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diff --git a/xpcom/base/CycleCollectedJSRuntime.h b/xpcom/base/CycleCollectedJSRuntime.h
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+++ b/xpcom/base/CycleCollectedJSRuntime.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/. */
+
+#ifndef mozilla_CycleCollectedJSRuntime_h
+#define mozilla_CycleCollectedJSRuntime_h
+
+#include "mozilla/CycleCollectedJSContext.h"
+#include "mozilla/DeferredFinalize.h"
+#include "mozilla/HashTable.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/SegmentedVector.h"
+#include "jsapi.h"
+#include "jsfriendapi.h"
+#include "js/TypeDecls.h"
+
+#include "nsCycleCollectionParticipant.h"
+#include "nsTHashMap.h"
+#include "nsHashKeys.h"
+#include "nsStringFwd.h"
+#include "nsTHashSet.h"
+
+class nsCycleCollectionNoteRootCallback;
+class nsIException;
+class nsWrapperCache;
+
+namespace mozilla {
+
+class JSGCThingParticipant : public nsCycleCollectionParticipant {
+ public:
+  constexpr JSGCThingParticipant() : nsCycleCollectionParticipant(false) {}
+
+  NS_IMETHOD_(void) Root(void*) override {
+    MOZ_ASSERT(false, "Don't call Root on GC things");
+  }
+
+  NS_IMETHOD_(void) Unlink(void*) override {
+    MOZ_ASSERT(false, "Don't call Unlink on GC things, as they may be dead");
+  }
+
+  NS_IMETHOD_(void) Unroot(void*) override {
+    MOZ_ASSERT(false, "Don't call Unroot on GC things, as they may be dead");
+  }
+
+  NS_IMETHOD_(void) DeleteCycleCollectable(void* aPtr) override {
+    MOZ_ASSERT(false, "Can't directly delete a cycle collectable GC thing");
+  }
+
+  NS_IMETHOD TraverseNative(void* aPtr,
+                            nsCycleCollectionTraversalCallback& aCb) override;
+
+  NS_DECL_CYCLE_COLLECTION_CLASS_NAME_METHOD(JSGCThingParticipant)
+};
+
+class JSZoneParticipant : public nsCycleCollectionParticipant {
+ public:
+  constexpr JSZoneParticipant() : nsCycleCollectionParticipant(false) {}
+
+  NS_IMETHOD_(void) Root(void*) override {
+    MOZ_ASSERT(false, "Don't call Root on GC things");
+  }
+
+  NS_IMETHOD_(void) Unlink(void*) override {
+    MOZ_ASSERT(false, "Don't call Unlink on GC things, as they may be dead");
+  }
+
+  NS_IMETHOD_(void) Unroot(void*) override {
+    MOZ_ASSERT(false, "Don't call Unroot on GC things, as they may be dead");
+  }
+
+  NS_IMETHOD_(void) DeleteCycleCollectable(void*) override {
+    MOZ_ASSERT(false, "Can't directly delete a cycle collectable GC thing");
+  }
+
+  NS_IMETHOD TraverseNative(void* aPtr,
+                            nsCycleCollectionTraversalCallback& aCb) override;
+
+  NS_DECL_CYCLE_COLLECTION_CLASS_NAME_METHOD(JSZoneParticipant)
+};
+
+class IncrementalFinalizeRunnable;
+
+// A map from JS holders to tracer objects, where the values are stored in
+// SegmentedVector to speed up iteration.
+class JSHolderMap {
+ public:
+  enum WhichHolders { AllHolders, HoldersRequiredForGrayMarking };
+
+  class Iter;
+
+  JSHolderMap();
+  ~JSHolderMap() { MOZ_RELEASE_ASSERT(!mHasIterator); }
+
+  bool Has(void* aHolder) const;
+  nsScriptObjectTracer* Get(void* aHolder) const;
+  nsScriptObjectTracer* Extract(void* aHolder);
+  void Put(void* aHolder, nsScriptObjectTracer* aTracer, JS::Zone* aZone);
+
+  size_t SizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const;
+
+ private:
+  struct Entry {
+    void* mHolder;
+    nsScriptObjectTracer* mTracer;
+#ifdef DEBUG
+    JS::Zone* mZone;
+#endif
+
+    Entry();
+    Entry(void* aHolder, nsScriptObjectTracer* aTracer, JS::Zone* aZone);
+  };
+
+  using EntryMap = mozilla::HashMap<void*, Entry*, DefaultHasher<void*>,
+                                    InfallibleAllocPolicy>;
+
+  using EntryVector = SegmentedVector<Entry, 256, InfallibleAllocPolicy>;
+
+  using EntryVectorMap =
+      mozilla::HashMap<JS::Zone*, UniquePtr<EntryVector>,
+                       DefaultHasher<JS::Zone*>, InfallibleAllocPolicy>;
+
+  class EntryVectorIter;
+
+  bool RemoveEntry(EntryVector& aJSHolders, Entry* aEntry);
+
+  // A map from a holder pointer to a pointer to an entry in a vector.
