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
//! Utilities for the array primitive type.
//!
//! *[See also the array primitive type](array).*

#![stable(feature = "core_array", since = "1.36.0")]

use crate::borrow::{Borrow, BorrowMut};
use crate::cmp::Ordering;
use crate::convert::Infallible;
use crate::error::Error;
use crate::fmt;
use crate::hash::{self, Hash};
use crate::iter::{repeat_n, UncheckedIterator};
use crate::mem::{self, MaybeUninit};
use crate::ops::{
    ChangeOutputType, ControlFlow, FromResidual, Index, IndexMut, NeverShortCircuit, Residual, Try,
};
use crate::slice::{Iter, IterMut};

mod ascii;
mod drain;
mod equality;
mod iter;

pub(crate) use drain::drain_array_with;
#[stable(feature = "array_value_iter", since = "1.51.0")]
pub use iter::IntoIter;

/// Creates an array of type `[T; N]` by repeatedly cloning a value.
///
/// This is the same as `[val; N]`, but it also works for types that do not
/// implement [`Copy`].
///
/// The provided value will be used as an element of the resulting array and
/// will be cloned N - 1 times to fill up the rest. If N is zero, the value
/// will be dropped.
///
/// # Example
///
/// Creating muliple copies of a `String`:
/// ```rust
/// #![feature(array_repeat)]
///
/// use std::array;
///
/// let string = "Hello there!".to_string();
/// let strings = array::repeat(string);
/// assert_eq!(strings, ["Hello there!", "Hello there!"]);
/// ```
#[inline]
#[unstable(feature = "array_repeat", issue = "126695")]
pub fn repeat<T: Clone, const N: usize>(val: T) -> [T; N] {
    from_trusted_iterator(repeat_n(val, N))
}

/// Creates an array of type [T; N], where each element `T` is the returned value from `cb`
/// using that element's index.
///
/// # Arguments
///
/// * `cb`: Callback where the passed argument is the current array index.
///
/// # Example
///
/// ```rust
/// // type inference is helping us here, the way `from_fn` knows how many
/// // elements to produce is the length of array down there: only arrays of
/// // equal lengths can be compared, so the const generic parameter `N` is
/// // inferred to be 5, thus creating array of 5 elements.
///
/// let array = core::array::from_fn(|i| i);
/// // indexes are:    0  1  2  3  4
/// assert_eq!(array, [0, 1, 2, 3, 4]);
///
/// let array2: [usize; 8] = core::array::from_fn(|i| i * 2);
/// // indexes are:     0  1  2  3  4  5   6   7
/// assert_eq!(array2, [0, 2, 4, 6, 8, 10, 12, 14]);
///
/// let bool_arr = core::array::from_fn::<_, 5, _>(|i| i % 2 == 0);
/// // indexes are:       0     1      2     3      4
/// assert_eq!(bool_arr, [true, false, true, false, true]);
/// ```
#[inline]
#[stable(feature = "array_from_fn", since = "1.63.0")]
pub fn from_fn<T, const N: usize, F>(cb: F) -> [T; N]
where
    F: FnMut(usize) -> T,
{
    try_from_fn(NeverShortCircuit::wrap_mut_1(cb)).0
}

/// Creates an array `[T; N]` where each fallible array element `T` is returned by the `cb` call.
/// Unlike [`from_fn`], where the element creation can't fail, this version will return an error
/// if any element creation was unsuccessful.
///
/// The return type of this function depends on the return type of the closure.
/// If you return `Result<T, E>` from the closure, you'll get a `Result<[T; N], E>`.
/// If you return `Option<T>` from the closure, you'll get an `Option<[T; N]>`.
///
/// # Arguments
///
/// * `cb`: Callback where the passed argument is the current array index.
///
/// # Example
///
/// ```rust
/// #![feature(array_try_from_fn)]
///
/// let array: Result<[u8; 5], _> = std::array::try_from_fn(|i| i.try_into());
/// assert_eq!(array, Ok([0, 1, 2, 3, 4]));
///
/// let array: Result<[i8; 200], _> = std::array::try_from_fn(|i| i.try_into());
/// assert!(array.is_err());
///
/// let array: Option<[_; 4]> = std::array::try_from_fn(|i| i.checked_add(100));
/// assert_eq!(array, Some([100, 101, 102, 103]));
///
/// let array: Option<[_; 4]> = std::array::try_from_fn(|i| i.checked_sub(100));
/// assert_eq!(array, None);
/// ```
#[inline]
#[unstable(feature = "array_try_from_fn", issue = "89379")]
pub fn try_from_fn<R, const N: usize, F>(cb: F) -> ChangeOutputType<R, [R::Output; N]>
where
    F: FnMut(usize) -> R,
    R: Try,
    R::Residual: Residual<[R::Output; N]>,
{
    let mut array = [const { MaybeUninit::uninit() }; N];
    match try_from_fn_erased(&mut array, cb) {
        ControlFlow::Break(r) => FromResidual::from_residual(r),
        ControlFlow::Continue(()) => {
            // SAFETY: All elements of the array were populated.
            try { unsafe { MaybeUninit::array_assume_init(array) } }
        }
    }
}

