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

It turns out that this visitor doesn't actually need a custom `NestedFilter`;
it just needs to override `visit_nested_item` and look up the item's span.

Author: Zalathar

compiler/rustc_mir_transform/src/coverage/mod.rs

@@ -10,7 +10,6 @@ mod unexpand;
 
 use rustc_hir as hir;
 use rustc_hir::intravisit::{Visitor, walk_expr};
-use rustc_middle::hir::nested_filter;
 use rustc_middle::mir::coverage::{
     CoverageKind, DecisionInfo, FunctionCoverageInfo, Mapping, MappingKind,
 };
@@ -352,19 +351,11 @@ 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;
-
-        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`.
         }