Limit how deep we visit items to find cfg'd out names

Author: estebank

compiler/rustc_expand/src/expand.rs

@@ -1260,9 +1260,10 @@ impl InvocationCollectorNode for P<ast::Item> {
         res
     }
     fn declared_names(&self) -> Vec<Ident> {
-        struct ItemNameVisitor(Vec<Ident>);
+        struct ItemNameVisitor(Vec<Ident>, u8);
         impl Visitor<'_> for ItemNameVisitor {
             fn visit_item(&mut self, i: &ast::Item) {
+                self.1 += 1;
                 if let ItemKind::Use(ut) = &i.kind {
                     fn collect_use_tree_leaves(ut: &ast::UseTree, idents: &mut Vec<Ident>) {
                         match &ut.kind {
@@ -1280,11 +1281,19 @@ impl InvocationCollectorNode for P<ast::Item> {
                 } else {
                     self.0.push(i.ident);
                 }
-                visit::walk_item(self, i);
+                if self.1 < 4 {
+                    // We only visit up to 3 levels of nesting from this item, like if we were
+                    // looking at `mod a`, we'd find item `a::b::c`. We have this limit to guard
+                    // against deeply nested modules behind `cfg` flags, where we could spend
+                    // significant time collecting this information purely for a potential
+                    // diagnostic improvement.
+                    visit::walk_item(self, i);
+                }
+                self.1 -= 1;
             }
         }
 
-        let mut v = ItemNameVisitor(vec![]);
+        let mut v = ItemNameVisitor(vec![], 0);
         v.visit_item(self);
         v.0
     }