“无法移动 FnOnce 类型的值"；移动盒装函数时 [英] "cannot move a value of type FnOnce" when moving a boxed function

问题描述

我正在尝试在 Rust 中进行一些更高阶的编程，但是我在处理闭包时遇到了一些困难.这是一个代码片段，说明了我遇到的问题之一:

I'm trying to do some higher order programming in Rust, but I'm having some difficulty dealing with closures. Here's a code snippet that illustrates one of the problems I'm having:

pub enum Foo {
    Bar(Box<FnOnce(i32)>),
}

pub fn app(i: i32, arg: Foo) {
    match arg {
        Foo::Bar(f) => f(i),
    }
}

当我编译这段代码时，我收到以下错误消息:

When I compile this piece of code I get the following error message:

error[E0161]: cannot move a value of type std::ops::FnOnce(i32) + 'static: the size of std::ops::FnOnce(i32) + 'static cannot be statically determined
 --> src/main.rs:7:24
  |
7 |         Foo::Bar(f) => f(i),
  |                        ^

因为我把函数放在一个Box中，我以为这样可以解决编译器不知道大小的问题.我怎样才能使上述程序编译?

Since I put the function in a Box, I would have thought that that would deal with the problem of the compiler not knowing the size. How can I make the above program compile?

推荐答案

这里是 FnOnce trait 的定义(稍微简化了一点):

Here's the FnOnce trait's definition (simplified a little):

pub trait FnOnce<Args> {
    type Output;

    fn call_once(self, args: Args) -> Self::Output;
}

要调用 FnOnce 闭包，您需要能够将闭包值本身移动到调用中.注意 self 必须是 actual 闭包类型；Box 是完全不同的类型.

To call a FnOnce closure, you need to be able to move the closure value itself into the invocation. Note that self has to be the actual closure type; a Box<dyn FnOnce> is a different type altogether.

Box 现在可以被调用；您的原始代码按原样运行.

Box<dyn FnOnce> is now able to be called; your original code works as-is.

在标准库中有一种类型可以解决这种情况:FnBox.不幸的是，它不稳定.

There is a type in the standard library for working around this situation: FnBox. Unfortunately, it's unstable.

您的替代选择是:

• 重构代码，以便保留实际闭包类型，而不是Box.
• 改用 Box.
• 等待 FnBox 稳定.
• 切换到夜间编译器.

• Refactor the code so that instead of Box<FnOnce>, you preserve the actual closure type.
• Use Box<FnMut> instead.
• Wait for FnBox to stabilise.
• Switch to a nightly compiler.

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