transient_heap_status_cstr() is used only for debugging

transient_heap.c

@@ -732,6 +732,7 @@ transient_heap_block_evacuate(struct transient_heap* theap, struct transient_hea
     }
 }
 
+#if USE_RUBY_DEBUG_LOG
 static const char *
 transient_heap_status_cstr(enum transient_heap_status status)
 {
@@ -740,8 +741,9 @@ transient_heap_status_cstr(enum transient_heap_status status)
       case transient_heap_marking: return "marking";
       case transient_heap_escaping: return "escaping";
     }
-    return NULL; // unreachable;
+    UNREACHABLE_RETURN(NULL);
 }
+#endif
 
 static void
 transient_heap_update_status(struct transient_heap* theap, enum transient_heap_status status)