error[E0596]: cannot borrow `x.0` as mutable, as `x` is not declared as mutable --> $DIR/borrowck-ref-mut-of-imm.rs:4:12 | LL | Some(ref mut v) => *v | ^^^^^^^^^ cannot borrow as mutable | help: consider changing this to be mutable | LL | fn destructure(mut x: Option) -> isize { | +++ error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0596`.