summaryrefslogtreecommitdiffstats
path: root/mfbt/Span.h
blob: bcbd492d35edbb02fb2a64b7bf60af8a02f3d1c7 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
///////////////////////////////////////////////////////////////////////////////
//
// Copyright (c) 2015 Microsoft Corporation. All rights reserved.
//
// This code is licensed under the MIT License (MIT).
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
//
///////////////////////////////////////////////////////////////////////////////

// Adapted from
// https://github.com/Microsoft/GSL/blob/3819df6e378ffccf0e29465afe99c3b324c2aa70/include/gsl/span
// and
// https://github.com/Microsoft/GSL/blob/3819df6e378ffccf0e29465afe99c3b324c2aa70/include/gsl/gsl_util

#ifndef mozilla_Span_h
#define mozilla_Span_h

#include <array>
#include <cstddef>
#include <cstdint>
#include <iterator>
#include <limits>
#include <string>
#include <type_traits>
#include <utility>

#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/Casting.h"
#include "mozilla/UniquePtr.h"

namespace mozilla {

template <typename T, size_t Length>
class Array;

// Stuff from gsl_util

// narrow_cast(): a searchable way to do narrowing casts of values
template <class T, class U>
inline constexpr T narrow_cast(U&& u) {
  return static_cast<T>(std::forward<U>(u));
}

// end gsl_util

// [views.constants], constants
// This was -1 in gsl::span, but using size_t for sizes instead of ptrdiff_t
// and reserving a magic value that realistically doesn't occur in
// compile-time-constant Span sizes makes things a lot less messy in terms of
// comparison between signed and unsigned.
constexpr const size_t dynamic_extent = std::numeric_limits<size_t>::max();

template <class ElementType, size_t Extent = dynamic_extent>
class Span;

// implementation details
namespace span_details {

template <class T>
struct is_span_oracle : std::false_type {};

template <class ElementType, size_t Extent>
struct is_span_oracle<mozilla::Span<ElementType, Extent>> : std::true_type {};

template <class T>
struct is_span : public is_span_oracle<std::remove_cv_t<T>> {};

template <class T>
struct is_std_array_oracle : std::false_type {};

template <class ElementType, size_t Extent>
struct is_std_array_oracle<std::array<ElementType, Extent>> : std::true_type {};

template <class T>
struct is_std_array : public is_std_array_oracle<std::remove_cv_t<T>> {};

template <size_t From, size_t To>
struct is_allowed_extent_conversion
    : public std::integral_constant<bool, From == To ||
                                              From == mozilla::dynamic_extent ||
                                              To == mozilla::dynamic_extent> {};

template <class From, class To>
struct is_allowed_element_type_conversion
    : public std::integral_constant<
          bool, std::is_convertible_v<From (*)[], To (*)[]>> {};

struct SpanKnownBounds {};

template <class SpanT, bool IsConst>
class span_iterator {
  using element_type_ = typename SpanT::element_type;

  template <class ElementType, size_t Extent>
  friend class ::mozilla::Span;

 public:
  using iterator_category = std::random_access_iterator_tag;
  using value_type = std::remove_const_t<element_type_>;
  using difference_type = ptrdiff_t;

  using reference =
      std::conditional_t<IsConst, const element_type_, element_type_>&;
  using pointer = std::add_pointer_t<reference>;

  constexpr span_iterator() : span_iterator(nullptr, 0, SpanKnownBounds{}) {}

  constexpr span_iterator(const SpanT* span, typename SpanT::index_type index)
      : span_(span), index_(index) {
    MOZ_RELEASE_ASSERT(span == nullptr ||
                       (index_ >= 0 && index <= span_->Length()));
  }

 private:
  // For whatever reason, the compiler doesn't like optimizing away the above
  // MOZ_RELEASE_ASSERT when `span_iterator` is constructed for
  // obviously-correct cases like `span.begin()` or `span.end()`.  We provide
  // this private constructor for such cases.
  constexpr span_iterator(const SpanT* span, typename SpanT::index_type index,
                          SpanKnownBounds)
      : span_(span), index_(index) {}

 public:
  // `other` is already correct by construction; we do not need to go through
  // the release assert above.  Put differently, this constructor is effectively
  // a copy constructor and therefore needs no assertions.
  friend class span_iterator<SpanT, true>;
  constexpr MOZ_IMPLICIT span_iterator(const span_iterator<SpanT, false>& other)
      : span_(other.span_), index_(other.index_) {}

