diff --git a/Makefile.in b/Makefile.in
index e64ac6a1f8e..bb7a5d4e0a1 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -29,7 +29,10 @@ B := $(CFG_BUILD_DIR)
 
 MKFILE_DEPS := config.stamp $(call rwildcard,$(S)mk/,*)
 
-CFG_RUSTC_FLAGS := $(RUSTFLAGS)
+# Enable debug!() etc even without configure --enable-debug
+# The evaluation of these prints & their arguments is controlled
+# at runtime by the environment variable RUST_LOG.
+CFG_RUSTC_FLAGS := $(RUSTFLAGS) --cfg debug
 
 ifdef CFG_DISABLE_OPTIMIZE
   $(info cfg: disabling rustc optimization (CFG_DISABLE_OPTIMIZE))