rust - 为什么不能将 Box 传递给以 &mut Trait 作为参数的函数

Question

我确定之前有人问过这个问题，但还没有遇到能准确描述此处情况的问题。我有以下代码:

let mut pool: Box<dyn redis::aio::ConnectionLike> = <...>
redis::cmd(COMMAND)
    .arg(LIST)
    .arg(value)
    .query_async(&mut pool)
    .await
    .unwrap();

这会返回错误:

error[E0277]: the trait bound `std::boxed::Box<dyn redis::aio::ConnectionLike>: redis::aio::ConnectionLike` is not satisfied
   --> svix-server/src/queue/redis.rs:66:30
    |
66  |                 .query_async(&mut pool)
    |                  ----------- ^^^^^^^^^ the trait `redis::aio::ConnectionLike` is not implemented for `std::boxed::Box<dyn redis::aio::ConnectionLike>`
    |                  |
    |                  required by a bound introduced by this call
    |

问题 1 -- 为什么错误提示未为 Box<dyn redis::aio::ConnectionLike> 实现特征？它不应该至少说&Box...吗？ ？

无论如何，如果我改为尝试通过 pool.as_mut()至 query_async ，我得到这个错误:

error[E0277]: the size for values of type `dyn redis::aio::ConnectionLike` cannot be known at compilation time
   --> svix-server/src/queue/redis.rs:66:30
    |
66  |                 .query_async(pool.as_mut())
    |                  ----------- ^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |                  |
    |                  required by a bound introduced by this call
    |
    = help: the trait `Sized` is not implemented for `dyn redis::aio::ConnectionLike`

问题 2 -- 为什么是 Sized这里需要吗？ Rust 中的引用不总是 Sized ？

问题 3 -- 有什么方法可以传递 dyn ConnectionLike对此

query_async方法，引用如下:

#[inline]
#[cfg(feature = "aio")]
pub async fn query_async<C, T: FromRedisValue>(&self, con: &mut C) -> RedisResult<T>
where
    C: crate::aio::ConnectionLike,
{
   ...
}

Answer 1

Question 1 -- Why does the error say the trait is not implemented for Box<dyn redis::aio::ConnectionLike>? Shouldn't it at least say &Box...?

不，因为引用不是泛型的一部分 C . Rust 不会命名整个 类型，它只会命名两个类型中不匹配的部分。该函数接受 &mut C和 C是匹配的通用部分。 Box type 是 C 的替代品，并且它没有实现这个特性。

Question 2 -- Why is Sized required here? Isn't a reference in Rust always Sized?

引用确实是Sized ，但如上所述，我们正在匹配类型 &mut dyn redis::aio::ConnectionLike反对&mut C .这导致 C正在dyn redis::aio::ConnectionLike .

在 Rust 中，泛型类型有 Sized默认绑定(bind) 。 您必须明确指定 ?Sized一定要放宽这个，这个功能的作者没有做到。因此，C必须实现 Sized , 和 dyn redis::aio::ConnectionLike没有。

Question 3 -- Is there any way to to pass a dyn ConnectionLike to this

可能，但不是直接的。您将需要实现一个类型来包装对 dyn ConnectionLike 的引用。或 Box<dyn ConnectionLike>并通过它。像这样的东西:

struct BoxedConnectionLike(Box<dyn ConnectionLike>);

impl ConnectionLike for BoxedConnectionLike {
    // Implement proxy functions for the trait.
}