  constexpr span_iterator<SpanT, IsConst>& operator=(
      const span_iterator<SpanT, IsConst>&) = default;

  constexpr reference operator*() const {
    MOZ_RELEASE_ASSERT(span_);
    return (*span_)[index_];
  }

  constexpr pointer operator->() const {
    MOZ_RELEASE_ASSERT(span_);
    return &((*span_)[index_]);
  }

  constexpr span_iterator& operator++() {
    ++index_;
    return *this;
  }

  constexpr span_iterator operator++(int) {
    auto ret = *this;
    ++(*this);
    return ret;
  }

  constexpr span_iterator& operator--() {
    --index_;
    return *this;
  }

  constexpr span_iterator operator--(int) {
    auto ret = *this;
    --(*this);
    return ret;
  }

  constexpr span_iterator operator+(difference_type n) const {
    auto ret = *this;
    return ret += n;
  }

  constexpr span_iterator& operator+=(difference_type n) {
    MOZ_RELEASE_ASSERT(span_ && (index_ + n) >= 0 &&
                       (index_ + n) <= span_->Length());
    index_ += n;
    return *this;
  }

  constexpr span_iterator operator-(difference_type n) const {
    auto ret = *this;
    return ret -= n;
  }

  constexpr span_iterator& operator-=(difference_type n) { return *this += -n; }

  constexpr difference_type operator-(const span_iterator& rhs) const {
    MOZ_RELEASE_ASSERT(span_ == rhs.span_);
    return index_ - rhs.index_;
  }

  constexpr reference operator[](difference_type n) const {
    return *(*this + n);
  }

  constexpr friend bool operator==(const span_iterator& lhs,
                                   const span_iterator& rhs) {
    // Iterators from different spans are uncomparable. A diagnostic assertion
    // should be enough to check this, though. To ensure that no iterators from
    // different spans are ever considered equal, still compare them in release
    // builds.
    MOZ_DIAGNOSTIC_ASSERT(lhs.span_ == rhs.span_);
    return lhs.index_ == rhs.index_ && lhs.span_ == rhs.span_;
  }

  constexpr friend bool operator!=(const span_iterator& lhs,
                                   const span_iterator& rhs) {
    return !(lhs == rhs);
  }

  constexpr friend bool operator<(const span_iterator& lhs,
                                  const span_iterator& rhs) {
    MOZ_DIAGNOSTIC_ASSERT(lhs.span_ == rhs.span_);
    return lhs.index_ < rhs.index_;
  }

  constexpr friend bool operator<=(const span_iterator& lhs,
                                   const span_iterator& rhs) {
    return !(rhs < lhs);
  }

  constexpr friend bool operator>(const span_iterator& lhs,
                                  const span_iterator& rhs) {
    return rhs < lhs;
  }

  constexpr friend bool operator>=(const span_iterator& lhs,
                                   const span_iterator& rhs) {
    return !(rhs > lhs);
  }

  void swap(span_iterator& rhs) {
    std::swap(index_, rhs.index_);
    std::swap(span_, rhs.span_);
  }

 protected:
  const SpanT* span_;
  size_t index_;
};

template <class Span, bool IsConst>
inline constexpr span_iterator<Span, IsConst> operator+(
    typename span_iterator<Span, IsConst>::difference_type n,
    const span_iterator<Span, IsConst>& rhs) {
  return rhs + n;
}

template <size_t Ext>
class extent_type {
 public:
  using index_type = size_t;

  static_assert(Ext >= 0, "A fixed-size Span must be >= 0 in size.");

  constexpr extent_type() = default;

  template <index_type Other>
  constexpr MOZ_IMPLICIT extent_type(extent_type<Other> ext) {
    static_assert(
        Other == Ext || Other == dynamic_extent,
        "Mismatch between fixed-size extent and size of initializing data.");
    MOZ_RELEASE_ASSERT(ext.size() == Ext);
  }

  constexpr MOZ_IMPLICIT extent_type(index_type length) {
    MOZ_RELEASE_ASSERT(length == Ext);
  }

  constexpr index_type size() const { return Ext; }
};