+  EntryMap mJSHolderMap;
+
+  // A vector of holders not associated with a particular zone or that can
+  // contain pointers to GC things in more than one zone.
+  EntryVector mAnyZoneJSHolders;
+
+  // A map from a zone to a vector of holders that only contain pointers to GC
+  // things in that zone.
+  //
+  // Currently this will only contain wrapper cache wrappers since these are the
+  // only holders to pass a zone parameter through to AddJSHolder.
+  EntryVectorMap mPerZoneJSHolders;
+
+  // Iterators can mutate the element vectors by removing stale elements. Allow
+  // at most one to exist at a time.
+  bool mHasIterator = false;
+};
+
+// An iterator over an EntryVector that skips over removed entries and removes
+// them from the map.
+class JSHolderMap::EntryVectorIter {
+ public:
+  EntryVectorIter(JSHolderMap& aMap, EntryVector& aVector)
+      : mHolderMap(aMap), mVector(aVector), mIter(aVector.Iter()) {
+    Settle();
+  }
+
+  const EntryVector& Vector() const { return mVector; }
+
+  bool Done() const { return mIter.Done(); }
+  const Entry& Get() const { return mIter.Get(); }
+  void Next() {
+    mIter.Next();
+    Settle();
+  }
+
+  operator const Entry*() const { return &Get(); }
+  const Entry* operator->() const { return &Get(); }
+
+ private:
+  void Settle();
+  friend class JSHolderMap::Iter;
+
+  JSHolderMap& mHolderMap;
+  EntryVector& mVector;
+  EntryVector::IterImpl mIter;
+};
+
+class JSHolderMap::Iter {
+ public:
+  explicit Iter(JSHolderMap& aMap, WhichHolders aWhich = AllHolders);
+
+  ~Iter() {
+    MOZ_RELEASE_ASSERT(mHolderMap.mHasIterator);
+    mHolderMap.mHasIterator = false;
+  }
+
+  bool Done() const { return mIter.Done(); }
+  const Entry& Get() const { return mIter.Get(); }
+  void Next() {
+    mIter.Next();
+    Settle();
+  }
+
+  // If the holders have been removed from the map while the iterator is live,
+  // then the iterator may point to a removed entry. Update the iterator to make
+  // sure it points to a valid entry or is done.
+  void UpdateForRemovals();
+
+  operator const Entry*() const { return &Get(); }
+  const Entry* operator->() const { return &Get(); }
+
+  JS::Zone* Zone() const { return mZone; }
+
+ private:
+  void Settle();
+
+  JSHolderMap& mHolderMap;
+  Vector<JS::Zone*, 1, InfallibleAllocPolicy> mZones;
+  JS::Zone* mZone = nullptr;
+  EntryVectorIter mIter;
+};
+
+class CycleCollectedJSRuntime {
+  friend class JSGCThingParticipant;
+  friend class JSZoneParticipant;
+  friend class IncrementalFinalizeRunnable;
+  friend class CycleCollectedJSContext;
+
+ protected:
+  CycleCollectedJSRuntime(JSContext* aMainContext);
+  virtual ~CycleCollectedJSRuntime();
+
+  virtual void Shutdown(JSContext* cx);
+
+  size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
+  void UnmarkSkippableJSHolders();
+
+  virtual void TraverseAdditionalNativeRoots(
+      nsCycleCollectionNoteRootCallback& aCb) {}
+  virtual void TraceAdditionalNativeGrayRoots(JSTracer* aTracer) {}
+
+  virtual void CustomGCCallback(JSGCStatus aStatus) {}
+  virtual void CustomOutOfMemoryCallback() {}
+
+  CycleCollectedJSContext* GetContext() { return mContext; }
+
+ private:
+  void DescribeGCThing(bool aIsMarked, JS::GCCellPtr aThing,
+                       nsCycleCollectionTraversalCallback& aCb) const;
+
+  virtual bool DescribeCustomObjects(JSObject* aObject, const JSClass* aClasp,
+                                     char (&aName)[72]) const {
+    return false;  // We did nothing.
