diff --git a/Makefile.in b/Makefile.in
index d531b9879a9a8..8ab704ebe1719 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -101,7 +101,7 @@ endif
 
 ifdef CFG_ENABLE_DEBUG
   $(info cfg: enabling more debugging (CFG_ENABLE_DEBUG))
-  CFG_RUSTC_FLAGS +=
+  CFG_RUSTC_FLAGS += --cfg debug
   CFG_GCCISH_CFLAGS += -DRUST_DEBUG
 else
   CFG_GCCISH_CFLAGS += -DRUST_NDEBUG