core/iter/adapters/
zip.rs

1use crate::cmp;
2use crate::fmt::{self, Debug};
3use crate::iter::{FusedIterator, InPlaceIterable, SourceIter, TrustedFused, TrustedLen};
4use crate::num::NonZero;
5
6/// An iterator that iterates two other iterators simultaneously.
7///
8/// This `struct` is created by [`zip`] or [`Iterator::zip`].
9/// See their documentation for more.
10#[derive(Clone)]
11#[must_use = "iterators are lazy and do nothing unless consumed"]
12#[stable(feature = "rust1", since = "1.0.0")]
13#[ferrocene::prevalidated]
14pub struct Zip<A, B> {
15    a: A,
16    b: B,
17    // index, len and a_len are only used by the specialized version of zip
18    index: usize,
19    len: usize,
20}
21impl<A: Iterator, B: Iterator> Zip<A, B> {
22    #[ferrocene::prevalidated]
23    pub(in crate::iter) fn new(a: A, b: B) -> Zip<A, B> {
24        ZipImpl::new(a, b)
25    }
26    #[ferrocene::prevalidated]
27    fn super_nth(&mut self, mut n: usize) -> Option<(A::Item, B::Item)> {
28        while let Some(x) = Iterator::next(self) {
29            if n == 0 {
30                return Some(x);
31            }
32            n -= 1;
33        }
34        None
35    }
36}
37
38/// Converts the arguments to iterators and zips them.
39///
40/// See the documentation of [`Iterator::zip`] for more.
41///
42/// # Examples
43///
44/// ```
45/// use std::iter::zip;
46///
47/// let xs = [1, 2, 3];
48/// let ys = [4, 5, 6];
49///
50/// let mut iter = zip(xs, ys);
51///
52/// assert_eq!(iter.next().unwrap(), (1, 4));
53/// assert_eq!(iter.next().unwrap(), (2, 5));
54/// assert_eq!(iter.next().unwrap(), (3, 6));
55/// assert!(iter.next().is_none());
56///
57/// // Nested zips are also possible:
58/// let zs = [7, 8, 9];
59///
60/// let mut iter = zip(zip(xs, ys), zs);
61///
62/// assert_eq!(iter.next().unwrap(), ((1, 4), 7));
63/// assert_eq!(iter.next().unwrap(), ((2, 5), 8));
64/// assert_eq!(iter.next().unwrap(), ((3, 6), 9));
65/// assert!(iter.next().is_none());
66/// ```
67#[stable(feature = "iter_zip", since = "1.59.0")]
68#[ferrocene::prevalidated]
69pub fn zip<A, B>(a: A, b: B) -> Zip<A::IntoIter, B::IntoIter>
70where
71    A: IntoIterator,
72    B: IntoIterator,
73{
74    ZipImpl::new(a.into_iter(), b.into_iter())
75}
76
77#[stable(feature = "rust1", since = "1.0.0")]
78impl<A, B> Iterator for Zip<A, B>
79where
80    A: Iterator,
81    B: Iterator,
82{
83    type Item = (A::Item, B::Item);
84
85    #[inline]
86    #[ferrocene::prevalidated]
87    fn next(&mut self) -> Option<Self::Item> {
88        ZipImpl::next(self)
89    }
90
91    #[inline]
92    #[ferrocene::prevalidated]
93    fn size_hint(&self) -> (usize, Option<usize>) {
94        ZipImpl::size_hint(self)
95    }
96
97    #[inline]
98    #[ferrocene::prevalidated]
99    fn nth(&mut self, n: usize) -> Option<Self::Item> {
100        ZipImpl::nth(self, n)
101    }
102
103    #[inline]
104    #[ferrocene::prevalidated]
105    fn fold<Acc, F>(self, init: Acc, f: F) -> Acc
106    where
107        F: FnMut(Acc, Self::Item) -> Acc,
108    {
109        ZipImpl::fold(self, init, f)
110    }
111
112    #[inline]
113    #[ferrocene::prevalidated]
114    unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item
115    where
116        Self: TrustedRandomAccessNoCoerce,
117    {
118        // SAFETY: `ZipImpl::__iterator_get_unchecked` has same safety
119        // requirements as `Iterator::__iterator_get_unchecked`.
