Adding upstream version 124.0.1.upstream/124.0.1

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

1 files changed, 3376 insertions, 0 deletions

diff --git a/xpcom/ds/nsTArray.h b/xpcom/ds/nsTArray.h
new file mode 100644
index 0000000000..f17624b721
--- /dev/null
+++ b/xpcom/ds/nsTArray.h
@@ -0,0 +1,3376 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsTArray_h__
+#define nsTArray_h__
+
+#include <string.h>
+
+#include <algorithm>
+#include <functional>
+#include <initializer_list>
+#include <iterator>
+#include <new>
+#include <ostream>
+#include <type_traits>
+#include <utility>
+
+#include "mozilla/Alignment.h"
+#include "mozilla/ArrayIterator.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/BinarySearch.h"
+#include "mozilla/CheckedInt.h"
+#include "mozilla/FunctionTypeTraits.h"
+#include "mozilla/MathAlgorithms.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/NotNull.h"
+#include "mozilla/Span.h"
+#include "mozilla/fallible.h"
+#include "mozilla/mozalloc.h"
+#include "nsAlgorithm.h"
+#include "nsDebug.h"
+#include "nsISupports.h"
+#include "nsRegionFwd.h"
+#include "nsTArrayForwardDeclare.h"
+
+namespace JS {
+template <class T>
+class Heap;
+} /* namespace JS */
+
+class nsCycleCollectionTraversalCallback;
+class nsRegion;
+
+namespace mozilla::a11y {
+class BatchData;
+}
+
+namespace mozilla {
+namespace layers {
+class Animation;
+class FrameStats;
+struct PropertyAnimationGroup;
+struct TileClient;
+}  // namespace layers
+}  // namespace mozilla
+
+namespace mozilla {
+struct SerializedStructuredCloneBuffer;
+class SourceBufferTask;
+}  // namespace mozilla
+
+namespace mozilla::dom::binding_detail {
+template <typename, typename>
+class RecordEntry;
+}
+
+namespace mozilla::dom::ipc {
+class StructuredCloneData;
+}  // namespace mozilla::dom::ipc
+
+namespace mozilla::dom {
+class ClonedMessageData;
+class MessageData;
+class MessagePortIdentifier;
+struct MozPluginParameter;
+template <typename T>
+struct Nullable;
+class OwningFileOrDirectory;
+class OwningStringOrBooleanOrObject;
+class OwningUTF8StringOrDouble;
+class Pref;
+class RefMessageData;
+class ResponsiveImageCandidate;
+class ServiceWorkerRegistrationData;
+namespace indexedDB {
+class SerializedStructuredCloneReadInfo;
+class ObjectStoreCursorResponse;
+class IndexCursorResponse;
+}  // namespace indexedDB
+}  // namespace mozilla::dom
+
+namespace mozilla::ipc {
+class ContentSecurityPolicy;
+template <class T>
+class Endpoint;
+}  // namespace mozilla::ipc
+
+class JSStructuredCloneData;
+
+template <class T>
+class RefPtr;
+
+//
+// nsTArray<E> is a resizable array class, like std::vector.
+//
+// Unlike std::vector, which follows C++'s construction/destruction rules,
+// By default, nsTArray assumes that instances of E can be relocated safely
+// using memory utils (memcpy/memmove).
+//
+// The public classes defined in this header are
+//
+//   nsTArray<E>,
+//   CopyableTArray<E>,
+//   FallibleTArray<E>,
+//   AutoTArray<E, N>,
+//   CopyableAutoTArray<E, N>
+//
+// nsTArray, CopyableTArray, AutoTArray and CopyableAutoTArray are infallible by
+// default. To opt-in to fallible behaviour, use the `mozilla::fallible`
+// parameter and check the return value.
+//
+// CopyableTArray and CopyableAutoTArray< are copy-constructible and
+// copy-assignable. Use these only when syntactically necessary to avoid implcit
+// unintentional copies. nsTArray/AutoTArray can be conveniently copied using
+// the Clone() member function. Consider using std::move where possible.
+//
+// If you just want to declare the nsTArray types (e.g., if you're in a header
+// file and don't need the full nsTArray definitions) consider including
+// nsTArrayForwardDeclare.h instead of nsTArray.h.
+//
+// The template parameter E specifies the type of the elements and has the
+// following requirements:
+//
+//   E MUST be safely memmove()'able.
+//   E MUST define a copy-constructor.
+//   E MAY define operator< for sorting.
+//   E MAY define operator== for searching.
+//
+// (Note that the memmove requirement may be relaxed for certain types - see
+// nsTArray_RelocationStrategy below.)
+//
+// There is a public type value_type defined as E within each array class, and
+// we reference the type under this name below.
+//
+// For member functions taking a Comparator instance, Comparator must be either
+// a functor with a tri-state comparison function with a signature compatible to
+//
+//   /** @return negative iff a < b, 0 iff a == b, positive iff a > b */
+//   int (const value_type& a, const value_type& b);
+//
+// or a class defining member functions with signatures compatible to:
+//
+//   class Comparator {
+//     public:
+//       /** @return True if the elements are equals; false otherwise. */
+//       bool Equals(const value_type& a, const value_type& b) const;
+//
+//       /** @return True if (a < b); false otherwise. */
+//       bool LessThan(const value_type& a, const value_type& b) const;
+//   };
+//
+// The Equals member function is used for searching, and the LessThan member
+// function is used for searching and sorting.  Note that some member functions,
+// e.g. Compare, are templates where a different type Item can be used for the
+// element to compare to. In that case, the signatures must be compatible to
+// allow those comparisons, but the details are not documented here.
+//
+
+//
+// nsTArrayFallibleResult and nsTArrayInfallibleResult types are proxy types
+// which are used because you cannot use a templated type which is bound to
+// void as an argument to a void function.  In order to work around that, we
+// encode either a void or a boolean inside these proxy objects, and pass them
+// to the aforementioned function instead, and then use the type information to
+// decide what to do in the function.
+//
+// Note that public nsTArray methods should never return a proxy type.  Such
+// types are only meant to be used in the internal nsTArray helper methods.
+// Public methods returning non-proxy types cannot be called from other
+// nsTArray members.
+//
+struct nsTArrayFallibleResult {
+  // Note: allows implicit conversions from and to bool
+  MOZ_IMPLICIT constexpr nsTArrayFallibleResult(bool aResult)
+      : mResult(aResult) {}
+
+  MOZ_IMPLICIT constexpr operator bool() { return mResult; }
+
+ private:
+  bool mResult;
+};
+
+struct nsTArrayInfallibleResult {};
+
+//
+// nsTArray*Allocators must all use the same |free()|, to allow swap()'ing
+// between fallible and infallible variants.
+//
+
+struct nsTArrayFallibleAllocatorBase {
+  typedef bool ResultType;
+  typedef nsTArrayFallibleResult ResultTypeProxy;
+
+  static constexpr ResultType Result(ResultTypeProxy aResult) {
+    return aResult;
+  }
+  static constexpr bool Successful(ResultTypeProxy aResult) { return aResult; }
+  static constexpr ResultTypeProxy SuccessResult() { return true; }
+  static constexpr ResultTypeProxy FailureResult() { return false; }
+  static constexpr ResultType ConvertBoolToResultType(bool aValue) {
+    return aValue;
+  }
+};
+
+struct nsTArrayInfallibleAllocatorBase {
+  typedef void ResultType;
+  typedef nsTArrayInfallibleResult ResultTypeProxy;
+
+  static constexpr ResultType Result(ResultTypeProxy aResult) {}
+  static constexpr bool Successful(ResultTypeProxy) { return true; }
+  static constexpr ResultTypeProxy SuccessResult() { return ResultTypeProxy(); }
+
+  [[noreturn]] static ResultTypeProxy FailureResult() {
+    MOZ_CRASH("Infallible nsTArray should never fail");
+  }
+
+  template <typename T>
+  static constexpr ResultType ConvertBoolToResultType(T aValue) {
+    if (!aValue) {
+      MOZ_CRASH("infallible nsTArray should never convert false to ResultType");
+    }
+  }
+
+  template <typename T>
+  static constexpr ResultType ConvertBoolToResultType(
+      const mozilla::NotNull<T>& aValue) {}
+};
+
+struct nsTArrayFallibleAllocator : nsTArrayFallibleAllocatorBase {
+  static void* Malloc(size_t aSize) { return malloc(aSize); }
+  static void* Realloc(void* aPtr, size_t aSize) {
+    return realloc(aPtr, aSize);
+  }
+
+  static void Free(void* aPtr) { free(aPtr); }
+  static void SizeTooBig(size_t) {}
+};
+
+struct nsTArrayInfallibleAllocator : nsTArrayInfallibleAllocatorBase {
+  static void* Malloc(size_t aSize) MOZ_NONNULL_RETURN {
+    return moz_xmalloc(aSize);
+  }
+  static void* Realloc(void* aPtr, size_t aSize) MOZ_NONNULL_RETURN {
+    return moz_xrealloc(aPtr, aSize);
+  }
+
+  static void Free(void* aPtr) { free(aPtr); }
+  static void SizeTooBig(size_t aSize) { NS_ABORT_OOM(aSize); }
+};
+
+// nsTArray_base stores elements into the space allocated beyond
+// sizeof(*this).  This is done to minimize the size of the nsTArray
+// object when it is empty.
+struct nsTArrayHeader {
+  uint32_t mLength;
+  uint32_t mCapacity : 31;
+  uint32_t mIsAutoArray : 1;
+};
+
+extern "C" {
+extern const nsTArrayHeader sEmptyTArrayHeader;
+}
+
+namespace detail {
+// nsTArray_CopyDisabler disables copy operations.
+class nsTArray_CopyDisabler {
+ public:
+  nsTArray_CopyDisabler() = default;
+
+  nsTArray_CopyDisabler(const nsTArray_CopyDisabler&) = delete;
+  nsTArray_CopyDisabler& operator=(const nsTArray_CopyDisabler&) = delete;
+};
+
+}  // namespace detail
+
+// This class provides a SafeElementAt method to nsTArray<E*> which does
+// not take a second default value parameter.
+template <class E, class Derived>
+struct nsTArray_SafeElementAtHelper : public ::detail::nsTArray_CopyDisabler {
+  typedef E* elem_type;
+  typedef size_t index_type;
+
+  // No implementation is provided for these two methods, and that is on
+  // purpose, since we don't support these functions on non-pointer type
+  // instantiations.
+  elem_type& SafeElementAt(index_type aIndex);
+  const elem_type& SafeElementAt(index_type aIndex) const;
+};
+
+template <class E, class Derived>
+struct nsTArray_SafeElementAtHelper<E*, Derived>
+    : public ::detail::nsTArray_CopyDisabler {
+  typedef E* elem_type;
+  // typedef const E* const_elem_type;   XXX: see below
+  typedef size_t index_type;
+
+  elem_type SafeElementAt(index_type aIndex) {
+    return static_cast<Derived*>(this)->SafeElementAt(aIndex, nullptr);
+  }
+
+  // XXX: Probably should return const_elem_type, but callsites must be fixed.
+  // Also, the use of const_elem_type for nsTArray<xpcGCCallback> in
+  // xpcprivate.h causes build failures on Windows because xpcGCCallback is a
+  // function pointer and MSVC doesn't like qualifying it with |const|.
+  elem_type SafeElementAt(index_type aIndex) const {
+    return static_cast<const Derived*>(this)->SafeElementAt(aIndex, nullptr);
+  }
+};
+
+// E is a smart pointer type; the
+// smart pointer can act as its element_type*.
+template <class E, class Derived>
+struct nsTArray_SafeElementAtSmartPtrHelper
+    : public ::detail::nsTArray_CopyDisabler {
+  typedef typename E::element_type* elem_type;
+  typedef const typename E::element_type* const_elem_type;
+  typedef size_t index_type;
+
+  elem_type SafeElementAt(index_type aIndex) {
+    auto* derived = static_cast<Derived*>(this);
+    if (aIndex < derived->Length()) {
+      return derived->Elements()[aIndex];
+    }
+    return nullptr;
+  }
+
+  // XXX: Probably should return const_elem_type, but callsites must be fixed.
+  elem_type SafeElementAt(index_type aIndex) const {
+    auto* derived = static_cast<const Derived*>(this);
+    if (aIndex < derived->Length()) {
+      return derived->Elements()[aIndex];
+    }
+    return nullptr;
+  }
+};
+
+template <class T>
+class nsCOMPtr;
+
+template <class E, class Derived>
+struct nsTArray_SafeElementAtHelper<nsCOMPtr<E>, Derived>
+    : public nsTArray_SafeElementAtSmartPtrHelper<nsCOMPtr<E>, Derived> {};
+
+template <class E, class Derived>
+struct nsTArray_SafeElementAtHelper<RefPtr<E>, Derived>
+    : public nsTArray_SafeElementAtSmartPtrHelper<RefPtr<E>, Derived> {};
+
+namespace mozilla {
+template <class T>
+class OwningNonNull;
+}  // namespace mozilla
+
+template <class E, class Derived>
+struct nsTArray_SafeElementAtHelper<mozilla::OwningNonNull<E>, Derived>
+    : public nsTArray_SafeElementAtSmartPtrHelper<mozilla::OwningNonNull<E>,
+                                                  Derived> {};
+
+// Servo bindings.
+extern "C" void Gecko_EnsureTArrayCapacity(void* aArray, size_t aCapacity,
+                                           size_t aElementSize);
+extern "C" void Gecko_ClearPODTArray(void* aArray, size_t aElementSize,
+                                     size_t aElementAlign);
+
+//
+// This class serves as a base class for nsTArray.  It shouldn't be used
+// directly.  It holds common implementation code that does not depend on the
+// element type of the nsTArray.
+//
+template <class Alloc, class RelocationStrategy>
+class nsTArray_base {
+  // Allow swapping elements with |nsTArray_base|s created using a
+  // different allocator.  This is kosher because all allocators use
+  // the same free().
+  template <class XAlloc, class XRelocationStrategy>
+  friend class nsTArray_base;
+
+  // Needed for AppendElements from an array with a different allocator, which
+  // calls ShiftData.
+  template <class E, class XAlloc>
+  friend class nsTArray_Impl;
+
+  friend void Gecko_EnsureTArrayCapacity(void* aArray, size_t aCapacity,
+                                         size_t aElemSize);
+  friend void Gecko_ClearPODTArray(void* aTArray, size_t aElementSize,
+                                   size_t aElementAlign);
+
+ protected:
+  typedef nsTArrayHeader Header;
+
+ public:
+  typedef size_t size_type;
+  typedef size_t index_type;
+
+  // @return The number of elements in the array.
+  size_type Length() const { return mHdr->mLength; }
+
+  // @return True if the array is empty or false otherwise.
+  bool IsEmpty() const { return Length() == 0; }
+
+  // @return The number of elements that can fit in the array without forcing
+  // the array to be re-allocated.  The length of an array is always less
+  // than or equal to its capacity.
+  size_type Capacity() const { return mHdr->mCapacity; }
+
+#ifdef DEBUG
+  void* DebugGetHeader() const { return mHdr; }
+#endif
+
+ protected:
+  nsTArray_base();
+
+  ~nsTArray_base();
+
+  nsTArray_base(const nsTArray_base&);
+  nsTArray_base& operator=(const nsTArray_base&);
+
+  // Resize the storage if necessary to achieve the requested capacity.
+  // @param aCapacity The requested number of array elements.
+  // @param aElemSize The size of an array element.
+  // @return False if insufficient memory is available; true otherwise.
+  template <typename ActualAlloc>
+  typename ActualAlloc::ResultTypeProxy EnsureCapacity(size_type aCapacity,
+                                                       size_type aElemSize) {
+    // Do this check here so that our callers can inline it.
+    if (aCapacity <= mHdr->mCapacity) {
+      return ActualAlloc::SuccessResult();
+    }
+    return EnsureCapacityImpl<ActualAlloc>(aCapacity, aElemSize);
+  }
+
+  // The rest of EnsureCapacity. Should only be called if aCapacity >
+  // mHdr->mCapacity.
+  template <typename ActualAlloc>
+  typename ActualAlloc::ResultTypeProxy EnsureCapacityImpl(size_type aCapacity,
+                                                           size_type aElemSize);
+
+  // Extend the storage to accommodate aCount extra elements.
+  // @param aLength The current size of the array.
+  // @param aCount The number of elements to add.
+  // @param aElemSize The size of an array element.
+  // @return False if insufficient memory is available or the new length
+  //   would overflow; true otherwise.
+  template <typename ActualAlloc>
+  typename ActualAlloc::ResultTypeProxy ExtendCapacity(size_type aLength,
+                                                       size_type aCount,
+                                                       size_type aElemSize);
+
+  // Tries to resize the storage to the minimum required amount. If this fails,
+  // the array is left as-is.