template <>
class extent_type<dynamic_extent> {
 public:
  using index_type = size_t;

  template <index_type Other>
  explicit constexpr extent_type(extent_type<Other> ext) : size_(ext.size()) {}

  explicit constexpr extent_type(index_type length) : size_(length) {}

  constexpr index_type size() const { return size_; }

 private:
  index_type size_;
};
}  // namespace span_details

/**
 * Span - slices for C++
 *
 * Span implements Rust's slice concept for C++. It's called "Span" instead of
 * "Slice" to follow the naming used in C++ Core Guidelines.
 *
 * A Span wraps a pointer and a length that identify a non-owning view to a
 * contiguous block of memory of objects of the same type. Various types,
 * including (pre-decay) C arrays, XPCOM strings, nsTArray, mozilla::Array,
 * mozilla::Range and contiguous standard-library containers, auto-convert
 * into Spans when attempting to pass them as arguments to methods that take
 * Spans. (Span itself autoconverts into mozilla::Range.)
 *
 * Like Rust's slices, Span provides safety against out-of-bounds access by
 * performing run-time bound checks. However, unlike Rust's slices, Span
 * cannot provide safety against use-after-free.
 *
 * (Note: Span is like Rust's slice only conceptually. Due to the lack of
 * ABI guarantees, you should still decompose spans/slices to raw pointer
 * and length parts when crossing the FFI. The Elements() and data() methods
 * are guaranteed to return a non-null pointer even for zero-length spans,
 * so the pointer can be used as a raw part of a Rust slice without further
 * checks.)
 *
 * In addition to having constructors (with the support of deduction guides)
 * that take various well-known types, a Span for an arbitrary type can be
 * constructed from a pointer and a length or a pointer and another pointer
 * pointing just past the last element.
 *
 * A Span<const char> or Span<const char16_t> can be obtained for const char*
 * or const char16_t pointing to a zero-terminated string using the
 * MakeStringSpan() function (which treats a nullptr argument equivalently
 * to the empty string). Corresponding implicit constructor does not exist
 * in order to avoid accidental construction in cases where const char* or
 * const char16_t* do not point to a zero-terminated string.
 *
 * Span has methods that follow the Mozilla naming style and methods that
 * don't. The methods that follow the Mozilla naming style are meant to be
 * used directly from Mozilla code. The methods that don't are meant for
 * integration with C++11 range-based loops and with meta-programming that
 * expects the same methods that are found on the standard-library
 * containers. For example, to decompose a Span into its parts in Mozilla
 * code, use Elements() and Length() (as with nsTArray) instead of data()
 * and size() (as with std::vector).
 *
 * The pointer and length wrapped by a Span cannot be changed after a Span has
 * been created. When new values are required, simply create a new Span. Span
 * has a method called Subspan() that works analogously to the Substring()
 * method of XPCOM strings taking a start index and an optional length. As a
 * Mozilla extension (relative to Microsoft's gsl::span that mozilla::Span is
 * based on), Span has methods From(start), To(end) and FromTo(start, end)
 * that correspond to Rust's &slice[start..], &slice[..end] and
 * &slice[start..end], respectively. (That is, the end index is the index of
 * the first element not to be included in the new subspan.)
 *
 * When indicating a Span that's only read from, const goes inside the type
 * parameter. Don't put const in front of Span. That is:
 * size_t ReadsFromOneSpanAndWritesToAnother(Span<const uint8_t> aReadFrom,
 *                                           Span<uint8_t> aWrittenTo);
 *
 * Any Span<const T> can be viewed as Span<const uint8_t> using the function
 * AsBytes(). Any Span<T> can be viewed as Span<uint8_t> using the function
 * AsWritableBytes().
 *
 * Note that iterators from different Span instances are uncomparable, even if
 * they refer to the same memory. This also applies to any spans derived via
 * Subspan etc.
 */
template <class ElementType, size_t Extent /* = dynamic_extent */>
class Span {
 public:
  // constants and types
  using element_type = ElementType;
  using value_type = std::remove_cv_t<element_type>;
  using index_type = size_t;
  using pointer = element_type*;
  using reference = element_type&;

  using iterator =
      span_details::span_iterator<Span<ElementType, Extent>, false>;
  using const_iterator =
      span_details::span_iterator<Span<ElementType, Extent>, true>;
  using reverse_iterator = std::reverse_iterator<iterator>;
  using const_reverse_iterator = std::reverse_iterator<const_iterator>;

  constexpr static const index_type extent = Extent;

