{"ast_node_type":"variable","tokens":[{"end":32,"kind":"keyword","start":0,"text":"let"}]}
    let val = attr.value_str()?;
fn parse_linkage_attr(tcx: TyCtxt<'_>, did: LocalDefId, attr: &Attribute) -> Option<Linkage> {
    let linkage = linkage_by_name(tcx, did, val.as_str());