{"ast_node_type":"variable","tokens":[{"end":84,"kind":"keyword","start":0,"text":"let"}]}
    let source_file = ast::SourceFile::parse(input, Edition::CURRENT).ok().unwrap();
fn check_dnf(input: &str, expect: Expect) {
    let tt = source_file.syntax().descendants().find_map(ast::TokenTree::cast).unwrap();