120        unsafe { ZipImpl::get_unchecked(self, idx) }
121    }
122}
123
124#[stable(feature = "rust1", since = "1.0.0")]
125impl<A, B> DoubleEndedIterator for Zip<A, B>
126where
127    A: DoubleEndedIterator + ExactSizeIterator,
128    B: DoubleEndedIterator + ExactSizeIterator,
129{
130    #[inline]
131    fn next_back(&mut self) -> Option<(A::Item, B::Item)> {
132        ZipImpl::next_back(self)
133    }
134}
135
136// Zip specialization trait
137trait ZipImpl<A, B> {
138    type Item;
139    fn new(a: A, b: B) -> Self;
140    fn next(&mut self) -> Option<Self::Item>;
141    fn size_hint(&self) -> (usize, Option<usize>);
142    fn nth(&mut self, n: usize) -> Option<Self::Item>;
143    fn next_back(&mut self) -> Option<Self::Item>
144    where
145        A: DoubleEndedIterator + ExactSizeIterator,
146        B: DoubleEndedIterator + ExactSizeIterator;
147    fn fold<Acc, F>(self, init: Acc, f: F) -> Acc
148    where
149        F: FnMut(Acc, Self::Item) -> Acc;
150    // This has the same safety requirements as `Iterator::__iterator_get_unchecked`
151    unsafe fn get_unchecked(&mut self, idx: usize) -> <Self as Iterator>::Item
152    where
153        Self: Iterator + TrustedRandomAccessNoCoerce;
154}
155
156// Work around limitations of specialization, requiring `default` impls to be repeated
157// in intermediary impls.
158macro_rules! zip_impl_general_defaults {
159    () => {
160        #[ferrocene::prevalidated]
161        default fn new(a: A, b: B) -> Self {
162            Zip {
163                a,
164                b,
165                index: 0, // unused
166                len: 0,   // unused
167            }
168        }
169
170        #[inline]
171        #[ferrocene::prevalidated]
172        default fn next(&mut self) -> Option<(A::Item, B::Item)> {
173            let x = self.a.next()?;
174            let y = self.b.next()?;
175            Some((x, y))
176        }
177
178        #[inline]
179        #[ferrocene::prevalidated]
180        default fn nth(&mut self, n: usize) -> Option<Self::Item> {
181            self.super_nth(n)
182        }
183
184        #[inline]
185        default fn next_back(&mut self) -> Option<(A::Item, B::Item)>
186        where
187            A: DoubleEndedIterator + ExactSizeIterator,
188            B: DoubleEndedIterator + ExactSizeIterator,
189        {
190            // The function body below only uses `self.a/b.len()` and `self.a/b.next_back()`
191            // and doesn’t call `next_back` too often, so this implementation is safe in
192            // the `TrustedRandomAccessNoCoerce` specialization
193
194            let a_sz = self.a.len();
195            let b_sz = self.b.len();
196            if a_sz != b_sz {
197                // Adjust a, b to equal length
198                if a_sz > b_sz {
199                    for _ in 0..a_sz - b_sz {
200                        self.a.next_back();
201                    }
202                } else {
203                    for _ in 0..b_sz - a_sz {
204                        self.b.next_back();
205                    }
206                }
207            }
208            match (self.a.next_back(), self.b.next_back()) {
209                (Some(x), Some(y)) => Some((x, y)),
210                (None, None) => None,
211                _ => unreachable!(),
212            }
213        }
214    };
215}
216
217// General Zip impl
218impl<A, B> ZipImpl<A, B> for Zip<A, B>
219where
220    A: Iterator,
221    B: Iterator,
222{
223    type Item = (A::Item, B::Item);
224
225    zip_impl_general_defaults! {}
226
227    #[inline]
228    #[ferrocene::prevalidated]
229    default fn size_hint(&self) -> (usize, Option<usize>) {
230        let (a_lower, a_upper) = self.a.size_hint();
231        let (b_lower, b_upper) = self.b.size_hint();
232
233        let lower = cmp::min(a_lower, b_lower);
234
235        let upper = match (a_upper, b_upper) {
236            (Some(x), Some(y)) => Some(cmp::min(x, y)),
237            (Some(x), None) => Some(x),
