syntax_data: {"ast_node_type":"other","tokens":[{"end":62,"kind":"keyword","start":0,"text":"assert_eq!(sm.span_to_snippet(span),"}]}
source_snippet:     assert_eq!(sm.span_to_snippet(span), Ok("a".to_string()));
context_before:     let span = Span::with_root_ctxt(BytePos(0), BytePos(1));
context_after:     let span = sm.next_point(span);