error[E0658]: the `#[register_attr]` attribute is an experimental feature
  --> $DIR/feature-gate-register_attr.rs:1:1
   |
LL | #![register_attr(attr)]
   | ^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/66080
   = help: add `#![feature(register_attr)]` to the crate attributes to enable

error: aborting due to previous error

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