diff --git a/common.mk b/common.mk
index 8f7fca4559..857f692fb5 100644
--- a/common.mk
+++ b/common.mk
@@ -672,6 +672,12 @@ rdoc-coverage: PHONY main srcs-doc
 	@echo Generating RDoc coverage report
 	$(Q) $(RDOC) --quiet -C $(RDOCFLAGS) "$(srcdir)"
 
+undocumented: PHONY main srcs-doc
+	$(Q) $(RDOC) --quiet -C $(RDOCFLAGS) "$(srcdir)" | \
+	sed -n \
+	-e '/^ *# in file /{' -e 's///;N;s/\n/: /p' -e '}' \
+	-e 's/^ *\(.*[^ ]\) *# in file \(.*\)/\2: \1/p'
+
 RDOCBENCHOUT=/tmp/rdocbench
 
 GCBENCH_ITEM=null