syntax_data: {"ast_node_type":"other","tokens":[{"end":39,"kind":"keyword","start":0,"text":"///"}]}
source_snippet:     /// first line, `None` is returned.
context_before:     /// number. If the source_file is empty or the position is located before the
context_after:     pub fn lookup_line(&self, pos: RelativeBytePos) -> Option<usize> {