      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

    use crate::fmt;
    use crate::marker;
    use crate::usize;

    use super::{FusedIterator, TrustedLen};

    /// An iterator that repeats an element endlessly.
    ///
    /// This `struct` is created by the [`repeat`] function. See its documentation for more.
    ///
    /// [`repeat`]: fn.repeat.html
    #[derive(Clone, Debug)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Repeat<A> {
        element: A
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A: Clone> Iterator for Repeat<A> {
        type Item = A;

        #[inline]
        fn next(&mut self) -> Option<A> { Some(self.element.clone()) }
        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) { (usize::MAX, None) }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A: Clone> DoubleEndedIterator for Repeat<A> {
        #[inline]
        fn next_back(&mut self) -> Option<A> { Some(self.element.clone()) }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl<A: Clone> FusedIterator for Repeat<A> {}

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<A: Clone> TrustedLen for Repeat<A> {}

    /// Creates a new iterator that endlessly repeats a single element.
    ///
    /// The `repeat()` function repeats a single value over and over again.
    ///
    /// Infinite iterators like `repeat()` are often used with adapters like
    /// [`take`], in order to make them finite.
    ///
    /// [`take`]: trait.Iterator.html#method.take
    ///
    /// If the element type of the iterator you need does not implement `Clone`,
    /// or if you do not want to keep the repeated element in memory, you can
    /// instead use the [`repeat_with`] function.
    ///
    /// [`repeat_with`]: fn.repeat_with.html
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::iter;
    ///
    /// // the number four 4ever:
    /// let mut fours = iter::repeat(4);
    ///
    /// assert_eq!(Some(4), fours.next());
    /// assert_eq!(Some(4), fours.next());
    /// assert_eq!(Some(4), fours.next());
    /// assert_eq!(Some(4), fours.next());
    /// assert_eq!(Some(4), fours.next());
    ///
    /// // yup, still four
    /// assert_eq!(Some(4), fours.next());
    /// ```
    ///
    /// Going finite with [`take`]:
    ///
    /// ```
    /// use std::iter;
    ///
    /// // that last example was too many fours. Let's only have four fours.
    /// let mut four_fours = iter::repeat(4).take(4);
    ///
    /// assert_eq!(Some(4), four_fours.next());
    /// assert_eq!(Some(4), four_fours.next());
    /// assert_eq!(Some(4), four_fours.next());
    /// assert_eq!(Some(4), four_fours.next());
    ///
    /// // ... and now we're done
    /// assert_eq!(None, four_fours.next());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn repeat<T: Clone>(elt: T) -> Repeat<T> {
        Repeat{element: elt}
    }

    /// An iterator that repeats elements of type `A` endlessly by
    /// applying the provided closure `F: FnMut() -> A`.
    ///
    /// This `struct` is created by the [`repeat_with`] function.
    /// See its documentation for more.
    ///
    /// [`repeat_with`]: fn.repeat_with.html
    #[derive(Copy, Clone, Debug)]
    #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
    pub struct RepeatWith<F> {
        repeater: F
    }

    #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
    impl<A, F: FnMut() -> A> Iterator for RepeatWith<F> {
        type Item = A;

        #[inline]
        fn next(&mut self) -> Option<A> { Some((self.repeater)()) }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) { (usize::MAX, None) }
    }

    #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
    impl<A, F: FnMut() -> A> FusedIterator for RepeatWith<F> {}

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<A, F: FnMut() -> A> TrustedLen for RepeatWith<F> {}