238            (None, Some(y)) => Some(y),
239            (None, None) => None,
240        };
241
242        (lower, upper)
243    }
244
245    default unsafe fn get_unchecked(&mut self, _idx: usize) -> <Self as Iterator>::Item
246    where
247        Self: TrustedRandomAccessNoCoerce,
248    {
249        unreachable!("Always specialized");
250    }
251
252    #[inline]
253    #[ferrocene::prevalidated]
254    default fn fold<Acc, F>(self, init: Acc, f: F) -> Acc
255    where
256        F: FnMut(Acc, Self::Item) -> Acc,
257    {
258        SpecFold::spec_fold(self, init, f)
259    }
260}
261
262impl<A, B> ZipImpl<A, B> for Zip<A, B>
263where
264    A: TrustedRandomAccessNoCoerce + Iterator,
265    B: TrustedRandomAccessNoCoerce + Iterator,
266{
267    zip_impl_general_defaults! {}
268
269    #[inline]
270    default fn size_hint(&self) -> (usize, Option<usize>) {
271        let size = cmp::min(self.a.size(), self.b.size());
272        (size, Some(size))
273    }
274
275    #[inline]
276    unsafe fn get_unchecked(&mut self, idx: usize) -> <Self as Iterator>::Item {
277        let idx = self.index + idx;
278        // SAFETY: the caller must uphold the contract for
279        // `Iterator::__iterator_get_unchecked`.
280        unsafe { (self.a.__iterator_get_unchecked(idx), self.b.__iterator_get_unchecked(idx)) }
281    }
282
283    #[inline]
284    fn fold<Acc, F>(mut self, init: Acc, mut f: F) -> Acc
285    where
286        F: FnMut(Acc, Self::Item) -> Acc,
287    {
288        let mut accum = init;
289        let len = ZipImpl::size_hint(&self).0;
290        for i in 0..len {
291            // SAFETY: since Self: TrustedRandomAccessNoCoerce we can trust the size-hint to
292            // calculate the length and then use that to do unchecked iteration.
293            // fold consumes the iterator so we don't need to fixup any state.
294            unsafe {
295                accum = f(accum, self.get_unchecked(i));
296            }
297        }
298        accum
299    }
300}
301
302impl<A, B> ZipImpl<A, B> for Zip<A, B>
303where
304    A: TrustedRandomAccess + Iterator,
305    B: TrustedRandomAccess + Iterator,
306{
307    #[ferrocene::prevalidated]
308    fn new(a: A, b: B) -> Self {
309        let len = cmp::min(a.size(), b.size());
310        Zip { a, b, index: 0, len }
311    }
312
313    #[inline]
314    #[ferrocene::prevalidated]
315    fn next(&mut self) -> Option<(A::Item, B::Item)> {
316        if self.index < self.len {
317            let i = self.index;
318            // since get_unchecked executes code which can panic we increment the counters beforehand
319            // so that the same index won't be accessed twice, as required by TrustedRandomAccess
320            self.index += 1;
321            // SAFETY: `i` is smaller than `self.len`, thus smaller than `self.a.len()` and `self.b.len()`
322            unsafe {
323                Some((self.a.__iterator_get_unchecked(i), self.b.__iterator_get_unchecked(i)))
324            }
325        } else {
326            None
327        }
328    }
329
330    #[inline]
331    fn size_hint(&self) -> (usize, Option<usize>) {
332        let len = self.len - self.index;
333        (len, Some(len))
334    }
335
336    #[inline]
337    fn nth(&mut self, n: usize) -> Option<Self::Item> {
338        let delta = cmp::min(n, self.len - self.index);
339        let end = self.index + delta;
340        while self.index < end {
341            let i = self.index;
342            // since get_unchecked executes code which can panic we increment the counters beforehand
343            // so that the same index won't be accessed twice, as required by TrustedRandomAccess
344            self.index += 1;
345            if A::MAY_HAVE_SIDE_EFFECT {
346                // SAFETY: the usage of `cmp::min` to calculate `delta`
347                // ensures that `end` is smaller than or equal to `self.len`,
348                // so `i` is also smaller than `self.len`.
