error: ~const can only be applied to `#[const_trait]` traits
  --> $DIR/unstable-const-fn-in-libcore.rs:19:39
   |
LL |     const fn unwrap_or_else<F: ~const FnOnce() -> T>(self, f: F) -> T {
   |                                       ^^^^^^^^^^^^^

error: aborting due to 1 previous error