Add regression test.

Author: jseyfried

src/test/compile-fail/cfg_attr_path.rs

@@ -8,5 +8,17 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#[cfg_attr(all(), path = "nonexistent_file.rs")] mod foo;
-//~^ ERROR nonexistent_file.rs
+#![feature(rustc_attrs)]
+#![allow(dead_code)]
+#![deny(unused_attributes)] // c.f #35584
+
+mod auxiliary {
+    #[cfg_attr(any(), path = "nonexistent_file.rs")] pub mod namespaced_enums;
+    #[cfg_attr(all(), path = "namespaced_enums.rs")] pub mod nonexistent_file;
+}
+
+#[rustc_error]
+fn main() { //~ ERROR compilation successful
+    let _ = auxiliary::namespaced_enums::Foo::A;
+    let _ = auxiliary::nonexistent_file::Foo::A;
+}