349                unsafe {
350                    self.a.__iterator_get_unchecked(i);
351                }
352            }
353            if B::MAY_HAVE_SIDE_EFFECT {
354                // SAFETY: same as above.
355                unsafe {
356                    self.b.__iterator_get_unchecked(i);
357                }
358            }
359        }
360
361        self.super_nth(n - delta)
362    }
363
364    #[inline]
365    fn next_back(&mut self) -> Option<(A::Item, B::Item)>
366    where
367        A: DoubleEndedIterator + ExactSizeIterator,
368        B: DoubleEndedIterator + ExactSizeIterator,
369    {
370        // No effects when the iterator is exhausted, to reduce the number of
371        // cases the unsafe code has to handle.
372        // See #137255 for a case where where too many epicycles lead to unsoundness.
373        if self.index < self.len {
374            let old_len = self.len;
375
376            // since get_unchecked and the side-effecting code can execute user code
377            // which can panic we decrement the counter beforehand
378            // so that the same index won't be accessed twice, as required by TrustedRandomAccess.
379            // Additionally this will ensure that the side-effects code won't run a second time.
380            self.len -= 1;
381
382            // Adjust a, b to equal length if we're iterating backwards.
383            if A::MAY_HAVE_SIDE_EFFECT || B::MAY_HAVE_SIDE_EFFECT {
384                // note if some forward-iteration already happened then these aren't the real
385                // remaining lengths of the inner iterators, so we have to relate them to
386                // Zip's internal length-tracking.
387                let sz_a = self.a.size();
388                let sz_b = self.b.size();
389                // This condition can and must only be true on the first `next_back` call,
390                // otherwise we will break the restriction on calls to `self.next_back()`
391                // after calling `get_unchecked()`.
392                if sz_a != sz_b && (old_len == sz_a || old_len == sz_b) {
393                    if A::MAY_HAVE_SIDE_EFFECT && sz_a > old_len {
394                        for _ in 0..sz_a - old_len {
395                            self.a.next_back();
396                        }
397                    }
398                    if B::MAY_HAVE_SIDE_EFFECT && sz_b > old_len {
399                        for _ in 0..sz_b - old_len {
400                            self.b.next_back();
401                        }
402                    }
403                    debug_assert_eq!(self.a.size(), self.b.size());
404                }
405            }
406            let i = self.len;
407            // SAFETY: `i` is smaller than the previous value of `self.len`,
408            // which is also smaller than or equal to `self.a.len()` and `self.b.len()`
409            unsafe {
410                Some((self.a.__iterator_get_unchecked(i), self.b.__iterator_get_unchecked(i)))
411            }
412        } else {
413            None
414        }
415    }
416}
417
418#[stable(feature = "rust1", since = "1.0.0")]
419impl<A, B> ExactSizeIterator for Zip<A, B>
420where
421    A: ExactSizeIterator,
422    B: ExactSizeIterator,
423{
424}
425
426#[doc(hidden)]
427#[unstable(feature = "trusted_random_access", issue = "none")]
428unsafe impl<A, B> TrustedRandomAccess for Zip<A, B>
429where
430    A: TrustedRandomAccess,
431    B: TrustedRandomAccess,
432{
433}
