syntax_data: {"ast_node_type":"variable","tokens":[{"end":69,"kind":"keyword","start":0,"text":"let"}]}
source_snippet:     let mut externs: BTreeMap<String, ExternEntry> = BTreeMap::new();
context_before:     let is_unstable_enabled = unstable_opts.unstable_options;
context_after:     for arg in matches.opt_strs("extern") {