+  // @param aElemSize  The size of an array element.
+  // @param aElemAlign The alignment in bytes of an array element.
+  void ShrinkCapacity(size_type aElemSize, size_t aElemAlign);
+
+  // Resizes the storage to 0. This may only be called when Length() is already
+  // 0.
+  // @param aElemSize  The size of an array element.
+  // @param aElemAlign The alignment in bytes of an array element.
+  void ShrinkCapacityToZero(size_type aElemSize, size_t aElemAlign);
+
+  // This method may be called to resize a "gap" in the array by shifting
+  // elements around.  It updates mLength appropriately.  If the resulting
+  // array has zero elements, then the array's memory is free'd.
+  // @param aStart     The starting index of the gap.
+  // @param aOldLen    The current length of the gap.
+  // @param aNewLen    The desired length of the gap.
+  // @param aElemSize  The size of an array element.
+  // @param aElemAlign The alignment in bytes of an array element.
+  template <typename ActualAlloc>
+  void ShiftData(index_type aStart, size_type aOldLen, size_type aNewLen,
+                 size_type aElemSize, size_t aElemAlign);
+
+  // This method may be called to swap elements from the end of the array to
+  // fill a "gap" in the array. If the resulting array has zero elements, then
+  // the array's memory is free'd.
+  // @param aStart     The starting index of the gap.
+  // @param aCount     The length of the gap.
+  // @param aElemSize  The size of an array element.
+  // @param aElemAlign The alignment in bytes of an array element.
+  template <typename ActualAlloc>
+  void SwapFromEnd(index_type aStart, size_type aCount, size_type aElemSize,
+                   size_t aElemAlign);
+
+  // This method increments the length member of the array's header.
+  // Note that mHdr may actually be sEmptyTArrayHeader in the case where a
+  // zero-length array is inserted into our array. But then aNum should
+  // always be 0.
+  void IncrementLength(size_t aNum) {
+    if (HasEmptyHeader()) {
+      if (MOZ_UNLIKELY(aNum != 0)) {
+        // Writing a non-zero length to the empty header would be extremely bad.
+        MOZ_CRASH();
+      }
+    } else {
+      mHdr->mLength += aNum;
+    }
+  }
+
+  // This method inserts blank slots into the array.
+  // @param aIndex the place to insert the new elements. This must be no
+  //               greater than the current length of the array.
+  // @param aCount the number of slots to insert
+  // @param aElementSize the size of an array element.
+  // @param aElemAlign the alignment in bytes of an array element.
+  template <typename ActualAlloc>
+  typename ActualAlloc::ResultTypeProxy InsertSlotsAt(index_type aIndex,
+                                                      size_type aCount,
+                                                      size_type aElementSize,
+                                                      size_t aElemAlign);
+
+  template <typename ActualAlloc, class Allocator>
+  typename ActualAlloc::ResultTypeProxy SwapArrayElements(
+      nsTArray_base<Allocator, RelocationStrategy>& aOther, size_type aElemSize,
+      size_t aElemAlign);
+
+  template <class Allocator>
+  void MoveConstructNonAutoArray(
+      nsTArray_base<Allocator, RelocationStrategy>& aOther, size_type aElemSize,
+      size_t aElemAlign);
+
+  template <class Allocator>
+  void MoveInit(nsTArray_base<Allocator, RelocationStrategy>& aOther,
+                size_type aElemSize, size_t aElemAlign);
+
+  // This is an RAII class used in SwapArrayElements.
+  class IsAutoArrayRestorer {
+   public:
+    IsAutoArrayRestorer(nsTArray_base<Alloc, RelocationStrategy>& aArray,
+                        size_t aElemAlign);
+    ~IsAutoArrayRestorer();
+
+   private:
+    nsTArray_base<Alloc, RelocationStrategy>& mArray;
+    size_t mElemAlign;
+    bool mIsAuto;
+  };
+
+  // Helper function for SwapArrayElements. Ensures that if the array
+  // is an AutoTArray that it doesn't use the built-in buffer.
+  template <typename ActualAlloc>
+  bool EnsureNotUsingAutoArrayBuffer(size_type aElemSize);
+
+  // Returns true if this nsTArray is an AutoTArray with a built-in buffer.
+  bool IsAutoArray() const { return mHdr->mIsAutoArray; }
+
+  // Returns a Header for the built-in buffer of this AutoTArray.
+  Header* GetAutoArrayBuffer(size_t aElemAlign) {
+    MOZ_ASSERT(IsAutoArray(), "Should be an auto array to call this");
+    return GetAutoArrayBufferUnsafe(aElemAlign);
+  }
+  const Header* GetAutoArrayBuffer(size_t aElemAlign) const {
+    MOZ_ASSERT(IsAutoArray(), "Should be an auto array to call this");
+    return GetAutoArrayBufferUnsafe(aElemAlign);
+  }
+
+  // Returns a Header for the built-in buffer of this AutoTArray, but doesn't
+  // assert that we are an AutoTArray.
+  Header* GetAutoArrayBufferUnsafe(size_t aElemAlign) {
+    return const_cast<Header*>(
+        static_cast<const nsTArray_base<Alloc, RelocationStrategy>*>(this)
+            ->GetAutoArrayBufferUnsafe(aElemAlign));
+  }
+  const Header* GetAutoArrayBufferUnsafe(size_t aElemAlign) const;
+
+  // Returns true if this is an AutoTArray and it currently uses the
+  // built-in buffer to store its elements.
+  bool UsesAutoArrayBuffer() const;
+
+  // The array's elements (prefixed with a Header).  This pointer is never
+  // null.  If the array is empty, then this will point to sEmptyTArrayHeader.
+  Header* mHdr;
+
+  Header* Hdr() const MOZ_NONNULL_RETURN { return mHdr; }
+  Header** PtrToHdr() MOZ_NONNULL_RETURN { return &mHdr; }
+  static Header* EmptyHdr() MOZ_NONNULL_RETURN {
+    return const_cast<Header*>(&sEmptyTArrayHeader);
+  }
+
+  [[nodiscard]] bool HasEmptyHeader() const { return mHdr == EmptyHdr(); }
+};
+
+namespace detail {
+
+// Used for argument checking in nsTArrayElementTraits::Emplace.
+template <typename... T>
+struct ChooseFirst;
+
+template <>
+struct ChooseFirst<> {
+  // Choose a default type that is guaranteed to not match E* for any
+  // nsTArray<E>.
+  typedef void Type;
+};
+
+template <typename A, typename... Args>
+struct ChooseFirst<A, Args...> {
+  typedef A Type;
+};
+
+}  // namespace detail
+
+//
+// This class defines convenience functions for element specific operations.
+// Specialize this template if necessary.
+//
+template <class E>
+class nsTArrayElementTraits {
+ public:
+  // Invoke the default constructor in place.
+  static inline void Construct(E* aE) {
+    // Do NOT call "E()"! That triggers C++ "default initialization"
+    // which zeroes out POD ("plain old data") types such as regular
+    // ints.  We don't want that because it can be a performance issue
+    // and people don't expect it; nsTArray should work like a regular
+    // C/C++ array in this respect.
+    new (static_cast<void*>(aE)) E;
+  }
+  // Invoke the copy-constructor in place.
+  template <class A>
+  static inline void Construct(E* aE, A&& aArg) {
+    using E_NoCV = std::remove_cv_t<E>;
+    using A_NoCV = std::remove_cv_t<A>;
+    static_assert(!std::is_same_v<E_NoCV*, A_NoCV>,
+                  "For safety, we disallow constructing nsTArray<E> elements "
+                  "from E* pointers. See bug 960591.");
+    new (static_cast<void*>(aE)) E(std::forward<A>(aArg));
+  }
+  // Construct in place.
+  template <class... Args>
+  static inline void Emplace(E* aE, Args&&... aArgs) {
+    using E_NoCV = std::remove_cv_t<E>;
+    using A_NoCV =
+        std::remove_cv_t<typename ::detail::ChooseFirst<Args...>::Type>;
+    static_assert(!std::is_same_v<E_NoCV*, A_NoCV>,
+                  "For safety, we disallow constructing nsTArray<E> elements "
+                  "from E* pointers. See bug 960591.");
+    new (static_cast<void*>(aE)) E(std::forward<Args>(aArgs)...);
+  }
+  // Invoke the destructor in place.
+  static inline void Destruct(E* aE) { aE->~E(); }
+};
+
+// The default comparator used by nsTArray
+template <class A, class B>
+class nsDefaultComparator {
+ public:
+  bool Equals(const A& aA, const B& aB) const { return aA == aB; }
+  bool LessThan(const A& aA, const B& aB) const { return aA < aB; }
+};
+
+template <bool IsTriviallyCopyConstructible, bool IsSameType>
+struct AssignRangeAlgorithm {
+  template <class Item, class ElemType, class IndexType, class SizeType>
+  static void implementation(ElemType* aElements, IndexType aStart,
+                             SizeType aCount, const Item* aValues) {
+    ElemType* iter = aElements + aStart;
+    ElemType* end = iter + aCount;
+    for (; iter != end; ++iter, ++aValues) {
+      nsTArrayElementTraits<ElemType>::Construct(iter, *aValues);
+    }
+  }
+};
+
+template <>
+struct AssignRangeAlgorithm<true, true> {
+  template <class Item, class ElemType, class IndexType, class SizeType>
+  static void implementation(ElemType* aElements, IndexType aStart,
+                             SizeType aCount, const Item* aValues) {
+    if (aValues) {
+      memcpy(aElements + aStart, aValues, aCount * sizeof(ElemType));
+    }
+  }
+};
+
+//
+// Normally elements are copied with memcpy and memmove, but for some element
+// types that is problematic.  The nsTArray_RelocationStrategy template class
+// can be specialized to ensure that copying calls constructors and destructors
+// instead, as is done below for JS::Heap<E> elements.
+//
+
+//
+// A class that defines how to copy elements using memcpy/memmove.
+//
+struct nsTArray_RelocateUsingMemutils {
+  const static bool allowRealloc = true;
+
+  static void RelocateNonOverlappingRegionWithHeader(void* aDest,
+                                                     const void* aSrc,
+                                                     size_t aCount,
+                                                     size_t aElemSize) {
+    memcpy(aDest, aSrc, sizeof(nsTArrayHeader) + aCount * aElemSize);
+  }
+
+  static void RelocateOverlappingRegion(void* aDest, void* aSrc, size_t aCount,
+                                        size_t aElemSize) {
+    memmove(aDest, aSrc, aCount * aElemSize);
+  }
+
+  static void RelocateNonOverlappingRegion(void* aDest, void* aSrc,
+                                           size_t aCount, size_t aElemSize) {
+    memcpy(aDest, aSrc, aCount * aElemSize);
+  }
+};
+
+//
+// A template class that defines how to relocate elements using the type's move
+// constructor and destructor appropriately.
+//
+template <class ElemType>
+struct nsTArray_RelocateUsingMoveConstructor {
+  typedef nsTArrayElementTraits<ElemType> traits;
+
+  const static bool allowRealloc = false;
+
+  static void RelocateNonOverlappingRegionWithHeader(void* aDest, void* aSrc,
+                                                     size_t aCount,
+                                                     size_t aElemSize) {
+    nsTArrayHeader* destHeader = static_cast<nsTArrayHeader*>(aDest);
+    nsTArrayHeader* srcHeader = static_cast<nsTArrayHeader*>(aSrc);
+    *destHeader = *srcHeader;
+    RelocateNonOverlappingRegion(
+        static_cast<uint8_t*>(aDest) + sizeof(nsTArrayHeader),
+        static_cast<uint8_t*>(aSrc) + sizeof(nsTArrayHeader), aCount,
+        aElemSize);
+  }
+
+  // RelocateNonOverlappingRegion and RelocateOverlappingRegion are defined by
+  // analogy with memmove and memcpy that are used for relocation of
+  // trivially-relocatable types through nsTArray_RelocateUsingMemutils. What
+  // they actually do is slightly different: RelocateOverlappingRegion checks to
+  // see which direction the movement needs to take place, whether from
+  // back-to-front of the range to be moved or from front-to-back.
+  // RelocateNonOverlappingRegion assumes that relocating front-to-back is
+  // always valid.  They use RelocateRegionForward and RelocateRegionBackward,
+  // which are analogous to std::move and std::move_backward respectively,
+  // except they don't move-assign the destination from the source but
+  // move-construct the destination from the source and destroy the source.
+  static void RelocateOverlappingRegion(void* aDest, void* aSrc, size_t aCount,
+                                        size_t aElemSize) {
+    ElemType* destBegin = static_cast<ElemType*>(aDest);
+    ElemType* srcBegin = static_cast<ElemType*>(aSrc);
+
+    // If destination and source are the same, this is a no-op.
+    // In practice, we don't do this.
+    if (destBegin == srcBegin) {
+      return;
+    }
+
+    ElemType* srcEnd = srcBegin + aCount;
+    ElemType* destEnd = destBegin + aCount;
+
+    // Figure out whether to relocate back-to-front or front-to-back.
+    if (srcEnd > destBegin && srcEnd < destEnd) {
+      RelocateRegionBackward(srcBegin, srcEnd, destEnd);
+    } else {
+      RelocateRegionForward(srcBegin, srcEnd, destBegin);
+    }
+  }
+
+  static void RelocateNonOverlappingRegion(void* aDest, void* aSrc,
+                                           size_t aCount, size_t aElemSize) {
+    ElemType* destBegin = static_cast<ElemType*>(aDest);
+    ElemType* srcBegin = static_cast<ElemType*>(aSrc);
+    ElemType* srcEnd = srcBegin + aCount;
+#ifdef DEBUG
+    ElemType* destEnd = destBegin + aCount;
+    MOZ_ASSERT(srcEnd <= destBegin || srcBegin >= destEnd);
+#endif
+    RelocateRegionForward(srcBegin, srcEnd, destBegin);
+  }
+
+ private:
+  static void RelocateRegionForward(ElemType* srcBegin, ElemType* srcEnd,
+                                    ElemType* destBegin) {
+    ElemType* srcElem = srcBegin;
+    ElemType* destElem = destBegin;
+
+    while (srcElem != srcEnd) {
+      RelocateElement(srcElem, destElem);
+      ++destElem;
+      ++srcElem;
+    }
+  }
+
+  static void RelocateRegionBackward(ElemType* srcBegin, ElemType* srcEnd,
+                                     ElemType* destEnd) {
+    ElemType* srcElem = srcEnd;
+    ElemType* destElem = destEnd;
+    while (srcElem != srcBegin) {
+      --destElem;
+      --srcElem;
+      RelocateElement(srcElem, destElem);
+    }
+  }
+
+  static void RelocateElement(ElemType* srcElem, ElemType* destElem) {
+    traits::Construct(destElem, std::move(*srcElem));
+    traits::Destruct(srcElem);
+  }
+};
+
+//
+// The default behaviour is to use memcpy/memmove for everything.
+//
+template <class E>
+struct MOZ_NEEDS_MEMMOVABLE_TYPE nsTArray_RelocationStrategy {
+  using Type = nsTArray_RelocateUsingMemutils;
+};
+
+//
+// Some classes require constructors/destructors to be called, so they are
+// specialized here.
+//
+#define MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(E)     \
+  template <>                                              \
+  struct nsTArray_RelocationStrategy<E> {                  \
+    using Type = nsTArray_RelocateUsingMoveConstructor<E>; \
+  };
+
+#define MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR_FOR_TEMPLATE(T) \
+  template <typename S>                                             \
+  struct nsTArray_RelocationStrategy<T<S>> {                        \
+    using Type = nsTArray_RelocateUsingMoveConstructor<T<S>>;       \
+  };
+
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR_FOR_TEMPLATE(JS::Heap)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR_FOR_TEMPLATE(std::function)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR_FOR_TEMPLATE(mozilla::ipc::Endpoint)
+
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(nsRegion)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(nsIntRegion)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(mozilla::layers::TileClient)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(
+    mozilla::SerializedStructuredCloneBuffer)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(
+    mozilla::dom::ipc::StructuredCloneData)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(mozilla::dom::ClonedMessageData)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(
+    mozilla::dom::indexedDB::ObjectStoreCursorResponse)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(
+    mozilla::dom::indexedDB::IndexCursorResponse)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(
+    mozilla::dom::indexedDB::SerializedStructuredCloneReadInfo);
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(JSStructuredCloneData)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(mozilla::dom::MessageData)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(mozilla::dom::RefMessageData)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(mozilla::SourceBufferTask)
+
+//
+// Base class for nsTArray_Impl that is templated on element type and derived
+// nsTArray_Impl class, to allow extra conversions to be added for specific
+// types.
