hir: Add some FIXMEs for future work

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),
     }