Doesn't that have an unneeded level of indirection? Why not make an instance variable of type os_unfair_lock?
final class UnfairLock {
func around<Answer>(_ body: () throws -> Answer) rethrows -> Answer {
os_unfair_lock_lock(&lock)
defer { os_unfair_lock_unlock(&lock) }
return try body()
}
private var lock = os_unfair_lock()
}