+//
+template <class E, class Derived>
+struct nsTArray_TypedBase : public nsTArray_SafeElementAtHelper<E, Derived> {};
+
+//
+// Specialization of nsTArray_TypedBase for arrays containing JS::Heap<E>
+// elements.
+//
+// These conversions are safe because JS::Heap<E> and E share the same
+// representation, and since the result of the conversions are const references
+// we won't miss any barriers.
+//
+// The static_cast is necessary to obtain the correct address for the derived
+// class since we are a base class used in multiple inheritance.
+//
+template <class E, class Derived>
+struct nsTArray_TypedBase<JS::Heap<E>, Derived>
+    : public nsTArray_SafeElementAtHelper<JS::Heap<E>, Derived> {
+  operator const nsTArray<E>&() {
+    static_assert(sizeof(E) == sizeof(JS::Heap<E>),
+                  "JS::Heap<E> must be binary compatible with E.");
+    Derived* self = static_cast<Derived*>(this);
+    return *reinterpret_cast<nsTArray<E>*>(self);
+  }
+
+  operator const FallibleTArray<E>&() {
+    Derived* self = static_cast<Derived*>(this);
+    return *reinterpret_cast<FallibleTArray<E>*>(self);
+  }
+};
+
+namespace detail {
+
+// These helpers allow us to differentiate between tri-state comparator
+// functions and classes with LessThan() and Equal() methods. If an object, when
+// called as a function with two instances of our element type, returns an int,
+// we treat it as a tri-state comparator.
+//
+// T is the type of the comparator object we want to check. U is the array
+// element type that we'll be comparing.
+//
+// V is never passed, and is only used to allow us to specialize on the return
+// value of the comparator function.
+template <typename T, typename U, typename V = int>
+struct IsCompareMethod : std::false_type {};
+
+template <typename T, typename U>
+struct IsCompareMethod<
+    T, U, decltype(std::declval<T>()(std::declval<U>(), std::declval<U>()))>
+    : std::true_type {};
+
+// These two wrappers allow us to use either a tri-state comparator, or an
+// object with Equals() and LessThan() methods interchangeably. They provide a
+// tri-state Compare() method, and Equals() method, and a LessThan() method.
+//
+// Depending on the type of the underlying comparator, they either pass these
+// through directly, or synthesize them from the methods available on the
+// comparator.
+//
+// Callers should always use the most-specific of these methods that match their
+// purpose.
+
+// Comparator wrapper for a tri-state comparator function
+template <typename T, typename U, bool IsCompare = IsCompareMethod<T, U>::value>
+struct CompareWrapper {
+#ifdef _MSC_VER
+#  pragma warning(push)
+#  pragma warning(disable : 4180) /* Silence "qualifier applied to function \
+                                     type has no meaning" warning */
+#endif
+  MOZ_IMPLICIT CompareWrapper(const T& aComparator)
+      : mComparator(aComparator) {}
+
+  template <typename A, typename B>
+  int Compare(A& aLeft, B& aRight) const {
+    return mComparator(aLeft, aRight);
+  }
+
+  template <typename A, typename B>
+  bool Equals(A& aLeft, B& aRight) const {
+    return Compare(aLeft, aRight) == 0;
+  }
+
+  template <typename A, typename B>
+  bool LessThan(A& aLeft, B& aRight) const {
+    return Compare(aLeft, aRight) < 0;
+  }
+
+  const T& mComparator;
+#ifdef _MSC_VER
+#  pragma warning(pop)
+#endif
+};
+
+// Comparator wrapper for a class with Equals() and LessThan() methods.
+template <typename T, typename U>
+struct CompareWrapper<T, U, false> {
+  MOZ_IMPLICIT CompareWrapper(const T& aComparator)
+      : mComparator(aComparator) {}
+
+  template <typename A, typename B>
+  int Compare(A& aLeft, B& aRight) const {
+    if (LessThan(aLeft, aRight)) {
+      return -1;
+    }
+    if (Equals(aLeft, aRight)) {
+      return 0;
+    }
+    return 1;
+  }
+
+  template <typename A, typename B>
+  bool Equals(A& aLeft, B& aRight) const {
+    return mComparator.Equals(aLeft, aRight);
+  }
+
+  template <typename A, typename B>
+  bool LessThan(A& aLeft, B& aRight) const {
+    return mComparator.LessThan(aLeft, aRight);
+  }
+
+  const T& mComparator;
+};
+
+}  // namespace detail
+
+//
+// nsTArray_Impl contains most of the guts supporting nsTArray, FallibleTArray,
+// AutoTArray.
+//
+// The only situation in which you might need to use nsTArray_Impl in your code
+// is if you're writing code which mutates a TArray which may or may not be
+// infallible.
+//
+// Code which merely reads from a TArray which may or may not be infallible can
+// simply cast the TArray to |const nsTArray&|; both fallible and infallible
+// TArrays can be cast to |const nsTArray&|.
+//
+template <class E, class Alloc>
+class nsTArray_Impl
+    : public nsTArray_base<Alloc,
+                           typename nsTArray_RelocationStrategy<E>::Type>,
+      public nsTArray_TypedBase<E, nsTArray_Impl<E, Alloc>> {
+ private:
+  friend class nsTArray<E>;
+
+  typedef nsTArrayFallibleAllocator FallibleAlloc;
+  typedef nsTArrayInfallibleAllocator InfallibleAlloc;
+
+ public:
+  typedef typename nsTArray_RelocationStrategy<E>::Type relocation_type;
+  typedef nsTArray_base<Alloc, relocation_type> base_type;
+  typedef typename base_type::size_type size_type;
+  typedef typename base_type::index_type index_type;
+  typedef E value_type;
+  typedef nsTArray_Impl<E, Alloc> self_type;
+  typedef nsTArrayElementTraits<E> elem_traits;
+  typedef nsTArray_SafeElementAtHelper<E, self_type> safeelementat_helper_type;
+  typedef mozilla::ArrayIterator<value_type&, self_type> iterator;
+  typedef mozilla::ArrayIterator<const value_type&, self_type> const_iterator;
+  typedef std::reverse_iterator<iterator> reverse_iterator;
+  typedef std::reverse_iterator<const_iterator> const_reverse_iterator;
+
+  using base_type::EmptyHdr;
+  using safeelementat_helper_type::SafeElementAt;
+
+  // A special value that is used to indicate an invalid or unknown index
+  // into the array.
+  static const index_type NoIndex = index_type(-1);
+
+  using base_type::Length;
+
+  //
+  // Finalization method
+  //
+
+  ~nsTArray_Impl() {
+    if (!base_type::IsEmpty()) {
+      ClearAndRetainStorage();
+    }
+    // mHdr cleanup will be handled by base destructor
+  }
+
+  //
+  // Initialization methods
+  //
+
+  nsTArray_Impl() = default;
+
+  // Initialize this array and pre-allocate some number of elements.
+  explicit nsTArray_Impl(size_type aCapacity) { SetCapacity(aCapacity); }
+
+  // Initialize this array with an r-value.
+  // Allow different types of allocators, since the allocator doesn't matter.
+  template <typename Allocator>
+  explicit nsTArray_Impl(nsTArray_Impl<E, Allocator>&& aOther) noexcept {
+    // We cannot be a (Copyable)AutoTArray because that overrides this ctor.
+    MOZ_ASSERT(!this->IsAutoArray());
+
+    // This does not use SwapArrayElements because that's unnecessarily complex.
+    this->MoveConstructNonAutoArray(aOther, sizeof(value_type),
+                                    MOZ_ALIGNOF(value_type));
+  }
+
+  // The array's copy-constructor performs a 'deep' copy of the given array.
+  // @param aOther The array object to copy.
+  //
+  // It's very important that we declare this method as taking |const
+  // self_type&| as opposed to taking |const nsTArray_Impl<E, OtherAlloc>| for
+  // an arbitrary OtherAlloc.
+  //
+  // If we don't declare a constructor taking |const self_type&|, C++ generates
+  // a copy-constructor for this class which merely copies the object's
+  // members, which is obviously wrong.
+  //
+  // You can pass an nsTArray_Impl<E, OtherAlloc> to this method because
+  // nsTArray_Impl<E, X> can be cast to const nsTArray_Impl<E, Y>&.  So the
+  // effect on the API is the same as if we'd declared this method as taking
+  // |const nsTArray_Impl<E, OtherAlloc>&|.
+  nsTArray_Impl(const nsTArray_Impl&) = default;
+
+  // Allow converting to a const array with a different kind of allocator,
+  // Since the allocator doesn't matter for const arrays
+  template <typename Allocator>
+  [[nodiscard]] operator const nsTArray_Impl<E, Allocator>&() const& {
+    return *reinterpret_cast<const nsTArray_Impl<E, Allocator>*>(this);
+  }
+  // And we have to do this for our subclasses too
+  [[nodiscard]] operator const nsTArray<E>&() const& {
+    return *reinterpret_cast<const nsTArray<E>*>(this);
+  }
+  [[nodiscard]] operator const FallibleTArray<E>&() const& {
+    return *reinterpret_cast<const FallibleTArray<E>*>(this);
+  }
+
+  // The array's assignment operator performs a 'deep' copy of the given
+  // array.  It is optimized to reuse existing storage if possible.
+  // @param aOther The array object to copy.
+  nsTArray_Impl& operator=(const nsTArray_Impl&) = default;
+
+  // The array's move assignment operator steals the underlying data from
+  // the other array.
+  // @param other  The array object to move from.
+  self_type& operator=(self_type&& aOther) {
+    if (this != &aOther) {
+      Clear();
+      this->MoveInit(aOther, sizeof(value_type), MOZ_ALIGNOF(value_type));
+    }
+    return *this;
+  }
+
+  // Return true if this array has the same length and the same
+  // elements as |aOther|.
+  template <typename Allocator>
+  [[nodiscard]] bool operator==(
+      const nsTArray_Impl<E, Allocator>& aOther) const {
+    size_type len = Length();
+    if (len != aOther.Length()) {
+      return false;
+    }
+
+    // XXX std::equal would be as fast or faster here
+    for (index_type i = 0; i < len; ++i) {
+      if (!(operator[](i) == aOther[i])) {
+        return false;
+      }
+    }
+
+    return true;
+  }
+
+  // Return true if this array does not have the same length and the same
+  // elements as |aOther|.
+  [[nodiscard]] bool operator!=(const self_type& aOther) const {
+    return !operator==(aOther);
+  }
+
+  // If Alloc == FallibleAlloc, ReplaceElementsAt might fail, without a way to
+  // signal this to the caller, so we disallow copying via operator=. Callers
+  // should use ReplaceElementsAt with a fallible argument instead, and check
+  // the result.
+  template <typename Allocator,
+            typename = std::enable_if_t<std::is_same_v<Alloc, InfallibleAlloc>,
+                                        Allocator>>
+  self_type& operator=(const nsTArray_Impl<E, Allocator>& aOther) {
+    AssignInternal<InfallibleAlloc>(aOther.Elements(), aOther.Length());
+    return *this;
+  }
+
+  template <typename Allocator>
+  self_type& operator=(nsTArray_Impl<E, Allocator>&& aOther) {
+    Clear();
+    this->MoveInit(aOther, sizeof(value_type), MOZ_ALIGNOF(value_type));
+    return *this;
+  }
+
+  // @return The amount of memory used by this nsTArray_Impl, excluding
+  // sizeof(*this). If you want to measure anything hanging off the array, you
+  // must iterate over the elements and measure them individually; hence the
+  // "Shallow" prefix.
+  [[nodiscard]] size_t ShallowSizeOfExcludingThis(
+      mozilla::MallocSizeOf aMallocSizeOf) const {
+    if (this->UsesAutoArrayBuffer() || this->HasEmptyHeader()) {
+      return 0;
+    }
+    return aMallocSizeOf(this->Hdr());
+  }
+
+  // @return The amount of memory used by this nsTArray_Impl, including
+  // sizeof(*this). If you want to measure anything hanging off the array, you
+  // must iterate over the elements and measure them individually; hence the
+  // "Shallow" prefix.
+  [[nodiscard]] size_t ShallowSizeOfIncludingThis(
+      mozilla::MallocSizeOf aMallocSizeOf) const {
+    return aMallocSizeOf(this) + ShallowSizeOfExcludingThis(aMallocSizeOf);
+  }
+
+  //
+  // Accessor methods
+  //
+
+  // This method provides direct access to the array elements.
+  // @return A pointer to the first element of the array.  If the array is
+  // empty, then this pointer must not be dereferenced.
+  [[nodiscard]] value_type* Elements() MOZ_NONNULL_RETURN {
+    return reinterpret_cast<value_type*>(Hdr() + 1);
+  }
+
+  // This method provides direct, readonly access to the array elements.
+  // @return A pointer to the first element of the array.  If the array is
+  // empty, then this pointer must not be dereferenced.
+  [[nodiscard]] const value_type* Elements() const MOZ_NONNULL_RETURN {
+    return reinterpret_cast<const value_type*>(Hdr() + 1);
+  }
+
+  // This method provides direct access to an element of the array. The given
+  // index must be within the array bounds.
+  // @param aIndex The index of an element in the array.
+  // @return A reference to the i'th element of the array.
+  [[nodiscard]] value_type& ElementAt(index_type aIndex) {
+    if (MOZ_UNLIKELY(aIndex >= Length())) {
+      mozilla::detail::InvalidArrayIndex_CRASH(aIndex, Length());
+    }
+    return Elements()[aIndex];
+  }
+
+  // This method provides direct, readonly access to an element of the array
+  // The given index must be within the array bounds.
+  // @param aIndex The index of an element in the array.
+  // @return A const reference to the i'th element of the array.
+  [[nodiscard]] const value_type& ElementAt(index_type aIndex) const {
+    if (MOZ_UNLIKELY(aIndex >= Length())) {
+      mozilla::detail::InvalidArrayIndex_CRASH(aIndex, Length());
+    }
+    return Elements()[aIndex];
+  }
+
+  // This method provides direct access to an element of the array in a bounds
+  // safe manner. If the requested index is out of bounds the provided default
+  // value is returned.
+  // @param aIndex The index of an element in the array.
+  // @param aDef   The value to return if the index is out of bounds.
+  [[nodiscard]] value_type& SafeElementAt(index_type aIndex, value_type& aDef) {
+    return aIndex < Length() ? Elements()[aIndex] : aDef;
+  }
+
+  // This method provides direct access to an element of the array in a bounds
+  // safe manner. If the requested index is out of bounds the provided default
+  // value is returned.
+  // @param aIndex The index of an element in the array.
+  // @param aDef   The value to return if the index is out of bounds.
+  [[nodiscard]] const value_type& SafeElementAt(index_type aIndex,
+                                                const value_type& aDef) const {
+    return aIndex < Length() ? Elements()[aIndex] : aDef;
+  }
+
+  // Shorthand for ElementAt(aIndex)
+  [[nodiscard]] value_type& operator[](index_type aIndex) {
+    return ElementAt(aIndex);
+  }
+
+  // Shorthand for ElementAt(aIndex)
+  [[nodiscard]] const value_type& operator[](index_type aIndex) const {
+    return ElementAt(aIndex);
+  }
+
+  // Shorthand for ElementAt(length - 1)
+  [[nodiscard]] value_type& LastElement() { return ElementAt(Length() - 1); }
+
+  // Shorthand for ElementAt(length - 1)
+  [[nodiscard]] const value_type& LastElement() const {
+    return ElementAt(Length() - 1);
+  }
+
+  // Shorthand for SafeElementAt(length - 1, def)
+  [[nodiscard]] value_type& SafeLastElement(value_type& aDef) {
+    return SafeElementAt(Length() - 1, aDef);
+  }
+
+  // Shorthand for SafeElementAt(length - 1, def)
+  [[nodiscard]] const value_type& SafeLastElement(
+      const value_type& aDef) const {
+    return SafeElementAt(Length() - 1, aDef);
+  }
+
+  // Methods for range-based for loops.
+  [[nodiscard]] iterator begin() { return iterator(*this, 0); }
+  [[nodiscard]] const_iterator begin() const {
+    return const_iterator(*this, 0);
+  }
+  [[nodiscard]] const_iterator cbegin() const { return begin(); }
+  [[nodiscard]] iterator end() { return iterator(*this, Length()); }
+  [[nodiscard]] const_iterator end() const {
+    return const_iterator(*this, Length());
+  }
+  [[nodiscard]] const_iterator cend() const { return end(); }
+
+  // Methods for reverse iterating.
