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diff --git a/mfbt/EnumSet.h b/mfbt/EnumSet.h
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+/* -*- 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/. */
+
+/* A set abstraction for enumeration values. */
+
+#ifndef mozilla_EnumSet_h
+#define mozilla_EnumSet_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/MathAlgorithms.h"
+
+#include <initializer_list>
+#include <type_traits>
+
+#include <stdint.h>
+
+namespace mozilla {
+
+/**
+ * EnumSet<T, U> is a set of values defined by an enumeration. It is implemented
+ * using a bit mask with the size of U for each value. It works both for enum
+ * and enum class types. EnumSet also works with U being a BitSet.
+ */
+template <typename T, typename Serialized = typename std::make_unsigned<
+                          typename std::underlying_type<T>::type>::type>
+class EnumSet {
+ public:
+  using valueType = T;
+  using serializedType = Serialized;
+
+  constexpr EnumSet() : mBitField() {}
+
+  constexpr MOZ_IMPLICIT EnumSet(T aEnum) : mBitField(bitFor(aEnum)) {}
+
+  constexpr EnumSet(T aEnum1, T aEnum2)
+      : mBitField(bitFor(aEnum1) | bitFor(aEnum2)) {}
+
+  constexpr EnumSet(T aEnum1, T aEnum2, T aEnum3)
+      : mBitField(bitFor(aEnum1) | bitFor(aEnum2) | bitFor(aEnum3)) {}
+
+  constexpr EnumSet(T aEnum1, T aEnum2, T aEnum3, T aEnum4)
+      : mBitField(bitFor(aEnum1) | bitFor(aEnum2) | bitFor(aEnum3) |
+                  bitFor(aEnum4)) {}
+
+  constexpr MOZ_IMPLICIT EnumSet(std::initializer_list<T> list) : mBitField() {
+    for (auto value : list) {
+      (*this) += value;
+    }
+  }
+
+#ifdef DEBUG
+  constexpr EnumSet(const EnumSet& aEnumSet) : mBitField(aEnumSet.mBitField) {}
+
+  constexpr EnumSet& operator=(const EnumSet& aEnumSet) {
+    mBitField = aEnumSet.mBitField;
+    incVersion();
+    return *this;
+  }
+#endif
+
+  /**
+   * Add an element
+   */
+  constexpr void operator+=(T aEnum) {
+    incVersion();
+    mBitField |= bitFor(aEnum);
+  }
+
+  /**
+   * Add an element
+   */
+  constexpr EnumSet operator+(T aEnum) const {
+    EnumSet result(*this);
+    result += aEnum;
+    return result;
+  }
+
+  /**
+   * Union
+   */
+  void operator+=(const EnumSet& aEnumSet) {
+    incVersion();
+    mBitField |= aEnumSet.mBitField;
+  }
+
+  /**
+   * Union
+   */
+  EnumSet operator+(const EnumSet& aEnumSet) const {
+    EnumSet result(*this);
+    result += aEnumSet;
+    return result;
+  }
+
+  /**
+   * Remove an element
+   */
+  void operator-=(T aEnum) {
+    incVersion();
+    mBitField &= ~(bitFor(aEnum));
+  }
+
+  /**
+   * Remove an element
+   */
+  EnumSet operator-(T aEnum) const {
+    EnumSet result(*this);
+    result -= aEnum;
+    return result;
+  }
+
+  /**
+   * Remove a set of elements
+   */
+  void operator-=(const EnumSet& aEnumSet) {
+    incVersion();
+    mBitField &= ~(aEnumSet.mBitField);
+  }
+
+  /**
+   * Remove a set of elements
+   */
+  EnumSet operator-(const EnumSet& aEnumSet) const {
+    EnumSet result(*this);
+    result -= aEnumSet;
+    return result;
+  }
+
+  /**
+   * Clear
+   */
+  void clear() {
+    incVersion();
+    mBitField = Serialized();
+  }
+
+  /**
+   * Intersection
+   */
+  void operator&=(const EnumSet& aEnumSet) {
+    incVersion();
+    mBitField &= aEnumSet.mBitField;
+  }
+
+  /**
+   * Intersection
+   */
+  EnumSet operator&(const EnumSet& aEnumSet) const {
+    EnumSet result(*this);
+    result &= aEnumSet;
+    return result;
+  }
+
+  /**
+   * Equality
+   */
+  bool operator==(const EnumSet& aEnumSet) const {
+    return mBitField == aEnumSet.mBitField;
+  }
+
+  /**
+   * Equality
+   */
+  bool operator==(T aEnum) const { return mBitField == bitFor(aEnum); }
+
+  /**
+   * Not equal
+   */
+  bool operator!=(const EnumSet& aEnumSet) const {
+    return !operator==(aEnumSet);
+  }
+
+  /**
+   * Not equal
+   */
+  bool operator!=(T aEnum) const { return !operator==(aEnum); }
+
+  /**
+   * Test is an element is contained in the set.
