Macro code 

macro_rules! code {
    (repeated $field_name:expr, $validator:expr) => { ... };
    (repeated $field_name:expr) => { ... };
    ($field_name:expr, $validator:expr) => { ... };
    ($field_name:expr) => { ... };
}

Expands to a FieldBuilder instance for a google.rpc.Code field.