我正在使用FuturesUnordered将异步工作负载排队到多线程时雄runner上。这些future返回各种不同类型的结果。为了区分它们，我将每个future的结果Map到一个自定义的Event类型。

enum Event {
   ResultTypeA {...},
   ResultTypeB {...},
   ResultTypeC {...},
   ResultTypeD {...}
}

let pending_futures: FuturesUnordered<Pin<Box<dyn Future<Output = Event> + Send>>> = FuturesUnordered::default()

loop {
    tokio::select! {
        Some(future) = workload_receiver.recv() => {
            pending_futures.push(future.boxed());
        },
        Some(event) = pending_futures.next() => process_event(event),
        else => break,
   }
}

上面的代码运行良好，但是，我想限制并行处理的pending_futures的数量。这就是buffered_unordered的用武之地。我的天真方法是：

loop {
    tokio::select! {
        Some(future) = workload_receiver.recv() => {
            pending_futures.push(future.boxed());
        },
        Some(event) = pending_futures.buffered(10).next() => process_event(event),
        else => break,
   }
}

这会掷回下列编译错误：

--> src/main.rs
     |
257  |     Some(event) = pending_futures.buffered(10).next() => process_event(event),
     |                                   ^^^^^^^^^^^^ `Event` is not a future
     |
     = help: the trait `futures::Future` is not implemented for `Event`
     = note: Event must be a future or must implement `IntoFuture` to be awaited
note: required by a bound in `buffered`
    --> futures-util-0.3.24/src/stream/stream/mod.rs:1359:21
     |
1359 |         Self::Item: Future,
     |                     ^^^^^^ required by this bound in `buffered`

如何限制FuturesUnordered在同一时间只处理其底层队列的N个future，但仍然允许动态地将新的future入队？