434
435#[doc(hidden)]
436#[unstable(feature = "trusted_random_access", issue = "none")]
437unsafe impl<A, B> TrustedRandomAccessNoCoerce for Zip<A, B>
438where
439    A: TrustedRandomAccessNoCoerce,
440    B: TrustedRandomAccessNoCoerce,
441{
442    const MAY_HAVE_SIDE_EFFECT: bool = A::MAY_HAVE_SIDE_EFFECT || B::MAY_HAVE_SIDE_EFFECT;
443}
444
445#[stable(feature = "fused", since = "1.26.0")]
446impl<A, B> FusedIterator for Zip<A, B>
447where
448    A: FusedIterator,
449    B: FusedIterator,
450{
451}
452
453#[unstable(issue = "none", feature = "trusted_fused")]
454unsafe impl<A, B> TrustedFused for Zip<A, B>
455where
456    A: TrustedFused,
457    B: TrustedFused,
458{
459}
460
461#[unstable(feature = "trusted_len", issue = "37572")]
462unsafe impl<A, B> TrustedLen for Zip<A, B>
463where
464    A: TrustedLen,
465    B: TrustedLen,
466{
467}
468
469// Arbitrarily selects the left side of the zip iteration as extractable "source"
470// it would require negative trait bounds to be able to try both
471#[unstable(issue = "none", feature = "inplace_iteration")]
472unsafe impl<A, B> SourceIter for Zip<A, B>
473where
474    A: SourceIter,
475{
476    type Source = A::Source;
477
478    #[inline]
479    unsafe fn as_inner(&mut self) -> &mut A::Source {
480        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
481        unsafe { SourceIter::as_inner(&mut self.a) }
482    }
483}
484
485// Since SourceIter forwards the left hand side we do the same here
486#[unstable(issue = "none", feature = "inplace_iteration")]
487unsafe impl<A: InPlaceIterable, B> InPlaceIterable for Zip<A, B> {
488    const EXPAND_BY: Option<NonZero<usize>> = A::EXPAND_BY;
489    const MERGE_BY: Option<NonZero<usize>> = A::MERGE_BY;
490}
491
492#[stable(feature = "rust1", since = "1.0.0")]
493impl<A: Debug, B: Debug> Debug for Zip<A, B> {
494    #[ferrocene::prevalidated]
495    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
496        ZipFmt::fmt(self, f)
497    }
498}
499
500trait ZipFmt<A, B> {
501    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result;
502}
503
504impl<A: Debug, B: Debug> ZipFmt<A, B> for Zip<A, B> {
505    #[ferrocene::prevalidated]
506    default fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
507        f.debug_struct("Zip").field("a", &self.a).field("b", &self.b).finish()
508    }
509}
510
511impl<A: Debug + TrustedRandomAccessNoCoerce, B: Debug + TrustedRandomAccessNoCoerce> ZipFmt<A, B>
512    for Zip<A, B>
513{
514    #[ferrocene::prevalidated]
515    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
516        // It's *not safe* to call fmt on the contained iterators, since once
517        // we start iterating they're in strange, potentially unsafe, states.
518        f.debug_struct("Zip").finish()
519    }
520}
521
522/// An iterator whose items are random-accessible efficiently
523///
524/// # Safety
525///
526/// The iterator's `size_hint` must be exact and cheap to call.
527///
528/// `TrustedRandomAccessNoCoerce::size` may not be overridden.
529///
530/// All subtypes and all supertypes of `Self` must also implement `TrustedRandomAccess`.
531/// In particular, this means that types with non-invariant parameters usually can not have
532/// an impl for `TrustedRandomAccess` that depends on any trait bounds on such parameters, except
533/// for bounds that come from the respective struct/enum definition itself, or bounds involving
534/// traits that themselves come with a guarantee similar to this one.
