changes from review: add FIXME to clippy and change subst_identity to skip_binder in mir subst methods

Author: kylematsuda

compiler/rustc_middle/src/ty/instance.rs

@@ -586,7 +586,7 @@ impl<'tcx> Instance<'tcx> {
         if let Some(substs) = self.substs_for_mir_body() {
             v.subst(tcx, substs)
         } else {
-            v.subst_identity()
+            v.skip_binder()
         }
     }
 
@@ -603,7 +603,7 @@ impl<'tcx> Instance<'tcx> {
         if let Some(substs) = self.substs_for_mir_body() {
             tcx.subst_and_normalize_erasing_regions(substs, param_env, v)
         } else {
-            tcx.normalize_erasing_regions(param_env, v.subst_identity())
+            tcx.normalize_erasing_regions(param_env, v.skip_binder())
         }
     }
 
@@ -620,7 +620,7 @@ impl<'tcx> Instance<'tcx> {
         if let Some(substs) = self.substs_for_mir_body() {
             tcx.try_subst_and_normalize_erasing_regions(substs, param_env, v)
         } else {
-            tcx.try_normalize_erasing_regions(param_env, v.subst_identity())
+            tcx.try_normalize_erasing_regions(param_env, v.skip_binder())
         }
     }
 

src/tools/clippy/clippy_lints/src/methods/unnecessary_to_owned.rs

@@ -385,6 +385,9 @@ fn can_change_type<'a>(cx: &LateContext<'a>, mut expr: &'a Expr<'a>, mut ty: Ty<
             Node::Expr(parent_expr) => {
                 if let Some((callee_def_id, call_substs, recv, call_args)) = get_callee_substs_and_args(cx, parent_expr)
                 {
+                    // FIXME: the `subst_identity()` below seems incorrect, since we eventually
+                    // call `tcx.try_subst_and_normalize_erasing_regions` further down
+                    // (i.e., we are explicitly not in the identity context).
                     let fn_sig = cx.tcx.fn_sig(callee_def_id).subst_identity().skip_binder();
                     if let Some(arg_index) = recv.into_iter().chain(call_args).position(|arg| arg.hir_id == expr.hir_id)
                         && let Some(param_ty) = fn_sig.inputs().get(arg_index)