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diff --git a/js/public/TracingAPI.h b/js/public/TracingAPI.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * 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 js_TracingAPI_h
+#define js_TracingAPI_h
+
+#include "jsalloc.h"
+
+#include "js/HashTable.h"
+#include "js/HeapAPI.h"
+#include "js/TraceKind.h"
+
+class JS_PUBLIC_API(JSTracer);
+
+namespace JS {
+class JS_PUBLIC_API(CallbackTracer);
+template <typename T> class Heap;
+template <typename T> class TenuredHeap;
+
+/** Returns a static string equivalent of |kind|. */
+JS_FRIEND_API(const char*)
+GCTraceKindToAscii(JS::TraceKind kind);
+
+} // namespace JS
+
+enum WeakMapTraceKind {
+    /**
+     * Do not trace into weak map keys or values during traversal. Users must
+     * handle weak maps manually.
+     */
+    DoNotTraceWeakMaps,
+
+    /**
+     * Do true ephemeron marking with a weak key lookup marking phase. This is
+     * the default for GCMarker.
+     */
+    ExpandWeakMaps,
+
+    /**
+     * Trace through to all values, irrespective of whether the keys are live
+     * or not. Used for non-marking tracers.
+     */
+    TraceWeakMapValues,
+
+    /**
+     * Trace through to all keys and values, irrespective of whether the keys
+     * are live or not. Used for non-marking tracers.
+     */
+    TraceWeakMapKeysValues
+};
+
+class JS_PUBLIC_API(JSTracer)
+{
+  public:
+    // Return the runtime set on the tracer.
+    JSRuntime* runtime() const { return runtime_; }
+
+    // Return the weak map tracing behavior currently set on this tracer.
+    WeakMapTraceKind weakMapAction() const { return weakMapAction_; }
+
+    enum class TracerKindTag {
+        // Marking path: a tracer used only for marking liveness of cells, not
+        // for moving them. The kind will transition to WeakMarking after
+        // everything reachable by regular edges has been marked.
+        Marking,
+
+        // Same as Marking, except we have now moved on to the "weak marking
+        // phase", in which every marked obj/script is immediately looked up to
+        // see if it is a weak map key (and therefore might require marking its
+        // weak map value).
+        WeakMarking,
+
+        // A tracer that traverses the graph for the purposes of moving objects
+        // from the nursery to the tenured area.
+        Tenuring,
+
+        // General-purpose traversal that invokes a callback on each cell.
+        // Traversing children is the responsibility of the callback.
+        Callback
+    };
+    bool isMarkingTracer() const { return tag_ == TracerKindTag::Marking || tag_ == TracerKindTag::WeakMarking; }
+    bool isWeakMarkingTracer() const { return tag_ == TracerKindTag::WeakMarking; }
+    bool isTenuringTracer() const { return tag_ == TracerKindTag::Tenuring; }
+    bool isCallbackTracer() const { return tag_ == TracerKindTag::Callback; }
+    inline JS::CallbackTracer* asCallbackTracer();
+#ifdef DEBUG
+    bool checkEdges() { return checkEdges_; }
+#endif
+
+  protected:
+    JSTracer(JSRuntime* rt, TracerKindTag tag,
+             WeakMapTraceKind weakTraceKind = TraceWeakMapValues)
+      : runtime_(rt)
+      , weakMapAction_(weakTraceKind)
+#ifdef DEBUG
+      , checkEdges_(true)
+#endif
+      , tag_(tag)
+    {}
+
+#ifdef DEBUG
+    // Set whether to check edges are valid in debug builds.