535///
536/// If `Self: ExactSizeIterator` then `self.len()` must always produce results consistent
537/// with `self.size()`.
538///
539/// If `Self: Iterator`, then `<Self as Iterator>::__iterator_get_unchecked(&mut self, idx)`
540/// must be safe to call provided the following conditions are met.
541///
542/// 1. `0 <= idx` and `idx < self.size()`.
543/// 2. If `Self: !Clone`, then `self.__iterator_get_unchecked(idx)` is never called with the same
544///    index on `self` more than once.
545/// 3. After `self.__iterator_get_unchecked(idx)` has been called, then `self.next_back()` will
546///    only be called at most `self.size() - idx - 1` times. If `Self: Clone` and `self` is cloned,
547///    then this number is calculated for `self` and its clone individually,
548///    but `self.next_back()` calls that happened before the cloning count for both `self` and the clone.
549/// 4. After `self.__iterator_get_unchecked(idx)` has been called, then only the following methods
550///    will be called on `self` or on any new clones of `self`:
551///     * `std::clone::Clone::clone`
552///     * `std::iter::Iterator::size_hint`
553///     * `std::iter::DoubleEndedIterator::next_back`
554///     * `std::iter::ExactSizeIterator::len`
555///     * `std::iter::Iterator::__iterator_get_unchecked`
556///     * `std::iter::TrustedRandomAccessNoCoerce::size`
557/// 5. If `Self` is a subtype of `T`, then `self` is allowed to be coerced
558///    to `T`. If `self` is coerced to `T` after `self.__iterator_get_unchecked(idx)` has already
559///    been called, then no methods except for the ones listed under 4. are allowed to be called
560///    on the resulting value of type `T`, either. Multiple such coercion steps are allowed.
561///    Regarding 2. and 3., the number of times `__iterator_get_unchecked(idx)` or `next_back()` is
562///    called on `self` and the resulting value of type `T` (and on further coercion results with
563///    super-supertypes) are added together and their sums must not exceed the specified bounds.
564///
565/// Further, given that these conditions are met, it must guarantee that:
566///
567/// * It does not change the value returned from `size_hint`
568/// * It must be safe to call the methods listed above on `self` after calling
569///   `self.__iterator_get_unchecked(idx)`, assuming that the required traits are implemented.
570/// * It must also be safe to drop `self` after calling `self.__iterator_get_unchecked(idx)`.
571/// * If `Self` is a subtype of `T`, then it must be safe to coerce `self` to `T`.
572//
573// FIXME: Clarify interaction with SourceIter/InPlaceIterable. Calling `SourceIter::as_inner`
574// after `__iterator_get_unchecked` is supposed to be allowed.
575#[doc(hidden)]
576#[unstable(feature = "trusted_random_access", issue = "none")]
577#[rustc_specialization_trait]
578pub unsafe trait TrustedRandomAccess: TrustedRandomAccessNoCoerce {}
579
580/// Like [`TrustedRandomAccess`] but without any of the requirements / guarantees around
581/// coercions to supertypes after `__iterator_get_unchecked` (they aren’t allowed here!), and
582/// without the requirement that subtypes / supertypes implement `TrustedRandomAccessNoCoerce`.
583///
584/// This trait was created in PR #85874 to fix soundness issue #85873 without performance regressions.
585/// It is subject to change as we might want to build a more generally useful (for performance
586/// optimizations) and more sophisticated trait or trait hierarchy that replaces or extends
587/// [`TrustedRandomAccess`] and `TrustedRandomAccessNoCoerce`.
588#[doc(hidden)]
589#[unstable(feature = "trusted_random_access", issue = "none")]
590#[rustc_specialization_trait]
591pub unsafe trait TrustedRandomAccessNoCoerce: Sized {
592    // Convenience method.
