rust - `Box` ed 类型的方法调用解析

Question

Rust playground :

trait FnBox {
    fn call_box(self: Box<Self>);
}

impl<F: FnOnce()> FnBox for F {
    fn call_box(self: Box<F>) {
        (*self)()
    }
}

fn main() {
    let job: Box<FnOnce()> = Box::new(|| {});
    // versions that compile
    // let job = Box::new(|| {});
    // let job: Box<FnBox> = Box::new(|| {});
    job.call_box();
}

job.call_box();不编译:

error[E0599]: no method named `call_box` found for type `std::boxed::Box<std::ops::FnOnce()>` in the current scope
  --> src/main.rs:16:9
   |
16 |     job.call_box();
   |         ^^^^^^^^
   |
   = note: job is a function, perhaps you wish to call it
   = note: the method `call_box` exists but the following trait bounds were not satisfied:
           `std::ops::FnOnce() : FnBox`
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `call_box`, perhaps you need to implement it:
           candidate #1: `FnBox`

为什么编译器看不到到 FnBox 的转换这里？

我想当我使用 let job = Box::new(|| {}); 时结果是 Box<closure>类型。编译器设法解析 job.call_box() ，即 Box<closure> -> Box<FnOnce()> -> Box<FnBox> (因为 FnBox 是为 FnOnce() 实现的，而不是为 closure 实现的)。为什么不只是 Box<FnOnce()> -> Box<FnBox>工作？

Answer 1

Author's note: Since Rust 1.35, an adapter trait like FnBox is not required to call boxed FnOnce closures; read to the end to see why.

FnBox 未为 FnOnce() 实现.

FnOnce()有两个含义:作为特征和作为类型。在 Box<FnOnce()> ，它是一个(动态的，未调整大小的)类型。 Since Rust 1.27, the type can be written more explicitly as dyn FnOnce() .对于这个答案的其余部分，我将使用 dyn为了弄清楚我什么时候在谈论特征，什么时候在谈论动态类型。

impl你写的没有实现 FnBox对于类型 dyn FnOnce() ，因为 generic parameters have an implicit Sized bound .如果我们按照该问题答案的建议添加 + ?Sized至 F , 编译器确实给出了一个稍微有用的错误信息:

impl<F: FnOnce() + ?Sized> FnBox for F {
    fn call_box(self: Box<F>) {
        (*self)()
    }
}

error[E0161]: cannot move a value of type F: the size of F cannot be statically determined
 --> src/main.rs:7:9
  |
7 |         (*self)()
  |         ^^^^^^^

哪个确实明确指出了Sized的地方需要 ness。

在 1.35 之前的 Rust 版本中，没有办法修复这个错误； FnOnce() 的特征对象只是没用。然而，今天Box<dyn FnOnce()>工具 FnOnce() , 所以你可以像普通闭包一样调用它，而不用 * :

impl<F: FnOnce() + ?Sized> FnBox for F {
    fn call_box(self: Box<F>) {
        self()
    }
}

另见

• What does "Box<Fn() + Send + 'static>" mean in Rust?
• Rust Trait object conversion (为什么你不能从一种特征对象类型转到另一种)