+    void setCheckEdges(bool check) {
+        checkEdges_ = check;
+    }
+#endif
+
+  private:
+    JSRuntime* runtime_;
+    WeakMapTraceKind weakMapAction_;
+#ifdef DEBUG
+    bool checkEdges_;
+#endif
+
+  protected:
+    TracerKindTag tag_;
+};
+
+namespace JS {
+
+class AutoTracingName;
+class AutoTracingIndex;
+class AutoTracingCallback;
+
+class JS_PUBLIC_API(CallbackTracer) : public JSTracer
+{
+  public:
+    CallbackTracer(JSRuntime* rt, WeakMapTraceKind weakTraceKind = TraceWeakMapValues)
+      : JSTracer(rt, JSTracer::TracerKindTag::Callback, weakTraceKind),
+        contextName_(nullptr), contextIndex_(InvalidIndex), contextFunctor_(nullptr)
+    {}
+    CallbackTracer(JSContext* cx, WeakMapTraceKind weakTraceKind = TraceWeakMapValues);
+
+    // Override these methods to receive notification when an edge is visited
+    // with the type contained in the callback. The default implementation
+    // dispatches to the fully-generic onChild implementation, so for cases that
+    // do not care about boxing overhead and do not need the actual edges,
+    // just override the generic onChild.
+    virtual void onObjectEdge(JSObject** objp) { onChild(JS::GCCellPtr(*objp)); }
+    virtual void onStringEdge(JSString** strp) { onChild(JS::GCCellPtr(*strp)); }
+    virtual void onSymbolEdge(JS::Symbol** symp) { onChild(JS::GCCellPtr(*symp)); }
+    virtual void onScriptEdge(JSScript** scriptp) { onChild(JS::GCCellPtr(*scriptp)); }
+    virtual void onShapeEdge(js::Shape** shapep) {
+        onChild(JS::GCCellPtr(*shapep, JS::TraceKind::Shape));
+    }
+    virtual void onObjectGroupEdge(js::ObjectGroup** groupp) {
+        onChild(JS::GCCellPtr(*groupp, JS::TraceKind::ObjectGroup));
+    }
+    virtual void onBaseShapeEdge(js::BaseShape** basep) {
+        onChild(JS::GCCellPtr(*basep, JS::TraceKind::BaseShape));
+    }
+    virtual void onJitCodeEdge(js::jit::JitCode** codep) {
+        onChild(JS::GCCellPtr(*codep, JS::TraceKind::JitCode));
+    }
+    virtual void onLazyScriptEdge(js::LazyScript** lazyp) {
+        onChild(JS::GCCellPtr(*lazyp, JS::TraceKind::LazyScript));
+    }
+    virtual void onScopeEdge(js::Scope** scopep) {
+        onChild(JS::GCCellPtr(*scopep, JS::TraceKind::Scope));
+    }
+
+    // Override this method to receive notification when a node in the GC
+    // heap graph is visited.
+    virtual void onChild(const JS::GCCellPtr& thing) = 0;
+
+    // Access to the tracing context:
+    // When tracing with a JS::CallbackTracer, we invoke the callback with the
+    // edge location and the type of target. This is useful for operating on
+    // the edge in the abstract or on the target thing, satisfying most common
+    // use cases.  However, some tracers need additional detail about the
+    // specific edge that is being traced in order to be useful. Unfortunately,
+    // the raw pointer to the edge that we provide is not enough information to
+    // infer much of anything useful about that edge.
+    //
+    // In order to better support use cases that care in particular about edges
+    // -- as opposed to the target thing -- tracing implementations are
+    // responsible for providing extra context information about each edge they
+    // trace, as it is traced. This contains, at a minimum, an edge name and,
+    // when tracing an array, the index. Further specialization can be achived
+    // (with some complexity), by associating a functor with the tracer so
+    // that, when requested, the user can generate totally custom edge
+    // descriptions.
+
+    // Returns the current edge's name. It is only valid to call this when
+    // inside the trace callback, however, the edge name will always be set.
+    const char* contextName() const { MOZ_ASSERT(contextName_); return contextName_; }
+
+    // Returns the current edge's index, if marked as part of an array of edges.
+    // This must be called only inside the trace callback. When not tracing an
+    // array, the value will be InvalidIndex.
+    const static size_t InvalidIndex = size_t(-1);
+    size_t contextIndex() const { return contextIndex_; }
+
+    // Build a description of this edge in the heap graph. This call may invoke
+    // the context functor, if set, which may inspect arbitrary areas of the
+    // heap. On the other hand, the description provided by this method may be
+    // substantially more accurate and useful than those provided by only the
+    // contextName and contextIndex.
