error[E0596]: cannot borrow `*x.1` as mutable, as it is behind a `&` reference --> $DIR/issue-61623.rs:6:19 | LL | f2(|| x.0, f1(x.1)) | ^^^ `x` is a `&` reference, so the data it refers to cannot be borrowed as mutable | help: consider changing this to be a mutable reference | LL | fn f3<'a>(x: &'a mut ((), &'a mut ())) { | +++ error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0596`.