error[E0658]: rust-call ABI is subject to change (see issue #29625)
  --> $DIR/feature-gate-unboxed-closures.rs:16:5
   |
LL | /     extern "rust-call" fn call_once(self, (a, b): (u32, u32)) -> u32 {
LL | |         a + b
LL | |     }
   | |_____^
   |
   = help: add #![feature(unboxed_closures)] to the crate attributes to enable

error: aborting due to previous error

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