syntax_data: {"ast_node_type":"other","tokens":[{"end":49,"kind":"keyword","start":0,"text":"$opt:ident,"}]}
source_snippet:             $opt:ident, $parse:ident, $t:ty, [] |
context_before:         @parse {$($eout:tt)*}, ($puser_value:ident){$($pout:tt)*};
context_after:         $($tail:tt)*