error[E0658]: rustc_allow_const_fn_unstable side-steps feature gating and stability checks --> $DIR/feature-gate-rustc-allow-const-fn-unstable.rs:3:1 | LL | #[rustc_allow_const_fn_unstable()] | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | = note: see issue #69399 for more information = help: add `#![feature(rustc_allow_const_fn_unstable)]` to the crate attributes to enable = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0658`.