message: Remove leading underscore from parameter name
  Summary:
  It is prohibited in the following diff
  And then no longer trims leading underscore.
diff: @@ -132,9 +132,10 @@ fn assert_star(builder: &mut crate::environment::GlobalsBuilder) {
  fn fails<'v>(
  f: Value<'v>,
  - _msg: &str,
  + msg: &str,
  eval: &mut Evaluator<'v, '_>,
  ) -> anyhow::Result<NoneType> {
  + let _ = msg;
  match f.invoke_pos(&[], eval) {
  Err(_e) => Ok(NoneType), // We don't actually check the message
  Ok(_) => Err(anyhow!("assert.fails: didn't fail")),
  