message: Add a FIXME.
diff: @@ -55,6 +55,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  let this = self.eval_context_mut();
  if !this.is_null(this.read_scalar(retval)?.not_undef()?)? {
  + // FIXME: implement reading the thread function's return place.
  throw_unsup_format!("Miri supports pthread_join only with retval==NULL");
  }
  