core/iter/adapters/
zip.rs

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
use crate::cmp;
use crate::fmt::{self, Debug};
use crate::iter::{
    FusedIterator, InPlaceIterable, SourceIter, TrustedFused, TrustedLen, UncheckedIterator,
};
use crate::num::NonZero;

/// An iterator that iterates two other iterators simultaneously.
///
/// This `struct` is created by [`zip`] or [`Iterator::zip`].
/// See their documentation for more.
#[derive(Clone)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Zip<A, B> {
    a: A,
    b: B,
    // index, len and a_len are only used by the specialized version of zip
    index: usize,
    len: usize,
    a_len: usize,
}
impl<A: Iterator, B: Iterator> Zip<A, B> {
    pub(in crate::iter) fn new(a: A, b: B) -> Zip<A, B> {
        ZipImpl::new(a, b)
    }
    fn super_nth(&mut self, mut n: usize) -> Option<(A::Item, B::Item)> {
        while let Some(x) = Iterator::next(self) {
            if n == 0 {
                return Some(x);
            }
            n -= 1;
        }
        None
    }
}

/// Converts the arguments to iterators and zips them.
///
/// See the documentation of [`Iterator::zip`] for more.
///
/// # Examples
///
/// ```
/// use std::iter::zip;
///
/// let xs = [1, 2, 3];
/// let ys = [4, 5, 6];
///
/// let mut iter = zip(xs, ys);
///
/// assert_eq!(iter.next().unwrap(), (1, 4));
/// assert_eq!(iter.next().unwrap(), (2, 5));
/// assert_eq!(iter.next().unwrap(), (3, 6));
/// assert!(iter.next().is_none());
///
/// // Nested zips are also possible:
/// let zs = [7, 8, 9];
///
/// let mut iter = zip(zip(xs, ys), zs);
///
/// assert_eq!(iter.next().unwrap(), ((1, 4), 7));
/// assert_eq!(iter.next().unwrap(), ((2, 5), 8));
/// assert_eq!(iter.next().unwrap(), ((3, 6), 9));
/// assert!(iter.next().is_none());
/// ```
#[stable(feature = "iter_zip", since = "1.59.0")]
pub fn zip<A, B>(a: A, b: B) -> Zip<A::IntoIter, B::IntoIter>
where
    A: IntoIterator,
    B: IntoIterator,
{
    ZipImpl::new(a.into_iter(), b.into_iter())
}

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

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

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        ZipImpl::size_hint(self)
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<Self::Item> {
        ZipImpl::nth(self, n)
    }

    #[inline]
    fn fold<Acc, F>(self, init: Acc, f: F) -> Acc
    where
        F: FnMut(Acc, Self::Item) -> Acc,
    {
        ZipImpl::fold(self, init, f)
    }

    #[inline]
    unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item
    where
        Self: TrustedRandomAccessNoCoerce,
    {
        // SAFETY: `ZipImpl::__iterator_get_unchecked` has same safety
        // requirements as `Iterator::__iterator_get_unchecked`.
        unsafe { ZipImpl::get_unchecked(self, idx) }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A, B> DoubleEndedIterator for Zip<A, B>
where
    A: DoubleEndedIterator + ExactSizeIterator,
    B: DoubleEndedIterator + ExactSizeIterator,
{
    #[inline]
    fn next_back(&mut self) -> Option<(A::Item, B::Item)> {
        ZipImpl::next_back(self)
    }
}

// Zip specialization trait
#[doc(hidden)]
trait ZipImpl<A, B> {
    type Item;
    fn new(a: A, b: B) -> Self;
    fn next(&mut self) -> Option<Self::Item>;
    fn size_hint(&self) -> (usize, Option<usize>);
    fn nth(&mut self, n: usize) -> Option<Self::Item>;
    fn next_back(&mut self) -> Option<Self::Item>
    where
        A: DoubleEndedIterator + ExactSizeIterator,
        B: DoubleEndedIterator + ExactSizeIterator;
    fn fold<Acc, F>(self, init: Acc, f: F) -> Acc
    where
        F: FnMut(Acc, Self::Item) -> Acc;
    // This has the same safety requirements as `Iterator::__iterator_get_unchecked`
    unsafe fn get_unchecked(&mut self, idx: usize) -> <Self as Iterator>::Item
    where
        Self: Iterator + TrustedRandomAccessNoCoerce;
}

// Work around limitations of specialization, requiring `default` impls to be repeated
// in intermediary impls.
macro_rules! zip_impl_general_defaults {
    () => {
        default fn new(a: A, b: B) -> Self {
            Zip {
                a,
                b,
                index: 0, // unused
                len: 0,   // unused
                a_len: 0, // unused
            }
        }