+    void getTracingEdgeName(char* buffer, size_t bufferSize);
+
+    // The trace implementation may associate a callback with one or more edges
+    // using AutoTracingDetails. This functor is called by getTracingEdgeName
+    // and is responsible for providing a textual representation of the
+    // currently being traced edge. The callback has access to the full heap,
+    // including the currently set tracing context.
+    class ContextFunctor {
+      public:
+        virtual void operator()(CallbackTracer* trc, char* buf, size_t bufsize) = 0;
+    };
+
+#ifdef DEBUG
+    enum class TracerKind { DoNotCare, Moving, GrayBuffering, VerifyTraceProtoAndIface };
+    virtual TracerKind getTracerKind() const { return TracerKind::DoNotCare; }
+#endif
+
+    // In C++, overriding a method hides all methods in the base class with
+    // that name, not just methods with that signature. Thus, the typed edge
+    // methods have to have distinct names to allow us to override them
+    // individually, which is freqently useful if, for example, we only want to
+    // process only one type of edge.
+    void dispatchToOnEdge(JSObject** objp) { onObjectEdge(objp); }
+    void dispatchToOnEdge(JSString** strp) { onStringEdge(strp); }
+    void dispatchToOnEdge(JS::Symbol** symp) { onSymbolEdge(symp); }
+    void dispatchToOnEdge(JSScript** scriptp) { onScriptEdge(scriptp); }
+    void dispatchToOnEdge(js::Shape** shapep) { onShapeEdge(shapep); }
+    void dispatchToOnEdge(js::ObjectGroup** groupp) { onObjectGroupEdge(groupp); }
+    void dispatchToOnEdge(js::BaseShape** basep) { onBaseShapeEdge(basep); }
+    void dispatchToOnEdge(js::jit::JitCode** codep) { onJitCodeEdge(codep); }
+    void dispatchToOnEdge(js::LazyScript** lazyp) { onLazyScriptEdge(lazyp); }
+    void dispatchToOnEdge(js::Scope** scopep) { onScopeEdge(scopep); }
+
+  private:
+    friend class AutoTracingName;
+    const char* contextName_;
+
+    friend class AutoTracingIndex;
+    size_t contextIndex_;
+
+    friend class AutoTracingDetails;
+    ContextFunctor* contextFunctor_;
+};
+
+// Set the name portion of the tracer's context for the current edge.
+class MOZ_RAII AutoTracingName
+{
+    CallbackTracer* trc_;
+    const char* prior_;
+
+  public:
+    AutoTracingName(CallbackTracer* trc, const char* name) : trc_(trc), prior_(trc->contextName_) {
+        MOZ_ASSERT(name);
+        trc->contextName_ = name;
+    }
+    ~AutoTracingName() {
+        MOZ_ASSERT(trc_->contextName_);
+        trc_->contextName_ = prior_;
+    }
+};
+
+// Set the index portion of the tracer's context for the current range.
+class MOZ_RAII AutoTracingIndex
+{
+    CallbackTracer* trc_;
+
+  public:
+    explicit AutoTracingIndex(JSTracer* trc, size_t initial = 0) : trc_(nullptr) {
+        if (trc->isCallbackTracer()) {
+            trc_ = trc->asCallbackTracer();
+            MOZ_ASSERT(trc_->contextIndex_ == CallbackTracer::InvalidIndex);
+            trc_->contextIndex_ = initial;
+        }
+    }
+    ~AutoTracingIndex() {
+        if (trc_) {
+            MOZ_ASSERT(trc_->contextIndex_ != CallbackTracer::InvalidIndex);
+            trc_->contextIndex_ = CallbackTracer::InvalidIndex;
+        }
+    }
+
+    void operator++() {
+        if (trc_) {
+            MOZ_ASSERT(trc_->contextIndex_ != CallbackTracer::InvalidIndex);
+            ++trc_->contextIndex_;
+        }
+    }
+};
+
+// Set a context callback for the trace callback to use, if it needs a detailed
+// edge description.
