syntax_data: {"ast_node_type":"variable","tokens":[{"end":51,"kind":"keyword","start":0,"text":"let"}]}
source_snippet:         let idx = self.lookup_source_file_idx(pos);
context_before:     pub fn lookup_source_file(&self, pos: BytePos) -> Arc<SourceFile> {
context_after:         Arc::clone(&(*self.files.borrow().source_files)[idx])