diff --git a/src/test/rustdoc/auxiliary/external-cross.rs b/src/test/rustdoc/auxiliary/external-cross.rs
index cb14fec7abe..767fb05313a 100644
--- a/src/test/rustdoc/auxiliary/external-cross.rs
+++ b/src/test/rustdoc/auxiliary/external-cross.rs
@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![feature(external_doc)]
+#![deny(missing_doc)]
 
 #[doc(include="external-cross-doc.md")]
 pub struct NeedMoreDocs;