{"ast_node_type":"other","tokens":[{"end":47,"kind":"keyword","start":0,"text":"//"}]}
    // `.i: pat` sets the current index to `i`.
    (@fields($($args:tt)*) $(,)?) => {};
    (@fields(idx:$_idx:expr, $($args:tt)*) , .$idx:literal : $($rest:tt)*) => {{