message: `u128` truncation and sign extension are not just interpreter related
diff: @@ -585,10 +585,7 @@ fn is_fat_ptr<'tcx>(
  .discriminant_for_variant(fx.tcx, *index)
  .unwrap();
  let discr = if discr.ty.is_signed() {
  - rustc_middle::mir::interpret::sign_extend(
  - discr.val,
  - fx.layout_of(discr.ty).size,
  - )
  + fx.layout_of(discr.ty).size.sign_extend(discr.val)
  } else {
  discr.val
  };
  