error[E0658]: the `#[rustc_variance]` attribute is just used for rustc unit tests and will never be stable
  --> $DIR/feature-gate-rustc-attrs-1.rs:3:1
   |
LL | #[rustc_variance]
   | ^^^^^^^^^^^^^^^^^
   |
   = help: add `#![feature(rustc_attrs)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date

error[E0658]: the `#[rustc_error]` attribute is just used for rustc unit tests and will never be stable
  --> $DIR/feature-gate-rustc-attrs-1.rs:4:1
   |
LL | #[rustc_error]
   | ^^^^^^^^^^^^^^
   |
   = help: add `#![feature(rustc_attrs)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date

error[E0658]: the `#[rustc_nonnull_optimization_guaranteed]` attribute is just used to document guaranteed niche optimizations in libcore and libstd and will never be stable
              (note that the compiler does not even check whether the type indeed is being non-null-optimized; it is your responsibility to ensure that the attribute is only used on types that are optimized)
  --> $DIR/feature-gate-rustc-attrs-1.rs:5:1
   |
LL | #[rustc_nonnull_optimization_guaranteed]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: add `#![feature(rustc_attrs)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date

error: aborting due to 3 previous errors

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