error[E0658]: C-varaidic functions are unstable (see issue #44930)
  --> $DIR/feature-gate-c_variadic.rs:3:1
   |
LL | pub unsafe extern "C" fn test(_: i32, ap: ...) { }
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: add #![feature(c_variadic)] to the crate attributes to enable

error: aborting due to previous error

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