error[E0594]: cannot assign to `nyan.how_hungry`, as `nyan` is not declared as mutable --> $DIR/mutable-class-fields.rs:15:3 | LL | nyan.how_hungry = 0; | ^^^^^^^^^^^^^^^^^^^ cannot assign | help: consider changing this to be mutable | LL | let mut nyan : Cat = cat(52, 99); | +++ error: aborting due to previous error For more information about this error, try `rustc --explain E0594`.