diff --git a/js/src/jsgc.cpp b/js/src/jsgc.cpp
index 774cd9c5ffd..57c2e356b2e 100644
--- a/js/src/jsgc.cpp
+++ b/js/src/jsgc.cpp
@@ -4058,6 +4058,25 @@ GCRuntime::markAllGrayReferences(gcstats::Phase phase)
 
 #ifdef DEBUG
 
+struct GCChunkHasher {
+    typedef gc::Chunk* Lookup;
+
+    /*
+     * Strip zeros for better distribution after multiplying by the golden
+     * ratio.
+     */
+    static HashNumber hash(gc::Chunk* chunk) {
+        MOZ_ASSERT(!(uintptr_t(chunk) & gc::ChunkMask));
+        return HashNumber(uintptr_t(chunk) >> gc::ChunkShift);
+    }
+
+    static bool match(gc::Chunk* k, gc::Chunk* l) {
+        MOZ_ASSERT(!(uintptr_t(k) & gc::ChunkMask));
+        MOZ_ASSERT(!(uintptr_t(l) & gc::ChunkMask));
+        return k == l;
+    }
+};
+
 class js::gc::MarkingValidator
 {
   public:
diff --git a/js/src/jsgc.h b/js/src/jsgc.h
index 6b65c48bee2..633ea403cf5 100644
--- a/js/src/jsgc.h
+++ b/js/src/jsgc.h
@@ -1100,27 +1100,6 @@ class GCParallelTask
     virtual void runFromHelperThread();
 };
 
-struct GCChunkHasher {
-    typedef gc::Chunk* Lookup;
-
-    /*
-     * Strip zeros for better distribution after multiplying by the golden
-     * ratio.
-     */
-    static HashNumber hash(gc::Chunk* chunk) {
-        MOZ_ASSERT(!(uintptr_t(chunk) & gc::ChunkMask));
-        return HashNumber(uintptr_t(chunk) >> gc::ChunkShift);
-    }
-
-    static bool match(gc::Chunk* k, gc::Chunk* l) {
-        MOZ_ASSERT(!(uintptr_t(k) & gc::ChunkMask));
-        MOZ_ASSERT(!(uintptr_t(l) & gc::ChunkMask));
-        return k == l;
-    }
-};
-
-typedef HashSet<js::gc::Chunk*, GCChunkHasher, SystemAllocPolicy> GCChunkSet;
-
 typedef void (*IterateChunkCallback)(JSRuntime* rt, void* data, gc::Chunk* chunk);
 typedef void (*IterateZoneCallback)(JSRuntime* rt, void* data, JS::Zone* zone);
 typedef void (*IterateArenaCallback)(JSRuntime* rt, void* data, gc::Arena* arena,