rust 为什么我得到了“不能借用'arr[_]'作为可变的一次超过一次”?

iqxoj9l9  于 2022-11-30  发布在  其他
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我有一个用Rust创建冒泡排序的编程任务。我在Rust方面没有太多的经验,所以这对我来说有点困难:

fn main() {
    println!("Sort numbers ascending");
    let num:[i32; 10] = [4, 65, 2, -31, 0, 99, 2, 83, 782, 1];
    println!("Before: {:?}", num);
    bubble_sort( num);
    println!("After: {:?}\n", num);
}

fn swap( a: &mut i32, b: &mut i32){
    let tmp: i32 = *b;
    *b=*a;
    *a=tmp;
}

fn bubble_sort(mut arr: [i32; 10]) {
    for i in 0..arr.len() {
        for j in 0..arr.len() - 1 - i {
            if arr[j] > arr[j + 1] {
                swap( &mut arr[j], &mut arr[j + 1]);
            }
        }
    }
}

错误:

Compiling playground v0.0.1 (/playground)
error[E0499]: cannot borrow `arr[_]` as mutable more than once at a time
  --> src/main.rs:19:24
   |
19 |                 swap( &mut arr[j], &mut arr[j + 1]);
   |                 ----  -----------  ^^^^^^^^^^^^^^^ second mutable borrow occurs here
   |                 |     |
   |                 |     first mutable borrow occurs here
   |                 first borrow later used by call
   |
   = help: consider using `.split_at_mut(position)` or similar method to obtain two mutable non-overlapping sub-slices

For more information about this error, try `rustc --explain E0499`.
error: could not compile `playground` due to previous error
i7uaboj4

i7uaboj41#

Rust借位检查器非常复杂,但它也有局限性。在这里,尽管对人类来说很明显没有可变别名,但借位检查器完全看不到这一事实。它看到&mut arr[j]&mut arr[j + 1]并得出结论:“您正尝试从arr可变地借用两次”。
错误消息确实指出了解决方案:
split_at_mut()就是为这种情况而提供的。在幕后,它使用unsafe以一种仍然合理并且不会导致UB的方式来弯曲上述规则。它接受一个索引来进行拆分,并返回2个不重叠的可变切片。这是来自文档的示例:

let mut v = [1, 0, 3, 0, 5, 6];
let (left, right) = v.split_at_mut(2);
assert_eq!(left, [1, 0]);
assert_eq!(right, [3, 0, 5, 6]);

因此,在您的情况下,不使用&mut arr[j]来获取对数字的可变引用,您可以执行以下操作:

let (start, end) = arr.split_at_mut(1);
swap(&mut start[0], &mut end[0]);

P.S.切片也有.swap(i, j):-D

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