Add mozilla::Span

1 files changed, 1041 insertions, 0 deletions

diff --git a/mfbt/Span.h b/mfbt/Span.h
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+++ b/mfbt/Span.h
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+///////////////////////////////////////////////////////////////////////////////
+//
+// 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 "mozilla/Array.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Casting.h"
+#include "mozilla/IntegerTypeTraits.h"
+#include "mozilla/Move.h"
+#include "mozilla/TypeTraits.h"
+#include "mozilla/UniquePtr.h"
+
+#include <algorithm>
+#include <array>
+#include <cstring>
+#include <iterator>
+
+// Classifications for reasons why constexpr was removed in C++14 to C++11
+// conversion. Once we upgrade compilers, we can try defining each of these
+// to constexpr to restore a category of constexprs at a time.
+#define MOZ_SPAN_ASSERTION_CONSTEXPR
+#define MOZ_SPAN_GCC_CONSTEXPR
+#define MOZ_SPAN_EXPLICITLY_DEFAULTED_CONSTEXPR
+#define MOZ_SPAN_CONSTEXPR_NOT_JUST_RETURN
+#define MOZ_SPAN_NON_CONST_CONSTEXPR
+
+#ifdef _MSC_VER
+#pragma warning(push)
+
+// turn off some warnings that are noisy about our MOZ_RELEASE_ASSERT statements
+#pragma warning(disable : 4127) // conditional expression is constant
+
+// blanket turn off warnings from CppCoreCheck for now
+// so people aren't annoyed by them when running the tool.
+// more targeted suppressions will be added in a future update to the GSL
+#pragma warning(disable : 26481 26482 26483 26485 26490 26491 26492 26493 26495)
+
+#if _MSC_VER < 1910
+#pragma push_macro("constexpr")
+#define constexpr /*constexpr*/
+
+#endif            // _MSC_VER < 1910
+#endif            // _MSC_VER
+
+namespace mozilla {
+
+// 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>(mozilla::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 = mozilla::MaxValue<size_t>::value;
+
+template<class ElementType, size_t Extent = dynamic_extent>
+class Span;
+
+// implementation details
+namespace span_details {
+
+// C++14 types that we don't have because we build as C++11.
+template<class T>
+using remove_cv_t = typename mozilla::RemoveCV<T>::Type;
+template<class T>
+using remove_const_t = typename mozilla::RemoveConst<T>::Type;
+template<bool B, class T, class F>
+using conditional_t = typename mozilla::Conditional<B, T, F>::Type;
+template<class T>
+using add_pointer_t = typename mozilla::AddPointer<T>::Type;
+template<bool B, class T = void>
+using enable_if_t = typename mozilla::EnableIf<B, T>::Type;
+
+template<class T>
+struct is_span_oracle : mozilla::FalseType
+{
+};
+
+template<class ElementType, size_t Extent>
+struct is_span_oracle<mozilla::Span<ElementType, Extent>> : mozilla::TrueType
+{
+};
+
+template<class T>
+struct is_span : public is_span_oracle<remove_cv_t<T>>
+{
+};
+
+template<class T>
+struct is_std_array_oracle : mozilla::FalseType
+{
+};
+
+template<class ElementType, size_t Extent>
+struct is_std_array_oracle<std::array<ElementType, Extent>> : mozilla::TrueType
+{
+};
+
+template<class T>
+struct is_std_array : public is_std_array_oracle<remove_cv_t<T>>
+{
+};
+
+template<size_t From, size_t To>
+struct is_allowed_extent_conversion
+  : public mozilla::IntegralConstant<bool,
+                                  From == To ||
+                                    From == mozilla::dynamic_extent ||
+                                    To == mozilla::dynamic_extent>
+{
+};
+
+template<class From, class To>
+struct is_allowed_element_type_conversion
+  : public mozilla::IntegralConstant<bool, mozilla::IsConvertible<From (*)[], To (*)[]>::value>
+{
+};
+
+template<class Span, bool IsConst>
+class span_iterator
+{
+  using element_type_ = typename Span::element_type;
+
+public:
+  using iterator_category = std::random_access_iterator_tag;
+  using value_type = remove_const_t<element_type_>;
+  using difference_type = typename Span::index_type;
+
+  using reference = conditional_t<IsConst, const element_type_, element_type_>&;
+  using pointer = add_pointer_t<reference>;
+
+  constexpr span_iterator() : span_iterator(nullptr, 0) {}
+
+  MOZ_SPAN_ASSERTION_CONSTEXPR span_iterator(const Span* span,
