message: Assert LocalNames.add_name is not called twice
  Summary:
  ... for the same name.
  Not possible after previous changes.
diff: @@ -84,15 +84,11 @@ impl ScopeNames {
  }
  fn add_name(&mut self, name: &str) -> LocalSlotId {
  - match self.mp.get(name) {
  - Some(v) => *v,
  - None => {
  let slot = self.next_slot();
  - self.mp.insert(name.to_owned(), slot);
  + let old_slot = self.mp.insert(name.to_owned(), slot);
  + assert!(old_slot.is_none());
  slot
  }
  - }
  - }
  fn add_scoped(&mut self, name: &str, unscope: &mut Unscope) -> LocalSlotId {
  match unscope.0.entry(name.to_owned()) {
  