    /// Creates a new iterator that repeats elements of type `A` endlessly by
    /// applying the provided closure, the repeater, `F: FnMut() -> A`.
    ///
    /// The `repeat_with()` function calls the repeater over and over again.
    ///
    /// Infinite iterators like `repeat_with()` are often used with adapters like
    /// [`take`], in order to make them finite.
    ///
    /// [`take`]: trait.Iterator.html#method.take
    ///
    /// If the element type of the iterator you need implements `Clone`, and
    /// it is OK to keep the source element in memory, you should instead use
    /// the [`repeat`] function.
    ///
    /// [`repeat`]: fn.repeat.html
    ///
    /// An iterator produced by `repeat_with()` is not a `DoubleEndedIterator`.
    /// If you need `repeat_with()` to return a `DoubleEndedIterator`,
    /// please open a GitHub issue explaining your use case.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::iter;
    ///
    /// // let's assume we have some value of a type that is not `Clone`
    /// // or which don't want to have in memory just yet because it is expensive:
    /// #[derive(PartialEq, Debug)]
    /// struct Expensive;
    ///
    /// // a particular value forever:
    /// let mut things = iter::repeat_with(|| Expensive);
    ///
    /// assert_eq!(Some(Expensive), things.next());
    /// assert_eq!(Some(Expensive), things.next());
    /// assert_eq!(Some(Expensive), things.next());
    /// assert_eq!(Some(Expensive), things.next());
    /// assert_eq!(Some(Expensive), things.next());
    /// ```
    ///
    /// Using mutation and going finite:
    ///
    /// ```rust
    /// use std::iter;
    ///
    /// // From the zeroth to the third power of two:
    /// let mut curr = 1;
    /// let mut pow2 = iter::repeat_with(|| { let tmp = curr; curr *= 2; tmp })
    ///                     .take(4);
    ///
    /// assert_eq!(Some(1), pow2.next());
    /// assert_eq!(Some(2), pow2.next());
    /// assert_eq!(Some(4), pow2.next());
    /// assert_eq!(Some(8), pow2.next());
    ///
    /// // ... and now we're done
    /// assert_eq!(None, pow2.next());
    /// ```
    #[inline]
    #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
    pub fn repeat_with<A, F: FnMut() -> A>(repeater: F) -> RepeatWith<F> {
        RepeatWith { repeater }
    }

    /// An iterator that yields nothing.
    ///
    /// This `struct` is created by the [`empty`] function. See its documentation for more.
    ///
    /// [`empty`]: fn.empty.html
    #[stable(feature = "iter_empty", since = "1.2.0")]
    pub struct Empty<T>(marker::PhantomData<T>);

    #[stable(feature = "core_impl_debug", since = "1.9.0")]
    impl<T> fmt::Debug for Empty<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.pad("Empty")
        }
    }

    #[stable(feature = "iter_empty", since = "1.2.0")]
    impl<T> Iterator for Empty<T> {
        type Item = T;

        fn next(&mut self) -> Option<T> {
            None
        }

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

    #[stable(feature = "iter_empty", since = "1.2.0")]
    impl<T> DoubleEndedIterator for Empty<T> {
        fn next_back(&mut self) -> Option<T> {
            None
        }
    }

    #[stable(feature = "iter_empty", since = "1.2.0")]
    impl<T> ExactSizeIterator for Empty<T> {
        fn len(&self) -> usize {
            0
        }
    }

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<T> TrustedLen for Empty<T> {}

    #[stable(feature = "fused", since = "1.26.0")]
    impl<T> FusedIterator for Empty<T> {}

    // not #[derive] because that adds a Clone bound on T,
    // which isn't necessary.
    #[stable(feature = "iter_empty", since = "1.2.0")]
    impl<T> Clone for Empty<T> {
        fn clone(&self) -> Empty<T> {
            Empty(marker::PhantomData)
        }
    }

    // not #[derive] because that adds a Default bound on T,
    // which isn't necessary.
    #[stable(feature = "iter_empty", since = "1.2.0")]
    impl<T> Default for Empty<T> {
        fn default() -> Empty<T> {
            Empty(marker::PhantomData)
        }
    }

    /// Creates an iterator that yields nothing.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::iter;
    ///
    /// // this could have been an iterator over i32, but alas, it's just not.
    /// let mut nope = iter::empty::<i32>();
    ///
    /// assert_eq!(None, nope.next());
    /// ```
    #[stable(feature = "iter_empty", since = "1.2.0")]
    pub const fn empty<T>() -> Empty<T> {
        Empty(marker::PhantomData)
    }

