error[E0594]: cannot assign to field `self.how_hungry` of immutable binding --> $DIR/mutable-class-fields-2.rs:19:5 | LL | pub fn eat(&self) { | ----- use `&mut self` here to make mutable LL | self.how_hungry -= 5; //~ ERROR cannot assign | ^^^^^^^^^^^^^^^^^^^^ cannot mutably borrow field of immutable binding error: aborting due to previous error For more information about this error, try `rustc --explain E0594`.