Deny internal lints for rustdoc

src/bootstrap/check.rs

@@ -320,6 +320,13 @@ macro_rules! tool_check_step {
                     cargo.arg("--all-targets");
                 }
 
+                // Enable internal lints for clippy and rustdoc
+                // NOTE: this intentionally doesn't enable lints for any other tools,
+                // see https://github.com/rust-lang/rust/pull/80573#issuecomment-754010776
+                if $path == "src/tools/rustdoc" || $path == "src/tools/clippy" {
+                    cargo.rustflag("-Zunstable-options");
+                }
+
                 builder.info(&format!(
                     "Checking stage{} {} artifacts ({} -> {})",
                     builder.top_stage,

src/librustdoc/lib.rs

@@ -18,6 +18,7 @@
 #![feature(str_split_once)]
 #![feature(iter_intersperse)]
 #![recursion_limit = "256"]
+#![deny(rustc::internal)]
 
 #[macro_use]
 extern crate lazy_static;