/// Converts a reference to `T` into a reference to an array of length 1 (without copying).
#[stable(feature = "array_from_ref", since = "1.53.0")]
#[rustc_const_stable(feature = "const_array_from_ref_shared", since = "1.63.0")]
pub const fn from_ref<T>(s: &T) -> &[T; 1] {
    // SAFETY: Converting `&T` to `&[T; 1]` is sound.
    unsafe { &*(s as *const T).cast::<[T; 1]>() }
}

/// Converts a mutable reference to `T` into a mutable reference to an array of length 1 (without copying).
#[stable(feature = "array_from_ref", since = "1.53.0")]
#[rustc_const_unstable(feature = "const_array_from_ref", issue = "90206")]
pub const fn from_mut<T>(s: &mut T) -> &mut [T; 1] {
    // SAFETY: Converting `&mut T` to `&mut [T; 1]` is sound.
    unsafe { &mut *(s as *mut T).cast::<[T; 1]>() }
}

/// The error type returned when a conversion from a slice to an array fails.
#[stable(feature = "try_from", since = "1.34.0")]
#[derive(Debug, Copy, Clone)]
pub struct TryFromSliceError(());

#[stable(feature = "core_array", since = "1.36.0")]
impl fmt::Display for TryFromSliceError {
    #[inline]
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        #[allow(deprecated)]
        self.description().fmt(f)
    }
}

#[stable(feature = "try_from", since = "1.34.0")]
impl Error for TryFromSliceError {
    #[allow(deprecated)]
    fn description(&self) -> &str {
        "could not convert slice to array"
    }
}

#[stable(feature = "try_from_slice_error", since = "1.36.0")]
impl From<Infallible> for TryFromSliceError {
    fn from(x: Infallible) -> TryFromSliceError {
        match x {}
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, const N: usize> AsRef<[T]> for [T; N] {
    #[inline]
    fn as_ref(&self) -> &[T] {
        &self[..]
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, const N: usize> AsMut<[T]> for [T; N] {
    #[inline]
    fn as_mut(&mut self) -> &mut [T] {
        &mut self[..]
    }
}

#[stable(feature = "array_borrow", since = "1.4.0")]
impl<T, const N: usize> Borrow<[T]> for [T; N] {
    fn borrow(&self) -> &[T] {
        self
    }
}

#[stable(feature = "array_borrow", since = "1.4.0")]
impl<T, const N: usize> BorrowMut<[T]> for [T; N] {
    fn borrow_mut(&mut self) -> &mut [T] {
        self
    }
}

/// Tries to create an array `[T; N]` by copying from a slice `&[T]`. Succeeds if
/// `slice.len() == N`.
///
/// ```
/// let bytes: [u8; 3] = [1, 0, 2];
///
/// let bytes_head: [u8; 2] = <[u8; 2]>::try_from(&bytes[0..2]).unwrap();
/// assert_eq!(1, u16::from_le_bytes(bytes_head));
///
/// let bytes_tail: [u8; 2] = bytes[1..3].try_into().unwrap();
/// assert_eq!(512, u16::from_le_bytes(bytes_tail));
/// ```
#[stable(feature = "try_from", since = "1.34.0")]
impl<T, const N: usize> TryFrom<&[T]> for [T; N]
where
    T: Copy,
{
    type Error = TryFromSliceError;

