message: use -Zmiri-start-fn as a clue to REQUIRE the lang item to be present
diff: @@ -138,8 +138,12 @@ fn after_analysis<'a, 'tcx>(state: &mut CompileState<'a, 'tcx>, use_start_fn: bo
  );
  } else if let Some((entry_node_id, _, _)) = *state.session.entry_fn.borrow() {
  let entry_def_id = tcx.hir.local_def_id(entry_node_id);
  - // Use start_fn lang item if it is available and we have -Zmiri-start-fn set
  - let start_wrapper = if use_start_fn { tcx.lang_items().start_fn() } else { None };
  + // Use start_fn lang item if we have -Zmiri-start-fn set
  + let start_wrapper = if use_start_fn {
  + Some(tcx.lang_items().start_fn().unwrap())
  + } else {
  + None
  + };
  miri::eval_main(tcx, entry_def_id, start_wrapper);
  state.session.abort_if_errors();
  