syntax_data: {"ast_node_type":"other","tokens":[{"end":55,"kind":"keyword","start":0,"text":".map_or(start,"}]}
source_snippet:                 .map_or(start, |(i, _)| start - i - 1);
context_before:                 .rfind(|&(_, c)| !f(c))
context_after:             Ok(span.with_lo(span.lo() - BytePos(n as u32)))