  // [Span.cons], Span constructors, copy, assignment, and destructor
  // "Dependent" is needed to make "std::enable_if_t<(Dependent ||
  //   Extent == 0 || Extent == dynamic_extent)>" SFINAE,
  // since
  // "std::enable_if_t<(Extent == 0 || Extent == dynamic_extent)>" is
  // ill-formed when Extent is neither of the extreme values.
  /**
   * Constructor with no args.
   */
  template <bool Dependent = false,
            class = std::enable_if_t<(Dependent || Extent == 0 ||
                                      Extent == dynamic_extent)>>
  constexpr Span() : storage_(nullptr, span_details::extent_type<0>()) {}

  /**
   * Constructor for nullptr.
   */
  constexpr MOZ_IMPLICIT Span(std::nullptr_t) : Span() {}

  /**
   * Constructor for pointer and length.
   */
  constexpr Span(pointer aPtr, index_type aLength) : storage_(aPtr, aLength) {}

  /**
   * Constructor for start pointer and pointer past end.
   */
  constexpr Span(pointer aStartPtr, pointer aEndPtr)
      : storage_(aStartPtr, std::distance(aStartPtr, aEndPtr)) {}

  /**
   * Constructor for pair of Span iterators.
   */
  template <typename OtherElementType, size_t OtherExtent, bool IsConst>
  constexpr Span(
      span_details::span_iterator<Span<OtherElementType, OtherExtent>, IsConst>
          aBegin,
      span_details::span_iterator<Span<OtherElementType, OtherExtent>, IsConst>
          aEnd)
      : storage_(aBegin == aEnd ? nullptr : &*aBegin, aEnd - aBegin) {}

  /**
   * Constructor for {iterator,size_t}
   */
  template <typename OtherElementType, size_t OtherExtent, bool IsConst>
  constexpr Span(
      span_details::span_iterator<Span<OtherElementType, OtherExtent>, IsConst>
          aBegin,
      index_type aLength)
      : storage_(!aLength ? nullptr : &*aBegin, aLength) {}

  /**
   * Constructor for C array.
   */
  template <size_t N>
  constexpr MOZ_IMPLICIT Span(element_type (&aArr)[N])
      : storage_(&aArr[0], span_details::extent_type<N>()) {}

  // Implicit constructors for char* and char16_t* pointers are deleted in order
  // to avoid accidental construction in cases where a pointer does not point to
  // a zero-terminated string. A Span<const char> or Span<const char16_t> can be
  // obtained for const char* or const char16_t pointing to a zero-terminated
  // string using the MakeStringSpan() function.
  // (This must be a template because otherwise it will prevent the previous
  // array constructor to match because an array decays to a pointer. This only
  // exists to point to the above explanation, since there's no other
  // constructor that would match.)
  template <
      typename T,
      typename = std::enable_if_t<
          std::is_pointer_v<T> &&
          (std::is_same_v<std::remove_const_t<std::decay_t<T>>, char> ||
           std::is_same_v<std::remove_const_t<std::decay_t<T>>, char16_t>)>>
  Span(T& aStr) = delete;

  /**
   * Constructor for std::array.
   */
  template <size_t N,
            class ArrayElementType = std::remove_const_t<element_type>>
  constexpr MOZ_IMPLICIT Span(std::array<ArrayElementType, N>& aArr)
      : storage_(&aArr[0], span_details::extent_type<N>()) {}

  /**
   * Constructor for const std::array.
   */
  template <size_t N>
  constexpr MOZ_IMPLICIT Span(
      const std::array<std::remove_const_t<element_type>, N>& aArr)
      : storage_(&aArr[0], span_details::extent_type<N>()) {}

  /**
   * Constructor for mozilla::Array.
   */
  template <size_t N,
            class ArrayElementType = std::remove_const_t<element_type>>
  constexpr MOZ_IMPLICIT Span(mozilla::Array<ArrayElementType, N>& aArr)
      : storage_(&aArr[0], span_details::extent_type<N>()) {}