    /// An iterator that yields an element exactly once.
    ///
    /// This `struct` is created by the [`once`] function. See its documentation for more.
    ///
    /// [`once`]: fn.once.html
    #[derive(Clone, Debug)]
    #[stable(feature = "iter_once", since = "1.2.0")]
    pub struct Once<T> {
        inner: crate::option::IntoIter<T>
    }

    #[stable(feature = "iter_once", since = "1.2.0")]
    impl<T> Iterator for Once<T> {
        type Item = T;

        fn next(&mut self) -> Option<T> {
            self.inner.next()
        }

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

    #[stable(feature = "iter_once", since = "1.2.0")]
    impl<T> DoubleEndedIterator for Once<T> {
        fn next_back(&mut self) -> Option<T> {
            self.inner.next_back()
        }
    }

    #[stable(feature = "iter_once", since = "1.2.0")]
    impl<T> ExactSizeIterator for Once<T> {
        fn len(&self) -> usize {
            self.inner.len()
        }
    }

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<T> TrustedLen for Once<T> {}

    #[stable(feature = "fused", since = "1.26.0")]
    impl<T> FusedIterator for Once<T> {}

    /// Creates an iterator that yields an element exactly once.
    ///
    /// This is commonly used to adapt a single value into a [`chain`] of other
    /// kinds of iteration. Maybe you have an iterator that covers almost
    /// everything, but you need an extra special case. Maybe you have a function
    /// which works on iterators, but you only need to process one value.
    ///
    /// [`chain`]: trait.Iterator.html#method.chain
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::iter;
    ///
    /// // one is the loneliest number
    /// let mut one = iter::once(1);
    ///
    /// assert_eq!(Some(1), one.next());
    ///
    /// // just one, that's all we get
    /// assert_eq!(None, one.next());
    /// ```
    ///
    /// Chaining together with another iterator. Let's say that we want to iterate
    /// over each file of the `.foo` directory, but also a configuration file,
    /// `.foorc`:
    ///
    /// ```no_run
    /// use std::iter;
    /// use std::fs;
    /// use std::path::PathBuf;
    ///
    /// let dirs = fs::read_dir(".foo").unwrap();
    ///
    /// // we need to convert from an iterator of DirEntry-s to an iterator of
    /// // PathBufs, so we use map
    /// let dirs = dirs.map(|file| file.unwrap().path());
    ///
    /// // now, our iterator just for our config file
    /// let config = iter::once(PathBuf::from(".foorc"));
    ///
    /// // chain the two iterators together into one big iterator
    /// let files = dirs.chain(config);
    ///
    /// // this will give us all of the files in .foo as well as .foorc
    /// for f in files {
    ///     println!("{:?}", f);
    /// }
    /// ```
    #[stable(feature = "iter_once", since = "1.2.0")]
    pub fn once<T>(value: T) -> Once<T> {
        Once { inner: Some(value).into_iter() }
    }

    /// An iterator that yields a single element of type `A` by
    /// applying the provided closure `F: FnOnce() -> A`.
    ///
    /// This `struct` is created by the [`once_with`] function.
    /// See its documentation for more.
    ///
    /// [`once_with`]: fn.once_with.html
    #[derive(Copy, Clone, Debug)]
    #[unstable(feature = "iter_once_with", issue = "57581")]
    pub struct OnceWith<F> {
        gen: Option<F>,
    }

    #[unstable(feature = "iter_once_with", issue = "57581")]
    impl<A, F: FnOnce() -> A> Iterator for OnceWith<F> {
        type Item = A;

        #[inline]
        fn next(&mut self) -> Option<A> {
            self.gen.take().map(|f| f())
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.gen.iter().size_hint()
        }
    }

    #[unstable(feature = "iter_once_with", issue = "57581")]
    impl<A, F: FnOnce() -> A> DoubleEndedIterator for OnceWith<F> {
        fn next_back(&mut self) -> Option<A> {
            self.next()
        }
    }

