rust - 为共享静态变量避免 `Sync`

Question

我有以下定义:

struct MyCustomFactory;

trait Factory {
    fn new<'a>(entity: &'a str) -> Arc<dyn Display + 'a>
    where
        Self: Sized;
}

impl Factory for MyCustomFactory {
    fn new<'a>(entity: &'a str) -> Arc<dyn Display + 'a>
    where
        Self: Sized,
    {
        Arc::new(entity)
    }
}

更高层次的问题:

我正在尝试创建一个全局静态(和不可变)字符串映射到实现 Factory 的结构特征。目标是对于映射的任何值成员，我都可以将其视为 Factory。 , 调用 new反对它，反过来又会得到东西(在这种情况下)实现了Display特质。

我有以下内容:

static ITEMS: Lazy<BTreeMap<&'static str, Arc<dyn Factory>>> =
    Lazy::new(|| {
        let mut map = BTreeMap::new();
        let value = Arc::new(MyCustomFactory) as Arc<dyn Factory>;
        map.insert("foo", value);
        map
    });

编译器提示:

77 | / static ITEMS: Lazy<BTreeMap<&'static str, Arc<dyn Factory>>> =
78 | |     Lazy::new(|| {
79 | |         let map = BTreeMap::new();
80 | |         let value = Arc::new(MyCustomFactory) as Arc<dyn Factory>;
...  |
88 | |     });
   | |_______^ `(dyn Factory + 'static)` cannot be shared between threads safely
   |
   = help: the trait `std::marker::Sync` is not implemented for `(dyn Factory + 'static)`
   = note: required because of the requirements on the impl of `std::marker::Send` for `std::sync::Arc<(dyn Factory + 'static)>`
   = note: required because of the requirements on the impl of `std::marker::Send` for `alloc::collections::btree::node::Root<&'static str, std::sync::Arc<(dyn Factory + 'static)>>`
   = note: required because it appears within the type `std::collections::BTreeMap<&'static str, std::sync::Arc<(dyn Factory + 'static)>>`
   = note: required because of the requirements on the impl of `std::marker::Sync` for `once_cell::imp::OnceCell<std::collections::BTreeMap<&'static str, std::sync::Arc<(dyn Factory + 'static)>>>`
   = note: required because it appears within the type `once_cell::sync::OnceCell<std::collections::BTreeMap<&'static str, std::sync::Arc<(dyn Factory + 'static)>>>`
   = note: required because of the requirements on the impl of `std::marker::Sync` for `once_cell::sync::Lazy<std::collections::BTreeMap<&'static str, std::sync::Arc<(dyn Factory + 'static)>>>`
   = note: shared static variables must have a type that implements `Sync`

我尝试过的事情:

• 实现 Sync反对特征:声明特征具有trait Factory : Sync , (意味着所有实现该特征的项目都必须实现 Sync )和/或定义:

     |
  66 | unsafe impl Sync for dyn Factory {}
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type
  

• 包装 Arc<dyn Factory> :我试过使用 Mutex/RefCell/OnceCell和其他人，都导致相同的错误。似乎应该有一种方法来处理底层 MyCustomFactory作为一个可以被锁定的单例。这个结构在全局范围内只有一次实例，这很好(也是意料之中的)。

Answer 1

您已接近添加 Sync。如果您仔细观察，您会发现该错误建议添加 Send:

= help: the trait `std::marker::Send` is not implemented for `(dyn Factory + 'static)`

同时添加 Send 和 Sync 并编译。你可以这样做:

trait Factory: Send + Sync {
    ...
}

或者，我的偏好是在静态 map 中专门要求它们，因为那是需要它们的地方:

static ITEMS: Lazy<BTreeMap<&'static str, Arc<dyn Factory + Send + Sync>>> =
    Lazy::new(|| {
        let mut map = BTreeMap::new();
        let value = Arc::new(MyCustomFactory) as Arc<dyn Factory + Send + Sync>;
        map.insert("foo", value);
        map
    });

您还可以通过让它推断第二个 Arc 来减少重复:

let value = Arc::new(MyCustomFactory) as Arc<_>;