+  }
+
+  void NoteGCThingJSChildren(JS::GCCellPtr aThing,
+                             nsCycleCollectionTraversalCallback& aCb) const;
+
+  void NoteGCThingXPCOMChildren(const JSClass* aClasp, JSObject* aObj,
+                                nsCycleCollectionTraversalCallback& aCb) const;
+
+  virtual bool NoteCustomGCThingXPCOMChildren(
+      const JSClass* aClasp, JSObject* aObj,
+      nsCycleCollectionTraversalCallback& aCb) const {
+    return false;  // We did nothing.
+  }
+
+  enum TraverseSelect { TRAVERSE_CPP, TRAVERSE_FULL };
+
+  void TraverseGCThing(TraverseSelect aTs, JS::GCCellPtr aThing,
+                       nsCycleCollectionTraversalCallback& aCb);
+
+  void TraverseZone(JS::Zone* aZone, nsCycleCollectionTraversalCallback& aCb);
+
+  static void TraverseObjectShim(void* aData, JS::GCCellPtr aThing,
+                                 const JS::AutoRequireNoGC& nogc);
+
+  void TraverseNativeRoots(nsCycleCollectionNoteRootCallback& aCb);
+
+  static void TraceBlackJS(JSTracer* aTracer, void* aData);
+
+  // Trace gray JS roots until budget is exceeded and return whether we
+  // finished.
+  static bool TraceGrayJS(JSTracer* aTracer, js::SliceBudget& budget,
+                          void* aData);
+
+  static void GCCallback(JSContext* aContext, JSGCStatus aStatus,
+                         JS::GCReason aReason, void* aData);
+  static void GCSliceCallback(JSContext* aContext, JS::GCProgress aProgress,
+                              const JS::GCDescription& aDesc);
+  static void GCNurseryCollectionCallback(JSContext* aContext,
+                                          JS::GCNurseryProgress aProgress,
+                                          JS::GCReason aReason);
+  static void OutOfMemoryCallback(JSContext* aContext, void* aData);
+
+  static bool ContextCallback(JSContext* aCx, unsigned aOperation, void* aData);
+
+  static void* BeforeWaitCallback(uint8_t* aMemory);
+  static void AfterWaitCallback(void* aCookie);
+
+  virtual void TraceNativeBlackRoots(JSTracer* aTracer){};
+
+#ifdef NS_BUILD_REFCNT_LOGGING
+  void TraceAllNativeGrayRoots(JSTracer* aTracer);
+#endif
+
+  bool TraceNativeGrayRoots(JSTracer* aTracer, JSHolderMap::WhichHolders aWhich,
+                            js::SliceBudget& aBudget);
+  bool TraceJSHolders(JSTracer* aTracer, JSHolderMap::Iter& aIter,
+                      js::SliceBudget& aBudget);
+
+ public:
+  enum DeferredFinalizeType {
+    // Never finalize immediately, because it would be unsafe.
+    FinalizeLater,
+    // Finalize later if we can, but it is okay to do it immediately.
+    FinalizeIncrementally,
+    // Finalize immediately, for shutdown or testing purposes.
+    FinalizeNow,
+  };
+
+  void FinalizeDeferredThings(DeferredFinalizeType aType);
+
+  virtual void PrepareForForgetSkippable() = 0;
+  virtual void BeginCycleCollectionCallback(mozilla::CCReason aReason) = 0;
+  virtual void EndCycleCollectionCallback(CycleCollectorResults& aResults) = 0;
+  virtual void DispatchDeferredDeletion(bool aContinuation,
+                                        bool aPurge = false) = 0;
+
+  // Two conditions, JSOutOfMemory and JSLargeAllocationFailure, are noted in
+  // crash reports. Here are the values that can appear in the reports:
+  enum class OOMState : uint32_t {
+    // The condition has never happened. No entry appears in the crash report.
