error[E0658]: use of unstable library feature 'unstable_test_feature' --> $DIR/issue-28388-3.rs:7:5 | LL | use lint_stability::UnstableEnum::{}; | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | = help: add `#![feature(unstable_test_feature)]` to the crate attributes to enable error: aborting due to previous error For more information about this error, try `rustc --explain E0658`.