syntax_data: {"ast_node_type":"variable","tokens":[{"end":33,"kind":"keyword","start":0,"text":"let"}]}
source_snippet:     let mut sf = SourceFile::new(
context_before:     let source = "abcdefghijklm\nabcdefghij\n...".to_owned();
context_after:         FileName::Anon(Hash64::ZERO),