{"ast_node_type":"other","tokens":[{"end":41,"kind":"keyword","start":0,"text":"///"}]}
    /// and return a reference to *that*.
    /// `vacant` and either forward its error, or add its result to the map
    fn get_mut_or<E>(&mut self, k: K, vacant: impl FnOnce() -> Result<V, E>) -> Result<&mut V, E>;