  /**
   * Constructor for const mozilla::Array.
   */
  template <size_t N>
  constexpr MOZ_IMPLICIT Span(
      const mozilla::Array<std::remove_const_t<element_type>, N>& aArr)
      : storage_(&aArr[0], span_details::extent_type<N>()) {}

  /**
   * Constructor for mozilla::UniquePtr holding an array and length.
   */
  template <class ArrayElementType = std::add_pointer<element_type>>
  constexpr Span(const mozilla::UniquePtr<ArrayElementType>& aPtr,
                 index_type aLength)
      : storage_(aPtr.get(), aLength) {}

  // NB: the SFINAE here uses .data() as a incomplete/imperfect proxy for the
  // requirement on Container to be a contiguous sequence container.
  /**
   * Constructor for standard-library containers.
   */
  template <
      class Container,
      class Dummy = std::enable_if_t<
          !std::is_const_v<Container> &&
              !span_details::is_span<Container>::value &&
              !span_details::is_std_array<Container>::value &&
              std::is_convertible_v<typename Container::pointer, pointer> &&
              std::is_convertible_v<typename Container::pointer,
                                    decltype(std::declval<Container>().data())>,
          Container>>
  constexpr MOZ_IMPLICIT Span(Container& cont, Dummy* = nullptr)
      : Span(cont.data(), ReleaseAssertedCast<index_type>(cont.size())) {}

  /**
   * Constructor for standard-library containers (const version).
   */
  template <
      class Container,
      class = std::enable_if_t<
          std::is_const_v<element_type> &&
          !span_details::is_span<Container>::value &&
          std::is_convertible_v<typename Container::pointer, pointer> &&
          std::is_convertible_v<typename Container::pointer,
                                decltype(std::declval<Container>().data())>>>
  constexpr MOZ_IMPLICIT Span(const Container& cont)
      : Span(cont.data(), ReleaseAssertedCast<index_type>(cont.size())) {}

  // NB: the SFINAE here uses .Elements() as a incomplete/imperfect proxy for
  // the requirement on Container to be a contiguous sequence container.
  /**
   * Constructor for contiguous Mozilla containers.
   */
  template <
      class Container,
      class = std::enable_if_t<
          !std::is_const_v<Container> &&
          !span_details::is_span<Container>::value &&
          !span_details::is_std_array<Container>::value &&
          std::is_convertible_v<typename Container::value_type*, pointer> &&
          std::is_convertible_v<
              typename Container::value_type*,
              decltype(std::declval<Container>().Elements())>>>
  constexpr MOZ_IMPLICIT Span(Container& cont, void* = nullptr)
      : Span(cont.Elements(), ReleaseAssertedCast<index_type>(cont.Length())) {}

  /**
   * Constructor for contiguous Mozilla containers (const version).
   */
  template <
      class Container,
      class = std::enable_if_t<
          std::is_const_v<element_type> &&
          !span_details::is_span<Container>::value &&
          std::is_convertible_v<typename Container::value_type*, pointer> &&
          std::is_convertible_v<
              typename Container::value_type*,
              decltype(std::declval<Container>().Elements())>>>
  constexpr MOZ_IMPLICIT Span(const Container& cont, void* = nullptr)
      : Span(cont.Elements(), ReleaseAssertedCast<index_type>(cont.Length())) {}

  /**
   * Constructor from other Span.
   */
  constexpr Span(const Span& other) = default;

  /**
   * Constructor from other Span.
   */
  constexpr Span(Span&& other) = default;

  /**
   * Constructor from other Span with conversion of element type.
   */
  template <
      class OtherElementType, size_t OtherExtent,
      class = std::enable_if_t<span_details::is_allowed_extent_conversion<
                                   OtherExtent, Extent>::value &&
                               span_details::is_allowed_element_type_conversion<
                                   OtherElementType, element_type>::value>>
  constexpr MOZ_IMPLICIT Span(const Span<OtherElementType, OtherExtent>& other)
      : storage_(other.data(),
                 span_details::extent_type<OtherExtent>(other.size())) {}

  /**
   * Constructor from other Span with conversion of element type.
   */
  template <
      class OtherElementType, size_t OtherExtent,
      class = std::enable_if_t<span_details::is_allowed_extent_conversion<
                                   OtherExtent, Extent>::value &&
                               span_details::is_allowed_element_type_conversion<
                                   OtherElementType, element_type>::value>>
  constexpr MOZ_IMPLICIT Span(Span<OtherElementType, OtherExtent>&& other)
      : storage_(other.data(),
                 span_details::extent_type<OtherExtent>(other.size())) {}

