diff --git a/src/libfmt_macros/lib.rs b/src/libfmt_macros/lib.rs
index aacd6cec565..2536121c7a3 100644
--- a/src/libfmt_macros/lib.rs
+++ b/src/libfmt_macros/lib.rs
@@ -9,6 +9,7 @@
        test(attr(deny(warnings))))]
 
 #![deny(rust_2018_idioms)]
+#![cfg_attr(not(stage0), deny(internal))]
 
 #![feature(nll)]
 #![feature(rustc_private)]
diff --git a/src/librustdoc/lib.rs b/src/librustdoc/lib.rs
index 2ebb465d53d..6cb937d9216 100644
--- a/src/librustdoc/lib.rs
+++ b/src/librustdoc/lib.rs
@@ -1,4 +1,5 @@
 #![deny(rust_2018_idioms)]
+#![cfg_attr(not(stage0), deny(internal))]
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/",
        html_playground_url = "https://play.rust-lang.org/")]