rust - 不是复制或克隆的全局const在Rust中如何工作？

Question

说我有以下片段(playground)

struct A {
    pub val: u32
}

const GLOBAL_A: A = A {val: 2};

fn main() {
    let some_a: A = GLOBAL_A;
    let other_a: A = GLOBAL_A;

    println!("double val = {}", some_a.val + other_a.val);
}

由于 A既不是 Clone也不是 Copy，我假设 GLOBAL_A的值将被 move 。对于const来说，这没有多大意义，而且由于它可以被“move ”两次，因此无论如何都不可能如此。

考虑到 A既不是 Clone也不是 Copy，则允许上述代码段工作的规则是什么？

Answer 1

常量总是内联的。您的示例与以下示例基本相同

struct A {
    pub val: u32
}

fn main() {
    let some_a: A = A {val: 2};
    let other_a: A = A {val: 2};

    println!("double val = {}", some_a.val + other_a.val);
}

该值被重建两次，因此它不必是 Copy或 Clone。

另一方面，不内联 static:

struct A {
    pub val: u32
}

static GLOBAL_A: A = A {val: 2};

fn main() {
    let some_a: A = GLOBAL_A;
}

结果是

error[E0507]: cannot move out of static item `GLOBAL_A`
 --> src/main.rs:8:21
  |
8 |     let some_a: A = GLOBAL_A;
  |                     ^^^^^^^^
  |                     |
  |                     move occurs because `GLOBAL_A` has type `A`, which does not implement the `Copy` trait
  |                     help: consider borrowing here: `&GLOBAL_A`