        #[inline]
        default fn next(&mut self) -> Option<(A::Item, B::Item)> {
            let x = self.a.next()?;
            let y = self.b.next()?;
            Some((x, y))
        }

        #[inline]
        default fn nth(&mut self, n: usize) -> Option<Self::Item> {
            self.super_nth(n)
        }

        #[inline]
        default fn next_back(&mut self) -> Option<(A::Item, B::Item)>
        where
            A: DoubleEndedIterator + ExactSizeIterator,
            B: DoubleEndedIterator + ExactSizeIterator,
        {
            // The function body below only uses `self.a/b.len()` and `self.a/b.next_back()`
            // and doesn’t call `next_back` too often, so this implementation is safe in
            // the `TrustedRandomAccessNoCoerce` specialization

            let a_sz = self.a.len();
            let b_sz = self.b.len();
            if a_sz != b_sz {
                // Adjust a, b to equal length
                if a_sz > b_sz {
                    for _ in 0..a_sz - b_sz {
                        self.a.next_back();
                    }
                } else {
                    for _ in 0..b_sz - a_sz {
                        self.b.next_back();
                    }
                }
            }
            match (self.a.next_back(), self.b.next_back()) {
                (Some(x), Some(y)) => Some((x, y)),
                (None, None) => None,
                _ => unreachable!(),
            }
        }
    };
}

// General Zip impl
#[doc(hidden)]
impl<A, B> ZipImpl<A, B> for Zip<A, B>
where
    A: Iterator,
    B: Iterator,
{
    type Item = (A::Item, B::Item);

    zip_impl_general_defaults! {}

    #[inline]
    default fn size_hint(&self) -> (usize, Option<usize>) {
        let (a_lower, a_upper) = self.a.size_hint();
        let (b_lower, b_upper) = self.b.size_hint();

        let lower = cmp::min(a_lower, b_lower);

        let upper = match (a_upper, b_upper) {
            (Some(x), Some(y)) => Some(cmp::min(x, y)),
            (Some(x), None) => Some(x),
            (None, Some(y)) => Some(y),
            (None, None) => None,
        };

        (lower, upper)
    }

    default unsafe fn get_unchecked(&mut self, _idx: usize) -> <Self as Iterator>::Item
    where
        Self: TrustedRandomAccessNoCoerce,
    {
        unreachable!("Always specialized");
    }

    #[inline]
    default fn fold<Acc, F>(self, init: Acc, f: F) -> Acc
    where
        F: FnMut(Acc, Self::Item) -> Acc,
    {
        SpecFold::spec_fold(self, init, f)
    }
}

#[doc(hidden)]
impl<A, B> ZipImpl<A, B> for Zip<A, B>
where
    A: TrustedRandomAccessNoCoerce + Iterator,
    B: TrustedRandomAccessNoCoerce + Iterator,
{
    zip_impl_general_defaults! {}

    #[inline]
    default fn size_hint(&self) -> (usize, Option<usize>) {
        let size = cmp::min(self.a.size(), self.b.size());
        (size, Some(size))
    }

    #[inline]
    unsafe fn get_unchecked(&mut self, idx: usize) -> <Self as Iterator>::Item {
        let idx = self.index + idx;
        // SAFETY: the caller must uphold the contract for
        // `Iterator::__iterator_get_unchecked`.
        unsafe { (self.a.__iterator_get_unchecked(idx), self.b.__iterator_get_unchecked(idx)) }
    }

    #[inline]
    fn fold<Acc, F>(mut self, init: Acc, mut f: F) -> Acc
    where
        F: FnMut(Acc, Self::Item) -> Acc,
    {
        let mut accum = init;
        let len = ZipImpl::size_hint(&self).0;
        for i in 0..len {
            // SAFETY: since Self: TrustedRandomAccessNoCoerce we can trust the size-hint to
            // calculate the length and then use that to do unchecked iteration.
            // fold consumes the iterator so we don't need to fixup any state.
            unsafe {
                accum = f(accum, self.get_unchecked(i));
            }
        }
        accum
    }
}

#[doc(hidden)]
impl<A, B> ZipImpl<A, B> for Zip<A, B>
where
    A: TrustedRandomAccess + Iterator,
    B: TrustedRandomAccess + Iterator,
{
    fn new(a: A, b: B) -> Self {
        let a_len = a.size();
        let len = cmp::min(a_len, b.size());
        Zip { a, b, index: 0, len, a_len }
    }

