{"ast_node_type":"variable","tokens":[{"end":43,"kind":"keyword","start":0,"text":"let"}]}
    let start = make_pos(file_text, start);
fn make_span(file_text: &str, start: &Position, end: &Position) -> Span {
    let end = make_pos(file_text, end) + end.string.len(); // just after matching thing ends