    #[inline]
    fn try_from(slice: &[T]) -> Result<[T; N], TryFromSliceError> {
        <&Self>::try_from(slice).copied()
    }
}

/// Tries to create an array `[T; N]` by copying from a mutable slice `&mut [T]`.
/// Succeeds if `slice.len() == N`.
///
/// ```
/// let mut bytes: [u8; 3] = [1, 0, 2];
///
/// let bytes_head: [u8; 2] = <[u8; 2]>::try_from(&mut bytes[0..2]).unwrap();
/// assert_eq!(1, u16::from_le_bytes(bytes_head));
///
/// let bytes_tail: [u8; 2] = (&mut bytes[1..3]).try_into().unwrap();
/// assert_eq!(512, u16::from_le_bytes(bytes_tail));
/// ```
#[stable(feature = "try_from_mut_slice_to_array", since = "1.59.0")]
impl<T, const N: usize> TryFrom<&mut [T]> for [T; N]
where
    T: Copy,
{
    type Error = TryFromSliceError;

    #[inline]
    fn try_from(slice: &mut [T]) -> Result<[T; N], TryFromSliceError> {
        <Self>::try_from(&*slice)
    }
}

/// Tries to create an array ref `&[T; N]` from a slice ref `&[T]`. Succeeds if
/// `slice.len() == N`.
///
/// ```
/// let bytes: [u8; 3] = [1, 0, 2];
///
/// let bytes_head: &[u8; 2] = <&[u8; 2]>::try_from(&bytes[0..2]).unwrap();
/// assert_eq!(1, u16::from_le_bytes(*bytes_head));
///
/// let bytes_tail: &[u8; 2] = bytes[1..3].try_into().unwrap();
/// assert_eq!(512, u16::from_le_bytes(*bytes_tail));
/// ```
#[stable(feature = "try_from", since = "1.34.0")]
impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N] {
    type Error = TryFromSliceError;

    #[inline]
    fn try_from(slice: &'a [T]) -> Result<&'a [T; N], TryFromSliceError> {
        if slice.len() == N {
            let ptr = slice.as_ptr() as *const [T; N];
            // SAFETY: ok because we just checked that the length fits
            unsafe { Ok(&*ptr) }
        } else {
            Err(TryFromSliceError(()))
        }
    }
}

/// Tries to create a mutable array ref `&mut [T; N]` from a mutable slice ref
/// `&mut [T]`. Succeeds if `slice.len() == N`.
///
/// ```
/// let mut bytes: [u8; 3] = [1, 0, 2];
///
/// let bytes_head: &mut [u8; 2] = <&mut [u8; 2]>::try_from(&mut bytes[0..2]).unwrap();
/// assert_eq!(1, u16::from_le_bytes(*bytes_head));
///
/// let bytes_tail: &mut [u8; 2] = (&mut bytes[1..3]).try_into().unwrap();
/// assert_eq!(512, u16::from_le_bytes(*bytes_tail));
/// ```
#[stable(feature = "try_from", since = "1.34.0")]
impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N] {
    type Error = TryFromSliceError;

    #[inline]
    fn try_from(slice: &'a mut [T]) -> Result<&'a mut [T; N], TryFromSliceError> {
        if slice.len() == N {
            let ptr = slice.as_mut_ptr() as *mut [T; N];
            // SAFETY: ok because we just checked that the length fits
            unsafe { Ok(&mut *ptr) }
        } else {
            Err(TryFromSliceError(()))
        }
    }
}

/// The hash of an array is the same as that of the corresponding slice,
/// as required by the `Borrow` implementation.
///
/// ```
/// use std::hash::BuildHasher;
///
/// let b = std::hash::RandomState::new();
/// let a: [u8; 3] = [0xa8, 0x3c, 0x09];
/// let s: &[u8] = &[0xa8, 0x3c, 0x09];
/// assert_eq!(b.hash_one(a), b.hash_one(s));
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Hash, const N: usize> Hash for [T; N] {
    fn hash<H: hash::Hasher>(&self, state: &mut H) {
        Hash::hash(&self[..], state)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: fmt::Debug, const N: usize> fmt::Debug for [T; N] {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&&self[..], f)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T, const N: usize> IntoIterator for &'a [T; N] {
    type Item = &'a T;
    type IntoIter = Iter<'a, T>;