+                                             typename Span::index_type index)
+    : span_(span)
+    , index_(index)
+  {
+    MOZ_RELEASE_ASSERT(span == nullptr ||
+                       (index_ >= 0 && index <= span_->Length()));
+  }
+
+  friend class span_iterator<Span, true>;
+  constexpr MOZ_IMPLICIT span_iterator(const span_iterator<Span, false>& other)
+    : span_iterator(other.span_, other.index_)
+  {
+  }
+
+  MOZ_SPAN_EXPLICITLY_DEFAULTED_CONSTEXPR span_iterator<Span, IsConst>&
+  operator=(const span_iterator<Span, IsConst>&) = default;
+
+  MOZ_SPAN_GCC_CONSTEXPR reference operator*() const
+  {
+    MOZ_RELEASE_ASSERT(span_);
+    return (*span_)[index_];
+  }
+
+  MOZ_SPAN_GCC_CONSTEXPR pointer operator->() const
+  {
+    MOZ_RELEASE_ASSERT(span_);
+    return &((*span_)[index_]);
+  }
+
+  MOZ_SPAN_NON_CONST_CONSTEXPR span_iterator& operator++()
+  {
+    MOZ_RELEASE_ASSERT(span_ && index_ >= 0 && index_ < span_->Length());
+    ++index_;
+    return *this;
+  }
+
+  MOZ_SPAN_NON_CONST_CONSTEXPR span_iterator operator++(int)
+  {
+    auto ret = *this;
+    ++(*this);
+    return ret;
+  }
+
+  MOZ_SPAN_NON_CONST_CONSTEXPR span_iterator& operator--()
+  {
+    MOZ_RELEASE_ASSERT(span_ && index_ > 0 && index_ <= span_->Length());
+    --index_;
+    return *this;
+  }
+
+  MOZ_SPAN_NON_CONST_CONSTEXPR span_iterator operator--(int)
+  {
+    auto ret = *this;
+    --(*this);
+    return ret;
+  }
+
+  MOZ_SPAN_CONSTEXPR_NOT_JUST_RETURN span_iterator
+  operator+(difference_type n) const
+  {
+    auto ret = *this;
+    return ret += n;
+  }
+
+  MOZ_SPAN_GCC_CONSTEXPR span_iterator& operator+=(difference_type n)
+  {
+    MOZ_RELEASE_ASSERT(span_ && (index_ + n) >= 0 &&
+                       (index_ + n) <= span_->Length());
+    index_ += n;
+    return *this;
+  }
+
+  MOZ_SPAN_CONSTEXPR_NOT_JUST_RETURN span_iterator
+  operator-(difference_type n) const
+  {
+    auto ret = *this;
+    return ret -= n;
+  }
+
+  MOZ_SPAN_NON_CONST_CONSTEXPR span_iterator& operator-=(difference_type n)
+
+  {
+    return *this += -n;
+  }
+
+  MOZ_SPAN_GCC_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)
+  {
+    return lhs.span_ == rhs.span_ && lhs.index_ == rhs.index_;
+  }
+
+  constexpr friend bool operator!=(const span_iterator& lhs,
+                                   const span_iterator& rhs)
+  {
+    return !(lhs == rhs);
+  }
+
+  MOZ_SPAN_GCC_CONSTEXPR friend bool operator<(const span_iterator& lhs,
+                                               const span_iterator& rhs)
+  {
+    MOZ_RELEASE_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 Span* 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() {}
+
+  template<index_type Other>
+  MOZ_SPAN_ASSERTION_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);
+  }
+
+  MOZ_SPAN_ASSERTION_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. MakeSpan() functions can be used for explicit conversion in other
+ * contexts. (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.)
+ *
+ * In addition to having constructors and MakeSpan() functions that take
+ * various well-known types, a Span for an arbitrary type can be constructed
+ * (via constructor or MakeSpan()) from a pointer and a length or a pointer
+ * and another pointer pointing just past the last element.
+ *
+ * A Span<const char> can be obtained for const char* pointing to a
+ * zero-terminated C string using the MakeCStringSpan() function. A
+ * corresponding implicit constructor does not exist in order to avoid
+ * accidental construction in cases where const char* does not point to a
+ * zero-terminated C 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().
+ */
+template<class ElementType, size_t Extent>
+class Span
+{
+public:
+  // constants and types
+  using element_type = ElementType;
+  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 "span_details::enable_if_t<(Dependent || Extent == 0 || Extent == mozilla::MaxValue<size_t>::value)>" SFINAE,
+  // since "span_details::enable_if_t<(Extent == 0 || Extent == mozilla::MaxValue<size_t>::value)>" is ill-formed when Extent is neither of the extreme values.
+  /**
+   * Constructor with no args.
