macro_rules! checked_feature { ("stable", $feature:ident, $current_protocol_version:expr) => { ... }; ($feature_name:tt, $feature:ident, $current_protocol_version:expr) => { ... }; ($feature_name:tt, $feature:ident, $current_protocol_version:expr, $feature_block:block) => { ... }; ($feature_name:tt, $feature:ident, $current_protocol_version:expr, $feature_block:block, $non_feature_block:block) => { ... }; }