error[E0596]: cannot borrow data in a `&` reference as mutable --> $DIR/index-mut-help-with-impl.rs:9:5 | LL | Index::index(&v, 1..2).make_ascii_uppercase(); | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot borrow as mutable error: aborting due to previous error For more information about this error, try `rustc --explain E0596`.