+class MOZ_RAII AutoTracingDetails
+{
+    CallbackTracer* trc_;
+
+  public:
+    AutoTracingDetails(JSTracer* trc, CallbackTracer::ContextFunctor& func) : trc_(nullptr) {
+        if (trc->isCallbackTracer()) {
+            trc_ = trc->asCallbackTracer();
+            MOZ_ASSERT(trc_->contextFunctor_ == nullptr);
+            trc_->contextFunctor_ = &func;
+        }
+    }
+    ~AutoTracingDetails() {
+        if (trc_) {
+            MOZ_ASSERT(trc_->contextFunctor_);
+            trc_->contextFunctor_ = nullptr;
+        }
+    }
+};
+
+} // namespace JS
+
+JS::CallbackTracer*
+JSTracer::asCallbackTracer()
+{
+    MOZ_ASSERT(isCallbackTracer());
+    return static_cast<JS::CallbackTracer*>(this);
+}
+
+namespace JS {
+
+// The JS::TraceEdge family of functions traces the given GC thing reference.
+// This performs the tracing action configured on the given JSTracer: typically
+// calling the JSTracer::callback or marking the thing as live.
+//
+// The argument to JS::TraceEdge is an in-out param: when the function returns,
+// the garbage collector might have moved the GC thing. In this case, the
+// reference passed to JS::TraceEdge will be updated to the thing's new
+// location. Callers of this method are responsible for updating any state that
+// is dependent on the object's address. For example, if the object's address
+// is used as a key in a hashtable, then the object must be removed and
+// re-inserted with the correct hash.
+//
+// Note that while |edgep| must never be null, it is fine for |*edgep| to be
+// nullptr.
+template <typename T>
+extern JS_PUBLIC_API(void)
+TraceEdge(JSTracer* trc, JS::Heap<T>* edgep, const char* name);
+
+extern JS_PUBLIC_API(void)
+TraceEdge(JSTracer* trc, JS::TenuredHeap<JSObject*>* edgep, const char* name);
+
+// Edges that are always traced as part of root marking do not require
+// incremental barriers. This function allows for marking non-barriered
+// pointers, but asserts that this happens during root marking.
+//
+// Note that while |edgep| must never be null, it is fine for |*edgep| to be
+// nullptr.
+template <typename T>
+extern JS_PUBLIC_API(void)
+UnsafeTraceRoot(JSTracer* trc, T* edgep, const char* name);
+
+extern JS_PUBLIC_API(void)
+TraceChildren(JSTracer* trc, GCCellPtr thing);
+
+using ZoneSet = js::HashSet<Zone*, js::DefaultHasher<Zone*>, js::SystemAllocPolicy>;
+using CompartmentSet = js::HashSet<JSCompartment*, js::DefaultHasher<JSCompartment*>,
+                                   js::SystemAllocPolicy>;
+
+/**
+ * Trace every value within |compartments| that is wrapped by a
+ * cross-compartment wrapper from a compartment that is not an element of
+ * |compartments|.
+ */
+extern JS_PUBLIC_API(void)
+TraceIncomingCCWs(JSTracer* trc, const JS::CompartmentSet& compartments);
+
+} // namespace JS
+
+extern JS_PUBLIC_API(void)
+JS_GetTraceThingInfo(char* buf, size_t bufsize, JSTracer* trc,
+                     void* thing, JS::TraceKind kind, bool includeDetails);
+
+namespace js {
+
+// Trace an edge that is not a GC root and is not wrapped in a barriered
+// wrapper for some reason.
+//
+// This method does not check if |*edgep| is non-null before tracing through
+// it, so callers must check any nullable pointer before calling this method.
+template <typename T>
+extern JS_PUBLIC_API(void)
+UnsafeTraceManuallyBarrieredEdge(JSTracer* trc, T* edgep, const char* name);
+
+namespace gc {
+
+// Return true if the given edge is not live and is about to be swept.
+template <typename T>
+extern JS_PUBLIC_API(bool)
+EdgeNeedsSweep(JS::Heap<T>* edgep);
+
+// Not part of the public API, but declared here so we can use it in GCPolicy
+// which is.
+template <typename T>
+bool
+IsAboutToBeFinalizedUnbarriered(T* thingp);
+
+} // namespace gc
+} // namespace js
+
+#endif /* js_TracingAPI_h */