+  [[nodiscard]] reverse_iterator rbegin() { return reverse_iterator(end()); }
+  [[nodiscard]] const_reverse_iterator rbegin() const {
+    return const_reverse_iterator(end());
+  }
+  [[nodiscard]] const_reverse_iterator crbegin() const { return rbegin(); }
+  [[nodiscard]] reverse_iterator rend() { return reverse_iterator(begin()); }
+  [[nodiscard]] const_reverse_iterator rend() const {
+    return const_reverse_iterator(begin());
+  }
+  [[nodiscard]] const_reverse_iterator crend() const { return rend(); }
+
+  // Span integration
+
+  [[nodiscard]] operator mozilla::Span<value_type>() {
+    return mozilla::Span<value_type>(Elements(), Length());
+  }
+
+  [[nodiscard]] operator mozilla::Span<const value_type>() const {
+    return mozilla::Span<const value_type>(Elements(), Length());
+  }
+
+  //
+  // Search methods
+  //
+
+  // This method searches for the first element in this array that is equal
+  // to the given element.
+  // @param aItem  The item to search for.
+  // @param aComp  The Comparator used to determine element equality.
+  // @return       true if the element was found.
+  template <class Item, class Comparator>
+  [[nodiscard]] bool Contains(const Item& aItem,
+                              const Comparator& aComp) const {
+    return ApplyIf(
+        aItem, 0, aComp, []() { return true; }, []() { return false; });
+  }
+
+  // Like Contains(), but assumes a sorted array.
+  template <class Item, class Comparator>
+  [[nodiscard]] bool ContainsSorted(const Item& aItem,
+                                    const Comparator& aComp) const {
+    return BinaryIndexOf(aItem, aComp) != NoIndex;
+  }
+
+  // This method searches for the first element in this array that is equal
+  // to the given element.  This method assumes that 'operator==' is defined
+  // for value_type.
+  // @param aItem  The item to search for.
+  // @return       true if the element was found.
+  template <class Item>
+  [[nodiscard]] bool Contains(const Item& aItem) const {
+    return Contains(aItem, nsDefaultComparator<value_type, Item>());
+  }
+
+  // Like Contains(), but assumes a sorted array.
+  template <class Item>
+  [[nodiscard]] bool ContainsSorted(const Item& aItem) const {
+    return BinaryIndexOf(aItem) != NoIndex;
+  }
+
+  // This method searches for the offset of the first element in this
+  // array that is equal to the given element.
+  // @param aItem  The item to search for.
+  // @param aStart The index to start from.
+  // @param aComp  The Comparator used to determine element equality.
+  // @return       The index of the found element or NoIndex if not found.
+  template <class Item, class Comparator>
+  [[nodiscard]] index_type IndexOf(const Item& aItem, index_type aStart,
+                                   const Comparator& aComp) const {
+    ::detail::CompareWrapper<Comparator, Item> comp(aComp);
+
+    const value_type* iter = Elements() + aStart;
+    const value_type* iend = Elements() + Length();
+    for (; iter != iend; ++iter) {
+      if (comp.Equals(*iter, aItem)) {
+        return index_type(iter - Elements());
+      }
+    }
+    return NoIndex;
+  }
+
+  // This method searches for the offset of the first element in this
+  // array that is equal to the given element.  This method assumes
+  // that 'operator==' is defined for value_type.
+  // @param aItem  The item to search for.
+  // @param aStart The index to start from.
+  // @return       The index of the found element or NoIndex if not found.
+  template <class Item>
+  [[nodiscard]] index_type IndexOf(const Item& aItem,
+                                   index_type aStart = 0) const {
+    return IndexOf(aItem, aStart, nsDefaultComparator<value_type, Item>());
+  }
+
+  // This method searches for the offset of the last element in this
+  // array that is equal to the given element.
+  // @param aItem  The item to search for.
+  // @param aStart The index to start from.  If greater than or equal to the
+  //               length of the array, then the entire array is searched.
+  // @param aComp  The Comparator used to determine element equality.
+  // @return       The index of the found element or NoIndex if not found.
+  template <class Item, class Comparator>
+  [[nodiscard]] index_type LastIndexOf(const Item& aItem, index_type aStart,
+                                       const Comparator& aComp) const {
+    ::detail::CompareWrapper<Comparator, Item> comp(aComp);
+
+    size_type endOffset = aStart >= Length() ? Length() : aStart + 1;
+    const value_type* iend = Elements() - 1;
+    const value_type* iter = iend + endOffset;
+    for (; iter != iend; --iter) {
+      if (comp.Equals(*iter, aItem)) {
+        return index_type(iter - Elements());
+      }
+    }
+    return NoIndex;
+  }
+
+  // This method searches for the offset of the last element in this
+  // array that is equal to the given element.  This method assumes
+  // that 'operator==' is defined for value_type.
+  // @param aItem  The item to search for.
+  // @param aStart The index to start from.  If greater than or equal to the
+  //               length of the array, then the entire array is searched.
+  // @return       The index of the found element or NoIndex if not found.
+  template <class Item>
+  [[nodiscard]] index_type LastIndexOf(const Item& aItem,
+                                       index_type aStart = NoIndex) const {
+    return LastIndexOf(aItem, aStart, nsDefaultComparator<value_type, Item>());
+  }
+
+  // This method searches for the offset for the element in this array
+  // that is equal to the given element. The array is assumed to be sorted.
+  // If there is more than one equivalent element, there is no guarantee
+  // on which one will be returned.
+  // @param aItem  The item to search for.
+  // @param aComp  The Comparator used.
+  // @return       The index of the found element or NoIndex if not found.
+  template <class Item, class Comparator>
+  [[nodiscard]] index_type BinaryIndexOf(const Item& aItem,
+                                         const Comparator& aComp) const {
+    using mozilla::BinarySearchIf;
+    ::detail::CompareWrapper<Comparator, Item> comp(aComp);
+
+    size_t index;
+    bool found = BinarySearchIf(
+        Elements(), 0, Length(),
+        // Note: We pass the Compare() args here in reverse order and negate the
+        // results for compatibility reasons. Some existing callers use Equals()
+        // functions with first arguments which match aElement but not aItem, or
+        // second arguments that match aItem but not aElement. To accommodate
+        // those callers, we preserve the argument order of the older version of
+        // this API. These callers, however, should be fixed, and this special
+        // case removed.
+        [&](const value_type& aElement) {
+          return -comp.Compare(aElement, aItem);
+        },
+        &index);
+    return found ? index : NoIndex;
+  }
+
+  // This method searches for the offset for the element in this array
+  // that is equal to the given element. The array is assumed to be sorted.
+  // This method assumes that 'operator==' and 'operator<' are defined.
+  // @param aItem  The item to search for.
+  // @return       The index of the found element or NoIndex if not found.
+  template <class Item>
+  [[nodiscard]] index_type BinaryIndexOf(const Item& aItem) const {
+    return BinaryIndexOf(aItem, nsDefaultComparator<value_type, Item>());
+  }
+
+  //
+  // Mutation methods
+  //
+ private:
+  template <typename ActualAlloc, class Item>
+  typename ActualAlloc::ResultType AssignInternal(const Item* aArray,
+                                                  size_type aArrayLen);
+
+ public:
+  template <class Allocator, typename ActualAlloc = Alloc>
+  [[nodiscard]] typename ActualAlloc::ResultType Assign(
+      const nsTArray_Impl<E, Allocator>& aOther) {
+    return AssignInternal<ActualAlloc>(aOther.Elements(), aOther.Length());
+  }
+
+  template <class Allocator>
+  [[nodiscard]] bool Assign(const nsTArray_Impl<E, Allocator>& aOther,
+                            const mozilla::fallible_t&) {
+    return Assign<Allocator, FallibleAlloc>(aOther);
+  }
+
+  template <class Allocator>
+  void Assign(nsTArray_Impl<E, Allocator>&& aOther) {
+    Clear();
+    this->MoveInit(aOther, sizeof(value_type), MOZ_ALIGNOF(value_type));
+  }
+
+  // This method call the destructor on each element of the array, empties it,
+  // but does not shrink the array's capacity.
+  // See also SetLengthAndRetainStorage.
+  // Make sure to call Compact() if needed to avoid keeping a huge array
+  // around.
+  void ClearAndRetainStorage() {
+    if (this->HasEmptyHeader()) {
+      return;
+    }
+
+    DestructRange(0, Length());
+    base_type::mHdr->mLength = 0;
+  }
+
+  // This method modifies the length of the array, but unlike SetLength
+  // it doesn't deallocate/reallocate the current internal storage.
+  // The new length MUST be shorter than or equal to the current capacity.
+  // If the new length is larger than the existing length of the array,
+  // then new elements will be constructed using value_type's default
+  // constructor.  If shorter, elements will be destructed and removed.
+  // See also ClearAndRetainStorage.
+  // @param aNewLen  The desired length of this array.
+  void SetLengthAndRetainStorage(size_type aNewLen) {
+    MOZ_ASSERT(aNewLen <= base_type::Capacity());
+    size_type oldLen = Length();
+    if (aNewLen > oldLen) {
+      /// XXX(Bug 1631367) SetLengthAndRetainStorage should be disabled for
+      /// FallibleTArray.
+      InsertElementsAtInternal<InfallibleAlloc>(oldLen, aNewLen - oldLen);
+      return;
+    }
+    if (aNewLen < oldLen) {
+      DestructRange(aNewLen, oldLen - aNewLen);
+      base_type::mHdr->mLength = aNewLen;
+    }
+  }
+
+  // This method replaces a range of elements in this array.
+  // @param aStart    The starting index of the elements to replace.
+  // @param aCount    The number of elements to replace.  This may be zero to
+  //                  insert elements without removing any existing elements.
+  // @param aArray    The values to copy into this array.  Must be non-null,
+  //                  and these elements must not already exist in the array
+  //                  being modified.
+  // @param aArrayLen The number of values to copy into this array.
+  // @return          A pointer to the new elements in the array, or null if
+  //                  the operation failed due to insufficient memory.
+ private:
+  template <typename ActualAlloc, class Item>
+  value_type* ReplaceElementsAtInternal(index_type aStart, size_type aCount,
+                                        const Item* aArray,
+                                        size_type aArrayLen);
+
+ public:
+  template <class Item>
+  [[nodiscard]] value_type* ReplaceElementsAt(index_type aStart,
+                                              size_type aCount,
+                                              const Item* aArray,
+                                              size_type aArrayLen,
+                                              const mozilla::fallible_t&) {
+    return ReplaceElementsAtInternal<FallibleAlloc>(aStart, aCount, aArray,
+                                                    aArrayLen);
+  }
+
+  // A variation on the ReplaceElementsAt method defined above.
+  template <class Item>
+  [[nodiscard]] value_type* ReplaceElementsAt(index_type aStart,
+                                              size_type aCount,
+                                              const nsTArray<Item>& aArray,
+                                              const mozilla::fallible_t&) {
+    return ReplaceElementsAtInternal<FallibleAlloc>(aStart, aCount, aArray);
+  }
+
+  template <class Item>
+  [[nodiscard]] value_type* ReplaceElementsAt(index_type aStart,
+                                              size_type aCount,
+                                              mozilla::Span<Item> aSpan,
+                                              const mozilla::fallible_t&) {
+    return ReplaceElementsAtInternal<FallibleAlloc>(aStart, aCount, aSpan);
+  }
+
+  // A variation on the ReplaceElementsAt method defined above.
+  template <class Item>
+  [[nodiscard]] value_type* ReplaceElementsAt(index_type aStart,
+                                              size_type aCount,
+                                              const Item& aItem,
+                                              const mozilla::fallible_t&) {
+    return ReplaceElementsAtInternal<FallibleAlloc>(aStart, aCount, aItem);
+  }
+
+  // A variation on the ReplaceElementsAt method defined above.
+  template <class Item>
+  mozilla::NotNull<value_type*> ReplaceElementAt(index_type aIndex,
+                                                 Item&& aItem) {
+    value_type* const elem = &ElementAt(aIndex);
+    elem_traits::Destruct(elem);
+    elem_traits::Construct(elem, std::forward<Item>(aItem));
+    return mozilla::WrapNotNullUnchecked(elem);
+  }
+
+  // InsertElementsAt is ReplaceElementsAt with 0 elements to replace.
+  // XXX Provide a proper documentation of InsertElementsAt.
+  template <class Item>
+  [[nodiscard]] value_type* InsertElementsAt(index_type aIndex,
+                                             const Item* aArray,
+                                             size_type aArrayLen,
+                                             const mozilla::fallible_t&) {
+    return ReplaceElementsAtInternal<FallibleAlloc>(aIndex, 0, aArray,
+                                                    aArrayLen);
+  }
+
+  template <class Item, class Allocator>
+  [[nodiscard]] value_type* InsertElementsAt(
+      index_type aIndex, const nsTArray_Impl<Item, Allocator>& aArray,
+      const mozilla::fallible_t&) {
+    return ReplaceElementsAtInternal<FallibleAlloc>(
+        aIndex, 0, aArray.Elements(), aArray.Length());
+  }
+
+  template <class Item>
+  [[nodiscard]] value_type* InsertElementsAt(index_type aIndex,
+                                             mozilla::Span<Item> aSpan,
+                                             const mozilla::fallible_t&) {
+    return ReplaceElementsAtInternal<FallibleAlloc>(aIndex, 0, aSpan.Elements(),
+                                                    aSpan.Length());
+  }
+
+ private:
+  template <typename ActualAlloc>
+  value_type* InsertElementAtInternal(index_type aIndex);
+
+  // Insert a new element without copy-constructing. This is useful to avoid
+  // temporaries.
+  // @return A pointer to the newly inserted element, or null on OOM.
+ public:
+  [[nodiscard]] value_type* InsertElementAt(index_type aIndex,
+                                            const mozilla::fallible_t&) {
+    return InsertElementAtInternal<FallibleAlloc>(aIndex);
+  }
+
+ private:
+  template <typename ActualAlloc, class Item>
+  value_type* InsertElementAtInternal(index_type aIndex, Item&& aItem);
+
+  // Insert a new element, move constructing if possible.
+ public:
+  template <class Item>
+  [[nodiscard]] value_type* InsertElementAt(index_type aIndex, Item&& aItem,
+                                            const mozilla::fallible_t&) {
+    return InsertElementAtInternal<FallibleAlloc>(aIndex,
+                                                  std::forward<Item>(aItem));
+  }
+
+  // Reconstruct the element at the given index, and return a pointer to the
+  // reconstructed element.  This will destroy the existing element and
+  // default-construct a new one, giving you a state much like what single-arg
+  // InsertElementAt(), or no-arg AppendElement() does, but without changing the
+  // length of the array.
+  //
+  // array[idx] = value_type()
+  //
+  // would accomplish the same thing as long as value_type has the appropriate
+  // moving operator=, but some types don't for various reasons.
+  mozilla::NotNull<value_type*> ReconstructElementAt(index_type aIndex) {
+    value_type* elem = &ElementAt(aIndex);
+    elem_traits::Destruct(elem);
+    elem_traits::Construct(elem);
+    return mozilla::WrapNotNullUnchecked(elem);
+  }
+
+  // This method searches for the smallest index of an element that is strictly
+  // greater than |aItem|. If |aItem| is inserted at this index, the array will
+  // remain sorted and |aItem| would come after all elements that are equal to
+  // it. If |aItem| is greater than or equal to all elements in the array, the
+  // array length is returned.
+  //
+  // Note that consumers who want to know whether there are existing items equal
+  // to |aItem| in the array can just check that the return value here is > 0
+  // and indexing into the previous slot gives something equal to |aItem|.
+  //
+  //
+  // @param aItem  The item to search for.
+  // @param aComp  The Comparator used.
+  // @return        The index of greatest element <= to |aItem|
+  // @precondition The array is sorted
+  template <class Item, class Comparator>
+  [[nodiscard]] index_type IndexOfFirstElementGt(
+      const Item& aItem, const Comparator& aComp) const {
+    using mozilla::BinarySearchIf;
+    ::detail::CompareWrapper<Comparator, Item> comp(aComp);
+
+    size_t index;
+    BinarySearchIf(
+        Elements(), 0, Length(),
+        [&](const value_type& aElement) {
+          return comp.Compare(aElement, aItem) <= 0 ? 1 : -1;
+        },
+        &index);
+    return index;
+  }
+
+  // A variation on the IndexOfFirstElementGt method defined above.
