error[E0594]: cannot assign to `*y`, as `y` is not declared as mutable
  --> $DIR/immut-function-arguments.rs:2:5
   |
LL | fn f(y: Box<isize>) {
   |      - help: consider changing this to be mutable: `mut y`
LL |     *y = 5;
   |     ^^^^^^ cannot assign

error[E0594]: cannot assign to `*q`, as `q` is not declared as mutable
  --> $DIR/immut-function-arguments.rs:6:35
   |
LL |     let _frob = |q: Box<isize>| { *q = 2; };
   |                  -                ^^^^^^ cannot assign
   |                  |
   |                  help: consider changing this to be mutable: `mut q`

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0594`.