    /// let result = block_on(rx.concat());
    ///
    /// assert_eq!(result, vec![7, 8, 9, 4, 5, 6, 1, 2, 3]);
    /// ```
    fn concat(self) -> Concat<Self>
    where Self: Sized,
          Self::Item: Extend<<<Self as Stream>::Item as IntoIterator>::Item> +
                      IntoIterator + Default,
    {
        Concat::new(self)
    }

    /// Execute an accumulating asynchronous computation over a stream,
    /// collecting all the values into one final result.
    ///
    /// This combinator will accumulate all values returned by this stream
    /// according to the closure provided. The initial state is also provided to
    /// this method and then is returned again by each execution of the closure.
    /// Once the entire stream has been exhausted the returned future will
    /// resolve to this value.
    ///
    /// # Examples
    ///
    /// ```
    /// use futures::executor::block_on;
    /// use futures::future;
    /// use futures::stream::{self, StreamExt};
    ///
    /// let number_stream = stream::iter(0..6);
    /// let sum = number_stream.fold(0, |acc, x| future::ready(acc + x));
    /// assert_eq!(block_on(sum), 15);
    /// ```
    fn fold<T, Fut, F>(self, init: T, f: F) -> Fold<Self, Fut, T, F>
        where F: FnMut(T, Self::Item) -> Fut,
              Fut: Future<Output = T>,
              Self: Sized
    {
        Fold::new(self, f, init)
    }

    /// Flattens a stream of streams into just one continuous stream.
    ///
    /// # Examples
    ///
    /// ```
    /// use futures::channel::mpsc;
    /// use futures::executor::block_on;
    /// use futures::stream::StreamExt;
    /// use std::thread;
    ///
    /// let (tx1, rx1) = mpsc::unbounded();
    /// let (tx2, rx2) = mpsc::unbounded();
    /// let (tx3, rx3) = mpsc::unbounded();
    ///
    /// thread::spawn(move || {
    ///     tx1.unbounded_send(1).unwrap();
    ///     tx1.unbounded_send(2).unwrap();
    /// });
    /// thread::spawn(move || {
    ///     tx2.unbounded_send(3).unwrap();
    ///     tx2.unbounded_send(4).unwrap();
    /// });
    /// thread::spawn(move || {
    ///     tx3.unbounded_send(rx1).unwrap();
    ///     tx3.unbounded_send(rx2).unwrap();
    /// });
    ///
    /// let output = block_on(rx3.flatten().collect::<Vec<i32>>());
    /// assert_eq!(output, vec![1, 2, 3, 4]);
    /// ```
    fn flatten(self) -> Flatten<Self>
        where Self::Item: Stream,
              Self: Sized
    {
        Flatten::new(self)
    }

    /// Skip elements on this stream while the provided asynchronous predicate
    /// resolves to `true`.
    ///
    /// This function, like `Iterator::skip_while`, will skip elements on the
    /// stream until the predicate `f` resolves to `false`. Once one element
    /// returns false all future elements will be returned from the underlying
    /// stream.
    ///
    /// # Examples
    ///
    /// ```
    /// use futures::executor::block_on;
    /// use futures::future;
    /// use futures::stream::{self, StreamExt};
    ///
    /// let stream = stream::iter(1..=10);
    ///
    /// let stream = stream.skip_while(|x| future::ready(*x <= 5));
    ///
    /// assert_eq!(vec![6, 7, 8, 9, 10], block_on(stream.collect::<Vec<_>>()));
    /// ```
    fn skip_while<Fut, F>(self, f: F) -> SkipWhile<Self, Fut, F>
        where F: FnMut(&Self::Item) -> Fut,
              Fut: Future<Output = bool>,
              Self: Sized
    {
        SkipWhile::new(self, f)
    }

    /// Take elements from this stream while the provided asynchronous 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
    ///
    /// ```
    /// use futures::executor::block_on;
    /// use futures::future;
    /// use futures::stream::{self, StreamExt};
    ///
    /// let stream = stream::iter(1..=10);
    ///
    /// let stream = stream.take_while(|x| future::ready(*x <= 5));
    ///
    /// assert_eq!(vec![1, 2, 3, 4, 5], block_on(stream.collect::<Vec<_>>()));
    /// ```
    fn take_while<Fut, F>(self, f: F) -> TakeWhile<Self, Fut, F>
        where F: FnMut(&Self::Item) -> Fut,
              Fut: Future<Output = bool>,
              Self: Sized
    {
        TakeWhile::new(self, f)
    }

