Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 973 insertions, 0 deletions

diff --git a/mfbt/Span.h b/mfbt/Span.h
new file mode 100644
index 0000000000..d9ba1af220
--- /dev/null
+++ b/mfbt/Span.h
@@ -0,0 +1,973 @@
+///////////////////////////////////////////////////////////////////////////////
+//
+// 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>,
+            class DeleterType>
+  constexpr Span(const mozilla::UniquePtr<ArrayElementType, DeleterType>& 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