error[E0594]: cannot assign to `self.how_hungry` which is behind a `&` reference --> $DIR/mutable-class-fields-2.rs:9:5 | LL | pub fn eat(&self) { | ----- help: consider changing this to be a mutable reference: `&mut self` LL | self.how_hungry -= 5; | ^^^^^^^^^^^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be written error: aborting due to previous error For more information about this error, try `rustc --explain E0594`.