    /// Runs this stream to completion, executing the provided asynchronous
    /// closure for each element on the stream.
    ///
    /// The closure provided will be called for each item this stream produces,
    /// yielding a future. That future will then be executed to completion
    /// before moving on to the next item.
    ///
    /// The returned value is a `Future` where the `Output` type is `()`; it is
    /// executed entirely for its side effects.
    ///
    /// To process each item in the stream and produce another stream instead
    /// of a single future, use `then` instead.
    ///
    /// # Examples
    ///
    /// ```
    /// use futures::executor::block_on;
    /// use futures::future;
    /// use futures::stream::{self, StreamExt};
    ///
    /// let mut x = 0;
    ///
    /// {
    ///     let fut = stream::repeat(1).take(3).for_each(|item| {
    ///         x += item;
    ///         future::ready(())
    ///     });
    ///     block_on(fut);
    /// }
    ///
    /// assert_eq!(x, 3);
    /// ```
    fn for_each<Fut, F>(self, f: F) -> ForEach<Self, Fut, F>
        where F: FnMut(Self::Item) -> Fut,
              Fut: Future<Output = ()>,
              Self: Sized
    {
        ForEach::new(self, f)
    }

    /// Runs this stream to completion, executing the provided asynchronous
    /// closure for each element on the stream concurrently as elements become
    /// available.
    ///
    /// This is similar to [`StreamExt::for_each`], but the futures
    /// produced by the closure are run concurrently (but not in parallel--
    /// this combinator does not introduce any threads).
    ///
    /// The closure provided will be called for each item this stream produces,
    /// yielding a future. That future will then be executed to completion
    /// concurrently with the other futures produced by the closure.
    ///
    /// The first argument is an optional limit on the number of concurrent
    /// futures. If this limit is not `None`, no more than `limit` futures
    /// will be run concurrently. The `limit` argument is of type
    /// `Into<Option<usize>>`, and so can be provided as either `None`,
    /// `Some(10)`, or just `10`. Note: a limit of zero is interpreted as
    /// no limit at all, and will have the same result as passing in `None`.
    ///
    /// This method is only available when the `std` feature of this
    /// library is activated, and it is activated by default.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(async_await, await_macro)]
    /// # futures::executor::block_on(async {
    /// use futures::channel::oneshot;
    /// use futures::stream::{self, StreamExt};
    ///
    /// let (tx1, rx1) = oneshot::channel();
    /// let (tx2, rx2) = oneshot::channel();
    /// let (tx3, rx3) = oneshot::channel();
    ///
    /// let fut = stream::iter(vec![rx1, rx2, rx3]).for_each_concurrent(
    ///     /* limit */ 2,
    ///     async move |rx| {
    ///         await!(rx).unwrap();
    ///     }
    /// );
    /// tx1.send(()).unwrap();
    /// tx2.send(()).unwrap();
    /// tx3.send(()).unwrap();
    /// await!(fut);
    /// # })
    /// ```
    #[cfg(feature = "std")]
    fn for_each_concurrent<Fut, F>(
        self,
        limit: impl Into<Option<usize>>,
        f: F,
    ) -> ForEachConcurrent<Self, Fut, F>
        where F: FnMut(Self::Item) -> Fut,
              Fut: Future<Output = ()>,
              Self: Sized,
    {
        ForEachConcurrent::new(self, limit.into(), f)
    }

    /// 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
    ///
    /// ```
    /// use futures::executor::block_on;
    /// use futures::stream::{self, StreamExt};
    ///
    /// let stream = stream::iter(1..=10).take(3);
    ///
    /// assert_eq!(vec![1, 2, 3], block_on(stream.collect::<Vec<_>>()));
    /// ```
    fn take(self, n: u64) -> Take<Self>
        where Self: Sized
    {
        Take::new(self, n)
    }

    /// Creates a new stream which skips `n` items of the underlying stream.
    ///
    /// Once `n` items have been skipped from this stream then it will always
    /// return the remaining items on this stream.
    ///
    /// # Examples
    ///
    /// ```
    /// use futures::executor::block_on;
    /// use futures::stream::{self, StreamExt};
    ///
    /// let stream = stream::iter(1..=10).skip(5);
    ///
    /// assert_eq!(vec![6, 7, 8, 9, 10], block_on(stream.collect::<Vec<_>>()));
    /// ```
    fn skip(self, n: u64) -> Skip<Self>
        where Self: Sized
    {
        Skip::new(self, n)
    }