    #[inline]
    fn next(&mut self) -> Option<(A::Item, B::Item)> {
        if self.index < self.len {
            let i = self.index;
            // since get_unchecked executes code which can panic we increment the counters beforehand
            // so that the same index won't be accessed twice, as required by TrustedRandomAccess
            self.index += 1;
            // SAFETY: `i` is smaller than `self.len`, thus smaller than `self.a.len()` and `self.b.len()`
            unsafe {
                Some((self.a.__iterator_get_unchecked(i), self.b.__iterator_get_unchecked(i)))
            }
        } else if A::MAY_HAVE_SIDE_EFFECT && self.index < self.a_len {
            let i = self.index;
            // as above, increment before executing code that may panic
            self.index += 1;
            self.len += 1;
            // match the base implementation's potential side effects
            // SAFETY: we just checked that `i` < `self.a.len()`
            unsafe {
                self.a.__iterator_get_unchecked(i);
            }
            None
        } else {
            None
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let len = self.len - self.index;
        (len, Some(len))
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<Self::Item> {
        let delta = cmp::min(n, self.len - self.index);
        let end = self.index + delta;
        while self.index < end {
            let i = self.index;
            // since get_unchecked executes code which can panic we increment the counters beforehand
            // so that the same index won't be accessed twice, as required by TrustedRandomAccess
            self.index += 1;
            if A::MAY_HAVE_SIDE_EFFECT {
                // SAFETY: the usage of `cmp::min` to calculate `delta`
                // ensures that `end` is smaller than or equal to `self.len`,
                // so `i` is also smaller than `self.len`.
                unsafe {
                    self.a.__iterator_get_unchecked(i);
                }
            }
            if B::MAY_HAVE_SIDE_EFFECT {
                // SAFETY: same as above.
                unsafe {
                    self.b.__iterator_get_unchecked(i);
                }
            }
        }

        self.super_nth(n - delta)
    }

    #[inline]
    fn next_back(&mut self) -> Option<(A::Item, B::Item)>
    where
        A: DoubleEndedIterator + ExactSizeIterator,
        B: DoubleEndedIterator + ExactSizeIterator,
    {
        if A::MAY_HAVE_SIDE_EFFECT || B::MAY_HAVE_SIDE_EFFECT {
            let sz_a = self.a.size();
            let sz_b = self.b.size();
            // Adjust a, b to equal length, make sure that only the first call
            // of `next_back` does this, otherwise we will break the restriction
            // on calls to `self.next_back()` after calling `get_unchecked()`.
            if sz_a != sz_b {
                let sz_a = self.a.size();
                if A::MAY_HAVE_SIDE_EFFECT && sz_a > self.len {
                    for _ in 0..sz_a - self.len {
                        // since next_back() may panic we increment the counters beforehand
                        // to keep Zip's state in sync with the underlying iterator source
                        self.a_len -= 1;
                        self.a.next_back();
                    }
                    debug_assert_eq!(self.a_len, self.len);
                }
                let sz_b = self.b.size();
                if B::MAY_HAVE_SIDE_EFFECT && sz_b > self.len {
                    for _ in 0..sz_b - self.len {
                        self.b.next_back();
                    }
                }
            }
        }
        if self.index < self.len {
            // since get_unchecked executes code which can panic we increment the counters beforehand
            // so that the same index won't be accessed twice, as required by TrustedRandomAccess
            self.len -= 1;
            self.a_len -= 1;
            let i = self.len;
            // SAFETY: `i` is smaller than the previous value of `self.len`,
            // which is also smaller than or equal to `self.a.len()` and `self.b.len()`
            unsafe {
                Some((self.a.__iterator_get_unchecked(i), self.b.__iterator_get_unchecked(i)))
            }
        } else {
            None
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A, B> ExactSizeIterator for Zip<A, B>
where
    A: ExactSizeIterator,
    B: ExactSizeIterator,
{
}

#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<A, B> TrustedRandomAccess for Zip<A, B>
where
    A: TrustedRandomAccess,
    B: TrustedRandomAccess,
{
}

#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<A, B> TrustedRandomAccessNoCoerce for Zip<A, B>
where
    A: TrustedRandomAccessNoCoerce,
    B: TrustedRandomAccessNoCoerce,
{
    const MAY_HAVE_SIDE_EFFECT: bool = A::MAY_HAVE_SIDE_EFFECT || B::MAY_HAVE_SIDE_EFFECT;
}

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

#[unstable(issue = "none", feature = "trusted_fused")]
unsafe impl<A, B> TrustedFused for Zip<A, B>
where
    A: TrustedFused,
    B: TrustedFused,
{
}

