{"ast_node_type":"function","tokens":[{"end":59,"kind":"keyword","start":0,"text":"fn"}]}
    fn root_vid(&self, ty: Ty<'tcx>) -> Option<ty::TyVid> {
    /// If `ty` is an unresolved type variable, returns its root vid.
        Some(self.root_var(self.shallow_resolve(ty).ty_vid()?))