593    #[ferrocene::prevalidated]
594    fn size(&self) -> usize
595    where
596        Self: Iterator,
597    {
598        self.size_hint().0
599    }
600    /// `true` if getting an iterator element may have side effects.
601    /// Remember to take inner iterators into account.
602    const MAY_HAVE_SIDE_EFFECT: bool;
603}
604
605/// Like `Iterator::__iterator_get_unchecked`, but doesn't require the compiler to
606/// know that `U: TrustedRandomAccess`.
607///
608/// ## Safety
609///
610/// Same requirements calling `get_unchecked` directly.
611#[doc(hidden)]
612#[inline]
613pub(in crate::iter::adapters) unsafe fn try_get_unchecked<I>(it: &mut I, idx: usize) -> I::Item
614where
615    I: Iterator,
616{
617    // SAFETY: the caller must uphold the contract for
618    // `Iterator::__iterator_get_unchecked`.
619    unsafe { it.try_get_unchecked(idx) }
620}
621
622unsafe trait SpecTrustedRandomAccess: Iterator {
623    /// If `Self: TrustedRandomAccess`, it must be safe to call
624    /// `Iterator::__iterator_get_unchecked(self, index)`.
625    unsafe fn try_get_unchecked(&mut self, index: usize) -> Self::Item;
626}
627
628unsafe impl<I: Iterator> SpecTrustedRandomAccess for I {
629    default unsafe fn try_get_unchecked(&mut self, _: usize) -> Self::Item {
630        panic!("Should only be called on TrustedRandomAccess iterators");
631    }
632}
633
634unsafe impl<I: Iterator + TrustedRandomAccessNoCoerce> SpecTrustedRandomAccess for I {
635    #[inline]
636    unsafe fn try_get_unchecked(&mut self, index: usize) -> Self::Item {
637        // SAFETY: the caller must uphold the contract for
638        // `Iterator::__iterator_get_unchecked`.
639        unsafe { self.__iterator_get_unchecked(index) }
640    }
641}
642
643trait SpecFold: Iterator {
644    fn spec_fold<B, F>(self, init: B, f: F) -> B
645    where
646        Self: Sized,
647        F: FnMut(B, Self::Item) -> B;
648}
649
650impl<A: Iterator, B: Iterator> SpecFold for Zip<A, B> {
651    // Adapted from default impl from the Iterator trait
652    #[inline]
653    #[ferrocene::prevalidated]
654    default fn spec_fold<Acc, F>(mut self, init: Acc, mut f: F) -> Acc
655    where
656        F: FnMut(Acc, Self::Item) -> Acc,
657    {
658        let mut accum = init;
659        while let Some(x) = ZipImpl::next(&mut self) {
660            accum = f(accum, x);
661        }
662        accum
663    }
664}
665
666impl<A: TrustedLen, B: TrustedLen> SpecFold for Zip<A, B> {
667    #[inline]
668    fn spec_fold<Acc, F>(mut self, init: Acc, mut f: F) -> Acc
669    where
670        F: FnMut(Acc, Self::Item) -> Acc,
671    {
672        let mut accum = init;
673        loop {
674            let (upper, more) = if let Some(upper) = ZipImpl::size_hint(&self).1 {
675                (upper, false)
676            } else {
677                // Per TrustedLen contract a None upper bound means more than usize::MAX items
678                (usize::MAX, true)
679            };
680
681            for _ in 0..upper {
682                let pair =
683                    // SAFETY: TrustedLen guarantees that at least `upper` many items are available
684                    // therefore we know they can't be None
685                    unsafe { (self.a.next().unwrap_unchecked(), self.b.next().unwrap_unchecked()) };
686                accum = f(accum, pair);
687            }
688
689            if !more {
690                break;
691            }
692        }
693        accum
694    }
695}
core/iter/adapters/zip.rs

core/iter/adapters/
zip.rs
