{"ast_node_type":"variable","tokens":[{"end":83,"kind":"keyword","start":0,"text":"let"}]}
                    let ty::Ref(_, _, mutbl) = method.sig.inputs()[0].kind() else {
                    // the Fn or FnMut trait, it should be one of those.
                        bug!("Expected `FnMut`/`Fn` to take receiver by-ref/by-mut")