error[E0596]: cannot borrow `*a` as mutable, as `a` is not declared as mutable --> $DIR/borrowck-borrow-immut-deref-of-box-as-mut.rs:12:5 | LL | a.foo(); | ^ cannot borrow as mutable | help: consider changing this to be mutable | LL | let mut a: Box<_> = Box::new(A); | +++ error: aborting due to previous error For more information about this error, try `rustc --explain E0596`.