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

error: aborting due to previous error

For more information about this error, try `rustc --explain E0596`.