+    OK,
+
+    // We are currently reporting the given condition.
+    //
+    // Suppose a crash report contains "JSLargeAllocationFailure:
+    // Reporting". This means we crashed while executing memory-pressure
+    // observers, trying to shake loose some memory. The large allocation in
+    // question did not return null: it is still on the stack. Had we not
+    // crashed, it would have been retried.
+    Reporting,
+
+    // The condition has been reported since the last GC.
+    //
+    // If a crash report contains "JSOutOfMemory: Reported", that means a small
+    // allocation failed, and then we crashed, probably due to buggy
+    // error-handling code that ran after allocation returned null.
+    //
+    // This contrasts with "Reporting" which means that no error-handling code
+    // had executed yet.
+    Reported,
+
+    // The condition has happened, but a GC cycle ended since then.
+    //
+    // GC is taken as a proxy for "we've been banging on the heap a good bit
+    // now and haven't crashed; the OOM was probably handled correctly".
+    Recovered
+  };
+
+  const char* OOMStateToString(const OOMState aOomState) const;
+
+  // Returns true if OOM was reported and a new successful GC cycle hasn't
+  // occurred since.
+  bool OOMReported();
+
+  void SetLargeAllocationFailure(OOMState aNewState);
+
+  void AnnotateAndSetOutOfMemory(OOMState* aStatePtr, OOMState aNewState);
+  void OnGC(JSContext* aContext, JSGCStatus aStatus, JS::GCReason aReason);
+  void OnOutOfMemory();
+  void OnLargeAllocationFailure();
+
+  JSRuntime* Runtime() { return mJSRuntime; }
+  const JSRuntime* Runtime() const { return mJSRuntime; }
+
+  bool HasPendingIdleGCTask() const {
+    // Idle GC task associates with JSRuntime.
+    MOZ_ASSERT_IF(mHasPendingIdleGCTask, Runtime());
+    return mHasPendingIdleGCTask;
+  }
+  void SetPendingIdleGCTask() {
+    // Idle GC task associates with JSRuntime.
+    MOZ_ASSERT(Runtime());
+    mHasPendingIdleGCTask = true;
+  }
+  void ClearPendingIdleGCTask() { mHasPendingIdleGCTask = false; }
+
+  void RunIdleTimeGCTask() {
+    if (HasPendingIdleGCTask()) {
+      JS::MaybeRunNurseryCollection(Runtime(),
+                                    JS::GCReason::EAGER_NURSERY_COLLECTION);
+      ClearPendingIdleGCTask();
+    }
+  }
+
+  bool IsIdleGCTaskNeeded() {
+    return !HasPendingIdleGCTask() && Runtime() &&
+           JS::WantEagerMinorGC(Runtime()) != JS::GCReason::NO_REASON;
+  }
+
+ public:
+  void AddJSHolder(void* aHolder, nsScriptObjectTracer* aTracer,
+                   JS::Zone* aZone);
+  void RemoveJSHolder(void* aHolder);
+#ifdef DEBUG
+  void AssertNoObjectsToTrace(void* aPossibleJSHolder);
+#endif
+
+  nsCycleCollectionParticipant* GCThingParticipant();
+  nsCycleCollectionParticipant* ZoneParticipant();
+
+  nsresult TraverseRoots(nsCycleCollectionNoteRootCallback& aCb);
+  virtual bool UsefulToMergeZones() const;
+  void FixWeakMappingGrayBits() const;
+  void CheckGrayBits() const;
+  bool AreGCGrayBitsValid() const;
+  void GarbageCollect(JS::GCOptions options, JS::GCReason aReason) const;
+
+  // This needs to be an nsWrapperCache, not a JSObject, because we need to know
+  // when our object gets moved.  But we can't trace it (and hence update our
+  // storage), because we do not want to keep it alive.  nsWrapperCache handles
+  // this for us via its "object moved" handling.
+  void NurseryWrapperAdded(nsWrapperCache* aCache);
+  void JSObjectsTenured();
+
+  void DeferredFinalize(DeferredFinalizeAppendFunction aAppendFunc,
+                        DeferredFinalizeFunction aFunc, void* aThing);
+  void DeferredFinalize(nsISupports* aSupports);
+
+  void DumpJSHeap(FILE* aFile);
+
+  // Add aZone to the set of zones waiting for a GC.