    /// Fuse a stream such that [`poll_next`](Stream::poll_next) will never
    /// again be called once it has finished.
    ///
    /// Normally, once a stream has returned [`None`] from
    /// [`poll_next`](Stream::poll_next) any further calls could exhibit bad
    /// behavior such as block forever, panic, never return, etc. If it is known
    /// that [`poll_next`](Stream::poll_next) may be called after stream
    /// has already finished, then this method can be used to ensure that it has
    /// defined semantics.
    ///
    /// The [`poll_next`](Stream::poll_next) method of a `fuse`d stream
    /// is guaranteed to return [`None`] after the underlying stream has
    /// finished.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(futures_api)]
    /// use futures::executor::block_on_stream;
    /// use futures::stream::{self, StreamExt};
    /// use futures::task::Poll;
    ///
    /// let mut x = 0;
    /// let stream = stream::poll_fn(|_| {
    ///     x += 1;
    ///     match x {
    ///         0..=2 => Poll::Ready(Some(x)),
    ///         3 => Poll::Ready(None),
    ///         _ => panic!("should not happen")
    ///     }
    /// }).fuse();
    ///
    /// let mut iter = block_on_stream(stream);
    /// assert_eq!(Some(1), iter.next());
    /// assert_eq!(Some(2), iter.next());
    /// assert_eq!(None, iter.next());
    /// assert_eq!(None, iter.next());
    /// // ...
    /// ```
    fn fuse(self) -> Fuse<Self>
        where Self: Sized
    {
        Fuse::new(self)
    }

    /// Borrows a stream, rather than consuming it.
    ///
    /// This is useful to allow applying stream adaptors while still retaining
    /// ownership of the original stream.
    ///
    /// # Examples
    ///
    /// ```
    /// use futures::executor::block_on;
    /// use futures::future;
    /// use futures::stream::{self, StreamExt};
    ///
    /// let mut stream = stream::iter(1..5);
    ///
    /// let sum = block_on(stream.by_ref()
    ///                          .take(2)
    ///                          .fold(0, |a, b| future::ready(a + b)));
    /// assert_eq!(sum, 3);
    ///
    /// // You can use the stream again
    /// let sum = block_on(stream.take(2).fold(0, |a, b| future::ready(a + b)));
    /// assert_eq!(sum, 7);
    /// ```
    fn by_ref(&mut self) -> &mut Self
        where Self: Sized
    {
        self
    }

    /// Catches unwinding panics while polling the stream.
    ///
    /// Caught panic (if any) will be the last element of the resulting stream.
    ///
    /// In general, panics within a stream can propagate all the way out to the
    /// task level. This combinator makes it possible to halt unwinding within
    /// the stream itself. It's most commonly used within task executors. This
    /// method should not be used for error handling.
    ///
    /// Note that this method requires the `UnwindSafe` bound from the standard
    /// library. This isn't always applied automatically, and the standard
    /// library provides an `AssertUnwindSafe` wrapper type to apply it
    /// after-the fact. To assist using this method, the [`Stream`] trait is
    /// also implemented for `AssertUnwindSafe<St>` where `St` implements
    /// [`Stream`].
    ///
    /// This method is only available when the `std` feature of this
    /// library is activated, and it is activated by default.
    ///
    /// # Examples
    ///
    /// ```
    /// use futures::executor::block_on;
    /// use futures::stream::{self, StreamExt};
    ///
    /// let stream = stream::iter(vec![Some(10), None, Some(11)]);
    /// // Panic on second element
    /// let stream_panicking = stream.map(|o| o.unwrap());
    /// // Collect all the results
    /// let stream = stream_panicking.catch_unwind();
    ///
    /// let results: Vec<Result<i32, _>> = block_on(stream.collect());
    /// match results[0] {
    ///     Ok(10) => {}
    ///     _ => panic!("unexpected result!"),
    /// }
    /// assert!(results[1].is_err());
    /// assert_eq!(results.len(), 2);
    /// ```
    #[cfg(feature = "std")]
    fn catch_unwind(self) -> CatchUnwind<Self>
        where Self: Sized + std::panic::UnwindSafe
    {
        CatchUnwind::new(self)
    }