    fn into_iter(self) -> Iter<'a, T> {
        self.iter()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T, const N: usize> IntoIterator for &'a mut [T; N] {
    type Item = &'a mut T;
    type IntoIter = IterMut<'a, T>;

    fn into_iter(self) -> IterMut<'a, T> {
        self.iter_mut()
    }
}

#[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
impl<T, I, const N: usize> Index<I> for [T; N]
where
    [T]: Index<I>,
{
    type Output = <[T] as Index<I>>::Output;

    #[inline]
    fn index(&self, index: I) -> &Self::Output {
        Index::index(self as &[T], index)
    }
}

#[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
impl<T, I, const N: usize> IndexMut<I> for [T; N]
where
    [T]: IndexMut<I>,
{
    #[inline]
    fn index_mut(&mut self, index: I) -> &mut Self::Output {
        IndexMut::index_mut(self as &mut [T], index)
    }
}

/// Implements comparison of arrays [lexicographically](Ord#lexicographical-comparison).
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: PartialOrd, const N: usize> PartialOrd for [T; N] {
    #[inline]
    fn partial_cmp(&self, other: &[T; N]) -> Option<Ordering> {
        PartialOrd::partial_cmp(&&self[..], &&other[..])
    }
    #[inline]
    fn lt(&self, other: &[T; N]) -> bool {
        PartialOrd::lt(&&self[..], &&other[..])
    }
    #[inline]
    fn le(&self, other: &[T; N]) -> bool {
        PartialOrd::le(&&self[..], &&other[..])
    }
    #[inline]
    fn ge(&self, other: &[T; N]) -> bool {
        PartialOrd::ge(&&self[..], &&other[..])
    }
    #[inline]
    fn gt(&self, other: &[T; N]) -> bool {
        PartialOrd::gt(&&self[..], &&other[..])
    }
}

/// Implements comparison of arrays [lexicographically](Ord#lexicographical-comparison).
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Ord, const N: usize> Ord for [T; N] {
    #[inline]
    fn cmp(&self, other: &[T; N]) -> Ordering {
        Ord::cmp(&&self[..], &&other[..])
    }
}

#[stable(feature = "copy_clone_array_lib", since = "1.58.0")]
impl<T: Copy, const N: usize> Copy for [T; N] {}

#[stable(feature = "copy_clone_array_lib", since = "1.58.0")]
impl<T: Clone, const N: usize> Clone for [T; N] {
    #[inline]
    fn clone(&self) -> Self {
        SpecArrayClone::clone(self)
    }

    #[inline]
    fn clone_from(&mut self, other: &Self) {
        self.clone_from_slice(other);
    }
}

trait SpecArrayClone: Clone {
    fn clone<const N: usize>(array: &[Self; N]) -> [Self; N];
}

impl<T: Clone> SpecArrayClone for T {
    #[inline]
    default fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
        from_trusted_iterator(array.iter().cloned())
    }
}

impl<T: Copy> SpecArrayClone for T {
    #[inline]
    fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
        *array
    }
}

// The Default impls cannot be done with const generics because `[T; 0]` doesn't
// require Default to be implemented, and having different impl blocks for
// different numbers isn't supported yet.

macro_rules! array_impl_default {
    {$n:expr, $t:ident $($ts:ident)*} => {
        #[stable(since = "1.4.0", feature = "array_default")]
        impl<T> Default for [T; $n] where T: Default {
            fn default() -> [T; $n] {
                [$t::default(), $($ts::default()),*]
            }
        }
        array_impl_default!{($n - 1), $($ts)*}
    };
    {$n:expr,} => {
        #[stable(since = "1.4.0", feature = "array_default")]
        impl<T> Default for [T; $n] {
            fn default() -> [T; $n] { [] }
        }
    };
}

array_impl_default! {32, T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T}