+  template <class Item>
+  [[nodiscard]] index_type IndexOfFirstElementGt(const Item& aItem) const {
+    return IndexOfFirstElementGt(aItem,
+                                 nsDefaultComparator<value_type, Item>());
+  }
+
+ private:
+  template <typename ActualAlloc, class Item, class Comparator>
+  value_type* InsertElementSortedInternal(Item&& aItem,
+                                          const Comparator& aComp) {
+    index_type index = IndexOfFirstElementGt<Item, Comparator>(aItem, aComp);
+    return InsertElementAtInternal<ActualAlloc>(index,
+                                                std::forward<Item>(aItem));
+  }
+
+  // Inserts |aItem| at such an index to guarantee that if the array
+  // was previously sorted, it will remain sorted after this
+  // insertion.
+ public:
+  template <class Item, class Comparator>
+  [[nodiscard]] value_type* InsertElementSorted(Item&& aItem,
+                                                const Comparator& aComp,
+                                                const mozilla::fallible_t&) {
+    return InsertElementSortedInternal<FallibleAlloc>(std::forward<Item>(aItem),
+                                                      aComp);
+  }
+
+  // A variation on the InsertElementSorted method defined above.
+ public:
+  template <class Item>
+  [[nodiscard]] value_type* InsertElementSorted(Item&& aItem,
+                                                const mozilla::fallible_t&) {
+    return InsertElementSortedInternal<FallibleAlloc>(
+        std::forward<Item>(aItem), nsDefaultComparator<value_type, Item>{});
+  }
+
+ private:
+  template <typename ActualAlloc, class Item>
+  value_type* AppendElementsInternal(const Item* aArray, size_type aArrayLen);
+
+  // This method appends elements to the end of this array.
+  // @param aArray    The elements to append to this array.
+  // @param aArrayLen The number of elements to append to this array.
+  // @return          A pointer to the new elements in the array, or null if
+  //                  the operation failed due to insufficient memory.
+ public:
+  template <class Item>
+  [[nodiscard]] value_type* AppendElements(const Item* aArray,
+                                           size_type aArrayLen,
+                                           const mozilla::fallible_t&) {
+    return AppendElementsInternal<FallibleAlloc>(aArray, aArrayLen);
+  }
+
+  template <class Item>
+  [[nodiscard]] value_type* AppendElements(mozilla::Span<Item> aSpan,
+                                           const mozilla::fallible_t&) {
+    return AppendElementsInternal<FallibleAlloc>(aSpan.Elements(),
+                                                 aSpan.Length());
+  }
+
+  // A variation on the AppendElements method defined above.
+  template <class Item, class Allocator>
+  [[nodiscard]] value_type* AppendElements(
+      const nsTArray_Impl<Item, Allocator>& aArray,
+      const mozilla::fallible_t&) {
+    return AppendElementsInternal<FallibleAlloc>(aArray.Elements(),
+                                                 aArray.Length());
+  }
+
+ private:
+  template <typename ActualAlloc, class Item, class Allocator>
+  value_type* AppendElementsInternal(nsTArray_Impl<Item, Allocator>&& aArray);
+
+  // Move all elements from another array to the end of this array.
+  // @return A pointer to the newly appended elements, or null on OOM.
+ public:
+  template <class Item, class Allocator>
+  [[nodiscard]] value_type* AppendElements(
+      nsTArray_Impl<Item, Allocator>&& aArray, const mozilla::fallible_t&) {
+    return AppendElementsInternal<FallibleAlloc>(std::move(aArray));
+  }
+
+  // Append a new element, constructed in place from the provided arguments.
+ protected:
+  template <typename ActualAlloc, class... Args>
+  value_type* EmplaceBackInternal(Args&&... aItem);
+
+ public:
+  template <class... Args>
+  [[nodiscard]] value_type* EmplaceBack(const mozilla::fallible_t&,
+                                        Args&&... aArgs) {
+    return EmplaceBackInternal<FallibleAlloc, Args...>(
+        std::forward<Args>(aArgs)...);
+  }
+
+ private:
+  template <typename ActualAlloc, class Item>
+  value_type* AppendElementInternal(Item&& aItem);
+
+  // Append a new element, move constructing if possible.
+ public:
+  template <class Item>
+  [[nodiscard]] value_type* AppendElement(Item&& aItem,
+                                          const mozilla::fallible_t&) {
+    return AppendElementInternal<FallibleAlloc>(std::forward<Item>(aItem));
+  }
+
+ private:
+  template <typename ActualAlloc>
+  value_type* AppendElementsInternal(size_type aCount) {
+    if (!ActualAlloc::Successful(this->template ExtendCapacity<ActualAlloc>(
+            Length(), aCount, sizeof(value_type)))) {
+      return nullptr;
+    }
+    value_type* elems = Elements() + Length();
+    size_type i;
+    for (i = 0; i < aCount; ++i) {
+      elem_traits::Construct(elems + i);
+    }
+    this->IncrementLength(aCount);
+    return elems;
+  }
+
+  // Append new elements without copy-constructing. This is useful to avoid
+  // temporaries.
+  // @return A pointer to the newly appended elements, or null on OOM.
+ public:
+  [[nodiscard]] value_type* AppendElements(size_type aCount,
+                                           const mozilla::fallible_t&) {
+    return AppendElementsInternal<FallibleAlloc>(aCount);
+  }
+
+ private:
+  // Append a new element without copy-constructing. This is useful to avoid
+  // temporaries.
+  // @return A pointer to the newly appended element, or null on OOM.
+ public:
+  [[nodiscard]] value_type* AppendElement(const mozilla::fallible_t&) {
+    return AppendElements(1, mozilla::fallible);
+  }
+
+  // This method removes a single element from this array, like
+  // std::vector::erase.
+  // @param pos to the element to remove
+  const_iterator RemoveElementAt(const_iterator pos) {
+    MOZ_ASSERT(pos.GetArray() == this);
+
+    RemoveElementAt(pos.GetIndex());
+    return pos;
+  }
+
+  // This method removes a range of elements from this array, like
+  // std::vector::erase.
+  // @param first iterator to the first of elements to remove
+  // @param last iterator to the last of elements to remove
+  const_iterator RemoveElementsRange(const_iterator first,
+                                     const_iterator last) {
+    MOZ_ASSERT(first.GetArray() == this);
+    MOZ_ASSERT(last.GetArray() == this);
+    MOZ_ASSERT(last.GetIndex() >= first.GetIndex());
+
+    RemoveElementsAt(first.GetIndex(), last.GetIndex() - first.GetIndex());
+    return first;
+  }
+
+  // This method removes a range of elements from this array.
+  // @param aStart The starting index of the elements to remove.
+  // @param aCount The number of elements to remove.
+  void RemoveElementsAt(index_type aStart, size_type aCount);
+
+ private:
+  // Remove a range of elements from this array, but do not check that
+  // the range is in bounds.
+  // @param aStart The starting index of the elements to remove.
+  // @param aCount The number of elements to remove.
+  void RemoveElementsAtUnsafe(index_type aStart, size_type aCount);
+
+ public:
+  // Similar to the above, but it removes just one element. This does bounds
+  // checking only in debug builds.
+  void RemoveElementAtUnsafe(index_type aIndex) {
+    MOZ_ASSERT(aIndex < Length(), "Trying to remove an invalid element");
+    RemoveElementsAtUnsafe(aIndex, 1);
+  }
+
+  // A variation on the RemoveElementsAt method defined above.
+  void RemoveElementAt(index_type aIndex) { RemoveElementsAt(aIndex, 1); }
+
+  // A variation on RemoveElementAt that removes the last element.
+  void RemoveLastElement() { RemoveLastElements(1); }
+
+  // A variation on RemoveElementsAt that removes the last 'aCount' elements.
+  void RemoveLastElements(const size_type aCount) {
+    // This assertion is redundant, but produces a better error message than the
+    // release assertion within TruncateLength.
+    MOZ_ASSERT(aCount <= Length());
+    TruncateLength(Length() - aCount);
+  }
+
+  // Removes the last element of the array and returns a copy of it.
+  [[nodiscard]] value_type PopLastElement() {
+    // This function intentionally does not call ElementsAt and calls
+    // TruncateLengthUnsafe directly to avoid multiple release checks for
+    // non-emptiness.
+    // This debug assertion is redundant, but produces a better error message
+    // than the release assertion below.
+    MOZ_ASSERT(!base_type::IsEmpty());
+    const size_type oldLen = Length();
+    if (MOZ_UNLIKELY(0 == oldLen)) {
+      mozilla::detail::InvalidArrayIndex_CRASH(1, 0);
+    }
+    value_type elem = std::move(Elements()[oldLen - 1]);
+    TruncateLengthUnsafe(oldLen - 1);
+    return elem;
+  }
+
+  // This method performs index-based removals from an array without preserving
+  // the order of the array. This is useful if you are using the array as a
+  // set-like data structure.
+  //
+  // These removals are efficient, as they move as few elements as possible. At
+  // most N elements, where N is the number of removed elements, will have to
+  // be relocated.
+  //
+  // ## Examples
+  //
+  // When removing an element from the end of the array, it can be removed in
+  // place, by destroying it and decrementing the length.
+  //
+  // [ 1, 2, 3 ] => [ 1, 2 ]
+  //         ^
+  //
+  // When removing any other single element, it is removed by swapping it with
+  // the last element, and then decrementing the length as before.
+  //
+  // [ 1, 2, 3, 4, 5, 6 ]  => [ 1, 6, 3, 4, 5 ]
+  //      ^
+  //
+  // This method also supports efficiently removing a range of elements. If they
+  // are at the end, then they can all be removed like in the one element case.
+  //
+  // [ 1, 2, 3, 4, 5, 6 ] => [ 1, 2 ]
+  //         ^--------^
+  //
+  // If more elements are removed than exist after the removed section, the
+  // remaining elements will be shifted down like in a normal removal.
+  //
+  // [ 1, 2, 3, 4, 5, 6, 7, 8 ] => [ 1, 2, 7, 8 ]
+  //         ^--------^
+  //
+  // And if fewer elements are removed than exist after the removed section,
+  // elements will be moved from the end of the array to fill the vacated space.
+  //
+  // [ 1, 2, 3, 4, 5, 6, 7, 8 ] => [ 1, 7, 8, 4, 5, 6 ]
+  //      ^--^
+  //
+  // @param aStart The starting index of the elements to remove. @param aCount
+  // The number of elements to remove.
+  void UnorderedRemoveElementsAt(index_type aStart, size_type aCount);
+
+  // A variation on the UnorderedRemoveElementsAt method defined above to remove
+  // a single element. This operation is sometimes called `SwapRemove`.
+  //
+  // This method is O(1), but does not preserve the order of the elements.
+  void UnorderedRemoveElementAt(index_type aIndex) {
+    UnorderedRemoveElementsAt(aIndex, 1);
+  }
+
+  void Clear() {
+    ClearAndRetainStorage();
+    base_type::ShrinkCapacityToZero(sizeof(value_type),
+                                    MOZ_ALIGNOF(value_type));
+  }
+
+  // This method removes elements based on the return value of the
+  // callback function aPredicate. If the function returns true for
+  // an element, the element is removed. aPredicate will be called
+  // for each element in order. It is not safe to access the array
+  // inside aPredicate.
+  //
+  // Returns the number of elements removed.
+  template <typename Predicate>
+  size_type RemoveElementsBy(Predicate aPredicate);
+
+  // This helper function combines IndexOf with RemoveElementAt to "search
+  // and destroy" the first element that is equal to the given element.
+  // @param aItem The item to search for.
+  // @param aComp The Comparator used to determine element equality.
+  // @return true if the element was found
+  template <class Item, class Comparator>
+  bool RemoveElement(const Item& aItem, const Comparator& aComp) {
+    index_type i = IndexOf(aItem, 0, aComp);
+    if (i == NoIndex) {
+      return false;
+    }
+
+    RemoveElementsAtUnsafe(i, 1);
+    return true;
+  }
+
+  // A variation on the RemoveElement method defined above that assumes
+  // that 'operator==' is defined for value_type.
+  template <class Item>
+  bool RemoveElement(const Item& aItem) {
+    return RemoveElement(aItem, nsDefaultComparator<value_type, Item>());
+  }
+
+  // This helper function combines IndexOfFirstElementGt with
+  // RemoveElementAt to "search and destroy" the last element that
+  // is equal to the given element.
+  // @param aItem The item to search for.
+  // @param aComp The Comparator used to determine element equality.
+  // @return true if the element was found
+  template <class Item, class Comparator>
+  bool RemoveElementSorted(const Item& aItem, const Comparator& aComp) {
+    index_type index = IndexOfFirstElementGt(aItem, aComp);
+    if (index > 0 && aComp.Equals(ElementAt(index - 1), aItem)) {
+      RemoveElementsAtUnsafe(index - 1, 1);
+      return true;
+    }
+    return false;
+  }
+
+  // A variation on the RemoveElementSorted method defined above.
+  template <class Item>
+  bool RemoveElementSorted(const Item& aItem) {
+    return RemoveElementSorted(aItem, nsDefaultComparator<value_type, Item>());
+  }
+
+  // This method causes the elements contained in this array and the given
+  // array to be swapped.
+  template <class Allocator>
+  void SwapElements(nsTArray_Impl<E, Allocator>& aOther) {
+    // The only case this might fail were if someone called this with a
+    // AutoTArray upcast to nsTArray_Impl, under the conditions mentioned in the
+    // overload for AutoTArray below.
+    this->template SwapArrayElements<InfallibleAlloc>(
+        aOther, sizeof(value_type), MOZ_ALIGNOF(value_type));
+  }
+
+  template <size_t N>
+  void SwapElements(AutoTArray<E, N>& aOther) {
+    // Allocation might fail if Alloc==FallibleAlloc and
+    // Allocator==InfallibleAlloc and aOther uses auto storage. Allow this for
+    // small inline sizes, and crash in the rare case of a small OOM error.
+    static_assert(!std::is_same_v<Alloc, FallibleAlloc> ||
+                  sizeof(E) * N <= 1024);
+    this->template SwapArrayElements<InfallibleAlloc>(
+        aOther, sizeof(value_type), MOZ_ALIGNOF(value_type));
+  }
+
+  template <class Allocator>
+  [[nodiscard]] auto SwapElements(nsTArray_Impl<E, Allocator>& aOther,
+                                  const mozilla::fallible_t&) {
+    // Allocation might fail if Alloc==FallibleAlloc and
+    // Allocator==InfallibleAlloc and aOther uses auto storage.
+    return FallibleAlloc::Result(
+        this->template SwapArrayElements<FallibleAlloc>(
+            aOther, sizeof(value_type), MOZ_ALIGNOF(value_type)));
+  }
+
+ private:
+  // Used by ApplyIf functions to invoke a callable that takes either:
+  // - Nothing: F(void)
+  // - Index only: F(size_t)
+  // - Reference to element only: F(maybe-const value_type&)
+  // - Both index and reference: F(size_t, maybe-const value_type&)
+  // `value_type` must be const when called from const method.
+  template <typename T, typename Param0, typename Param1>
+  struct InvokeWithIndexAndOrReferenceHelper {
+    static constexpr bool valid = false;
+  };
+  template <typename T>
+  struct InvokeWithIndexAndOrReferenceHelper<T, void, void> {
+    static constexpr bool valid = true;
+    template <typename F>
+    static auto Invoke(F&& f, size_t, T&) {
+      return f();
+    }
+  };
+  template <typename T>
+  struct InvokeWithIndexAndOrReferenceHelper<T, size_t, void> {
+    static constexpr bool valid = true;
+    template <typename F>
+    static auto Invoke(F&& f, size_t i, T&) {
+      return f(i);
+    }
+  };
+  template <typename T>
+  struct InvokeWithIndexAndOrReferenceHelper<T, T&, void> {
+    static constexpr bool valid = true;
+    template <typename F>
+    static auto Invoke(F&& f, size_t, T& e) {
+      return f(e);
+    }
+  };
+  template <typename T>
+  struct InvokeWithIndexAndOrReferenceHelper<T, const T&, void> {
+    static constexpr bool valid = true;
+    template <typename F>
+    static auto Invoke(F&& f, size_t, T& e) {
+      return f(e);
+    }
+  };
+  template <typename T>
+  struct InvokeWithIndexAndOrReferenceHelper<T, size_t, T&> {
+    static constexpr bool valid = true;
+    template <typename F>
+    static auto Invoke(F&& f, size_t i, T& e) {
+      return f(i, e);
+    }
+  };
+  template <typename T>
+  struct InvokeWithIndexAndOrReferenceHelper<T, size_t, const T&> {
+    static constexpr bool valid = true;
+    template <typename F>
+    static auto Invoke(F&& f, size_t i, T& e) {
+      return f(i, e);
+    }
+  };
+  template <typename T, typename F>
+  static auto InvokeWithIndexAndOrReference(F&& f, size_t i, T& e) {
+    using Invoker = InvokeWithIndexAndOrReferenceHelper<
+        T, typename mozilla::FunctionTypeTraits<F>::template ParameterType<0>,
+        typename mozilla::FunctionTypeTraits<F>::template ParameterType<1>>;
+    static_assert(Invoker::valid,
+                  "ApplyIf's Function parameters must match either: (void), "
+                  "(size_t), (maybe-const value_type&), or "
+                  "(size_t, maybe-const value_type&)");
+    return Invoker::Invoke(std::forward<F>(f), i, e);
+  }
+
+ public:
+  // 'Apply' family of methods.
