{"ast_node_type":"variable","tokens":[{"end":37,"kind":"keyword","start":0,"text":"let"}]}
        let vid = self.root_var(vid);
    pub(crate) fn instantiate(&mut self, vid: ty::TyVid, ty: Ty<'tcx>) {
        debug_assert!(!ty.is_ty_var(), "instantiating ty var with var: {vid:?} {ty:?}");