  ~Span() = default;
  constexpr Span& operator=(const Span& other) = default;

  constexpr Span& operator=(Span&& other) = default;

  // [Span.sub], Span subviews
  /**
   * Subspan with first N elements with compile-time N.
   */
  template <size_t Count>
  constexpr Span<element_type, Count> First() const {
    MOZ_RELEASE_ASSERT(Count <= size());
    return {data(), Count};
  }

  /**
   * Subspan with last N elements with compile-time N.
   */
  template <size_t Count>
  constexpr Span<element_type, Count> Last() const {
    const size_t len = size();
    MOZ_RELEASE_ASSERT(Count <= len);
    return {data() + (len - Count), Count};
  }

  /**
   * Subspan with compile-time start index and length.
   */
  template <size_t Offset, size_t Count = dynamic_extent>
  constexpr Span<element_type, Count> Subspan() const {
    const size_t len = size();
    MOZ_RELEASE_ASSERT(Offset <= len &&
                       (Count == dynamic_extent || (Offset + Count <= len)));
    return {data() + Offset, Count == dynamic_extent ? len - Offset : Count};
  }

  /**
   * Subspan with first N elements with run-time N.
   */
  constexpr Span<element_type, dynamic_extent> First(index_type aCount) const {
    MOZ_RELEASE_ASSERT(aCount <= size());
    return {data(), aCount};
  }

  /**
   * Subspan with last N elements with run-time N.
   */
  constexpr Span<element_type, dynamic_extent> Last(index_type aCount) const {
    const size_t len = size();
    MOZ_RELEASE_ASSERT(aCount <= len);
    return {data() + (len - aCount), aCount};
  }

  /**
   * Subspan with run-time start index and length.
   */
  constexpr Span<element_type, dynamic_extent> Subspan(
      index_type aStart, index_type aLength = dynamic_extent) const {
    const size_t len = size();
    MOZ_RELEASE_ASSERT(aStart <= len && (aLength == dynamic_extent ||
                                         (aStart + aLength <= len)));
    return {data() + aStart,
            aLength == dynamic_extent ? len - aStart : aLength};
  }

  /**
   * Subspan with run-time start index. (Rust's &foo[start..])
   */
  constexpr Span<element_type, dynamic_extent> From(index_type aStart) const {
    return Subspan(aStart);
  }

  /**
   * Subspan with run-time exclusive end index. (Rust's &foo[..end])
   */
  constexpr Span<element_type, dynamic_extent> To(index_type aEnd) const {
    return Subspan(0, aEnd);
  }

  /// std::span-compatible method name
  constexpr auto subspan(index_type aStart,
                         index_type aLength = dynamic_extent) const {
    return Subspan(aStart, aLength);
  }
  /// std::span-compatible method name
  constexpr auto from(index_type aStart) const { return From(aStart); }
  /// std::span-compatible method name
  constexpr auto to(index_type aEnd) const { return To(aEnd); }

  /**
   * Subspan with run-time start index and exclusive end index.
   * (Rust's &foo[start..end])
   */
  constexpr Span<element_type, dynamic_extent> FromTo(index_type aStart,
                                                      index_type aEnd) const {
    MOZ_RELEASE_ASSERT(aStart <= aEnd);
    return Subspan(aStart, aEnd - aStart);
  }

  // [Span.obs], Span observers
  /**
   * Number of elements in the span.
   */
  constexpr index_type Length() const { return size(); }

  /**
   * Number of elements in the span (standard-libray duck typing version).
   */
  constexpr index_type size() const { return storage_.size(); }

  /**
   * Size of the span in bytes.
   */
  constexpr index_type LengthBytes() const { return size_bytes(); }

  /**
   * Size of the span in bytes (standard-library naming style version).
   */
  constexpr index_type size_bytes() const {
    return size() * narrow_cast<index_type>(sizeof(element_type));
  }

  /**
   * Checks if the the length of the span is zero.
   */
  constexpr bool IsEmpty() const { return empty(); }