+  //
+  // The advantages of using Apply methods with lambdas include:
+  // - Safety of accessing elements from within the call, when the array cannot
+  //   have been modified between the iteration and the subsequent access.
+  // - Avoiding moot conversions: pointer->index during a search, followed by
+  //   index->pointer after the search when accessing the element.
+  // - Embedding your code into the algorithm, giving the compiler more chances
+  //   to optimize.
+
+  // Search for the first element comparing equal to aItem with the given
+  // comparator (`==` by default).
+  // If such an element exists, return the result of evaluating either:
+  // - `aFunction()`
+  // - `aFunction(index_type)`
+  // - `aFunction(maybe-const? value_type&)`
+  // - `aFunction(index_type, maybe-const? value_type&)`
+  // (`aFunction` must have one of the above signatures with these exact types,
+  //  including references; implicit conversions or generic types not allowed.
+  //  If `this` array is const, the referenced `value_type` must be const too;
+  //  otherwise it may be either const or non-const.)
+  // But if the element is not found, return the result of evaluating
+  // `aFunctionElse()`.
+  template <class Item, class Comparator, class Function, class FunctionElse>
+  auto ApplyIf(const Item& aItem, index_type aStart, const Comparator& aComp,
+               Function&& aFunction, FunctionElse&& aFunctionElse) const {
+    static_assert(
+        std::is_same_v<
+            typename mozilla::FunctionTypeTraits<Function>::ReturnType,
+            typename mozilla::FunctionTypeTraits<FunctionElse>::ReturnType>,
+        "ApplyIf's `Function` and `FunctionElse` must return the same type.");
+
+    ::detail::CompareWrapper<Comparator, Item> comp(aComp);
+
+    const value_type* const elements = Elements();
+    const value_type* const iend = elements + Length();
+    for (const value_type* iter = elements + aStart; iter != iend; ++iter) {
+      if (comp.Equals(*iter, aItem)) {
+        return InvokeWithIndexAndOrReference<const value_type>(
+            std::forward<Function>(aFunction), iter - elements, *iter);
+      }
+    }
+    return aFunctionElse();
+  }
+  template <class Item, class Comparator, class Function, class FunctionElse>
+  auto ApplyIf(const Item& aItem, index_type aStart, const Comparator& aComp,
+               Function&& aFunction, FunctionElse&& aFunctionElse) {
+    static_assert(
+        std::is_same_v<
+            typename mozilla::FunctionTypeTraits<Function>::ReturnType,
+            typename mozilla::FunctionTypeTraits<FunctionElse>::ReturnType>,
+        "ApplyIf's `Function` and `FunctionElse` must return the same type.");
+
+    ::detail::CompareWrapper<Comparator, Item> comp(aComp);
+
+    value_type* const elements = Elements();
+    value_type* const iend = elements + Length();
+    for (value_type* iter = elements + aStart; iter != iend; ++iter) {
+      if (comp.Equals(*iter, aItem)) {
+        return InvokeWithIndexAndOrReference<value_type>(
+            std::forward<Function>(aFunction), iter - elements, *iter);
+      }
+    }
+    return aFunctionElse();
+  }
+  template <class Item, class Function, class FunctionElse>
+  auto ApplyIf(const Item& aItem, index_type aStart, Function&& aFunction,
+               FunctionElse&& aFunctionElse) const {
+    return ApplyIf(aItem, aStart, nsDefaultComparator<value_type, Item>(),
+                   std::forward<Function>(aFunction),
+                   std::forward<FunctionElse>(aFunctionElse));
+  }
+  template <class Item, class Function, class FunctionElse>
+  auto ApplyIf(const Item& aItem, index_type aStart, Function&& aFunction,
+               FunctionElse&& aFunctionElse) {
+    return ApplyIf(aItem, aStart, nsDefaultComparator<value_type, Item>(),
+                   std::forward<Function>(aFunction),
+                   std::forward<FunctionElse>(aFunctionElse));
+  }
+  template <class Item, class Function, class FunctionElse>
+  auto ApplyIf(const Item& aItem, Function&& aFunction,
+               FunctionElse&& aFunctionElse) const {
+    return ApplyIf(aItem, 0, std::forward<Function>(aFunction),
+                   std::forward<FunctionElse>(aFunctionElse));
+  }
+  template <class Item, class Function, class FunctionElse>
+  auto ApplyIf(const Item& aItem, Function&& aFunction,
+               FunctionElse&& aFunctionElse) {
+    return ApplyIf(aItem, 0, std::forward<Function>(aFunction),
+                   std::forward<FunctionElse>(aFunctionElse));
+  }
+
+  //
+  // Allocation
+  //
+
+  // This method may increase the capacity of this array object to the
+  // specified amount.  This method may be called in advance of several
+  // AppendElement operations to minimize heap re-allocations.  This method
+  // will not reduce the number of elements in this array.
+  // @param aCapacity The desired capacity of this array.
+  // @return True if the operation succeeded; false if we ran out of memory
+ protected:
+  template <typename ActualAlloc = Alloc>
+  typename ActualAlloc::ResultType SetCapacity(size_type aCapacity) {
+    return ActualAlloc::Result(this->template EnsureCapacity<ActualAlloc>(
+        aCapacity, sizeof(value_type)));
+  }
+
+ public:
+  [[nodiscard]] bool SetCapacity(size_type aCapacity,
+                                 const mozilla::fallible_t&) {
+    return SetCapacity<FallibleAlloc>(aCapacity);
+  }
+
+  // This method modifies the length of the array.  If the new length is
+  // larger than the existing length of the array, then new elements will be
+  // constructed using value_type's default constructor.  Otherwise, this call
+  // removes elements from the array (see also RemoveElementsAt).
+  // @param aNewLen The desired length of this array.
+  // @return True if the operation succeeded; false otherwise.
+  // See also TruncateLength for a more efficient variant if the new length is
+  // guaranteed to be smaller than the old.
+ protected:
+  template <typename ActualAlloc = Alloc>
+  typename ActualAlloc::ResultType SetLength(size_type aNewLen) {
+    const size_type oldLen = Length();
+    if (aNewLen > oldLen) {
+      return ActualAlloc::ConvertBoolToResultType(
+          InsertElementsAtInternal<ActualAlloc>(oldLen, aNewLen - oldLen) !=
+          nullptr);
+    }
+
+    TruncateLengthUnsafe(aNewLen);
+    return ActualAlloc::ConvertBoolToResultType(true);
+  }
+
+ public:
+  [[nodiscard]] bool SetLength(size_type aNewLen, const mozilla::fallible_t&) {
+    return SetLength<FallibleAlloc>(aNewLen);
+  }
+
+  // This method modifies the length of the array, but may only be
+  // called when the new length is shorter than the old.  It can
+  // therefore be called when value_type has no default constructor,
+  // unlike SetLength.  It removes elements from the array (see also
+  // RemoveElementsAt).
+  // @param aNewLen The desired length of this array.
+  void TruncateLength(size_type aNewLen) {
+    // This assertion is redundant, but produces a better error message than the
+    // release assertion below.
+    MOZ_ASSERT(aNewLen <= Length(), "caller should use SetLength instead");
+
+    if (MOZ_UNLIKELY(aNewLen > Length())) {
+      mozilla::detail::InvalidArrayIndex_CRASH(aNewLen, Length());
+    }
+
+    TruncateLengthUnsafe(aNewLen);
+  }
+
+ private:
+  void TruncateLengthUnsafe(size_type aNewLen) {
+    const size_type oldLen = Length();
+    if (oldLen) {
+      DestructRange(aNewLen, oldLen - aNewLen);
+      base_type::mHdr->mLength = aNewLen;
+    }
+  }
+
+  // This method ensures that the array has length at least the given
+  // length.  If the current length is shorter than the given length,
+  // then new elements will be constructed using value_type's default
+  // constructor.
+  // @param aMinLen The desired minimum length of this array.
+  // @return True if the operation succeeded; false otherwise.
+ protected:
+  template <typename ActualAlloc = Alloc>
+  typename ActualAlloc::ResultType EnsureLengthAtLeast(size_type aMinLen) {
+    size_type oldLen = Length();
+    if (aMinLen > oldLen) {
+      return ActualAlloc::ConvertBoolToResultType(
+          !!InsertElementsAtInternal<ActualAlloc>(oldLen, aMinLen - oldLen));
+    }
+    return ActualAlloc::ConvertBoolToResultType(true);
+  }
+
+ public:
+  [[nodiscard]] bool EnsureLengthAtLeast(size_type aMinLen,
+                                         const mozilla::fallible_t&) {
+    return EnsureLengthAtLeast<FallibleAlloc>(aMinLen);
+  }
+
+  // This method inserts elements into the array, constructing
+  // them using value_type's default constructor.
+  // @param aIndex the place to insert the new elements. This must be no
+  //               greater than the current length of the array.
+  // @param aCount the number of elements to insert
+ private:
+  template <typename ActualAlloc>
+  value_type* InsertElementsAtInternal(index_type aIndex, size_type aCount) {
+    if (!ActualAlloc::Successful(this->template InsertSlotsAt<ActualAlloc>(
+            aIndex, aCount, sizeof(value_type), MOZ_ALIGNOF(value_type)))) {
+      return nullptr;
+    }
+
+    // Initialize the extra array elements
+    value_type* iter = Elements() + aIndex;
+    value_type* iend = iter + aCount;
+    for (; iter != iend; ++iter) {
+      elem_traits::Construct(iter);
+    }
+
+    return Elements() + aIndex;
+  }
+
+ public:
+  [[nodiscard]] value_type* InsertElementsAt(index_type aIndex,
+                                             size_type aCount,
+                                             const mozilla::fallible_t&) {
+    return InsertElementsAtInternal<FallibleAlloc>(aIndex, aCount);
+  }
+
+  // This method inserts elements into the array, constructing them
+  // value_type's copy constructor (or whatever one-arg constructor
+  // happens to match the Item type).
+  // @param aIndex the place to insert the new elements. This must be no
+  //               greater than the current length of the array.
+  // @param aCount the number of elements to insert.
+  // @param aItem the value to use when constructing the new elements.
+ private:
+  template <typename ActualAlloc, class Item>
+  value_type* InsertElementsAtInternal(index_type aIndex, size_type aCount,
+                                       const Item& aItem);
+
+ public:
+  template <class Item>
+  [[nodiscard]] value_type* InsertElementsAt(index_type aIndex,
+                                             size_type aCount,
+                                             const Item& aItem,
+                                             const mozilla::fallible_t&) {
+    return InsertElementsAt<Item, FallibleAlloc>(aIndex, aCount, aItem);
+  }
+
+  // This method may be called to minimize the memory used by this array.
+  void Compact() {
+    ShrinkCapacity(sizeof(value_type), MOZ_ALIGNOF(value_type));
+  }
+
+  //
+  // Sorting
+  //
+
+  // This method sorts the elements of the array.  It uses the LessThan
+  // method defined on the given Comparator object to collate elements or
+  // it wraps a tri-state comparison lambda into such a comparator.
+  // It uses std::sort. It expects value_type to be move assignable and
+  // constructible and uses those always, regardless of the chosen
+  // nsTArray_RelocationStrategy.
+  //
+  // @param aComp The Comparator used to collate elements.
+  template <class Comparator>
+  void Sort(const Comparator& aComp) {
+    static_assert(std::is_move_assignable_v<value_type>);
+    static_assert(std::is_move_constructible_v<value_type>);
+
+    ::detail::CompareWrapper<Comparator, value_type> comp(aComp);
+    std::sort(begin(), end(), [&comp](const auto& left, const auto& right) {
+      return comp.LessThan(left, right);
+    });
+  }
+
+  // A variation on the Sort method defined above that assumes that
+  // 'operator<' is defined for 'value_type'.
+  void Sort() { Sort(nsDefaultComparator<value_type, value_type>()); }
+
+  // This method sorts the elements of the array in a stable way (i.e. not
+  // changing the relative order of elements considered equal by the
+  // Comparator).  It uses the LessThan method defined on the given Comparator
+  // object to collate elements.
+  // It uses std::stable_sort. It expects value_type to be move assignable and
+  // constructible and uses those always, regardless of the chosen
+  // nsTArray_RelocationStrategy.
+  //
+  // @param aComp The Comparator used to collate elements.
+  template <class Comparator>
+  void StableSort(const Comparator& aComp) {
+    static_assert(std::is_move_assignable_v<value_type>);
+    static_assert(std::is_move_constructible_v<value_type>);
+
+    const ::detail::CompareWrapper<Comparator, value_type> comp(aComp);
+    std::stable_sort(Elements(), Elements() + Length(),
+                     [&comp](const auto& lhs, const auto& rhs) {
+                       return comp.LessThan(lhs, rhs);
+                     });
+  }
+
+  // A variation on the StableSort method defined above that assumes that
+  // 'operator<' is defined for 'value_type'.
+  void StableSort() {
+    StableSort(nsDefaultComparator<value_type, value_type>());
+  }
+
+  // This method reverses the array in place.
+  void Reverse() {
+    value_type* elements = Elements();
+    const size_type len = Length();
+    for (index_type i = 0, iend = len / 2; i < iend; ++i) {
+      std::swap(elements[i], elements[len - i - 1]);
+    }
+  }
+
+ protected:
+  using base_type::Hdr;
+  using base_type::ShrinkCapacity;
+
+  // This method invokes value_type's destructor on a range of elements.
+  // @param aStart The index of the first element to destroy.
+  // @param aCount The number of elements to destroy.
+  void DestructRange(index_type aStart, size_type aCount) {
+    value_type* iter = Elements() + aStart;
+    value_type* iend = iter + aCount;
+    for (; iter != iend; ++iter) {
+      elem_traits::Destruct(iter);
+    }
+  }
+
+  // This method invokes value_type's copy-constructor on a range of elements.
+  // @param aStart  The index of the first element to construct.
+  // @param aCount  The number of elements to construct.
+  // @param aValues The array of elements to copy.
+  template <class Item>
+  void AssignRange(index_type aStart, size_type aCount, const Item* aValues) {
+    AssignRangeAlgorithm<
+        std::is_trivially_copy_constructible_v<Item>,
+        std::is_same_v<Item, value_type>>::implementation(Elements(), aStart,
+                                                          aCount, aValues);
+  }
+};
+
+template <typename E, class Alloc>
+template <typename ActualAlloc, class Item>
+auto nsTArray_Impl<E, Alloc>::AssignInternal(const Item* aArray,
+                                             size_type aArrayLen) ->
+    typename ActualAlloc::ResultType {
+  static_assert(std::is_same_v<ActualAlloc, InfallibleAlloc> ||
+                std::is_same_v<ActualAlloc, FallibleAlloc>);
+
+  if constexpr (std::is_same_v<ActualAlloc, InfallibleAlloc>) {
+    ClearAndRetainStorage();
+  }
+  // Adjust memory allocation up-front to catch errors in the fallible case.
+  // We might relocate the elements to be destroyed unnecessarily. This could be
+  // optimized, but would make things more complicated.
+  if (!ActualAlloc::Successful(this->template EnsureCapacity<ActualAlloc>(
+          aArrayLen, sizeof(value_type)))) {
+    return ActualAlloc::ConvertBoolToResultType(false);
+  }
+
+  MOZ_ASSERT_IF(this->HasEmptyHeader(), aArrayLen == 0);
+  if (!this->HasEmptyHeader()) {
+    if constexpr (std::is_same_v<ActualAlloc, FallibleAlloc>) {
+      ClearAndRetainStorage();
+    }
+    AssignRange(0, aArrayLen, aArray);
+    base_type::mHdr->mLength = aArrayLen;
+  }
+
+  return ActualAlloc::ConvertBoolToResultType(true);
+}
+
+template <typename E, class Alloc>
+template <typename ActualAlloc, class Item>
+auto nsTArray_Impl<E, Alloc>::ReplaceElementsAtInternal(index_type aStart,
+                                                        size_type aCount,
+                                                        const Item* aArray,
+                                                        size_type aArrayLen)
+    -> value_type* {
+  if (MOZ_UNLIKELY(aStart > Length())) {
+    mozilla::detail::InvalidArrayIndex_CRASH(aStart, Length());
+  }
+  if (MOZ_UNLIKELY(aCount > Length() - aStart)) {
+    mozilla::detail::InvalidArrayIndex_CRASH(aStart + aCount, Length());
+  }
+
+  // Adjust memory allocation up-front to catch errors.
