hir: Add some FIXMEs for future work

compiler/rustc_ast_lowering/src/index.rs

@@ -265,6 +265,7 @@ impl<'a, 'hir> Visitor<'hir> for NodeCollector<'a, 'hir> {
     }
 
     fn visit_path_segment(&mut self, path_segment: &'hir PathSegment<'hir>) {
+        // FIXME: walk path segment with `path_segment.hir_id` parent.
         self.insert(path_segment.ident.span, path_segment.hir_id, Node::PathSegment(path_segment));
         intravisit::walk_path_segment(self, path_segment);
     }

compiler/rustc_hir/src/hir.rs

@@ -3493,6 +3493,7 @@ pub enum Node<'hir> {
     Crate(&'hir Mod<'hir>),
     Infer(&'hir InferArg),
     WhereBoundPredicate(&'hir WhereBoundPredicate<'hir>),
+    // FIXME: Merge into `Node::Infer`.
     ArrayLenInfer(&'hir InferArg),
     // Span by reference to minimize `Node`'s size
     #[allow(rustc::pass_by_value)]

compiler/rustc_hir/src/intravisit.rs

@@ -669,6 +669,7 @@ pub fn walk_pat_field<'v, V: Visitor<'v>>(visitor: &mut V, field: &'v PatField<'
 
 pub fn walk_array_len<'v, V: Visitor<'v>>(visitor: &mut V, len: &'v ArrayLen) {
     match len {
+        // FIXME: Use `visit_infer` here.
         ArrayLen::Infer(InferArg { hir_id, span: _ }) => visitor.visit_id(*hir_id),
         ArrayLen::Body(c) => visitor.visit_anon_const(c),
     }

compiler/rustc_middle/src/hir/map/mod.rs

@@ -222,6 +222,9 @@ impl<'hir> Map<'hir> {
     /// If calling repeatedly and iterating over parents, prefer [`Map::parent_iter`].
     pub fn opt_parent_id(self, id: HirId) -> Option<HirId> {
         if id.local_id == ItemLocalId::from_u32(0) {
+            // FIXME: This function never returns `None` right now, and the parent chain end is
+            // determined by checking for `parent(id) == id`. This function should return `None`
+            // for the crate root instead.
             Some(self.tcx.hir_owner_parent(id.owner))
         } else {
             let owner = self.tcx.hir_owner_nodes(id.owner);