syntax_data: {"ast_node_type":"function","tokens":[{"end":73,"kind":"keyword","start":0,"text":"fn"}]}
source_snippet:     fn lookup_file_pos(&self, pos: RelativeBytePos) -> (usize, CharPos) {
context_before:     /// given `RelativeBytePos`.
context_after:         let chpos = self.bytepos_to_file_charpos(pos);