Fix quadratic behaviour in the MissingDoc lint.

The `MissingDoc` lint has quadratic behaviour when processing doc comments.
This is a problem for large doc comments (e.g. 1000+ lines) when
`deny(missing_code)` is enabled.

A 1000-line doc comment using `//!` comments is represented as 1000 attributes
on an item. The lint machinery iterates over each attribute with
`visit_attribute`. `MissingDoc`'s impl of that function calls
`with_lint_attrs`, which calls `enter_attrs`, which iterates over all 1000
attributes looking for a `doc(hidden)` attribute. I.e. for every attribute we
iterate over all the other attributes.

The fix is simple: don't call `with_lint_attrs` on attributes. This makes
sense: `with_lint_attrs` is intended to iterate over the attributes on a
language fragment like a statement or expression, but it doesn't need to
be called on attributes themselves.

Author: nnethercote

compiler/rustc_lint/src/late.rs

@@ -337,10 +337,8 @@ impl<'tcx, T: LateLintPass<'tcx>> hir_visit::Visitor<'tcx> for LateContextAndPas
         hir_visit::walk_path(self, p);
     }
 
-    fn visit_attribute(&mut self, hir_id: hir::HirId, attr: &'tcx ast::Attribute) {
-        self.with_lint_attrs(hir_id, |cx| {
-            lint_callback!(cx, check_attribute, attr);
-        })
+    fn visit_attribute(&mut self, _hir_id: hir::HirId, attr: &'tcx ast::Attribute) {
+        lint_callback!(self, check_attribute, attr);
     }
 }