error[E0658]: transparent unions are unstable
  --> $DIR/feature-gate-transparent_unions.rs:2:1
   |
LL | union OkButUnstableUnion {
   | ^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/60405
   = help: add #![feature(transparent_unions)] to the crate attributes to enable

error: aborting due to previous error

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