Use invalid local id for zeroth node parent.

compiler/rustc_ast_lowering/src/index.rs

@@ -47,7 +47,10 @@ pub(super) fn index_hir<'hir>(
     bodies: &IndexVec<ItemLocalId, Option<&'hir Body<'hir>>>,
 ) -> (IndexVec<ItemLocalId, Option<ParentedNode<'hir>>>, FxHashMap<LocalDefId, ItemLocalId>) {
     let mut nodes = IndexVec::new();
-    nodes.push(Some(ParentedNode { parent: ItemLocalId::new(0), node: item.into() }));
+    // This node's parent should never be accessed: the owner's parent is computed by the
+    // hir_owner_parent query.  Make it invalid (= ItemLocalId::MAX) to force an ICE whenever it is
+    // used.
+    nodes.push(Some(ParentedNode { parent: ItemLocalId::INVALID, node: item.into() }));
     let mut collector = NodeCollector {
         source_map: sess.source_map(),
         definitions,

compiler/rustc_hir/src/hir.rs

@@ -696,7 +696,9 @@ pub struct OwnerNodes<'tcx> {
     /// Pre-computed hash of the item signature, sithout recursing into the body.
     pub hash_without_bodies: Fingerprint,
     /// Full HIR for the current owner.
-    // The zeroth node's parent is trash, but is never accessed.
+    // The zeroth node's parent should never be accessed: the owner's parent is computed by the
+    // hir_owner_parent query.  It is set to `ItemLocalId::INVALID` to force an ICE if accidentally
+    // used.
     pub nodes: IndexVec<ItemLocalId, Option<ParentedNode<'tcx>>>,
     /// Content of local bodies.
     pub bodies: IndexVec<ItemLocalId, Option<&'tcx Body<'tcx>>>,

compiler/rustc_hir/src/hir_id.rs

@@ -56,6 +56,10 @@ rustc_index::newtype_index! {
     pub struct ItemLocalId { .. }
 }
 rustc_data_structures::impl_stable_hash_via_hash!(ItemLocalId);
+impl ItemLocalId {
+    /// Signal local id which should never be used.
+    pub const INVALID: ItemLocalId = ItemLocalId::MAX;
+}
 
 /// The `HirId` corresponding to `CRATE_NODE_ID` and `CRATE_DEF_INDEX`.
 pub const CRATE_HIR_ID: HirId = HirId {