error[E0658]: use of an internal attribute
  --> $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
   = note: the `#[rustc_on_unimplemented]` attribute is an internal implementation detail that will never be stable
   = note: see `#[diagnostic::on_unimplemented]` for the stable equivalent of this attribute

error: aborting due to 1 previous error

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