syntax_data: {"ast_node_type":"other","tokens":[{"end":93,"kind":"keyword","start":0,"text":"///"}]}
source_snippet:     /// - if next_point reached the end of source, return a span exceeding the end of source,
context_before:     /// - if span is a dummy one, returns the same span
context_after:     ///   which means sm.span_to_snippet(next_point) will get `Err`