    #[unstable(feature = "iter_once_with", issue = "57581")]
    impl<A, F: FnOnce() -> A> ExactSizeIterator for OnceWith<F> {
        fn len(&self) -> usize {
            self.gen.iter().len()
        }
    }

    #[unstable(feature = "iter_once_with", issue = "57581")]
    impl<A, F: FnOnce() -> A> FusedIterator for OnceWith<F> {}

    #[unstable(feature = "iter_once_with", issue = "57581")]
    unsafe impl<A, F: FnOnce() -> A> TrustedLen for OnceWith<F> {}

    /// Creates an iterator that lazily generates a value exactly once by invoking
    /// the provided closure.
    ///
    /// This is commonly used to adapt a single value generator into a [`chain`] of
    /// other kinds of iteration. Maybe you have an iterator that covers almost
    /// everything, but you need an extra special case. Maybe you have a function
    /// which works on iterators, but you only need to process one value.
    ///
    /// Unlike [`once`], this function will lazily generate the value on request.
    ///
    /// [`once`]: fn.once.html
    /// [`chain`]: trait.Iterator.html#method.chain
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// #![feature(iter_once_with)]
    ///
    /// use std::iter;
    ///
    /// // one is the loneliest number
    /// let mut one = iter::once_with(|| 1);
    ///
    /// assert_eq!(Some(1), one.next());
    ///
    /// // just one, that's all we get
    /// assert_eq!(None, one.next());
    /// ```
    ///
    /// Chaining together with another iterator. Let's say that we want to iterate
    /// over each file of the `.foo` directory, but also a configuration file,
    /// `.foorc`:
    ///
    /// ```no_run
    /// #![feature(iter_once_with)]
    ///
    /// use std::iter;
    /// use std::fs;
    /// use std::path::PathBuf;
    ///
    /// let dirs = fs::read_dir(".foo").unwrap();
    ///
    /// // we need to convert from an iterator of DirEntry-s to an iterator of
    /// // PathBufs, so we use map
    /// let dirs = dirs.map(|file| file.unwrap().path());
    ///
    /// // now, our iterator just for our config file
    /// let config = iter::once_with(|| PathBuf::from(".foorc"));
    ///
    /// // chain the two iterators together into one big iterator
    /// let files = dirs.chain(config);
    ///
    /// // this will give us all of the files in .foo as well as .foorc
    /// for f in files {
    ///     println!("{:?}", f);
    /// }
    /// ```
    #[inline]
    #[unstable(feature = "iter_once_with", issue = "57581")]
    pub fn once_with<A, F: FnOnce() -> A>(gen: F) -> OnceWith<F> {
        OnceWith { gen: Some(gen) }
    }

    /// Creates a new iterator where each iteration calls the provided closure
    /// `F: FnMut() -> Option<T>`.
    ///
    /// This allows creating a custom iterator with any behavior
    /// without using the more verbose syntax of creating a dedicated type
    /// and implementing the `Iterator` trait for it.
    ///
    /// Note that the `FromFn` iterator doesn’t make assumptions about the behavior of the closure,
    /// and therefore conservatively does not implement [`FusedIterator`],
    /// or override [`Iterator::size_hint`] from its default `(0, None)`.
    ///
    /// [`FusedIterator`]: trait.FusedIterator.html
    /// [`Iterator::size_hint`]: trait.Iterator.html#method.size_hint
    ///
    /// The closure can use captures and its environment to track state across iterations. Depending on
    /// how the iterator is used, this may require specifying the `move` keyword on the closure.
    ///
    /// # Examples
    ///
    /// Let’s re-implement the counter iterator from [module-level documentation]:
    ///
    /// [module-level documentation]: index.html
    ///
    /// ```
    /// let mut count = 0;
    /// let counter = std::iter::from_fn(move || {
    ///     // Increment our count. This is why we started at zero.
    ///     count += 1;
    ///
    ///     // Check to see if we've finished counting or not.
    ///     if count < 6 {
    ///         Some(count)
    ///     } else {
    ///         None
    ///     }
    /// });
    /// assert_eq!(counter.collect::<Vec<_>>(), &[1, 2, 3, 4, 5]);
    /// ```
    #[inline]
    #[stable(feature = "iter_from_fn", since = "1.34.0")]
    pub fn from_fn<T, F>(f: F) -> FromFn<F>
        where F: FnMut() -> Option<T>
    {
        FromFn(f)
    }