+   */
+  template<
+    bool Dependent = false,
+    class = span_details::enable_if_t<
+      (Dependent || Extent == 0 || Extent == mozilla::MaxValue<size_t>::value)>>
+  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 C array.
+   */
+  template<size_t N>
+  constexpr MOZ_IMPLICIT Span(element_type (&aArr)[N])
+    : storage_(&aArr[0], span_details::extent_type<N>())
+  {
+  }
+
+  /**
+   * Constructor for std::array.
+   */
+  template<size_t N,
+           class ArrayElementType = span_details::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<span_details::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 = span_details::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<span_details::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 = span_details::enable_if_t<
+      !span_details::is_span<Container>::value &&
+      !span_details::is_std_array<Container>::value &&
+      mozilla::IsConvertible<typename Container::pointer, pointer>::value &&
+      mozilla::IsConvertible<typename Container::pointer,
+                          decltype(mozilla::DeclVal<Container>().data())>::value>>
+  constexpr MOZ_IMPLICIT Span(Container& cont)
+    : Span(cont.data(), ReleaseAssertedCast<index_type>(cont.size()))
+  {
+  }
+
+  /**
+   * Constructor for standard-library containers (const version).
+   */
+  template<
+    class Container,
+    class = span_details::enable_if_t<
+      mozilla::IsConst<element_type>::value &&
+      !span_details::is_span<Container>::value &&
+      mozilla::IsConvertible<typename Container::pointer, pointer>::value &&
+      mozilla::IsConvertible<typename Container::pointer,
+                          decltype(mozilla::DeclVal<Container>().data())>::value>>
+  constexpr MOZ_IMPLICIT Span(const Container& cont)
+    : Span(cont.data(), ReleaseAssertedCast<index_type>(cont.size()))
+  {
+  }
+
+  /**
+   * 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 = span_details::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 = span_details::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;
+  MOZ_SPAN_EXPLICITLY_DEFAULTED_CONSTEXPR Span& operator=(const Span& other)
+    = default;
+
+  MOZ_SPAN_EXPLICITLY_DEFAULTED_CONSTEXPR Span& operator=(Span&& other)
+    = default;
+
+  // [Span.sub], Span subviews
+  /**
+   * Subspan with first N elements with compile-time N.
+   */
+  template<size_t Count>
+  MOZ_SPAN_GCC_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>
+  MOZ_SPAN_GCC_CONSTEXPR Span<element_type, Count> Last() const
+  {
+    MOZ_RELEASE_ASSERT(Count <= size());
+    return { data() + (size() - Count), Count };
+  }
+
+  /**
+   * Subspan with compile-time start index and length.
+   */
+  template<size_t Offset, size_t Count = dynamic_extent>
+  MOZ_SPAN_GCC_CONSTEXPR Span<element_type, Count> Subspan() const
+  {
+    MOZ_RELEASE_ASSERT(Offset <= size() &&
+      (Count == dynamic_extent || (Offset + Count <= size())));
+    return { data() + Offset,
+             Count == dynamic_extent ? size() - Offset : Count };
+  }
+
+  /**
+   * Subspan with first N elements with run-time N.
+   */
+  MOZ_SPAN_GCC_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.
+   */
+  MOZ_SPAN_GCC_CONSTEXPR Span<element_type, dynamic_extent> Last(
+    index_type aCount) const
+  {
+    MOZ_RELEASE_ASSERT(aCount <= size());
+    return { data() + (size() - aCount), aCount };
+  }
+
+  /**
+   * Subspan with run-time start index and length.
+   */
+  MOZ_SPAN_GCC_CONSTEXPR Span<element_type, dynamic_extent> Subspan(
+    index_type aStart,
+    index_type aLength = dynamic_extent) const
+  {
+    MOZ_RELEASE_ASSERT(aStart <= size() &&
+                       (aLength == dynamic_extent ||
+                        (aStart + aLength <= size())));
+    return { data() + aStart,
+             aLength == dynamic_extent ? size() - aStart : aLength };
+  }
+
+  /**
+   * Subspan with run-time start index. (Rust's &foo[start..])
+   */
+  MOZ_SPAN_GCC_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])
+   */
+  MOZ_SPAN_GCC_CONSTEXPR Span<element_type, dynamic_extent> To(
+    index_type aEnd) const
+  {
+    return Subspan(0, aEnd);
+  }
+
+  /**
+   * Subspan with run-time start index and exclusive end index.
+   * (Rust's &foo[start..end])
+   */
+  MOZ_SPAN_GCC_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
+  MOZ_SPAN_GCC_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.
+   */
+  constexpr pointer Elements() const { return data(); }
+
+  /**
+   * Pointer to the first element of the span (standard-libray duck typing version).
