syntax_data: {"ast_node_type":"other","tokens":[{"end":67,"kind":"keyword","start":0,"text":"///"}]}
source_snippet:     ///   which means sm.span_to_snippet(next_point) will get `Err`
context_before:     /// - if next_point reached the end of source, return a span exceeding the end of source,
context_after:     /// - respect multi-byte characters