Rollup merge of rust-lang#35617 - jseyfried:fix_unused_cfg_attr_path, r=eddyb

Fix incorrect unused import warnings on `cfg_attr`ed `path` attributes

Fixes rust-lang#35584.
r? @eddyb

Author: Manishearth

src/libsyntax/config.rs

@@ -62,12 +62,16 @@ impl<'a> StripUnconfigured<'a> {
         };
 
         if attr::cfg_matches(self.config, &cfg, self.sess, self.features) {
-            self.process_cfg_attr(respan(mi.span, ast::Attribute_ {
+            let inner_attr = respan(mi.span, ast::Attribute_ {
                 id: attr::mk_attr_id(),
                 style: attr.node.style,
                 value: mi.clone(),
                 is_sugared_doc: false,
-            }))
+            });
+            if attr::is_used(&attr) {
+                attr::mark_used(&inner_attr);
+            }
+            self.process_cfg_attr(inner_attr)
         } else {
             None
         }

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;
+}