rust - 错误[E0502] : cannot borrow `char_array` as mutable because it is also borrowed as immutable

Question

我正在编写一个小程序来识别字符串中第一个重复出现的字符:

use std::io;

fn main() {
    let mut main_string = String::new();

    println!("Please enter the string : ");
    io::stdin()
        .read_line(&mut main_string)
        .expect("Failed to read the input value");

    main_string = main_string.trim().to_string();
    println!("The trimmed string is : {}", main_string);

    let repeating_character = recurring_string_parser(main_string);

    println!(
        "The character which is first repeating is {}",
        repeating_character
    );
}

fn recurring_string_parser(main_string: String) -> char {
    let mut char_array = Vec::new();
    for each_char in main_string.chars() {
        let mut some_val = char_array.iter().find(|&&c| c == each_char);

        match some_val {
            Some(ch) => return each_char,
            _ => println!("do nothing"),
        }
        char_array.push(each_char);
        println!(" The charcater is {:?}", some_val);
    }

    return 'a';
}

错误:

error[E0502]: cannot borrow `char_array` as mutable because it is also borrowed as immutable
  --> src/main.rs:31:9
   |
25 |         let mut some_val = char_array.iter().find(|&&c| c == each_char);
   |                            ---------- immutable borrow occurs here
...
31 |         char_array.push(each_char);
   |         ^^^^^^^^^^ mutable borrow occurs here
32 |         println!(" The charcater is {:?}", some_val);
33 |     }
   |     - immutable borrow ends here

我做错了什么？如果有人也能解释一下，那将会很有帮助，因为我发现很难理解可变借用的概念。

Answer 1

char_array.iter().find(|&&c| c == each_char) 的返回类型是一个 Option<&char> ;指向 char_array 中原始项目的引用.然后您尝试修改 char_array这可能会使任何引用无效。之后，您尝试通过打印来访问现在可能无效的值。

在 C 或 C++ 等语言中，此代码是允许的，但偶尔会崩溃、损坏数据、允许任意代码执行或吃掉你的衣服。

最懒惰的解决方案是重新排序 println并在 some_val 周围添加一组额外的大括号应该住(将来我们将拥有非词法生命周期，并且不需要额外的大括号):

{
    let mut some_val = char_array.iter().find(|&&c| c == each_char);

    match some_val {
        Some(ch) => return each_char,
        _ => println!("do nothing"),
    }
    println!(" The character is {:?}", some_val);
}

char_array.push(each_char);

您还可以克隆该值，打破 some_val 之间的关系和 char_array :

let mut some_val = char_array.iter().find(|&&c| c == each_char).cloned();

另见:

• Unable to borrow an iterator as mutable more than once at a time
• Why does iter() borrow the list as immutable?