+   */
+  constexpr pointer data() const { return storage_.data(); }
+
+  // [Span.iter], Span iterator support
+  iterator begin() const { return { this, 0 }; }
+  iterator end() const { return { this, Length() }; }
+
+  const_iterator cbegin() const { return { this, 0 }; }
+  const_iterator cend() const { return { this, Length() }; }
+
+  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() };
+  }
+
+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>
+    MOZ_SPAN_ASSERTION_CONSTEXPR storage_type(pointer elements,
+                                              OtherExtentType ext)
+      : ExtentType(ext)
+      , data_(elements)
+    {
+      MOZ_RELEASE_ASSERT(
+        (!elements && ExtentType::size() == 0) ||
+        (elements && ExtentType::size() != mozilla::MaxValue<size_t>::value));
+    }
+
+    constexpr pointer data() const { return data_; }
+
+  private:
+    pointer data_;
+  };
+
+  storage_type<span_details::extent_type<Extent>> storage_;
+};
+
+// [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.begin(), l.end(), r.begin());
+}
+
+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.begin(), l.end(), r.begin(), r.end());
+}
+
+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
+  : mozilla::IntegralConstant<size_t,
+                           static_cast<size_t>(sizeof(ElementType) *
+                                               static_cast<size_t>(Extent))>
+{
+};
+
+template<class ElementType>
+struct calculate_byte_size<ElementType, dynamic_extent>
+  : mozilla::IntegralConstant<size_t, dynamic_extent>
+{
+};
+}
+
+// [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 = span_details::enable_if_t<!mozilla::IsConst<ElementType>::value>>
+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() };
+}
+
+//
+// MakeSpan() - Utility functions for creating Spans
+//
+/**
+ * Create span from pointer and length.
+ */
+template<class ElementType>
+Span<ElementType>
+MakeSpan(ElementType* aPtr, typename Span<ElementType>::index_type aLength)
+{
+  return Span<ElementType>(aPtr, aLength);
+}
+
+/**
+ * Create span from start pointer and pointer past end.
+ */
+template<class ElementType>
+Span<ElementType>
+MakeSpan(ElementType* aStartPtr, ElementType* aEndPtr)
+{
+  return Span<ElementType>(aStartPtr, aEndPtr);
+}
+
+/**
+ * Create span from C array.
+ */
+template<class ElementType, size_t N>
+Span<ElementType> MakeSpan(ElementType (&aArr)[N])
+{
+  return Span<ElementType>(aArr);
+}
+
+/**
+ * Create span from mozilla::Array.
+ */
+template<class ElementType, size_t N>
+Span<ElementType>
+MakeSpan(mozilla::Array<ElementType, N>& aArr)
+{
+  return aArr;
+}
+
+/**
+ * Create span from const mozilla::Array.
+ */
+template<class ElementType, size_t N>
+Span<const ElementType>
+MakeSpan(const mozilla::Array<ElementType, N>& arr)
+{
+  return arr;
+}
+
+/**
+ * Create span from standard-library container.
+ */
+template<class Container>
+Span<typename Container::value_type>
+MakeSpan(Container& cont)
+{
+  return Span<typename Container::value_type>(cont);
+}
+
+/**
+ * Create span from standard-library container (const version).
+ */
+template<class Container>
+Span<const typename Container::value_type>
+MakeSpan(const Container& cont)
+{
+  return Span<const typename Container::value_type>(cont);
+}
+
+/**
+ * Create span from smart pointer and length.
+ */
+template<class Ptr>
+Span<typename Ptr::element_type>
+MakeSpan(Ptr& aPtr, size_t aLength)
+{
+  return Span<typename Ptr::element_type>(aPtr, aLength);
+}
+
+/**
+ * Create span from C string.
+ */
+inline Span<const char>
+MakeCStringSpan(const char* aStr)
+{
+  return Span<const char>(aStr, std::strlen(aStr));
+}
+
+} // namespace mozilla
+
+#ifdef _MSC_VER
+#if _MSC_VER < 1910
+#undef constexpr
+#pragma pop_macro("constexpr")
+
+#endif // _MSC_VER < 1910
+
+#pragma warning(pop)
+#endif // _MSC_VER
+
+#undef MOZ_SPAN_ASSERTION_CONSTEXPR
+#undef MOZ_SPAN_GCC_CONSTEXPR
+#undef MOZ_SPAN_EXPLICITLY_DEFAULTED_CONSTEXPR
+#undef MOZ_SPAN_CONSTEXPR_NOT_JUST_RETURN
+#undef MOZ_SPAN_NON_CONST_CONSTEXPR
+
+#endif // mozilla_Span_h