mfbt/Span.h
author Petru Lingurar <petru.lingurar@softvision.ro>
Fri, 21 Dec 2018 08:56:47 +0000
changeset 501492 65621d0fe1262af0643cec37c23b2d9ec42588ad
parent 478069 7910d46af606344992f232d710794e5a7c686daa
child 507697 b4662b6db1b34414494d070e33481193625403d1
permissions -rw-r--r--
Bug 1513938 - Enforce a Bundle size limit and drop `privateSession` if exceeds it. r=JanH, a=jcristau The `privateSession` key would normally allow persisting the Private Browsing session across OOMs in Activity's Bundle. We need to do that to avoid storing private, sensible data on disk like we do with the normal browsing session. In some cases `privateSession` would contain a lot of data which, along with other possible concurrent transactions could overflow Binder's buffer which has a limited fixed size, currently 1Mb. To avoid this, we will drop `privateSession` from the Bundle if the resulting size is greater than a _speculative_ size of 300KBs which would mean that in the case of an OOM all Private Browsing state would be lost. Bug 1515592 is filed to investigate for a better solution. Differential Revision: https://phabricator.services.mozilla.com/D15067

///////////////////////////////////////////////////////////////////////////////
//
// 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>

#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>(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 = mozilla::MaxValue<size_t>::value;

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

// implementation details
namespace span_details {

inline size_t strlen16(const char16_t* aZeroTerminated) {
  size_t len = 0;
  while (*(aZeroTerminated++)) {
    len++;
  }
  return len;
}

// 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) {}

  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_)
  {
  }

  constexpr span_iterator<Span, IsConst>&
  operator=(const span_iterator<Span, 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++()
  {
    MOZ_RELEASE_ASSERT(span_ && index_ >= 0 && index_ < span_->Length());
    ++index_;
    return *this;
  }

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

  constexpr span_iterator& operator--()
  {
    MOZ_RELEASE_ASSERT(span_ && index_ > 0 && index_ <= span_->Length());
    --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)
  {
    return lhs.span_ == rhs.span_ && lhs.index_ == rhs.index_;
  }

  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_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>
  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. 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. 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 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> 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().
 */
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>())
  {
  }

  // 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.
  Span(char* aStr) = delete;
  Span(const char* aStr) = delete;
  Span(char16_t* aStr) = delete;
  Span(const char16_t* aStr) = delete;

  /**
   * 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;
  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);
  }

  /**
   * 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 }; }
  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>
    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 != 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.
 * MakeSpan() does not permit creating Span objects from string literals (const
 * char or char16_t arrays) because the Span length would include the zero
 * terminator, which may surprise callers. Use MakeStringSpan() to create a
 * Span whose length that excludes the string literal's zero terminator or use
 * the MakeSpan() overload that accepts a pointer and length and specify the
 * string literal's full length.
 */
template<class ElementType, size_t N,
         class = span_details::enable_if_t<
                   !IsSame<ElementType, const char>::value &&
                   !IsSame<ElementType, const char16_t>::value>>
Span<ElementType> MakeSpan(ElementType (&aArr)[N])
{
  return Span<ElementType>(aArr, N);
}

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

/**
 * Create span from a zero-terminated UTF-16 C string. nullptr is
 * treated as the empty string.
 */
inline Span<const char16_t>
MakeStringSpan(const char16_t* aZeroTerminated)
{
  if (!aZeroTerminated) {
    return Span<const char16_t>();
  }
  return Span<const char16_t>(aZeroTerminated, span_details::strlen16(aZeroTerminated));
}

} // 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

#endif // mozilla_Span_h