如何修正Rust代码使HashMap统计不可变vector元素出现次数？

目标是用HashMap统计aux_vec_i32中各元素的出现次数，以下是原代码、编译错误分析及修复方案：

原代码 #

use std::collections::HashMap;

fn main(){
    struct Data{
        the_data: i32,
    }
    let aux_vec_i32 = vec![1, 2, 3, 1];
    let mut hmap_for_data: HashMap<i32, Data>=HashMap::new();

    for i in &aux_vec_i32 {
        match hmap_for_data.get(&mut aux_vec_i32[*i as usize]) {
            None => {
                hmap_for_data.insert(
                    aux_vec_i32[*i as usize], 
                    Data {
                        the_data: 1,
                    }
                );
            }
            Some(mut data_to_increment) => {
                data_to_increment.the_data += 1;
            }
        }
    }
}

编译错误信息 #

error[E0502]: cannot borrow `aux_vec_i32` as mutable because it is also borrowed as immutable
  --> src/main.rs:11:38
   |
10 |     for i in &aux_vec_i32 {
   |              ------------
   |              |
   |              immutable borrow occurs here
   |              immutable borrow later used here
11 |         match hmap_for_data.get(&mut aux_vec_i32[*i as usize]) {
   |                                      ^^^^^^^^^^^ mutable borrow occurs here

error[E0596]: cannot borrow `aux_vec_i32` as mutable, as it is not declared as mutable
  --> src/main.rs:11:38
   |
11 |         match hmap_for_data.get(&mut aux_vec_i32[*i as usize]) {
   |                                      ^^^^^^^^^^^ cannot borrow as mutable
   |
help: consider changing this to be mutable
   |
7  |     let mut aux_vec_i32 = vec![1, 2, 3, 1];
   |         +++

error[E0594]: cannot assign to `data_to_increment.the_data`, which is behind a `&` reference
  --> src/main.rs:21:17
   |
20 |             Some(mut data_to_increment) => {
   |                  --------------------- consider changing this binding's type to be: `&mut Data`
21 |                 data_to_increment.the_data += 1;
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `data_to_increment` is a `&` reference, so the data it refers to cannot be written

错误核心原因 #

• 完全无需操作原数组aux_vec_i32获取哈希表key，遍历得到的i就是目标统计元素，直接用它作为key即可。
• HashMap::get返回不可变引用，无法修改内部数据；要修改需用get_mut，或更高效的entry API。
• 错误尝试对不可变的aux_vec_i32进行可变借用，违反Rust借用规则。

修复后的代码 #

use std::collections::HashMap;

fn main(){
    struct Data{
        the_data: i32,
    }
    let aux_vec_i32 = vec![1, 2, 3, 1];
    let mut hmap_for_data: HashMap<i32, Data> = HashMap::new();

    for &num in &aux_vec_i32 {
        // 用entry API统一处理key存在/不存在的情况
        let data = hmap_for_data.entry(num).or_insert(Data { the_data: 0 });
        data.the_data += 1;
    }

    // 可选：验证统计结果
    for (key, val) in hmap_for_data {
        println!("元素{}出现{}次", key, val.the_data);
    }
}

修复说明 #

1. 遍历直接取&num，拿到数组元素值作为哈希表key，无需操作原数组。
2. HashMap::entry方法：若key不存在则插入初始值Data { the_data: 0 }，若存在则返回对应值的可变引用。
3. 直接对可变引用指向的the_data自增，符合Rust借用规则，同时代码更简洁高效。

内容的提问来源于stack exchange，提问作者branco