    /// Wrap the stream in a Box, pinning it.
    #[cfg(feature = "std")]
    fn boxed(self) -> Pin<Box<Self>>
        where Self: Sized
    {
        Box::pinned(self)
    }

    /// An adaptor for creating a buffered list of pending futures.
    ///
    /// If this stream's item can be converted into a future, then this adaptor
    /// will buffer up to at most `n` futures and then return the outputs in the
    /// same order as the underlying stream. No more than `n` futures will be
    /// buffered at any point in time, and less than `n` may also be buffered
    /// depending on the state of each future.
    ///
    /// The returned stream will be a stream of each future's output.
    ///
    /// This method is only available when the `std` feature of this
    /// library is activated, and it is activated by default.
    #[cfg(feature = "std")]
    fn buffered(self, n: usize) -> Buffered<Self>
        where Self::Item: Future,
              Self: Sized
    {
        Buffered::new(self, n)
    }

    /// An adaptor for creating a buffered list of pending futures (unordered).
    ///
    /// If this stream's item can be converted into a future, then this adaptor
    /// will buffer up to `n` futures and then return the outputs in the order
    /// in which they complete. No more than `n` futures will be buffered at
    /// any point in time, and less than `n` may also be buffered depending on
    /// the state of each future.
    ///
    /// The returned stream will be a stream of each future's output.
    ///
    /// This method is only available when the `std` feature of this
    /// library is activated, and it is activated by default.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(async_await, await_macro)]
    /// # futures::executor::block_on(async {
    /// use futures::channel::oneshot;
    /// use futures::stream::{self, StreamExt};
    ///
    /// let (send_one, recv_one) = oneshot::channel();
    /// let (send_two, recv_two) = oneshot::channel();
    ///
    /// let stream_of_futures = stream::iter(vec![recv_one, recv_two]);
    /// let mut buffered = stream_of_futures.buffer_unordered(10);
    ///
    /// send_two.send(2i32);
    /// assert_eq!(await!(buffered.next()), Some(Ok(2i32)));
    ///
    /// send_one.send(1i32);
    /// assert_eq!(await!(buffered.next()), Some(Ok(1i32)));
    ///
    /// assert_eq!(await!(buffered.next()), None);
    /// # })
    /// ```
    #[cfg(feature = "std")]
    fn buffer_unordered(self, n: usize) -> BufferUnordered<Self>
        where Self::Item: Future,
              Self: Sized
    {
        BufferUnordered::new(self, n)
    }

    /// An adapter for zipping two streams together.
    ///
    /// The zipped stream waits for both streams to produce an item, and then
    /// returns that pair. If either stream ends then the zipped stream will
    /// also end.
    ///
    /// # Examples
    ///
    /// ```
    /// use futures::executor::block_on;
    /// use futures::stream::{self, StreamExt};
    ///
    /// let mut stream1 = stream::iter(1..=3);
    /// let mut stream2 = stream::iter(5..=10);
    ///
    /// let vec = block_on(stream1.zip(stream2)
    ///                           .collect::<Vec<_>>());
    /// assert_eq!(vec![(1, 5), (2, 6), (3, 7)], vec);
    /// ```
    ///
    fn zip<St>(self, other: St) -> Zip<Self, St>
        where St: Stream,
              Self: Sized,
    {
        Zip::new(self, other)
    }

    /// Adapter for chaining two stream.
    ///
    /// The resulting stream emits elements from the first stream, and when
    /// first stream reaches the end, emits the elements from the second stream.
    ///
    /// ```
    /// use futures::executor::block_on;
    /// use futures::stream::{self, StreamExt};
    ///
    /// let stream1 = stream::iter(vec![Ok(10), Err(false)]);
    /// let stream2 = stream::iter(vec![Err(true), Ok(20)]);
    ///
    /// let stream = stream1.chain(stream2);
    ///
    /// let result: Vec<_> = block_on(stream.collect());
    /// assert_eq!(result, vec![
    ///     Ok(10),
    ///     Err(false),
    ///     Err(true),
    ///     Ok(20),
    /// ]);
    /// ```
    fn chain<St>(self, other: St) -> Chain<Self, St>
        where St: Stream<Item = Self::Item>,
              Self: Sized
    {
        Chain::new(self, other)
    }

    /// Creates a new stream which exposes a `peek` method.
    ///
    /// Calling `peek` returns a reference to the next item in the stream.
    fn peekable(self) -> Peekable<Self>
        where Self: Sized
    {
        Peekable::new(self)
    }

