error[E0596]: cannot borrow `b` as mutable, as it is not declared as mutable
  --> $DIR/mut-borrow-of-mut-ref.rs:8:7
   |
LL | fn f(b: &mut i32) {
   |      - help: consider changing this to be mutable: `mut b`
LL |     g(&mut b)
   |       ^^^^^^ cannot borrow as mutable

error: aborting due to previous error

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