假设我有一个返回Vec<Box<dyn Subscriber>>的函数，我想将它们组合成一个单独的函数，并将其设置为默认订阅者。如何做到这一点？
我尝试做类似的事情，但我无法使类型匹配：

pub fn init_log(subscribers: Vec<Box<dyn Subscriber>>) -> Result<(), Error> {
    use tracing_subscriber::prelude::*;
    let mut layers: Vec<Box<dyn Layer<dyn Subscriber>>> = Vec::new();

    for subscriber in subscribers.iter().drain(..) {
        let layer: Box<dyn Layer<dyn Subscriber>> =
          Box::new(tracing_subscriber::layer::Identity::new().with_subscriber(subscriber));

        layers.push(layer);
    }

    let init_layer: Box<dyn Layer<dyn Subscriber>> =
        Box::new(tracing_subscriber::layer::Identity::new());

    let acc_subscriber: Layered<Box<dyn Layer<dyn Subscriber>>, _> =
        tracing_subscriber::fmt().finish().with(init_layer);
    
    let composed = layers
        .drain(..)
        .into_iter()
        .fold(acc_subscriber, |acc, layer| acc.with(layer));
    
    tracing::subscriber::set_global_default(composed);

    Ok(())
}

error[E0277]: the trait bound `Box<dyn tracing::Subscriber>: tracing::Subscriber` is not satisfied
  --> jormungandr\src\settings\logging.rs:97:85
   |
97 |                 Box::new(tracing_subscriber::layer::Identity::new().with_subscriber(subscriber));
   |                                                                                     ^^^^^^^^^^ the trait `tracing::Subscriber` is not implemented for `Box<dyn tracing::Subscriber>`