impl<T, const N: usize> [T; N] {
    /// Returns an array of the same size as `self`, with function `f` applied to each element
    /// in order.
    ///
    /// If you don't necessarily need a new fixed-size array, consider using
    /// [`Iterator::map`] instead.
    ///
    ///
    /// # Note on performance and stack usage
    ///
    /// Unfortunately, usages of this method are currently not always optimized
    /// as well as they could be. This mainly concerns large arrays, as mapping
    /// over small arrays seem to be optimized just fine. Also note that in
    /// debug mode (i.e. without any optimizations), this method can use a lot
    /// of stack space (a few times the size of the array or more).
    ///
    /// Therefore, in performance-critical code, try to avoid using this method
    /// on large arrays or check the emitted code. Also try to avoid chained
    /// maps (e.g. `arr.map(...).map(...)`).
    ///
    /// In many cases, you can instead use [`Iterator::map`] by calling `.iter()`
    /// or `.into_iter()` on your array. `[T; N]::map` is only necessary if you
    /// really need a new array of the same size as the result. Rust's lazy
    /// iterators tend to get optimized very well.
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// let x = [1, 2, 3];
    /// let y = x.map(|v| v + 1);
    /// assert_eq!(y, [2, 3, 4]);
    ///
    /// let x = [1, 2, 3];
    /// let mut temp = 0;
    /// let y = x.map(|v| { temp += 1; v * temp });
    /// assert_eq!(y, [1, 4, 9]);
    ///
    /// let x = ["Ferris", "Bueller's", "Day", "Off"];
    /// let y = x.map(|v| v.len());
    /// assert_eq!(y, [6, 9, 3, 3]);
    /// ```
    #[stable(feature = "array_map", since = "1.55.0")]
    pub fn map<F, U>(self, f: F) -> [U; N]
    where
        F: FnMut(T) -> U,
    {
        self.try_map(NeverShortCircuit::wrap_mut_1(f)).0
    }

    /// A fallible function `f` applied to each element on array `self` in order to
    /// return an array the same size as `self` or the first error encountered.
    ///
    /// The return type of this function depends on the return type of the closure.
    /// If you return `Result<T, E>` from the closure, you'll get a `Result<[T; N], E>`.
    /// If you return `Option<T>` from the closure, you'll get an `Option<[T; N]>`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(array_try_map)]
    ///
    /// let a = ["1", "2", "3"];
    /// let b = a.try_map(|v| v.parse::<u32>()).unwrap().map(|v| v + 1);
    /// assert_eq!(b, [2, 3, 4]);
    ///
    /// let a = ["1", "2a", "3"];
    /// let b = a.try_map(|v| v.parse::<u32>());
    /// assert!(b.is_err());
    ///
    /// use std::num::NonZero;
    ///
    /// let z = [1, 2, 0, 3, 4];
    /// assert_eq!(z.try_map(NonZero::new), None);
    ///
    /// let a = [1, 2, 3];
    /// let b = a.try_map(NonZero::new);
    /// let c = b.map(|x| x.map(NonZero::get));
    /// assert_eq!(c, Some(a));
    /// ```
    #[unstable(feature = "array_try_map", issue = "79711")]
    pub fn try_map<R>(self, f: impl FnMut(T) -> R) -> ChangeOutputType<R, [R::Output; N]>
    where
        R: Try<Residual: Residual<[R::Output; N]>>,
    {
        drain_array_with(self, |iter| try_from_trusted_iterator(iter.map(f)))
    }

    /// Returns a slice containing the entire array. Equivalent to `&s[..]`.
    #[stable(feature = "array_as_slice", since = "1.57.0")]
    #[rustc_const_stable(feature = "array_as_slice", since = "1.57.0")]
    pub const fn as_slice(&self) -> &[T] {
        self
    }

    /// Returns a mutable slice containing the entire array. Equivalent to
    /// `&mut s[..]`.
    #[stable(feature = "array_as_slice", since = "1.57.0")]
    pub fn as_mut_slice(&mut self) -> &mut [T] {
        self
    }

