message: only print the index part
diff: @@ -1367,7 +1367,7 @@ impl<'a, 'tcx> EvalContext<'a, 'tcx> {
  if let Lvalue::Local { frame, local, field } = lvalue {
  let mut allocs = Vec::new();
  let mut msg = format!("{:?}", local);
  - if let Some(field) = field {
  + if let Some((field, _)) = field {
  write!(msg, ".{}", field).unwrap();
  }
  let last_frame = self.stack.len() - 1;
  