error[E0594]: cannot assign to field `self.how_hungry` of immutable binding
  --> $DIR/mutable-class-fields-2.rs:9:5
   |
LL |   pub fn eat(&self) {
   |              ----- use `&mut self` here to make mutable
LL |     self.how_hungry -= 5;
   |     ^^^^^^^^^^^^^^^^^^^^ cannot mutably borrow field of immutable binding

error: aborting due to previous error