Bug 1377008 - Teach GCPolicy about Maybe;r=jonco

MozReview-Commit-ID: CdfNBojdx3K

--HG--
extra : rebase_source : 1db545ca96345ed9b82c986b70cad794a6d764c2

js/public/GCPolicyAPI.h

@@ -40,6 +40,7 @@
 #ifndef GCPolicyAPI_h
 #define GCPolicyAPI_h
 
+#include "mozilla/Maybe.h"
 #include "mozilla/UniquePtr.h"
 
 #include "js/TraceKind.h"
@@ -159,6 +160,23 @@ struct GCPolicy<mozilla::UniquePtr<T, D>>
     }
 };
 
+// GCPolicy<Maybe<T>> forwards tracing/sweeping to GCPolicy<T*> if
+// when the Maybe<T> is full.
+template <typename T>
+struct GCPolicy<mozilla::Maybe<T>>
+{
+    static mozilla::Maybe<T> initial() { return mozilla::Maybe<T>(); }
+    static void trace(JSTracer* trc, mozilla::Maybe<T>* tp, const char* name) {
+        if (tp->isSome())
+            GCPolicy<T>::trace(trc, tp->ptr(), name);
+    }
+    static bool needsSweep(mozilla::Maybe<T>* tp) {
+        if (tp->isSome())
+            return GCPolicy<T>::needsSweep(tp->ptr());
+        return false;
+    }
+};
+
 } // namespace JS
 
 #endif // GCPolicyAPI_h