syntax_data: {"ast_node_type":"other","tokens":[{"end":89,"kind":"keyword","start":0,"text":"$opt:ident"}]}
source_snippet:         $opt:ident : $t:ty [$dep_tracking_marker:ident $( $tmod:ident $variant:ident )?],
context_before:         $( #[$attr:meta] )*
context_after:     )* } ) => (