syntax_data: {"ast_node_type":"other","tokens":[{"end":91,"kind":"keyword","start":0,"text":"\"only"}]}
source_snippet:         "only allow the listed language features to be enabled in code (comma separated)"),
context_before:     allow_features: Option<Vec<String>> = (None, parse_opt_comma_list, [TRACKED],
context_after:     always_encode_mir: bool = (false, parse_bool, [TRACKED],