error[E0596]: cannot borrow data in an index of `Container` as mutable --> $DIR/issue-44405.rs:21:5 | LL | container[&mut val].test(); | ^^^^^^^^^^^^^^^^^^^ cannot borrow as mutable | = help: trait `IndexMut` is required to modify indexed content, but it is not implemented for `Container` error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0596`.