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diff --git a/js/src/irregexp/util/VectorShim.h b/js/src/irregexp/util/VectorShim.h
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+++ b/js/src/irregexp/util/VectorShim.h
@@ -0,0 +1,231 @@
+// Copyright 2014 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef V8_UTIL_VECTOR_H_
+#define V8_UTIL_VECTOR_H_
+
+#include <algorithm>
+#include <cstring>
+#include <iterator>
+#include <memory>
+
+#include "js/AllocPolicy.h"
+#include "js/Utility.h"
+#include "js/Vector.h"
+
+namespace v8 {
+namespace internal {
+
+//////////////////////////////////////////////////
+
+// Adapted from:
+// https://github.com/v8/v8/blob/5f69bbc233c2d1baf149faf869a7901603929914/src/utils/allocation.h#L36-L58
+
+template <typename T>
+T* NewArray(size_t size) {
+  static_assert(std::is_pod<T>::value, "");
+  js::AutoEnterOOMUnsafeRegion oomUnsafe;
+  T* result = static_cast<T*>(js_malloc(size * sizeof(T)));
+  if (!result) {
+    oomUnsafe.crash("Irregexp NewArray");
+  }
+  return result;
+}
+
+template <typename T>
+void DeleteArray(T* array) {
+  js_free(array);
+}
+
+}  // namespace internal
+
+namespace base {
+
+//////////////////////////////////////////////////
+
+// A non-resizable vector containing a pointer and a length.
+// The Vector may or may not own the pointer, depending on context.
+// Origin:
+// https://github.com/v8/v8/blob/5f69bbc233c2d1baf149faf869a7901603929914/src/utils/vector.h#L20-L134
+
+template <typename T>
+class Vector {
+ public:
+  constexpr Vector() : start_(nullptr), length_(0) {}
+
+  constexpr Vector(T* data, size_t length) : start_(data), length_(length) {
+    MOZ_ASSERT_IF(length != 0, data != nullptr);
+  }
+
+  static Vector<T> New(size_t length) {
+    return Vector<T>(v8::internal::NewArray<T>(length), length);
+  }
+
+  // Returns a vector using the same backing storage as this one,
+  // spanning from and including 'from', to but not including 'to'.
+  Vector<T> SubVector(size_t from, size_t to) const {
+    MOZ_ASSERT(from <= to);
+    MOZ_ASSERT(to <= length_);
+    return Vector<T>(begin() + from, to - from);
+  }
+
+  // Returns the length of the vector. Only use this if you really need an
+  // integer return value. Use {size()} otherwise.
+  int length() const {
+    MOZ_ASSERT(length_ <= static_cast<size_t>(std::numeric_limits<int>::max()));
+    return static_cast<int>(length_);
+  }
+
+  // Returns the length of the vector as a size_t.
+  constexpr size_t size() const { return length_; }
+
+  // Returns whether or not the vector is empty.
+  constexpr bool empty() const { return length_ == 0; }
+
+  // Access individual vector elements - checks bounds in debug mode.
+  T& operator[](size_t index) const {
+    MOZ_ASSERT(index < length_);
+    return start_[index];
+  }
+
+  const T& at(size_t index) const { return operator[](index); }
+
+  T& first() { return start_[0]; }
+
+  T& last() {
+    MOZ_ASSERT(length_ > 0);
+    return start_[length_ - 1];
+  }
+
+  // Returns a pointer to the start of the data in the vector.
+  constexpr T* begin() const { return start_; }
+
+  // Returns a pointer past the end of the data in the vector.
+  constexpr T* end() const { return start_ + length_; }
+
+  // Returns a clone of this vector with a new backing store.
+  Vector<T> Clone() const {
+    T* result = v8::internal::NewArray<T>(length_);
+    for (size_t i = 0; i < length_; i++) result[i] = start_[i];
+    return Vector<T>(result, length_);
+  }
+
+  void Truncate(size_t length) {
+    MOZ_ASSERT(length <= length_);
+    length_ = length;
+  }
+
+  // Releases the array underlying this vector. Once disposed the
+  // vector is empty.