  /**
   * Checks if the the length of the span is zero (standard-libray duck
   * typing version).
   */
  constexpr bool empty() const { return size() == 0; }

  // [Span.elem], Span element access
  constexpr reference operator[](index_type idx) const {
    MOZ_RELEASE_ASSERT(idx < storage_.size());
    return data()[idx];
  }

  /**
   * Access element of span by index (standard-library duck typing version).
   */
  constexpr reference at(index_type idx) const { return this->operator[](idx); }

  constexpr reference operator()(index_type idx) const {
    return this->operator[](idx);
  }

  /**
   * Pointer to the first element of the span. The return value is never
   * nullptr, not ever for zero-length spans, so it can be passed as-is
   * to std::slice::from_raw_parts() in Rust.
   */
  constexpr pointer Elements() const { return data(); }

  /**
   * Pointer to the first element of the span (standard-libray duck typing
   * version). The return value is never nullptr, not ever for zero-length
   * spans, so it can be passed as-is to std::slice::from_raw_parts() in Rust.
   */
  constexpr pointer data() const { return storage_.data(); }

  // [Span.iter], Span iterator support
  iterator begin() const { return {this, 0, span_details::SpanKnownBounds{}}; }
  iterator end() const {
    return {this, Length(), span_details::SpanKnownBounds{}};
  }

  const_iterator cbegin() const {
    return {this, 0, span_details::SpanKnownBounds{}};
  }
  const_iterator cend() const {
    return {this, Length(), span_details::SpanKnownBounds{}};
  }

  reverse_iterator rbegin() const { return reverse_iterator{end()}; }
  reverse_iterator rend() const { return reverse_iterator{begin()}; }

  const_reverse_iterator crbegin() const {
    return const_reverse_iterator{cend()};
  }
  const_reverse_iterator crend() const {
    return const_reverse_iterator{cbegin()};
  }

  template <size_t SplitPoint>
  constexpr std::pair<Span<ElementType, SplitPoint>,
                      Span<ElementType, Extent - SplitPoint>>
  SplitAt() const {
    static_assert(Extent != dynamic_extent);
    static_assert(SplitPoint <= Extent);
    return {First<SplitPoint>(), Last<Extent - SplitPoint>()};
  }

  constexpr std::pair<Span<ElementType, dynamic_extent>,
                      Span<ElementType, dynamic_extent>>
  SplitAt(const index_type aSplitPoint) const {
    MOZ_RELEASE_ASSERT(aSplitPoint <= Length());
    return {First(aSplitPoint), Last(Length() - aSplitPoint)};
  }

  constexpr Span<std::add_const_t<ElementType>, Extent> AsConst() const {
    return {Elements(), Length()};
  }

 private:
  // this implementation detail class lets us take advantage of the
  // empty base class optimization to pay for only storage of a single
  // pointer in the case of fixed-size Spans
  template <class ExtentType>
  class storage_type : public ExtentType {
   public:
    template <class OtherExtentType>
    constexpr storage_type(pointer elements, OtherExtentType ext)
        : ExtentType(ext)
          // Replace nullptr with aligned bogus pointer for Rust slice
          // compatibility. See
          // https://doc.rust-lang.org/std/slice/fn.from_raw_parts.html
          ,
          data_(elements ? elements
                         : reinterpret_cast<pointer>(alignof(element_type))) {
      const size_t extentSize = ExtentType::size();
      MOZ_RELEASE_ASSERT((!elements && extentSize == 0) ||
                         (elements && extentSize != dynamic_extent));
    }

    constexpr pointer data() const { return data_; }

   private:
    pointer data_;
  };

  storage_type<span_details::extent_type<Extent>> storage_;
};

template <typename T, size_t OtherExtent, bool IsConst>
Span(span_details::span_iterator<Span<T, OtherExtent>, IsConst> aBegin,
     span_details::span_iterator<Span<T, OtherExtent>, IsConst> aEnd)
    -> Span<std::conditional_t<IsConst, std::add_const_t<T>, T>>;

template <typename T, size_t Extent>
Span(T (&)[Extent]) -> Span<T, Extent>;

template <class Container>
Span(Container&) -> Span<typename Container::value_type>;

template <class Container>
Span(const Container&) -> Span<const typename Container::value_type>;

template <typename T, size_t Extent>
Span(mozilla::Array<T, Extent>&) -> Span<T, Extent>;

template <typename T, size_t Extent>
Span(const mozilla::Array<T, Extent>&) -> Span<const T, Extent>;

