error[E0658]: this is an internal attribute that will never be stable --> $DIR/feature-gate-on-unimplemented.rs:3:1 | LL | #[rustc_on_unimplemented = "test error `{Self}` with `{Bar}`"] | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | = help: add `#![feature(rustc_attrs)]` to the crate attributes to enable error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0658`.