+  void Dispose() {
+    DeleteArray(start_);
+    start_ = nullptr;
+    length_ = 0;
+  }
+
+  Vector<T> operator+(size_t offset) const {
+    MOZ_ASSERT(offset <= length_);
+    return Vector<T>(start_ + offset, length_ - offset);
+  }
+
+  Vector<T> operator+=(size_t offset) {
+    MOZ_ASSERT(offset <= length_);
+    start_ += offset;
+    length_ -= offset;
+    return *this;
+  }
+
+  // Implicit conversion from Vector<T> to Vector<const T>.
+  inline operator Vector<const T>() const {
+    return Vector<const T>::cast(*this);
+  }
+
+  template <typename S>
+  static constexpr Vector<T> cast(Vector<S> input) {
+    return Vector<T>(reinterpret_cast<T*>(input.begin()),
+                     input.length() * sizeof(S) / sizeof(T));
+  }
+
+  bool operator==(const Vector<const T> other) const {
+    if (length_ != other.length_) return false;
+    if (start_ == other.start_) return true;
+    for (size_t i = 0; i < length_; ++i) {
+      if (start_[i] != other.start_[i]) {
+        return false;
+      }
+    }
+    return true;
+  }
+
+ private:
+  T* start_;
+  size_t length_;
+};
+
+// The resulting vector does not contain a null-termination byte. If you want
+// the null byte, use ArrayVector("foo").
+inline Vector<const char> CStrVector(const char* data) {
+  return Vector<const char>(data, strlen(data));
+}
+
+// Construct a Vector from a start pointer and a size.
+template <typename T>
+inline constexpr Vector<T> VectorOf(T* start, size_t size) {
+  return {start, size};
+}
+
+class DefaultAllocator {
+ public:
+  using Policy = js::SystemAllocPolicy;
+  Policy policy() const { return js::SystemAllocPolicy(); }
+};
+
+// SmallVector uses inline storage first, and reallocates when full.
+// It is basically equivalent to js::Vector, and is implemented
+// as a thin wrapper.
+// V8's implementation:
+// https://github.com/v8/v8/blob/main/src/base/small-vector.h
+template <typename T, size_t kSize, typename Allocator = DefaultAllocator>
+class SmallVector {
+ public:
+  explicit SmallVector(const Allocator& allocator = DefaultAllocator())
+      : inner_(allocator.policy()) {}
+  SmallVector(size_t size) { resize_no_init(size); }
+
+  inline bool empty() const { return inner_.empty(); }
+  inline const T& back() const { return inner_.back(); }
+  inline void pop_back() { inner_.popBack(); };
+  template <typename... Args>
+  inline void emplace_back(Args&&... args) {
+    js::AutoEnterOOMUnsafeRegion oomUnsafe;
+    if (!inner_.emplaceBack(args...)) {
+      oomUnsafe.crash("Irregexp SmallVector emplace_back");
+    }
+  };
+  inline size_t size() const { return inner_.length(); }
+  inline const T& at(size_t index) const { return inner_[index]; }
+  T* data() { return inner_.begin(); }
+  T* begin() { return inner_.begin(); }
+
+  T& operator[](size_t index) { return inner_[index]; }
+  const T& operator[](size_t index) const { return inner_[index]; }
+
+  inline void clear() { inner_.clear(); }
+
+  void resize_no_init(size_t new_size) {
+    js::AutoEnterOOMUnsafeRegion oomUnsafe;
+    if (!inner_.resizeUninitialized(new_size)) {
+      oomUnsafe.crash("Irregexp SmallVector resize");
+    }
+  }
+
+ private:
+  js::Vector<T, kSize, typename Allocator::Policy> inner_;
+};
+
+}  // namespace base
+
+}  // namespace v8
+
+#endif  // V8_UTIL_VECTOR_H_