    /// Borrows each element and returns an array of references with the same
    /// size as `self`.
    ///
    ///
    /// # Example
    ///
    /// ```
    /// let floats = [3.1, 2.7, -1.0];
    /// let float_refs: [&f64; 3] = floats.each_ref();
    /// assert_eq!(float_refs, [&3.1, &2.7, &-1.0]);
    /// ```
    ///
    /// This method is particularly useful if combined with other methods, like
    /// [`map`](#method.map). This way, you can avoid moving the original
    /// array if its elements are not [`Copy`].
    ///
    /// ```
    /// let strings = ["Ferris".to_string(), "♥".to_string(), "Rust".to_string()];
    /// let is_ascii = strings.each_ref().map(|s| s.is_ascii());
    /// assert_eq!(is_ascii, [true, false, true]);
    ///
    /// // We can still access the original array: it has not been moved.
    /// assert_eq!(strings.len(), 3);
    /// ```
    #[stable(feature = "array_methods", since = "1.77.0")]
    pub fn each_ref(&self) -> [&T; N] {
        from_trusted_iterator(self.iter())
    }

    /// Borrows each element mutably and returns an array of mutable references
    /// with the same size as `self`.
    ///
    ///
    /// # Example
    ///
    /// ```
    ///
    /// let mut floats = [3.1, 2.7, -1.0];
    /// let float_refs: [&mut f64; 3] = floats.each_mut();
    /// *float_refs[0] = 0.0;
    /// assert_eq!(float_refs, [&mut 0.0, &mut 2.7, &mut -1.0]);
    /// assert_eq!(floats, [0.0, 2.7, -1.0]);
    /// ```
    #[stable(feature = "array_methods", since = "1.77.0")]
    pub fn each_mut(&mut self) -> [&mut T; N] {
        from_trusted_iterator(self.iter_mut())
    }

    /// Divides one array reference into two at an index.
    ///
    /// The first will contain all indices from `[0, M)` (excluding
    /// the index `M` itself) and the second will contain all
    /// indices from `[M, N)` (excluding the index `N` itself).
    ///
    /// # Panics
    ///
    /// Panics if `M > N`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(split_array)]
    ///
    /// let v = [1, 2, 3, 4, 5, 6];
    ///
    /// {
    ///    let (left, right) = v.split_array_ref::<0>();
    ///    assert_eq!(left, &[]);
    ///    assert_eq!(right, &[1, 2, 3, 4, 5, 6]);
    /// }
    ///
    /// {
    ///     let (left, right) = v.split_array_ref::<2>();
    ///     assert_eq!(left, &[1, 2]);
    ///     assert_eq!(right, &[3, 4, 5, 6]);
    /// }
    ///
    /// {
    ///     let (left, right) = v.split_array_ref::<6>();
    ///     assert_eq!(left, &[1, 2, 3, 4, 5, 6]);
    ///     assert_eq!(right, &[]);
    /// }
    /// ```
    #[unstable(
        feature = "split_array",
        reason = "return type should have array as 2nd element",
        issue = "90091"
    )]
    #[inline]
    pub fn split_array_ref<const M: usize>(&self) -> (&[T; M], &[T]) {
        (&self[..]).split_first_chunk::<M>().unwrap()
    }

    /// Divides one mutable array reference into two at an index.
    ///
    /// The first will contain all indices from `[0, M)` (excluding
    /// the index `M` itself) and the second will contain all
    /// indices from `[M, N)` (excluding the index `N` itself).
    ///
    /// # Panics
    ///
    /// Panics if `M > N`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(split_array)]
    ///
    /// let mut v = [1, 0, 3, 0, 5, 6];
    /// let (left, right) = v.split_array_mut::<2>();
    /// assert_eq!(left, &mut [1, 0][..]);
    /// assert_eq!(right, &mut [3, 0, 5, 6]);
    /// left[1] = 2;
    /// right[1] = 4;
    /// assert_eq!(v, [1, 2, 3, 4, 5, 6]);
    /// ```
    #[unstable(
        feature = "split_array",
        reason = "return type should have array as 2nd element",
        issue = "90091"
    )]
    #[inline]
    pub fn split_array_mut<const M: usize>(&mut self) -> (&mut [T; M], &mut [T]) {
        (&mut self[..]).split_first_chunk_mut::<M>().unwrap()
    }

