coverage: Get hole spans from nested items without fully visiting them

It turns out that this visitor doesn't actually need `nested_filter::All` to
handle nested items; it just needs to override `visit_nested_item` and look up
the item's span.

Author: Zalathar

Diff for: compiler/rustc_mir_transform/src/coverage/mod.rs

@@ -352,19 +352,20 @@ fn extract_hole_spans_from_hir<'tcx>(
     }
 
     impl<'hir, F: FnMut(Span)> Visitor<'hir> for HolesVisitor<'hir, F> {
-        /// - We need `NestedFilter::INTRA = true` so that `visit_item` will be called.
-        /// - Bodies of nested items don't actually get visited, because of the
-        ///   `visit_item` override.
-        /// - For nested bodies that are not part of an item, we do want to visit any
-        ///   items contained within them.
-        type NestedFilter = nested_filter::All;
+        /// We have special handling for nested items, but we still want to
+        /// traverse into nested bodies of things that are not considered items,
+        /// such as "anon consts" (e.g. array lengths).
+        type NestedFilter = nested_filter::OnlyBodies;
 
         fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
             self.tcx
         }
 
-        fn visit_item(&mut self, item: &'hir hir::Item<'hir>) {
-            (self.visit_hole_span)(item.span);
+        /// We override `visit_nested_item` instead of `visit_item` because we
+        /// only need the item's span, not the item itself.
+        fn visit_nested_item(&mut self, id: hir::ItemId) -> Self::Result {
+            let span = self.tcx.def_span(id.owner_id.def_id);
+            (self.visit_hole_span)(span);
             // Having visited this item, we don't care about its children,
             // so don't call `walk_item`.
         }