syntax_data: {"ast_node_type":"other","tokens":[{"end":43,"kind":"keyword","start":0,"text":"for"}]}
source_snippet:     for arg in matches.opt_strs("extern") {
context_before:     let mut externs: BTreeMap<String, ExternEntry> = BTreeMap::new();
context_after:         let ExternOpt { crate_name: name, path, options } =