error[E0658]: the `#[pattern_complexity]` attribute is just used for rustc unit tests and will never be stable
  --> $DIR/feature-gate-pattern-complexity.rs:3:1
   |
LL | #![pattern_complexity = "42"]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = 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   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date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 1 previous error

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