message: no need for `format!` in `panic!`
diff: @@ -42,7 +42,7 @@ where
  }
  fn missing_value(key: &Q::Key) -> Q::Value {
  - panic!(format!("no value set for {:?}({:?})", Q::default(), key))
  + panic!("no value set for {:?}({:?})", Q::default(), key)
  }
  }
  