    /// An adaptor for chunking up items of the stream inside a vector.
    ///
    /// This combinator will attempt to pull items from this stream and buffer
    /// them into a local vector. At most `capacity` items will get buffered
    /// before they're yielded from the returned stream.
    ///
    /// Note that the vectors returned from this iterator may not always have
    /// `capacity` elements. If the underlying stream ended and only a partial
    /// vector was created, it'll be returned. Additionally if an error happens
    /// from the underlying stream then the currently buffered items will be
    /// yielded.
    ///
    /// This method is only available when the `std` feature of this
    /// library is activated, and it is activated by default.
    ///
    /// # Panics
    ///
    /// This method will panic of `capacity` is zero.
    #[cfg(feature = "std")]
    fn chunks(self, capacity: usize) -> Chunks<Self>
        where Self: Sized
    {
        Chunks::new(self, capacity)
    }

    /// This combinator will attempt to pull items from both streams. Each
    /// stream will be polled in a round-robin fashion, and whenever a stream is
    /// ready to yield an item that item is yielded.
    ///
    /// After one of the two input stream completes, the remaining one will be
    /// polled exclusively. The returned stream completes when both input
    /// streams have completed.
    ///
    /// Note that this method consumes both streams and returns a wrapped
    /// version of them.
    fn select<St>(self, other: St) -> Select<Self, St>
        where St: Stream<Item = Self::Item>,
              Self: Sized,
    {
        Select::new(self, other)
    }

    /// A future that completes after the given stream has been fully processed
    /// into the sink, including flushing.
    ///
    /// This future will drive the stream to keep producing items until it is
    /// exhausted, sending each item to the sink. It will complete once both the
    /// stream is exhausted and the sink has received and flushed all items.
    /// Note that the sink is **not** closed.
    ///
    /// On completion, the sink is returned.
    ///
    /// Note that this combinator is only usable with `Unpin` sinks.
    /// Sinks that are not `Unpin` will need to be pinned in order to be used
    /// with `forward`.
    fn forward<S>(self, sink: S) -> Forward<Self, S>
    where
        S: Sink + Unpin,
        Self: Stream<Item = Result<S::SinkItem, S::SinkError>> + Sized,
    {
        Forward::new(self, sink)
    }

    /// Splits this `Stream + Sink` object into separate `Stream` and `Sink`
    /// objects.
    ///
    /// This can be useful when you want to split ownership between tasks, or
    /// allow direct interaction between the two objects (e.g. via
    /// `Sink::send_all`).
    ///
    /// This method is only available when the `std` feature of this
    /// library is activated, and it is activated by default.
    #[cfg(feature = "std")]
    fn split(self) -> (SplitSink<Self>, SplitStream<Self>)
        where Self: Sink + Sized
    {
        split::split(self)
    }

    /// Do something with each item of this stream, afterwards passing it on.
    ///
    /// This is similar to the `Iterator::inspect` method in the standard
    /// library where it allows easily inspecting each value as it passes
    /// through the stream, for example to debug what's going on.
    fn inspect<F>(self, f: F) -> Inspect<Self, F>
        where F: FnMut(&Self::Item),
              Self: Sized,
    {
        Inspect::new(self, f)
    }

    /// Wrap this stream in an `Either` stream, making it the left-hand variant
    /// of that `Either`.
    ///
    /// This can be used in combination with the `right_stream` method to write `if`
    /// statements that evaluate to different streams in different branches.
    fn left_stream<B>(self) -> Either<Self, B>
        where B: Stream<Item = Self::Item>,
              Self: Sized
    {
        Either::Left(self)
    }

    /// Wrap this stream in an `Either` stream, making it the right-hand variant
    /// of that `Either`.
    ///
    /// This can be used in combination with the `left_stream` method to write `if`
    /// statements that evaluate to different streams in different branches.
    fn right_stream<B>(self) -> Either<B, Self>
        where B: Stream<Item = Self::Item>,
              Self: Sized
    {
        Either::Right(self)
    }

    /// A convenience method for calling [`Stream::poll_next`] on [`Unpin`]
    /// stream types.
    fn poll_next_unpin(
        &mut self,
        lw: &LocalWaker
    ) -> Poll<Option<Self::Item>>
    where Self: Unpin + Sized
    {
        Pin::new(self).poll_next(lw)
    }
}