+   */
+  bool contains(T aEnum) const {
+    return static_cast<bool>(mBitField & bitFor(aEnum));
+  }
+
+  /**
+   * Test if a set is contained in the set.
+   */
+  bool contains(const EnumSet& aEnumSet) const {
+    return (mBitField & aEnumSet.mBitField) == aEnumSet.mBitField;
+  }
+
+  /**
+   * Return the number of elements in the set.
+   */
+  size_t size() const {
+    if constexpr (std::is_unsigned_v<Serialized>) {
+      if constexpr (kMaxBits > 32) {
+        return CountPopulation64(mBitField);
+      } else {
+        return CountPopulation32(mBitField);
+      }
+    } else {
+      return mBitField.Count();
+    }
+  }
+
+  bool isEmpty() const {
+    if constexpr (std::is_unsigned_v<Serialized>) {
+      return mBitField == 0;
+    } else {
+      return mBitField.IsEmpty();
+    }
+  }
+
+  Serialized serialize() const { return mBitField; }
+
+  void deserialize(Serialized aValue) {
+    incVersion();
+    mBitField = aValue;
+  }
+
+  class ConstIterator {
+    const EnumSet* mSet;
+    uint32_t mPos;
+#ifdef DEBUG
+    uint64_t mVersion;
+#endif
+
+    void checkVersion() const {
+      // Check that the set has not been modified while being iterated.
+      MOZ_ASSERT_IF(mSet, mSet->mVersion == mVersion);
+    }
+
+   public:
+    ConstIterator(const EnumSet& aSet, uint32_t aPos)
+        : mSet(&aSet), mPos(aPos) {
+#ifdef DEBUG
+      mVersion = mSet->mVersion;
+#endif
+      MOZ_ASSERT(aPos <= kMaxBits);
+      if (aPos != kMaxBits && !mSet->contains(T(mPos))) {
+        ++*this;
+      }
+    }
+
+    ConstIterator(const ConstIterator& aOther)
+        : mSet(aOther.mSet), mPos(aOther.mPos) {
+#ifdef DEBUG
+      mVersion = aOther.mVersion;
+      checkVersion();
+#endif
+    }
+
+    ConstIterator(ConstIterator&& aOther)
+        : mSet(aOther.mSet), mPos(aOther.mPos) {
+#ifdef DEBUG
+      mVersion = aOther.mVersion;
+      checkVersion();
+#endif
+      aOther.mSet = nullptr;
+    }
+
+    ~ConstIterator() { checkVersion(); }
+
+    bool operator==(const ConstIterator& other) const {
+      MOZ_ASSERT(mSet == other.mSet);
+      checkVersion();
+      return mPos == other.mPos;
+    }
+
+    bool operator!=(const ConstIterator& other) const {
+      return !(*this == other);
+    }
+
+    T operator*() const {
+      MOZ_ASSERT(mSet);
+      MOZ_ASSERT(mPos < kMaxBits);
+      MOZ_ASSERT(mSet->contains(T(mPos)));
+      checkVersion();
+      return T(mPos);
+    }
+
+    ConstIterator& operator++() {
+      MOZ_ASSERT(mSet);
+      MOZ_ASSERT(mPos < kMaxBits);
+      checkVersion();
+      do {
+        mPos++;
+      } while (mPos < kMaxBits && !mSet->contains(T(mPos)));
+      return *this;
+    }
+  };
+
+  ConstIterator begin() const { return ConstIterator(*this, 0); }
+
+  ConstIterator end() const { return ConstIterator(*this, kMaxBits); }
+
+ private:
+  constexpr static Serialized bitFor(T aEnum) {
+    auto bitNumber = static_cast<size_t>(aEnum);
+    MOZ_DIAGNOSTIC_ASSERT(bitNumber < kMaxBits);
+    if constexpr (std::is_unsigned_v<Serialized>) {
+      return static_cast<Serialized>(Serialized{1} << bitNumber);
+    } else {
+      Serialized bitField;
+      bitField[bitNumber] = true;
+      return bitField;
+    }
+  }
+
+  constexpr void incVersion() {
+#ifdef DEBUG
+    mVersion++;
+#endif
+  }
+
+  static constexpr size_t MaxBits() {
+    if constexpr (std::is_unsigned_v<Serialized>) {
+      return sizeof(Serialized) * 8;
+    } else {
+      return Serialized::Size();
+    }
+  }
+
+  static constexpr size_t kMaxBits = MaxBits();
+
+  Serialized mBitField;
+
+#ifdef DEBUG
+  uint64_t mVersion = 0;
+#endif
+};
+
+}  // namespace mozilla
+
+#endif /* mozilla_EnumSet_h_*/