Note discrepancies in constness calculation

compiler/rustc_ty_utils/src/ty.rs

@@ -137,6 +137,7 @@ fn param_env(tcx: TyCtxt<'_>, def_id: DefId) -> ty::ParamEnv<'_> {
     let local_did = def_id.as_local();
     let hir_id = local_did.map(|def_id| tcx.hir().local_def_id_to_hir_id(def_id));
 
+    // FIXME(consts): This is not exactly in line with the constness query.
     let constness = match hir_id {
         Some(hir_id) => match tcx.hir().get(hir_id) {
             hir::Node::TraitItem(hir::TraitItem { kind: hir::TraitItemKind::Fn(..), .. })
@@ -201,6 +202,10 @@ fn param_env(tcx: TyCtxt<'_>, def_id: DefId) -> ty::ParamEnv<'_> {
 
             _ => hir::Constness::NotConst,
         },
+        // FIXME(consts): It's suspicious that a param-env for a foreign item
+        // will always have NotConst param-env, though we don't typically use
+        // that param-env for anything meaningful right now, so it's likely
+        // not an issue.
         None => hir::Constness::NotConst,
     };