error[E0658]: the `#[custom_mir]` attribute is just used for the Rust test suite
  --> $DIR/feature-gate-custom_mir.rs:5:1
   |
LL | #[custom_mir(dialect = "built")]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: add `#![feature(custom_mir)]` 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   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date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`.