// [Span.comparison], Span comparison operators
template <class ElementType, size_t FirstExtent, size_t SecondExtent>
inline constexpr bool operator==(const Span<ElementType, FirstExtent>& l,
                                 const Span<ElementType, SecondExtent>& r) {
  return (l.size() == r.size()) &&
         std::equal(l.data(), l.data() + l.size(), r.data());
}

template <class ElementType, size_t Extent>
inline constexpr bool operator!=(const Span<ElementType, Extent>& l,
                                 const Span<ElementType, Extent>& r) {
  return !(l == r);
}

template <class ElementType, size_t Extent>
inline constexpr bool operator<(const Span<ElementType, Extent>& l,
                                const Span<ElementType, Extent>& r) {
  return std::lexicographical_compare(l.data(), l.data() + l.size(), r.data(),
                                      r.data() + r.size());
}

template <class ElementType, size_t Extent>
inline constexpr bool operator<=(const Span<ElementType, Extent>& l,
                                 const Span<ElementType, Extent>& r) {
  return !(l > r);
}

template <class ElementType, size_t Extent>
inline constexpr bool operator>(const Span<ElementType, Extent>& l,
                                const Span<ElementType, Extent>& r) {
  return r < l;
}

template <class ElementType, size_t Extent>
inline constexpr bool operator>=(const Span<ElementType, Extent>& l,
                                 const Span<ElementType, Extent>& r) {
  return !(l < r);
}

namespace span_details {
// if we only supported compilers with good constexpr support then
// this pair of classes could collapse down to a constexpr function

// we should use a narrow_cast<> to go to size_t, but older compilers may not
// see it as constexpr and so will fail compilation of the template
template <class ElementType, size_t Extent>
struct calculate_byte_size
    : std::integral_constant<size_t,
                             static_cast<size_t>(sizeof(ElementType) *
                                                 static_cast<size_t>(Extent))> {
};

template <class ElementType>
struct calculate_byte_size<ElementType, dynamic_extent>
    : std::integral_constant<size_t, dynamic_extent> {};
}  // namespace span_details

// [Span.objectrep], views of object representation
/**
 * View span as Span<const uint8_t>.
 */
template <class ElementType, size_t Extent>
Span<const uint8_t,
     span_details::calculate_byte_size<ElementType, Extent>::value>
AsBytes(Span<ElementType, Extent> s) {
  return {reinterpret_cast<const uint8_t*>(s.data()), s.size_bytes()};
}

/**
 * View span as Span<uint8_t>.
 */
template <class ElementType, size_t Extent,
          class = std::enable_if_t<!std::is_const_v<ElementType>>>
Span<uint8_t, span_details::calculate_byte_size<ElementType, Extent>::value>
AsWritableBytes(Span<ElementType, Extent> s) {
  return {reinterpret_cast<uint8_t*>(s.data()), s.size_bytes()};
}

/**
 * View a span of uint8_t as a span of char.
 */
inline Span<const char> AsChars(Span<const uint8_t> s) {
  return {reinterpret_cast<const char*>(s.data()), s.size()};
}

/**
 * View a writable span of uint8_t as a span of char.
 */
inline Span<char> AsWritableChars(Span<uint8_t> s) {
  return {reinterpret_cast<char*>(s.data()), s.size()};
}

/**
 * Create span from a zero-terminated C string. nullptr is
 * treated as the empty string.
 */
constexpr Span<const char> MakeStringSpan(const char* aZeroTerminated) {
  if (!aZeroTerminated) {
    return Span<const char>();
  }
  return Span<const char>(aZeroTerminated,
                          std::char_traits<char>::length(aZeroTerminated));
}

/**
 * Create span from a zero-terminated UTF-16 C string. nullptr is
 * treated as the empty string.
 */
constexpr Span<const char16_t> MakeStringSpan(const char16_t* aZeroTerminated) {
  if (!aZeroTerminated) {
    return Span<const char16_t>();
  }
  return Span<const char16_t>(
      aZeroTerminated, std::char_traits<char16_t>::length(aZeroTerminated));
}

}  // namespace mozilla

#endif  // mozilla_Span_h