我有以下简化的代码。

use async_trait::async_trait; // 0.1.36
use std::error::Error;

#[async_trait]
trait Metric: Send {
    type Output;
    type Error: Error;

    async fn refresh_metric(&mut self) -> Result<Self::Output, Self::Error>;
}

#[derive(Default)]
struct StaticMetric;

#[async_trait]
impl Metric for StaticMetric {
    type Output = ();
    type Error = Box<dyn Error>;

    async fn refresh_metric(&mut self) -> Result<Self::Output, Self::Error> {
        Ok(())
    }
}

struct LocalSystemData<T> {
    inner: T,
}

impl<T> LocalSystemData<T>
where
    T: Metric,
    <T as Metric>::Error: 'static,
{
    fn new(inner: T) -> LocalSystemData<T> {
        LocalSystemData { inner }
    }

    async fn refresh_all(&mut self) -> Result<(), Box<dyn Error>> {
        self.inner.refresh_metric().await?;
        Ok(())
    }
}

#[tokio::main]
async fn main() -> Result<(), Box<dyn Error>> {
    let mut sys_data = LocalSystemData::new(StaticMetric::default());
    sys_data.refresh_all().await?;

    Ok(())
}

Playground
编译器抛出以下错误

error[E0277]: the size for values of type `(dyn std::error::Error + 'static)` cannot be known at compilation time
  --> src/main.rs:18:18
   |
5  | trait Metric: Send {
   |       ------ required by a bound in this
6  |     type Output;
7  |     type Error: Error;
   |                 ----- required by this bound in `Metric`
...
18 |     type Error = Box<dyn Error>;
   |                  ^^^^^^^^^^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `(dyn std::error::Error + 'static)`
   = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
   = note: required because of the requirements on the impl of `std::error::Error` for `std::boxed::Box<(dyn std::error::Error + 'static)>`

我不确定我是否正确理解了这个问题。
我使用Box<dyn Error>是因为我没有一个具体的类型，并且将一个错误装箱可以处理所有的错误。在我的LocaSystemData实现中，我添加了<T as Metric>::Error: 'static（编译器给了我这个提示，不是我的主意）。在我添加了'static要求之后，编译器抱怨大小未知。发生这种情况是因为'static类型的大小应该在编译时总是已知的，因为static的含义。
我改变了Metric trait，这样就去掉了type Error; trait。现在我有了下面的异步trait函数，代码编译完成了
Playground

async fn refresh_metric(&mut self) -> Result<Self::Output, Box<dyn Error>>;

为什么我的第二个版本可以编译，而第一个版本不行？对我来说，作为一个人类，代码做的完全一样。我觉得我欺骗了编译器：-）。