syntax_data: {"ast_node_type":"variable","tokens":[{"end":60,"kind":"keyword","start":0,"text":"let"}]}
source_snippet:     let span = Span::with_root_ctxt(BytePos(0), BytePos(1));
context_before:     // Span advance respect multi-byte character
context_after:     assert_eq!(sm.span_to_snippet(span), Ok("a".to_string()));