call

        fn call(
            closure: *Group.AsyncClosure,
            message: *const SwitchMessage,
        ) callconv(.withStackAlign(.c, @alignOf(Group.AsyncClosure))) noreturn {
            const ev = closure.evented;
            const fiber = closure.fiber;
            message.handle(ev);
            closure.start(closure.contextPointer());
            if (closure.group.removeFiber(ev, fiber)) |awaiter| ev.queue.async(awaiter, &Fiber.@"resume");
            ev.yield(.destroy);
            unreachable; // switched to dead fiber
        }