{"ast_node_type":"variable","tokens":[{"end":66,"kind":"keyword","start":0,"text":"let"}]}
        let tmp = mem::replace(ty, Substitution::empty(Interner));
    fn fill_subst(&mut self, ty: &mut Substitution) -> Result<(), MirLowerError> {
        *ty = tmp.try_fold_with(self, DebruijnIndex::INNERMOST)?;