Add regression test for rust-lang#126796

Author: GuillaumeGomez

tests/rustdoc/doc-hidden-crate.rs

@@ -0,0 +1,27 @@
+// Regression test for <https://github.com/rust-lang/rust/issues/126796>.
+// `doc(hidden)` should still be able to hide extern crates, only the local crates
+// cannot be hidden because we still need to generate its `index.html` file.
+
+#![crate_name = "foo"]
+#![doc(hidden)]
+
+//@ has 'foo/index.html'
+// First we check that the page contains the crate name (`foo`).
+//@ has - '//*' 'foo'
+// But doesn't contain any of the other items.
+//@ !has - '//*' 'other'
+//@ !has - '//*' 'marker'
+//@ !has - '//*' 'PhantomData'
+
+#[doc(inline)]
+pub use std as other;
+
+#[doc(inline)]
+pub use std::marker;
+
+#[doc(inline)]
+pub use std::marker::PhantomData;
+
+//@ !has - '//*' 'myself'
+#[doc(inline)]
+pub use crate as myself;