Function contract_check_ensures
const fn contract_check_ensures<C: Fn(&Ret) -> bool + Copy, Ret>(cond: Option<C>, ret: Ret) -> Ret
Check if the post-condition cond has been met.
By default, if contract_checks is enabled, this will panic with no unwind if the condition
returns false.
If cond is None, then no postcondition checking is performed.
Note that this function is a no-op during constant evaluation.