+  if (!ActualAlloc::Successful(this->template EnsureCapacity<ActualAlloc>(
+          Length() + aArrayLen - aCount, sizeof(value_type)))) {
+    return nullptr;
+  }
+  DestructRange(aStart, aCount);
+  this->template ShiftData<ActualAlloc>(
+      aStart, aCount, aArrayLen, sizeof(value_type), MOZ_ALIGNOF(value_type));
+  AssignRange(aStart, aArrayLen, aArray);
+  return Elements() + aStart;
+}
+
+template <typename E, class Alloc>
+void nsTArray_Impl<E, Alloc>::RemoveElementsAt(index_type aStart,
+                                               size_type aCount) {
+  MOZ_ASSERT(aCount == 0 || aStart < Length(), "Invalid aStart index");
+
+  mozilla::CheckedInt<index_type> rangeEnd = aStart;
+  rangeEnd += aCount;
+
+  if (MOZ_UNLIKELY(!rangeEnd.isValid() || rangeEnd.value() > Length())) {
+    mozilla::detail::InvalidArrayIndex_CRASH(aStart, Length());
+  }
+
+  RemoveElementsAtUnsafe(aStart, aCount);
+}
+
+template <typename E, class Alloc>
+void nsTArray_Impl<E, Alloc>::RemoveElementsAtUnsafe(index_type aStart,
+                                                     size_type aCount) {
+  DestructRange(aStart, aCount);
+  this->template ShiftData<InfallibleAlloc>(
+      aStart, aCount, 0, sizeof(value_type), MOZ_ALIGNOF(value_type));
+}
+
+template <typename E, class Alloc>
+void nsTArray_Impl<E, Alloc>::UnorderedRemoveElementsAt(index_type aStart,
+                                                        size_type aCount) {
+  MOZ_ASSERT(aCount == 0 || aStart < Length(), "Invalid aStart index");
+
+  mozilla::CheckedInt<index_type> rangeEnd = aStart;
+  rangeEnd += aCount;
+
+  if (MOZ_UNLIKELY(!rangeEnd.isValid() || rangeEnd.value() > Length())) {
+    mozilla::detail::InvalidArrayIndex_CRASH(aStart, Length());
+  }
+
+  // Destroy the elements which are being removed, and then swap elements in to
+  // replace them from the end. See the docs on the declaration of this
+  // function.
+  DestructRange(aStart, aCount);
+  this->template SwapFromEnd<InfallibleAlloc>(
+      aStart, aCount, sizeof(value_type), MOZ_ALIGNOF(value_type));
+}
+
+template <typename E, class Alloc>
+template <typename Predicate>
+auto nsTArray_Impl<E, Alloc>::RemoveElementsBy(Predicate aPredicate)
+    -> size_type {
+  if (this->HasEmptyHeader()) {
+    return 0;
+  }
+
+  index_type j = 0;
+  const index_type len = Length();
+  value_type* const elements = Elements();
+  for (index_type i = 0; i < len; ++i) {
+    const bool result = aPredicate(elements[i]);
+
+    // Check that the array has not been modified by the predicate.
+    MOZ_DIAGNOSTIC_ASSERT(len == base_type::mHdr->mLength &&
+                          elements == Elements());
+
+    if (result) {
+      elem_traits::Destruct(elements + i);
+    } else {
+      if (j < i) {
+        relocation_type::RelocateNonOverlappingRegion(
+            elements + j, elements + i, 1, sizeof(value_type));
+      }
+      ++j;
+    }
+  }
+
+  base_type::mHdr->mLength = j;
+  return len - j;
+}
+
+template <typename E, class Alloc>
+template <typename ActualAlloc, class Item>
+auto nsTArray_Impl<E, Alloc>::InsertElementsAtInternal(index_type aIndex,
+                                                       size_type aCount,
+                                                       const Item& aItem)
+    -> value_type* {
+  if (!ActualAlloc::Successful(this->template InsertSlotsAt<ActualAlloc>(
+          aIndex, aCount, sizeof(value_type), MOZ_ALIGNOF(value_type)))) {
+    return nullptr;
+  }
+
+  // Initialize the extra array elements
+  value_type* iter = Elements() + aIndex;
+  value_type* iend = iter + aCount;
+  for (; iter != iend; ++iter) {
+    elem_traits::Construct(iter, aItem);
+  }
+
+  return Elements() + aIndex;
+}
+
+template <typename E, class Alloc>
+template <typename ActualAlloc>
+auto nsTArray_Impl<E, Alloc>::InsertElementAtInternal(index_type aIndex)
+    -> value_type* {
+  if (MOZ_UNLIKELY(aIndex > Length())) {
+    mozilla::detail::InvalidArrayIndex_CRASH(aIndex, Length());
+  }
+
+  // Length() + 1 is guaranteed to not overflow, so EnsureCapacity is OK.
+  if (!ActualAlloc::Successful(this->template EnsureCapacity<ActualAlloc>(
+          Length() + 1, sizeof(value_type)))) {
+    return nullptr;
+  }
+  this->template ShiftData<ActualAlloc>(aIndex, 0, 1, sizeof(value_type),
+                                        MOZ_ALIGNOF(value_type));
+  value_type* elem = Elements() + aIndex;
+  elem_traits::Construct(elem);
+  return elem;
+}
+
+template <typename E, class Alloc>
+template <typename ActualAlloc, class Item>
+auto nsTArray_Impl<E, Alloc>::InsertElementAtInternal(index_type aIndex,
+                                                      Item&& aItem)
+    -> value_type* {
+  if (MOZ_UNLIKELY(aIndex > Length())) {
+    mozilla::detail::InvalidArrayIndex_CRASH(aIndex, Length());
+  }
+
+  // Length() + 1 is guaranteed to not overflow, so EnsureCapacity is OK.
+  if (!ActualAlloc::Successful(this->template EnsureCapacity<ActualAlloc>(
+          Length() + 1, sizeof(value_type)))) {
+    return nullptr;
+  }
+  this->template ShiftData<ActualAlloc>(aIndex, 0, 1, sizeof(value_type),
+                                        MOZ_ALIGNOF(value_type));
+  value_type* elem = Elements() + aIndex;
+  elem_traits::Construct(elem, std::forward<Item>(aItem));
+  return elem;
+}
+
+template <typename E, class Alloc>
+template <typename ActualAlloc, class Item>
+auto nsTArray_Impl<E, Alloc>::AppendElementsInternal(const Item* aArray,
+                                                     size_type aArrayLen)
+    -> value_type* {
+  if (!ActualAlloc::Successful(this->template ExtendCapacity<ActualAlloc>(
+          Length(), aArrayLen, sizeof(value_type)))) {
+    return nullptr;
+  }
+  index_type len = Length();
+  AssignRange(len, aArrayLen, aArray);
+  this->IncrementLength(aArrayLen);
+  return Elements() + len;
+}
+
+template <typename E, class Alloc>
+template <typename ActualAlloc, class Item, class Allocator>
+auto nsTArray_Impl<E, Alloc>::AppendElementsInternal(
+    nsTArray_Impl<Item, Allocator>&& aArray) -> value_type* {
+  if constexpr (std::is_same_v<Alloc, Allocator>) {
+    MOZ_ASSERT(&aArray != this, "argument must be different aArray");
+  }
+  if (Length() == 0) {
+    // XXX This might still be optimized. If aArray uses auto-storage but we
+    // won't, we might better retain our storage if it's sufficiently large.
+    this->ShrinkCapacityToZero(sizeof(value_type), MOZ_ALIGNOF(value_type));
+    this->MoveInit(aArray, sizeof(value_type), MOZ_ALIGNOF(value_type));
+    return Elements();
+  }
+
+  index_type len = Length();
+  index_type otherLen = aArray.Length();
+  if (!ActualAlloc::Successful(this->template ExtendCapacity<ActualAlloc>(
+          len, otherLen, sizeof(value_type)))) {
+    return nullptr;
+  }
+  relocation_type::RelocateNonOverlappingRegion(
+      Elements() + len, aArray.Elements(), otherLen, sizeof(value_type));
+  this->IncrementLength(otherLen);
+  aArray.template ShiftData<ActualAlloc>(0, otherLen, 0, sizeof(value_type),
+                                         MOZ_ALIGNOF(value_type));
+  return Elements() + len;
+}
+
+template <typename E, class Alloc>
+template <typename ActualAlloc, class Item>
+auto nsTArray_Impl<E, Alloc>::AppendElementInternal(Item&& aItem)
+    -> value_type* {
+  // Length() + 1 is guaranteed to not overflow, so EnsureCapacity is OK.
+  if (!ActualAlloc::Successful(this->template EnsureCapacity<ActualAlloc>(
+          Length() + 1, sizeof(value_type)))) {
+    return nullptr;
+  }
+  value_type* elem = Elements() + Length();
+  elem_traits::Construct(elem, std::forward<Item>(aItem));
+  this->mHdr->mLength += 1;
+  return elem;
+}
+
+template <typename E, class Alloc>
+template <typename ActualAlloc, class... Args>
+auto nsTArray_Impl<E, Alloc>::EmplaceBackInternal(Args&&... aArgs)
+    -> value_type* {
+  // Length() + 1 is guaranteed to not overflow, so EnsureCapacity is OK.
+  if (!ActualAlloc::Successful(this->template EnsureCapacity<ActualAlloc>(
+          Length() + 1, sizeof(value_type)))) {
+    return nullptr;
+  }
+  value_type* elem = Elements() + Length();
+  elem_traits::Emplace(elem, std::forward<Args>(aArgs)...);
+  this->mHdr->mLength += 1;
+  return elem;
+}
+
+template <typename E, typename Alloc>
+inline void ImplCycleCollectionUnlink(nsTArray_Impl<E, Alloc>& aField) {
+  aField.Clear();
+}
+
+namespace detail {
+// This is defined in the cpp file to avoid including
+// nsCycleCollectionNoteChild.h in this header file.
+void SetCycleCollectionArrayFlag(uint32_t& aFlags);
+}  // namespace detail
+
+template <typename E, typename Alloc>
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback,
+    nsTArray_Impl<E, Alloc>& aField, const char* aName, uint32_t aFlags = 0) {
+  ::detail::SetCycleCollectionArrayFlag(aFlags);
+  size_t length = aField.Length();
+  E* elements = aField.Elements();
+  for (size_t i = 0; i < length; ++i) {
+    ImplCycleCollectionTraverse(aCallback, elements[i], aName, aFlags);
+  }
+}
+
+//
+// nsTArray is an infallible vector class.  See the comment at the top of this
+// file for more details.
+//
+template <class E>
+class nsTArray : public nsTArray_Impl<E, nsTArrayInfallibleAllocator> {
+ public:
+  using InfallibleAlloc = nsTArrayInfallibleAllocator;
+  using base_type = nsTArray_Impl<E, InfallibleAlloc>;
+  using self_type = nsTArray<E>;
+  using typename base_type::index_type;
+  using typename base_type::size_type;
+  using typename base_type::value_type;
+
+  nsTArray() {}
+  explicit nsTArray(size_type aCapacity) : base_type(aCapacity) {}
+  MOZ_IMPLICIT nsTArray(std::initializer_list<E> aIL) {
+    AppendElements(aIL.begin(), aIL.size());
+  }
+
+  template <class Item>
+  nsTArray(const Item* aArray, size_type aArrayLen) {
+    AppendElements(aArray, aArrayLen);
+  }
+
+  template <class Item>
+  explicit nsTArray(mozilla::Span<Item> aSpan) {
+    AppendElements(aSpan);
+  }
+
+  template <class Allocator>
+  explicit nsTArray(const nsTArray_Impl<E, Allocator>& aOther)
+      : base_type(aOther) {}
+  template <class Allocator>
+  MOZ_IMPLICIT nsTArray(nsTArray_Impl<E, Allocator>&& aOther)
+      : base_type(std::move(aOther)) {}
+
+  template <class Allocator>
+  self_type& operator=(const nsTArray_Impl<E, Allocator>& aOther) {
+    base_type::operator=(aOther);
+    return *this;
+  }
+  template <class Allocator>
+  self_type& operator=(nsTArray_Impl<E, Allocator>&& aOther) {
+    // This is quite complex, since we don't know if we are an AutoTArray. While
+    // AutoTArray overrides this operator=, this might be called on a nsTArray&
+    // bound to an AutoTArray.
+    base_type::operator=(std::move(aOther));
+    return *this;
+  }
+
+  using base_type::AppendElement;
+  using base_type::AppendElements;
+  using base_type::EmplaceBack;
+  using base_type::EnsureLengthAtLeast;
+  using base_type::InsertElementAt;
+  using base_type::InsertElementsAt;
+  using base_type::InsertElementSorted;
+  using base_type::ReplaceElementsAt;
+  using base_type::SetCapacity;
+  using base_type::SetLength;
+
+  template <class Item>
+  mozilla::NotNull<value_type*> AppendElements(const Item* aArray,
+                                               size_type aArrayLen) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template AppendElementsInternal<InfallibleAlloc>(aArray,
+                                                               aArrayLen));
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> AppendElements(mozilla::Span<Item> aSpan) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template AppendElementsInternal<InfallibleAlloc>(aSpan.Elements(),
+                                                               aSpan.Length()));
+  }
+
+  template <class Item, class Allocator>
+  mozilla::NotNull<value_type*> AppendElements(
+      const nsTArray_Impl<Item, Allocator>& aArray) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template AppendElementsInternal<InfallibleAlloc>(
+            aArray.Elements(), aArray.Length()));
+  }
+
+  template <class Item, class Allocator>
+  mozilla::NotNull<value_type*> AppendElements(
+      nsTArray_Impl<Item, Allocator>&& aArray) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template AppendElementsInternal<InfallibleAlloc>(
+            std::move(aArray)));
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> AppendElement(Item&& aItem) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template AppendElementInternal<InfallibleAlloc>(
+            std::forward<Item>(aItem)));
+  }
+
+  mozilla::NotNull<value_type*> AppendElements(size_type aCount) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template AppendElementsInternal<InfallibleAlloc>(aCount));
+  }
+
+  mozilla::NotNull<value_type*> AppendElement() {
+    return mozilla::WrapNotNullUnchecked(
+        this->template AppendElementsInternal<InfallibleAlloc>(1));
+  }
+
+  self_type Clone() const {
+    self_type result;
+    result.Assign(*this);
+    return result;
+  }
+
+  mozilla::NotNull<value_type*> InsertElementsAt(index_type aIndex,
+                                                 size_type aCount) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template InsertElementsAtInternal<InfallibleAlloc>(aIndex,
+                                                                 aCount));
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> InsertElementsAt(index_type aIndex,
+                                                 size_type aCount,
+                                                 const Item& aItem) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template InsertElementsAtInternal<InfallibleAlloc>(aIndex, aCount,
+                                                                 aItem));
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> InsertElementsAt(index_type aIndex,
+                                                 const Item* aArray,
+                                                 size_type aArrayLen) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template ReplaceElementsAtInternal<InfallibleAlloc>(
+            aIndex, 0, aArray, aArrayLen));
+  }
+
+  template <class Item, class Allocator>
+  mozilla::NotNull<value_type*> InsertElementsAt(
+      index_type aIndex, const nsTArray_Impl<Item, Allocator>& aArray) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template ReplaceElementsAtInternal<InfallibleAlloc>(
+            aIndex, 0, aArray.Elements(), aArray.Length()));
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> InsertElementsAt(index_type aIndex,
+                                                 mozilla::Span<Item> aSpan) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template ReplaceElementsAtInternal<InfallibleAlloc>(
+            aIndex, 0, aSpan.Elements(), aSpan.Length()));
+  }
+
+  mozilla::NotNull<value_type*> InsertElementAt(index_type aIndex) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template InsertElementAtInternal<InfallibleAlloc>(aIndex));
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> InsertElementAt(index_type aIndex,
+                                                Item&& aItem) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template InsertElementAtInternal<InfallibleAlloc>(
+            aIndex, std::forward<Item>(aItem)));
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> ReplaceElementsAt(index_type aStart,
+                                                  size_type aCount,
+                                                  const Item* aArray,
+                                                  size_type aArrayLen) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template ReplaceElementsAtInternal<InfallibleAlloc>(
+            aStart, aCount, aArray, aArrayLen));
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> ReplaceElementsAt(
+      index_type aStart, size_type aCount, const nsTArray<Item>& aArray) {
+    return ReplaceElementsAt(aStart, aCount, aArray.Elements(),
+                             aArray.Length());
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> ReplaceElementsAt(index_type aStart,
+                                                  size_type aCount,
+                                                  mozilla::Span<Item> aSpan) {
+    return ReplaceElementsAt(aStart, aCount, aSpan.Elements(), aSpan.Length());
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> ReplaceElementsAt(index_type aStart,
+                                                  size_type aCount,
+                                                  const Item& aItem) {
+    return ReplaceElementsAt(aStart, aCount, &aItem, 1);
+  }
+
+  template <class Item, class Comparator>
+  mozilla::NotNull<value_type*> InsertElementSorted(Item&& aItem,
+                                                    const Comparator& aComp) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template InsertElementSortedInternal<InfallibleAlloc>(
+            std::forward<Item>(aItem), aComp));
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> InsertElementSorted(Item&& aItem) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template InsertElementSortedInternal<InfallibleAlloc>(
+            std::forward<Item>(aItem),
+            nsDefaultComparator<value_type, Item>{}));
+  }
+
+  template <class... Args>
+  mozilla::NotNull<value_type*> EmplaceBack(Args&&... aArgs) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template EmplaceBackInternal<InfallibleAlloc, Args...>(
+            std::forward<Args>(aArgs)...));
+  }
+};
+
+template <class E>
+class CopyableTArray : public nsTArray<E> {
+ public:
+  using nsTArray<E>::nsTArray;
+
+  CopyableTArray(const CopyableTArray& aOther) : nsTArray<E>() {
+    this->Assign(aOther);
+  }
+  CopyableTArray& operator=(const CopyableTArray& aOther) {
+    if (this != &aOther) {
+      this->Assign(aOther);
+    }
+    return *this;
+  }
+  template <typename Allocator>
+  MOZ_IMPLICIT CopyableTArray(const nsTArray_Impl<E, Allocator>& aOther) {
+    this->Assign(aOther);
+  }
+  template <typename Allocator>
+  CopyableTArray& operator=(const nsTArray_Impl<E, Allocator>& aOther) {
+    if constexpr (std::is_same_v<Allocator, nsTArrayInfallibleAllocator>) {
+      if (this == &aOther) {
+        return *this;
+      }
+    }
+    this->Assign(aOther);
+    return *this;
+  }
+  template <typename Allocator>
+  MOZ_IMPLICIT CopyableTArray(nsTArray_Impl<E, Allocator>&& aOther)
+      : nsTArray<E>{std::move(aOther)} {}
+  template <typename Allocator>
+  CopyableTArray& operator=(nsTArray_Impl<E, Allocator>&& aOther) {
+    static_cast<nsTArray<E>&>(*this) = std::move(aOther);
+    return *this;
+  }
+
+  CopyableTArray(CopyableTArray&&) = default;
+  CopyableTArray& operator=(CopyableTArray&&) = default;
+};
+
+//
+// FallibleTArray is a fallible vector class.
