Function rs_es::json::serialize_map_optional_kv

pub fn serialize_map_optional_kv<S, K, V>(serializer: &mut S,
                                          state: &mut S::MapState,
                                          key: K,
                                          value: &Option<V>)
                                          -> Result<(), S::Error> where S: Serializer, K: Serialize, V: Serialize