    /// Divides one array reference into two at an index from the end.
    ///
    /// The first will contain all indices from `[0, N - M)` (excluding
    /// the index `N - M` itself) and the second will contain all
    /// indices from `[N - M, N)` (excluding the index `N` itself).
    ///
    /// # Panics
    ///
    /// Panics if `M > N`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(split_array)]
    ///
    /// let v = [1, 2, 3, 4, 5, 6];
    ///
    /// {
    ///    let (left, right) = v.rsplit_array_ref::<0>();
    ///    assert_eq!(left, &[1, 2, 3, 4, 5, 6]);
    ///    assert_eq!(right, &[]);
    /// }
    ///
    /// {
    ///     let (left, right) = v.rsplit_array_ref::<2>();
    ///     assert_eq!(left, &[1, 2, 3, 4]);
    ///     assert_eq!(right, &[5, 6]);
    /// }
    ///
    /// {
    ///     let (left, right) = v.rsplit_array_ref::<6>();
    ///     assert_eq!(left, &[]);
    ///     assert_eq!(right, &[1, 2, 3, 4, 5, 6]);
    /// }
    /// ```
    #[unstable(
        feature = "split_array",
        reason = "return type should have array as 2nd element",
        issue = "90091"
    )]
    #[inline]
    pub fn rsplit_array_ref<const M: usize>(&self) -> (&[T], &[T; M]) {
        (&self[..]).split_last_chunk::<M>().unwrap()
    }

    /// Divides one mutable array reference into two at an index from the end.
    ///
    /// The first will contain all indices from `[0, N - M)` (excluding
    /// the index `N - M` itself) and the second will contain all
    /// indices from `[N - M, N)` (excluding the index `N` itself).
    ///
    /// # Panics
    ///
    /// Panics if `M > N`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(split_array)]
    ///
    /// let mut v = [1, 0, 3, 0, 5, 6];
    /// let (left, right) = v.rsplit_array_mut::<4>();
    /// assert_eq!(left, &mut [1, 0]);
    /// assert_eq!(right, &mut [3, 0, 5, 6][..]);
    /// left[1] = 2;
    /// right[1] = 4;
    /// assert_eq!(v, [1, 2, 3, 4, 5, 6]);
    /// ```
    #[unstable(
        feature = "split_array",
        reason = "return type should have array as 2nd element",
        issue = "90091"
    )]
    #[inline]
    pub fn rsplit_array_mut<const M: usize>(&mut self) -> (&mut [T], &mut [T; M]) {
        (&mut self[..]).split_last_chunk_mut::<M>().unwrap()
    }
}

/// Populate an array from the first `N` elements of `iter`
///
/// # Panics
///
/// If the iterator doesn't actually have enough items.
///
/// By depending on `TrustedLen`, however, we can do that check up-front (where
/// it easily optimizes away) so it doesn't impact the loop that fills the array.
#[inline]
fn from_trusted_iterator<T, const N: usize>(iter: impl UncheckedIterator<Item = T>) -> [T; N] {
    try_from_trusted_iterator(iter.map(NeverShortCircuit)).0
}

#[inline]
fn try_from_trusted_iterator<T, R, const N: usize>(
    iter: impl UncheckedIterator<Item = R>,
) -> ChangeOutputType<R, [T; N]>
where
    R: Try<Output = T>,
    R::Residual: Residual<[T; N]>,
{
    assert!(iter.size_hint().0 >= N);
    fn next<T>(mut iter: impl UncheckedIterator<Item = T>) -> impl FnMut(usize) -> T {
        move |_| {
            // SAFETY: We know that `from_fn` will call this at most N times,
            // and we checked to ensure that we have at least that many items.
            unsafe { iter.next_unchecked() }
        }
    }

    try_from_fn(next(iter))
}

/// Version of [`try_from_fn`] using a passed-in slice in order to avoid
/// needing to monomorphize for every array length.
///
/// This takes a generator rather than an iterator so that *at the type level*
/// it never needs to worry about running out of items.  When combined with
/// an infallible `Try` type, that means the loop canonicalizes easily, allowing
/// it to optimize well.
///
/// It would be *possible* to unify this and [`iter_next_chunk_erased`] into one
/// function that does the union of both things, but last time it was that way
/// it resulted in poor codegen from the "are there enough source items?" checks
/// not optimizing away.  So if you give it a shot, make sure to watch what
/// happens in the codegen tests.
#[inline]
fn try_from_fn_erased<T, R>(
    buffer: &mut [MaybeUninit<T>],
    mut generator: impl FnMut(usize) -> R,
) -> ControlFlow<R::Residual>
where
    R: Try<Output = T>,
{
    let mut guard = Guard { array_mut: buffer, initialized: 0 };

    while guard.initialized < guard.array_mut.len() {
        let item = generator(guard.initialized).branch()?;

