error[E0596]: cannot borrow `*x` as mutable, as `x` is not declared as mutable --> $DIR/issue-36400.rs:5:7 | LL | f(&mut *x); | ^^^^^^^ cannot borrow as mutable | help: consider changing this to be mutable | LL | let mut x = Box::new(3); | +++ error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0596`.