Bug 501840 - Avoid a warning when a cycle collection participant doesn't use tmp. r+sr=peterv

xpcom/glue/nsCycleCollectionParticipant.h

@@ -481,7 +481,11 @@ public:
 #define NS_IMPL_CYCLE_COLLECTION_TRACE_JS_MEMBER_CALLBACK(_field)              \
   NS_IMPL_CYCLE_COLLECTION_TRACE_JS_CALLBACK(tmp->_field)
 
+// NB: The (void)tmp; hack in the TRACE_END macro exists to support
+// implementations that don't need to do anything in their Trace method.
+// Without this hack, some compilers warn about the unused tmp local.
 #define NS_IMPL_CYCLE_COLLECTION_TRACE_END                                     \
+      (void)tmp;                                                               \
   }
 
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