{"ast_node_type":"variable","tokens":[{"end":84,"kind":"keyword","start":0,"text":"let"}]}
    let fn_sig_span = maybe_fn_sig.map(|fn_sig| fn_sig.span).filter(|&fn_sig_span| {
    // filename as the body, and precedes the body.
        let source_map = tcx.sess.source_map();