    /// An iterator where each iteration calls the provided closure `F: FnMut() -> Option<T>`.
    ///
    /// This `struct` is created by the [`iter::from_fn`] function.
    /// See its documentation for more.
    ///
    /// [`iter::from_fn`]: fn.from_fn.html
    #[derive(Clone)]
    #[stable(feature = "iter_from_fn", since = "1.34.0")]
    pub struct FromFn<F>(F);

    #[stable(feature = "iter_from_fn", since = "1.34.0")]
    impl<T, F> Iterator for FromFn<F>
        where F: FnMut() -> Option<T>
    {
        type Item = T;

        #[inline]
        fn next(&mut self) -> Option<Self::Item> {
            (self.0)()
        }
    }

    #[stable(feature = "iter_from_fn", since = "1.34.0")]
    impl<F> fmt::Debug for FromFn<F> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("FromFn").finish()
        }
    }

    /// Creates a new iterator where each successive item is computed based on the preceding one.
    ///
    /// The iterator starts with the given first item (if any)
    /// and calls the given `FnMut(&T) -> Option<T>` closure to compute each item’s successor.
    ///
    /// ```
    /// use std::iter::successors;
    ///
    /// let powers_of_10 = successors(Some(1_u16), |n| n.checked_mul(10));
    /// assert_eq!(powers_of_10.collect::<Vec<_>>(), &[1, 10, 100, 1_000, 10_000]);
    /// ```
    #[stable(feature = "iter_successors", since = "1.34.0")]
    pub fn successors<T, F>(first: Option<T>, succ: F) -> Successors<T, F>
        where F: FnMut(&T) -> Option<T>
    {
        // If this function returned `impl Iterator<Item=T>`
        // it could be based on `unfold` and not need a dedicated type.
        // However having a named `Successors<T, F>` type allows it to be `Clone` when `T` and `F` are.
        Successors {
            next: first,
            succ,
        }
    }

    /// An new iterator where each successive item is computed based on the preceding one.
    ///
    /// This `struct` is created by the [`successors`] function.
    /// See its documentation for more.
    ///
    /// [`successors`]: fn.successors.html
    #[derive(Clone)]
    #[stable(feature = "iter_successors", since = "1.34.0")]
    pub struct Successors<T, F> {
        next: Option<T>,
        succ: F,
    }

    #[stable(feature = "iter_successors", since = "1.34.0")]
    impl<T, F> Iterator for Successors<T, F>
        where F: FnMut(&T) -> Option<T>
    {
        type Item = T;

        #[inline]
        fn next(&mut self) -> Option<Self::Item> {
            self.next.take().map(|item| {
                self.next = (self.succ)(&item);
                item
            })
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            if self.next.is_some() {
                (1, None)
            } else {
                (0, Some(0))
            }
        }
    }

    #[stable(feature = "iter_successors", since = "1.34.0")]
    impl<T, F> FusedIterator for Successors<T, F>
        where F: FnMut(&T) -> Option<T>
    {}

    #[stable(feature = "iter_successors", since = "1.34.0")]
    impl<T: fmt::Debug, F> fmt::Debug for Successors<T, F> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Successors")
                .field("next", &self.next)
                .finish()
        }
    }

# Help

## Keyboard Shortcuts

`?`
  Show this help dialog
`S`
  Focus the search field
`↑`
  Move up in search results
`↓`
  Move down in search results
`↹`
  Switch tab
`⏎`
  Go to active search result
`+`
  Expand all sections
`-`
  Collapse all sections

## Search Tricks

Prefix searches with a type followed by a colon (e.g., `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g., `vec -> usize` or `* -> vec`)

Search multiple things at once by splitting your query with comma (e.g., `str,u8` or `String,struct:Vec,test`)