{"ast_node_type":"variable","tokens":[{"end":51,"kind":"keyword","start":0,"text":"let"}]}
        let sig_ty = ClosureSubst(substs).sig_ty();
    ) -> chalk_ir::Binders<rust_ir::FnDefInputsAndOutputDatum<Interner>> {
        let sig = &sig_ty.callable_sig(self.db).expect("first closure param should be fn ptr");