State

    const State = enum(usize) {
        unlocked = 1,
        /// This value is intentionally 0 so that `waiter` returns `null`.
        locked_once = 0,
        /// Contended; value is a `*Waiter`.
        _,
        /// Returns the head of the waiter list. Illegal to call if `s == .unlocked`.
        fn waiter(s: State) ?*Waiter {
            return @ptrFromInt(@backingInt(s));
        }
        /// Returns a locked state where `w` is contending the lock.
        /// If `w` is `null`, returns `.locked_once`.
        fn fromWaiter(w: ?*Waiter) State {
            return @fromBackingInt(@intCast(@intFromPtr(w)));
        }
    }