diff --git a/src/rt/memory_region.cpp b/src/rt/memory_region.cpp
index fb19620f8b95..482202902701 100644
--- a/src/rt/memory_region.cpp
+++ b/src/rt/memory_region.cpp
@@ -1,7 +1,10 @@
 #include "rust_internal.h"
 #include "memory_region.h"
 
-#define TRACK_ALLOCATIONS
+// NB: please do not commit code with this uncommented. It's
+// hugely expensive and should only be used as a last resort.
+//
+// #define TRACK_ALLOCATIONS
 
 memory_region::memory_region(rust_srv *srv, bool synchronized) :
     _srv(srv), _parent(NULL), _live_allocations(0),