error[E0308]: mismatched types
  --> $DIR/mut-cross-borrowing.rs:7:7
   |
LL |     f(x)
   |       ^
   |       |
   |       expected `&mut isize`, found struct `Box`
   |       help: consider mutably borrowing here: `&mut x`
   |
   = note: expected mutable reference `&mut isize`
                         found struct `Box<{integer}>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.