        // SAFETY: The loop condition ensures we have space to push the item
        unsafe { guard.push_unchecked(item) };
    }

    mem::forget(guard);
    ControlFlow::Continue(())
}

/// Panic guard for incremental initialization of arrays.
///
/// Disarm the guard with `mem::forget` once the array has been initialized.
///
/// # Safety
///
/// All write accesses to this structure are unsafe and must maintain a correct
/// count of `initialized` elements.
///
/// To minimize indirection fields are still pub but callers should at least use
/// `push_unchecked` to signal that something unsafe is going on.
struct Guard<'a, T> {
    /// The array to be initialized.
    pub array_mut: &'a mut [MaybeUninit<T>],
    /// The number of items that have been initialized so far.
    pub initialized: usize,
}

impl<T> Guard<'_, T> {
    /// Adds an item to the array and updates the initialized item counter.
    ///
    /// # Safety
    ///
    /// No more than N elements must be initialized.
    #[inline]
    pub unsafe fn push_unchecked(&mut self, item: T) {
        // SAFETY: If `initialized` was correct before and the caller does not
        // invoke this method more than N times then writes will be in-bounds
        // and slots will not be initialized more than once.
        unsafe {
            self.array_mut.get_unchecked_mut(self.initialized).write(item);
            self.initialized = self.initialized.unchecked_add(1);
        }
    }
}

impl<T> Drop for Guard<'_, T> {
    #[inline]
    fn drop(&mut self) {
        debug_assert!(self.initialized <= self.array_mut.len());

        // SAFETY: this slice will contain only initialized objects.
        unsafe {
            crate::ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(
                self.array_mut.get_unchecked_mut(..self.initialized),
            ));
        }
    }
}

/// Pulls `N` items from `iter` and returns them as an array. If the iterator
/// yields fewer than `N` items, `Err` is returned containing an iterator over
/// the already yielded items.
///
/// Since the iterator is passed as a mutable reference and this function calls
/// `next` at most `N` times, the iterator can still be used afterwards to
/// retrieve the remaining items.
///
/// If `iter.next()` panicks, all items already yielded by the iterator are
/// dropped.
///
/// Used for [`Iterator::next_chunk`].
#[inline]
pub(crate) fn iter_next_chunk<T, const N: usize>(
    iter: &mut impl Iterator<Item = T>,
) -> Result<[T; N], IntoIter<T, N>> {
    let mut array = [const { MaybeUninit::uninit() }; N];
    let r = iter_next_chunk_erased(&mut array, iter);
    match r {
        Ok(()) => {
            // SAFETY: All elements of `array` were populated.
            Ok(unsafe { MaybeUninit::array_assume_init(array) })
        }
        Err(initialized) => {
            // SAFETY: Only the first `initialized` elements were populated
            Err(unsafe { IntoIter::new_unchecked(array, 0..initialized) })
        }
    }
}

/// Version of [`iter_next_chunk`] using a passed-in slice in order to avoid
/// needing to monomorphize for every array length.
///
/// Unfortunately this loop has two exit conditions, the buffer filling up
/// or the iterator running out of items, making it tend to optimize poorly.
#[inline]
fn iter_next_chunk_erased<T>(
    buffer: &mut [MaybeUninit<T>],
    iter: &mut impl Iterator<Item = T>,
) -> Result<(), usize> {
    let mut guard = Guard { array_mut: buffer, initialized: 0 };
    while guard.initialized < guard.array_mut.len() {
        let Some(item) = iter.next() else {
            // Unlike `try_from_fn_erased`, we want to keep the partial results,
            // so we need to defuse the guard instead of using `?`.
            let initialized = guard.initialized;
            mem::forget(guard);
            return Err(initialized);
        };

        // SAFETY: The loop condition ensures we have space to push the item
        unsafe { guard.push_unchecked(item) };
    }

    mem::forget(guard);
    Ok(())
}