+//
+template <class E>
+class FallibleTArray : public nsTArray_Impl<E, nsTArrayFallibleAllocator> {
+ public:
+  typedef nsTArray_Impl<E, nsTArrayFallibleAllocator> base_type;
+  typedef FallibleTArray<E> self_type;
+  typedef typename base_type::size_type size_type;
+
+  FallibleTArray() = default;
+  explicit FallibleTArray(size_type aCapacity) : base_type(aCapacity) {}
+
+  template <class Allocator>
+  explicit FallibleTArray(const nsTArray_Impl<E, Allocator>& aOther)
+      : base_type(aOther) {}
+  template <class Allocator>
+  explicit FallibleTArray(nsTArray_Impl<E, Allocator>&& aOther)
+      : base_type(std::move(aOther)) {}
+
+  template <class Allocator>
+  self_type& operator=(const nsTArray_Impl<E, Allocator>& aOther) {
+    base_type::operator=(aOther);
+    return *this;
+  }
+  template <class Allocator>
+  self_type& operator=(nsTArray_Impl<E, Allocator>&& aOther) {
+    base_type::operator=(std::move(aOther));
+    return *this;
+  }
+};
+
+//
+// AutoTArray<E, N> is like nsTArray<E>, but with N elements of inline storage.
+// Storing more than N elements is fine, but it will cause a heap allocation.
+//
+template <class E, size_t N>
+class MOZ_NON_MEMMOVABLE AutoTArray : public nsTArray<E> {
+  static_assert(N != 0, "AutoTArray<E, 0> should be specialized");
+
+ public:
+  typedef AutoTArray<E, N> self_type;
+  typedef nsTArray<E> base_type;
+  typedef typename base_type::Header Header;
+  typedef typename base_type::value_type value_type;
+
+  AutoTArray() : mAlign() { Init(); }
+
+  AutoTArray(self_type&& aOther) : nsTArray<E>() {
+    Init();
+    this->MoveInit(aOther, sizeof(value_type), MOZ_ALIGNOF(value_type));
+  }
+
+  explicit AutoTArray(base_type&& aOther) : mAlign() {
+    Init();
+    this->MoveInit(aOther, sizeof(value_type), MOZ_ALIGNOF(value_type));
+  }
+
+  template <typename Allocator>
+  explicit AutoTArray(nsTArray_Impl<value_type, Allocator>&& aOther) {
+    Init();
+    this->MoveInit(aOther, sizeof(value_type), MOZ_ALIGNOF(value_type));
+  }
+
+  MOZ_IMPLICIT AutoTArray(std::initializer_list<E> aIL) : mAlign() {
+    Init();
+    this->AppendElements(aIL.begin(), aIL.size());
+  }
+
+  self_type& operator=(self_type&& aOther) {
+    base_type::operator=(std::move(aOther));
+    return *this;
+  }
+
+  template <typename Allocator>
+  self_type& operator=(nsTArray_Impl<value_type, Allocator>&& aOther) {
+    base_type::operator=(std::move(aOther));
+    return *this;
+  }
+
+  // Intentionally hides nsTArray_Impl::Clone to make clones usually be
+  // AutoTArray as well.
+  self_type Clone() const {
+    self_type result;
+    result.Assign(*this);
+    return result;
+  }
+
+ private:
+  // nsTArray_base casts itself as an nsAutoArrayBase in order to get a pointer
+  // to mAutoBuf.
+  template <class Allocator, class RelocationStrategy>
+  friend class nsTArray_base;
+
+  void Init() {
+    static_assert(MOZ_ALIGNOF(value_type) <= 8,
+                  "can't handle alignments greater than 8, "
+                  "see nsTArray_base::UsesAutoArrayBuffer()");
+    // Temporary work around for VS2012 RC compiler crash
+    Header** phdr = base_type::PtrToHdr();
+    *phdr = reinterpret_cast<Header*>(&mAutoBuf);
+    (*phdr)->mLength = 0;
+    (*phdr)->mCapacity = N;
+    (*phdr)->mIsAutoArray = 1;
+
+    MOZ_ASSERT(base_type::GetAutoArrayBuffer(MOZ_ALIGNOF(value_type)) ==
+                   reinterpret_cast<Header*>(&mAutoBuf),
+               "GetAutoArrayBuffer needs to be fixed");
+  }
+
+  // Declare mAutoBuf aligned to the maximum of the header's alignment and
+  // value_type's alignment.  We need to use a union rather than
+  // MOZ_ALIGNED_DECL because GCC is picky about what goes into
+  // __attribute__((aligned(foo))).
+  union {
+    char mAutoBuf[sizeof(nsTArrayHeader) + N * sizeof(value_type)];
+    // Do the max operation inline to ensure that it is a compile-time constant.
+    mozilla::AlignedElem<(MOZ_ALIGNOF(Header) > MOZ_ALIGNOF(value_type))
+                             ? MOZ_ALIGNOF(Header)
+                             : MOZ_ALIGNOF(value_type)>
+        mAlign;
+  };
+};
+
+//
+// Specialization of AutoTArray<E, N> for the case where N == 0.
+// AutoTArray<E, 0> behaves exactly like nsTArray<E>, but without this
+// specialization, it stores a useless inline header.
+//
+// We do have many AutoTArray<E, 0> objects in memory: about 2,000 per tab as
+// of May 2014. These are typically not explicitly AutoTArray<E, 0> but rather
+// AutoTArray<E, N> for some value N depending on template parameters, in
+// generic code.
+//
+// For that reason, we optimize this case with the below partial specialization,
+// which ensures that AutoTArray<E, 0> is just like nsTArray<E>, without any
+// inline header overhead.
+//
+template <class E>
+class AutoTArray<E, 0> : public nsTArray<E> {
+  using nsTArray<E>::nsTArray;
+};
+
+template <class E, size_t N>
+struct nsTArray_RelocationStrategy<AutoTArray<E, N>> {
+  using Type = nsTArray_RelocateUsingMoveConstructor<AutoTArray<E, N>>;
+};
+
+template <class E, size_t N>
+class CopyableAutoTArray : public AutoTArray<E, N> {
+ public:
+  typedef CopyableAutoTArray<E, N> self_type;
+  using AutoTArray<E, N>::AutoTArray;
+
+  CopyableAutoTArray(const CopyableAutoTArray& aOther) : AutoTArray<E, N>() {
+    this->Assign(aOther);
+  }
+  CopyableAutoTArray& operator=(const CopyableAutoTArray& aOther) {
+    if (this != &aOther) {
+      this->Assign(aOther);
+    }
+    return *this;
+  }
+  template <typename Allocator>
+  MOZ_IMPLICIT CopyableAutoTArray(const nsTArray_Impl<E, Allocator>& aOther) {
+    this->Assign(aOther);
+  }
+  template <typename Allocator>
+  CopyableAutoTArray& operator=(const nsTArray_Impl<E, Allocator>& aOther) {
+    if constexpr (std::is_same_v<Allocator, nsTArrayInfallibleAllocator>) {
+      if (this == &aOther) {
+        return *this;
+      }
+    }
+    this->Assign(aOther);
+    return *this;
+  }
+  template <typename Allocator>
+  MOZ_IMPLICIT CopyableAutoTArray(nsTArray_Impl<E, Allocator>&& aOther)
+      : AutoTArray<E, N>{std::move(aOther)} {}
+  template <typename Allocator>
+  CopyableAutoTArray& operator=(nsTArray_Impl<E, Allocator>&& aOther) {
+    static_cast<AutoTArray<E, N>&>(*this) = std::move(aOther);
+    return *this;
+  }
+
+  // CopyableTArray exists for cases where an explicit Clone is not possible.
+  // These uses should not be mixed, so we delete Clone() here.
+  self_type Clone() const = delete;
+
+  CopyableAutoTArray(CopyableAutoTArray&&) = default;
+  CopyableAutoTArray& operator=(CopyableAutoTArray&&) = default;
+};
+
+namespace mozilla {
+template <typename E, typename ArrayT>
+class nsTArrayBackInserter {
+  ArrayT* mArray;
+
+  class Proxy {
+    ArrayT& mArray;
+
+   public:
+    explicit Proxy(ArrayT& aArray) : mArray{aArray} {}
+
+    template <typename E2>
+    void operator=(E2&& aValue) {
+      mArray.AppendElement(std::forward<E2>(aValue));
+    }
+  };
+
+ public:
+  using iterator_category = std::output_iterator_tag;
+  using value_type = void;
+  using difference_type = void;
+  using pointer = void;
+  using reference = void;
+  explicit nsTArrayBackInserter(ArrayT& aArray) : mArray{&aArray} {}
+
+  // Return a proxy so that nsTArrayBackInserter has the default special member
+  // functions, and the operator= template is defined in Proxy rather than this
+  // class (which otherwise breaks with recent MS STL versions).
+  // See also Bug 1331137, comment 11.
+  Proxy operator*() { return Proxy(*mArray); }
+
+  nsTArrayBackInserter& operator++() { return *this; }
+  nsTArrayBackInserter& operator++(int) { return *this; }
+};
+}  // namespace mozilla
+
+template <typename E>
+auto MakeBackInserter(nsTArray<E>& aArray) {
+  return mozilla::nsTArrayBackInserter<E, nsTArray<E>>{aArray};
+}
+
+// Span integration
+namespace mozilla {
+template <typename E, class Alloc>
+Span(nsTArray_Impl<E, Alloc>&) -> Span<E>;
+
+template <typename E, class Alloc>
+Span(const nsTArray_Impl<E, Alloc>&) -> Span<const E>;
+
+// Provides a view on a nsTArray through which the existing array elements can
+// be accessed in a non-const way, but the array itself cannot be modified, so
+// that references to elements are guaranteed to be stable.
+template <typename E>
+class nsTArrayView {
+ public:
+  using element_type = E;
+  using pointer = element_type*;
+  using reference = element_type&;
+  using index_type = typename Span<element_type>::index_type;
+  using size_type = typename Span<element_type>::index_type;
+
+  explicit nsTArrayView(nsTArray<element_type> aArray)
+      : mArray(std::move(aArray)), mSpan(mArray) {}
+
+  element_type& operator[](index_type aIndex) { return mSpan[aIndex]; }
+
+  const element_type& operator[](index_type aIndex) const {
+    return mSpan[aIndex];
+  }
+
+  size_type Length() const { return mSpan.Length(); }
+
+  auto begin() { return mSpan.begin(); }
+  auto end() { return mSpan.end(); }
+  auto begin() const { return mSpan.begin(); }
+  auto end() const { return mSpan.end(); }
+  auto cbegin() const { return mSpan.cbegin(); }
+  auto cend() const { return mSpan.cend(); }
+
+  Span<element_type> AsSpan() { return mSpan; }
+  Span<const element_type> AsSpan() const { return mSpan; }
+
+ private:
+  nsTArray<element_type> mArray;
+  const Span<element_type> mSpan;
+};
+
+template <typename Range, typename = std::enable_if_t<std::is_same_v<
+                              typename std::iterator_traits<
+                                  typename Range::iterator>::iterator_category,
+                              std::random_access_iterator_tag>>>
+auto RangeSize(const Range& aRange) {
+  // See https://en.cppreference.com/w/cpp/iterator/begin, section 'User-defined
+  // overloads'.
+  using std::begin;
+  using std::end;
+
+  return std::distance(begin(aRange), end(aRange));
+}
+
+/**
+ * Materialize a range as a nsTArray (or a compatible variant, like AutoTArray)
+ * of an explicitly specified type. The array value type must be implicitly
+ * convertible from the range's value type.
+ */
+template <typename Array, typename Range>
+auto ToTArray(const Range& aRange) {
+  using std::begin;
+  using std::end;
+
+  Array res;
+  res.SetCapacity(RangeSize(aRange));
+  std::copy(begin(aRange), end(aRange), MakeBackInserter(res));
+  return res;
+}
+
+/**
+ * Materialize a range as a nsTArray of its (decayed) value type.
+ */
+template <typename Range>
+auto ToArray(const Range& aRange) {
+  return ToTArray<nsTArray<std::decay_t<
+      typename std::iterator_traits<typename Range::iterator>::value_type>>>(
+      aRange);
+}
+
+/**
+ * Appends all elements from a range to an array.
+ */
+template <typename Array, typename Range>
+void AppendToArray(Array& aArray, const Range& aRange) {
+  using std::begin;
+  using std::end;
+
+  aArray.SetCapacity(aArray.Length() + RangeSize(aRange));
+  std::copy(begin(aRange), end(aRange), MakeBackInserter(aArray));
+}
+
+}  // namespace mozilla
+
+// MOZ_DBG support
+
+template <class E, class Alloc>
+std::ostream& operator<<(std::ostream& aOut,
+                         const nsTArray_Impl<E, Alloc>& aTArray) {
+  return aOut << mozilla::Span(aTArray);
+}
+
+// Assert that AutoTArray doesn't have any extra padding inside.
+//
+// It's important that the data stored in this auto array takes up a multiple of
+// 8 bytes; e.g. AutoTArray<uint32_t, 1> wouldn't work.  Since AutoTArray
+// contains a pointer, its size must be a multiple of alignof(void*).  (This is
+// because any type may be placed into an array, and there's no padding between
+// elements of an array.)  The compiler pads the end of the structure to
+// enforce this rule.
+//
+// If we used AutoTArray<uint32_t, 1> below, this assertion would fail on a
+// 64-bit system, where the compiler inserts 4 bytes of padding at the end of
+// the auto array to make its size a multiple of alignof(void*) == 8 bytes.
+
+static_assert(sizeof(AutoTArray<uint32_t, 2>) ==
+                  sizeof(void*) + sizeof(nsTArrayHeader) + sizeof(uint32_t) * 2,
+              "AutoTArray shouldn't contain any extra padding, "
+              "see the comment");
+
+// Definitions of nsTArray_Impl methods
+#include "nsTArray-inl.h"
+
+#endif  // nsTArray_h__