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}`"]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
   = help: add `#![feature(rustc_attrs)]` to the crate attributes to enable

error: aborting due to previous error

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