+  void AddZoneWaitingForGC(JS::Zone* aZone) {
+    mZonesWaitingForGC.Insert(aZone);
+  }
+
+  static void OnZoneDestroyed(JS::GCContext* aGcx, JS::Zone* aZone);
+
+  // Prepare any zones for GC that have been passed to AddZoneWaitingForGC()
+  // since the last GC or since the last call to PrepareWaitingZonesForGC(),
+  // whichever was most recent. If there were no such zones, prepare for a
+  // full GC.
+  void PrepareWaitingZonesForGC();
+
+  // Get the current thread's CycleCollectedJSRuntime.  Returns null if there
+  // isn't one.
+  static CycleCollectedJSRuntime* Get();
+
+  void SetContext(CycleCollectedJSContext* aContext);
+
+#ifdef NIGHTLY_BUILD
+  bool GetRecentDevError(JSContext* aContext,
+                         JS::MutableHandle<JS::Value> aError);
+  void ClearRecentDevError();
+#endif  // defined(NIGHTLY_BUILD)
+
+ private:
+  CycleCollectedJSContext* mContext;
+
+  JSGCThingParticipant mGCThingCycleCollectorGlobal;
+
+  JSZoneParticipant mJSZoneCycleCollectorGlobal;
+
+  JSRuntime* mJSRuntime;
+  bool mHasPendingIdleGCTask;
+
+  JS::GCSliceCallback mPrevGCSliceCallback;
+  JS::GCNurseryCollectionCallback mPrevGCNurseryCollectionCallback;
+
+  mozilla::TimeStamp mLatestNurseryCollectionStart;
+
+  JSHolderMap mJSHolders;
+  Maybe<JSHolderMap::Iter> mHolderIter;
+
+  typedef nsTHashMap<nsFuncPtrHashKey<DeferredFinalizeFunction>, void*>
+      DeferredFinalizerTable;
+  DeferredFinalizerTable mDeferredFinalizerTable;
+
+  RefPtr<IncrementalFinalizeRunnable> mFinalizeRunnable;
+
+  OOMState mOutOfMemoryState;
+  OOMState mLargeAllocationFailureState;
+
+  static const size_t kSegmentSize = 512;
+  SegmentedVector<nsWrapperCache*, kSegmentSize, InfallibleAllocPolicy>
+      mNurseryObjects;
+
+  nsTHashSet<JS::Zone*> mZonesWaitingForGC;
+
+  struct EnvironmentPreparer : public js::ScriptEnvironmentPreparer {
+    void invoke(JS::Handle<JSObject*> global, Closure& closure) override;
+  };
+  EnvironmentPreparer mEnvironmentPreparer;
+
+#ifdef DEBUG
+  bool mShutdownCalled;
+#endif
+
+#ifdef NIGHTLY_BUILD
+  // Implementation of the error interceptor.
+  // Built on nightly only to avoid any possible performance impact on release
+
+  struct ErrorInterceptor final : public JSErrorInterceptor {
+    virtual void interceptError(JSContext* cx,
+                                JS::Handle<JS::Value> exn) override;
+    void Shutdown(JSRuntime* rt);
+
+    // Copy of the details of the exception.
+    // We store this rather than the exception itself to avoid dealing with
+    // complicated garbage-collection scenarios, e.g. a JSContext being killed
+    // while we still hold onto an exception thrown from it.
+    struct ErrorDetails {
+      nsString mFilename;
+      nsString mMessage;
+      nsString mStack;
+      JSExnType mType;
+      uint32_t mLine;
+      uint32_t mColumn;
+    };
+
+    // If we have encountered at least one developer error,
+    // the first error we have encountered. Otherwise, or
+    // if we have reset since the latest error, `None`.
+    Maybe<ErrorDetails> mThrownError;
+  };
+  ErrorInterceptor mErrorInterceptor;
+
+#endif  // defined(NIGHTLY_BUILD)
+};
+
+void TraceScriptHolder(nsISupports* aHolder, JSTracer* aTracer);
+
+}  // namespace mozilla
+
+#endif  // mozilla_CycleCollectedJSRuntime_h