error[E0658]: the `#[rustc_on_unimplemented]` attribute is an experimental feature (see issue #29628)
  --> $DIR/feature-gate-on-unimplemented.rs:14:1
   |
LL | #[rustc_on_unimplemented = "test error `{Self}` with `{Bar}`"]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: add #![feature(on_unimplemented)] to the crate attributes to enable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.