syntax_data: {"ast_node_type":"function","tokens":[{"end":70,"kind":"keyword","start":0,"text":"pub"}]}
source_snippet:     pub fn lookup_line(&self, pos: RelativeBytePos) -> Option<usize> {
context_before:     /// first line, `None` is returned.
context_after:         self.lines().partition_point(|x| x <= &pos).checked_sub(1)