leave

    fn leave(cancelable: *Cancelable, fiber: *Fiber) RequestedError!void {
        const function = cancelable.cancel;
        assert(function != is_requested);
        if (function == is_blocked) {
            @branchHint(.unlikely);
            return;
        }
        const cancel_status = @atomicRmw(Fiber.CancelStatus, &fiber.cancel_status, .And, .{
            .requested = true,
            .awaiting = .nothing,
        }, .monotonic);
        assert(cancel_status.awaiting.toCancelable() == cancelable);
        if (cancel_status.requested) return error.CancelRequested;
    }