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

impl<A, B> UncheckedIterator for Zip<A, B>
where
    A: UncheckedIterator,
    B: UncheckedIterator,
{
}

// Arbitrarily selects the left side of the zip iteration as extractable "source"
// it would require negative trait bounds to be able to try both
#[unstable(issue = "none", feature = "inplace_iteration")]
unsafe impl<A, B> SourceIter for Zip<A, B>
where
    A: SourceIter,
{
    type Source = A::Source;

    #[inline]
    unsafe fn as_inner(&mut self) -> &mut A::Source {
        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
        unsafe { SourceIter::as_inner(&mut self.a) }
    }
}

// Since SourceIter forwards the left hand side we do the same here
#[unstable(issue = "none", feature = "inplace_iteration")]
unsafe impl<A: InPlaceIterable, B> InPlaceIterable for Zip<A, B> {
    const EXPAND_BY: Option<NonZero<usize>> = A::EXPAND_BY;
    const MERGE_BY: Option<NonZero<usize>> = A::MERGE_BY;
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A: Debug, B: Debug> Debug for Zip<A, B> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        ZipFmt::fmt(self, f)
    }
}

trait ZipFmt<A, B> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result;
}

impl<A: Debug, B: Debug> ZipFmt<A, B> for Zip<A, B> {
    default fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Zip").field("a", &self.a).field("b", &self.b).finish()
    }
}

impl<A: Debug + TrustedRandomAccessNoCoerce, B: Debug + TrustedRandomAccessNoCoerce> ZipFmt<A, B>
    for Zip<A, B>
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        // It's *not safe* to call fmt on the contained iterators, since once
        // we start iterating they're in strange, potentially unsafe, states.
        f.debug_struct("Zip").finish()
    }
}

/// An iterator whose items are random-accessible efficiently
///
/// # Safety
///
/// The iterator's `size_hint` must be exact and cheap to call.
///
/// `TrustedRandomAccessNoCoerce::size` may not be overridden.
///
/// All subtypes and all supertypes of `Self` must also implement `TrustedRandomAccess`.
/// In particular, this means that types with non-invariant parameters usually can not have
/// an impl for `TrustedRandomAccess` that depends on any trait bounds on such parameters, except
/// for bounds that come from the respective struct/enum definition itself, or bounds involving
/// traits that themselves come with a guarantee similar to this one.
///
/// If `Self: ExactSizeIterator` then `self.len()` must always produce results consistent
/// with `self.size()`.
///
/// If `Self: Iterator`, then `<Self as Iterator>::__iterator_get_unchecked(&mut self, idx)`
/// must be safe to call provided the following conditions are met.
///
/// 1. `0 <= idx` and `idx < self.size()`.
/// 2. If `Self: !Clone`, then `self.__iterator_get_unchecked(idx)` is never called with the same
///    index on `self` more than once.
/// 3. After `self.__iterator_get_unchecked(idx)` has been called, then `self.next_back()` will
///    only be called at most `self.size() - idx - 1` times. If `Self: Clone` and `self` is cloned,
///    then this number is calculated for `self` and its clone individually,
///    but `self.next_back()` calls that happened before the cloning count for both `self` and the clone.
/// 4. After `self.__iterator_get_unchecked(idx)` has been called, then only the following methods
///    will be called on `self` or on any new clones of `self`:
///     * `std::clone::Clone::clone`
///     * `std::iter::Iterator::size_hint`
///     * `std::iter::DoubleEndedIterator::next_back`
///     * `std::iter::ExactSizeIterator::len`
///     * `std::iter::Iterator::__iterator_get_unchecked`
///     * `std::iter::TrustedRandomAccessNoCoerce::size`
/// 5. If `T` is a subtype of `Self`, then `self` is allowed to be coerced
///    to `T`. If `self` is coerced to `T` after `self.__iterator_get_unchecked(idx)` has already
///    been called, then no methods except for the ones listed under 4. are allowed to be called
///    on the resulting value of type `T`, either. Multiple such coercion steps are allowed.
///    Regarding 2. and 3., the number of times `__iterator_get_unchecked(idx)` or `next_back()` is
///    called on `self` and the resulting value of type `T` (and on further coercion results with
///    sub-subtypes) are added together and their sums must not exceed the specified bounds.
///
/// Further, given that these conditions are met, it must guarantee that:
///
/// * It does not change the value returned from `size_hint`
/// * It must be safe to call the methods listed above on `self` after calling
///   `self.__iterator_get_unchecked(idx)`, assuming that the required traits are implemented.
/// * It must also be safe to drop `self` after calling `self.__iterator_get_unchecked(idx)`.
/// * If `T` is a subtype of `Self`, then it must be safe to coerce `self` to `T`.
//
// FIXME: Clarify interaction with SourceIter/InPlaceIterable. Calling `SourceIter::as_inner`
// after `__iterator_get_unchecked` is supposed to be allowed.
#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
#[rustc_specialization_trait]
pub unsafe trait TrustedRandomAccess: TrustedRandomAccessNoCoerce {}

