Rust 为什么 ops：：Range 不实现 Copy，即使 T 是 Copy

发布于04月15日

最近，我想为3D投影编写一个包含参数的类型:

use std::ops::Range;

#[derive(Clone, Copy)]
struct CamProj {
    /// Near and far plane
    proj_range: Range<f32>,
    /// Field of view
    fov: cgmath::Rad<f32>,     // `Rad` derives `Copy`
    /// Width divided by height
    aspect_ratio: f32,       
}

然而，我得到了这个错误:

error[E0204]: the trait `Copy` may not be implemented for this type
 --> <anon>:3:21
  |
3 |     #[derive(Clone, Copy)]
  |                     ^^^^
...
6 |         proj_range: Range<f32>,
  |         ---------------------- this field does not implement `Copy`

显然，Range<T>永远不会实现Copy，即使T是Copy，就像f32是.Why is that?我以为Range<T>就是T？那么它肯定能实现Copy？

推荐答案

因为Range<T>经常用作迭代器，迭代器是Copy was discovered to be a footgun.One specific example与认为迭代器是高级的有关，而实际上迭代器是高级的副本:

for x in it {  // a *copy* of the iterator is used here
    // ..
}

match it.next() {  // the original iterator is used here
    // ..
}

Another example:

fn main() {
    let stream = "Hello, world!".chars().cycle();
    for _ in 0..10 {
        let chunk: String = stream.take(3).collect();
        println!("{}", chunk);
    }
}

还有一个问题:Using the same iterator multiple times in Rust

人们认为，通过clone显式复制迭代器有助于防止这些情况

具体来说，将Copy添加到Range was proposed and rejected.有人提出了一个潜在的解决办法: