如何初始化Weak<dyn T + Send + Sync>为空？Rust开发问题

我正在实现如下发布/订阅模型：

trait EventObserver { ... }

struct Publisher {
    observer: Mutex<Weak<dyn EventObserver + Send + Sync>>,
}

尝试用以下代码初始化Publisher时：

impl Publisher {
    pub fn new() -> Publisher {
        Publisher {
            observer: Mutex::new(Weak::new()),
        }
    }
}

编译器报错：

error[E0283]: type annotations needed
  --> src/lib.rs:12:34
   |
12 |             observer: Mutex::new(Weak::new()),
   |                                  ^^^^^^^^^ cannot infer type of the type parameter `T` declared on the struct `Weak`
   |
   = note: cannot satisfy `_: Unsize<dyn EventObserver + Send + Sync>`
   = note: required for the cast from `std::sync::Weak<_>` to `std::sync::Weak<(dyn EventObserver + Send + Sync + 'static)>`
help: consider specifying the generic argument
   |
12 |             observer: Mutex::new(Weak::<T>::new()),
   |                                      +++++

如果显式指定类型为dyn EventObserver + Send + Sync：

impl Publisher {
    pub fn new() -> Publisher {
        Publisher {
            observer: Mutex::new(Weak::<dyn EventObserver + Send + Sync>::new()),
        }
    }
}

编译器依然报错：

error[E0599]: the function or associated item `new` exists for struct `Weak<dyn EventObserver + Send + Sync>`, but its trait bounds were not satisfied
  --> src/lib.rs:12:75
   |
3  | trait EventObserver {}
   | ------------------- doesn't satisfy `dyn EventObserver + Send + Sync: Sized`
...
12 |             observer: Mutex::new(Weak::<dyn EventObserver + Send + Sync>::new()),
   |                                                                           ^^^ function or associated item cannot be called on `Weak<dyn EventObserver + Send + Sync>` due to unsatisfied trait bounds
   |
   = note: the following trait bounds were not satisfied:
           `dyn EventObserver + Send + Sync: Sized`

我的需求很明确：只需要将Weak初始化为空状态，让observer.upgrade()在初始时返回None，调用subscribe()后能成功升级。目前我知道两种替代方案：用Option包裹Weak，或者创建一个实现EventObserver的空结构体。有没有更简洁的方式直接初始化空的Weak<dyn EventObserver + Send + Sync>？

解决方案：类型强制转换构造空Weak #

可以通过将空的Weak<()>强制转换为目标类型来实现，这是最简洁的方式，不需要额外包装或空结构体：

use std::sync::{Mutex, Weak};

trait EventObserver {}

struct Publisher {
    observer: Mutex<Weak<dyn EventObserver + Send + Sync>>,
}

impl Publisher {
    pub fn new() -> Publisher {
        Publisher {
            // 先创建空的Weak<()>，再强制转换为目标dyn trait类型
            observer: Mutex::new(Weak::<()>::new() as Weak<dyn EventObserver + Send + Sync>),
        }
    }
}

原理说明 #

空的Weak指针本质上不指向任何堆内存，因此其内部的具体类型信息不影响upgrade()的行为——无论原类型是什么，空Weak调用upgrade()都会返回None。通过这种强制转换，我们绕过了Weak::new()对Sized类型的要求，同时让编译器明确了目标类型，完美满足初始化需求。

内容的提问来源于stack exchange，提问作者OLNG