/// Like [`TrustedRandomAccess`] but without any of the requirements / guarantees around
/// coercions to subtypes after `__iterator_get_unchecked` (they aren’t allowed here!), and
/// without the requirement that subtypes / supertypes implement `TrustedRandomAccessNoCoerce`.
///
/// This trait was created in PR #85874 to fix soundness issue #85873 without performance regressions.
/// It is subject to change as we might want to build a more generally useful (for performance
/// optimizations) and more sophisticated trait or trait hierarchy that replaces or extends
/// [`TrustedRandomAccess`] and `TrustedRandomAccessNoCoerce`.
#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
#[rustc_specialization_trait]
pub unsafe trait TrustedRandomAccessNoCoerce: Sized {
    // Convenience method.
    fn size(&self) -> usize
    where
        Self: Iterator,
    {
        self.size_hint().0
    }
    /// `true` if getting an iterator element may have side effects.
    /// Remember to take inner iterators into account.
    const MAY_HAVE_SIDE_EFFECT: bool;
}

/// Like `Iterator::__iterator_get_unchecked`, but doesn't require the compiler to
/// know that `U: TrustedRandomAccess`.
///
/// ## Safety
///
/// Same requirements calling `get_unchecked` directly.
#[doc(hidden)]
#[inline]
pub(in crate::iter::adapters) unsafe fn try_get_unchecked<I>(it: &mut I, idx: usize) -> I::Item
where
    I: Iterator,
{
    // SAFETY: the caller must uphold the contract for
    // `Iterator::__iterator_get_unchecked`.
    unsafe { it.try_get_unchecked(idx) }
}

unsafe trait SpecTrustedRandomAccess: Iterator {
    /// If `Self: TrustedRandomAccess`, it must be safe to call
    /// `Iterator::__iterator_get_unchecked(self, index)`.
    unsafe fn try_get_unchecked(&mut self, index: usize) -> Self::Item;
}

unsafe impl<I: Iterator> SpecTrustedRandomAccess for I {
    default unsafe fn try_get_unchecked(&mut self, _: usize) -> Self::Item {
        panic!("Should only be called on TrustedRandomAccess iterators");
    }
}

unsafe impl<I: Iterator + TrustedRandomAccessNoCoerce> SpecTrustedRandomAccess for I {
    #[inline]
    unsafe fn try_get_unchecked(&mut self, index: usize) -> Self::Item {
        // SAFETY: the caller must uphold the contract for
        // `Iterator::__iterator_get_unchecked`.
        unsafe { self.__iterator_get_unchecked(index) }
    }
}

trait SpecFold: Iterator {
    fn spec_fold<B, F>(self, init: B, f: F) -> B
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B;
}

impl<A: Iterator, B: Iterator> SpecFold for Zip<A, B> {
    // Adapted from default impl from the Iterator trait
    #[inline]
    default fn spec_fold<Acc, F>(mut self, init: Acc, mut f: F) -> Acc
    where
        F: FnMut(Acc, Self::Item) -> Acc,
    {
        let mut accum = init;
        while let Some(x) = ZipImpl::next(&mut self) {
            accum = f(accum, x);
        }
        accum
    }
}

impl<A: TrustedLen, B: TrustedLen> SpecFold for Zip<A, B> {
    #[inline]
    fn spec_fold<Acc, F>(mut self, init: Acc, mut f: F) -> Acc
    where
        F: FnMut(Acc, Self::Item) -> Acc,
    {
        let mut accum = init;
        loop {
            let (upper, more) = if let Some(upper) = ZipImpl::size_hint(&self).1 {
                (upper, false)
            } else {
                // Per TrustedLen contract a None upper bound means more than usize::MAX items
                (usize::MAX, true)
            };

            for _ in 0..upper {
                let pair =
                    // SAFETY: TrustedLen guarantees that at least `upper` many items are available
                    // therefore we know they can't be None
                    unsafe { (self.a.next().unwrap_unchecked(), self.b.next().unwrap_unchecked()) };
                accum = f(accum, pair);
            }

            if !more {
                break;
            }
        }
        accum
    }
}