Result<(), Box<(dyn SomeTrait + 'static)>> 不满足 Trait Bound

Question

use once_cell::sync::OnceCell;

pub trait SomeTrait {}
pub struct Impl1 {}

impl SomeTrait for Impl1 {}

pub static GLOBAL_THING: OnceCell<Box<dyn SomeTrait>> = OnceCell::new();

pub fn main() {
    GLOBAL_THING.set(Box::new(Impl1 {})).unwrap();
}

我收到此错误，不知道如何解释或修复它。

error[E0599]: the method `unwrap` exists for enum `Result<(), Box<(dyn SomeTrait + 'static)>>`, but its trait bounds were not satisfied
   --> src/main.rs:11:42
    |
11  |       GLOBAL_THING.set(Box::new(Impl1 {})).unwrap();
    |                                            ^^^^^^ method cannot be called on `Result<(), Box<(dyn SomeTrait + 'static)>>` due to unsatisfied trait bounds
    |
    = note: the following trait bounds were not satisfied:
            `Box<dyn SomeTrait>: Debug`

一个简单的解决方法是只使用if let Err() = GLOBAL_THING.set() {panic!(...)} 而不是使用unwrap()，但我想了解这里发生了什么并尽可能修复。

Answer 1

如果您查看unwrap method 文档，您会发现它不是为所有Results 定义的，而是只为E: Debug 定义的：

impl<T, E> Result<T, E>
where
    E: Debug, 
{
    pub fn unwrap(self) -> T;
}

它需要错误类型E来实现Debug，以便在解包失败时打印错误。

错误信息显示结果类型为Result<(), Box<(dyn SomeTrait + 'static)>>，所以E = Box<(dyn SomeTrait + 'static)>。您可以通过使用SomeTrait: Debug 使此错误类型可调试，这要求实现SomeTrait 的任何类型也必须实现Debug。

pub trait SomeTrait: Debug {}

#[derive(Debug)]
pub struct Impl1 {}

一旦你这样做，你会遇到下一个错误：

error[E0277]: `dyn SomeTrait` cannot be shared between threads safely
  --> src/main.rs:11:1
   |
11 | pub static GLOBAL_THING: OnceCell<Box<dyn SomeTrait>> = OnceCell::new();
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `dyn SomeTrait` cannot be shared between threads safely
   |
   = help: the trait `Sync` is not implemented for `dyn SomeTrait`
   = note: required because of the requirements on the impl of `Sync` for `Unique<dyn SomeTrait>`
   = note: required because it appears within the type `Box<dyn SomeTrait>`
   = note: required because of the requirements on the impl of `Sync` for `once_cell::imp::OnceCell<Box<dyn SomeTrait>>`
   = note: required because it appears within the type `once_cell::sync::OnceCell<Box<dyn SomeTrait>>`
   = note: shared static variables must have a type that implements `Sync`

要解决此问题，您还需要创建盒装特征对象Send + Sync，以便它可以跨线程共享。

pub static GLOBAL_THING: OnceCell<Box<dyn SomeTrait + Send + Sync>> = OnceCell::new();

你可以在Playground看到最终结果。