错误[E0597]:借来的值在While循环中存活的时间不够长

问题描述

我对 Rust 真的很陌生，我在解决这个错误时遇到了麻烦，但只有在我注释掉 while 语句时才会发生，基本上我是从控制台询问值并将其存储在 HashMap 中:

I am really new to Rust, I am having trouble solving this error, but it only happens if I comment out the while statement , basicly I am asking values from the console and storing it in a HashMap:

use std::collections::HashMap;
use std::io;

fn main() {
    let mut customers = HashMap::new();
    let mut next_customer = true;
    while next_customer {
        let mut input_string = String::new();
        let mut temp_vec = Vec::with_capacity(3);
        let mut vec = Vec::with_capacity(2);
        println!("Insert new customer f.e = customer id,name,address:");
        io::stdin().read_line(&mut input_string);
        input_string = input_string.trim().to_string();
        for s in input_string.split(",") {
            temp_vec.push(s);
        }
        vec.push(temp_vec[1]);
        vec.push(temp_vec[2]);
        let mut key_value = temp_vec[0].parse::<i32>().unwrap();
        customers.insert(key_value, vec);
        next_customer = false;
    }
    println!("DONE");
}

代码导致错误

error[E0597]: `input_string` does not live long enough
  --> src/main.rs:14:18
   |
14 |         for s in input_string.split(",") {
   |                  ^^^^^^^^^^^^ borrowed value does not live long enough
...
20 |         customers.insert(key_value, vec);
   |         --------- borrow later used here
21 |         next_customer = false;
22 |     }
   |     - `input_string` dropped here while still borrowed

推荐答案

正如其他人所说，问题在于放入客户地图的值的生命周期和/或类型.

As others have said the problem lies with the lifetime and/or type of the values getting put into the customers map.

customers.insert(key_value, vec);
   |         --------- borrow later used here

当编译器决定给一个对象一个你没有预料到的类型时，通常会发生这种情况.要了解它在做什么，您可以强制类型，并查看它如何抱怨.将代码更改为:

Often this happens when the compiler has decided to give an object a type that you didn't expect. To find out what it's doing you can force the type, and see how it complains. Changing the code to:

    let mut customers: HashMap<(),()> = HashMap::new();

给我们两个相关的错误:

Gives us two relevant errors:

20 |         customers.insert(key_value, vec);
   |                          ^^^^^^^^^ expected `()`, found `i32`
...
20 |         customers.insert(key_value, vec);
   |                                     ^^^ expected `()`, found struct `std::vec::Vec`
   |
   = note: expected unit type `()`
                 found struct `std::vec::Vec<&str>`

所以编译器想要给我们的客户对象的类型是 HashMap

So the type that the compiler wants to give our customers object is HashMap<i32, Vec<&str>>

问题是 &str 生命周期必须在块内，因为我们没有将 String 存储在任何地方，而且它们不能有'static 生命周期，因为它们是用户输入.

The problem is that the &str lifetime has got to be inside the block as we don't store the Strings anywhere, and they can't have 'static lifetime since they're user input.

这意味着我们可能需要一个 HashMap>.

This means we probably want a HashMap<i32,Vec<String>>.

更改代码以使用其中之一会给我们一个关于 vec 没有正确类型的错误:它被推导出为 Vec，但是我们想要一个 Vec.

Changing the code to use one of those gives us an error about vec not having the right type: It's getting deduced as a Vec<&str>, but we want a Vec<String>.

我们有两个选择.

1. 在我们使用 customers.insert(key_value, vec.iter().map(|s| s.to_string()).collect 将 vec 插入地图之前，将其转换为正确的类型()).(尽管为了清楚起见，您可能希望将其提取为变量).

1. Convert the vec to the right type just before we insert it into the map using customers.insert(key_value, vec.iter().map(|s| s.to_string()).collect()). (Though you may want to extract it to a variable for clarity).

明确将vec的类型改为Vec

Explicitly change the type of vec to Vec<String>

选项 1有效".虽然选项 2 使我们走上了一条越来越接近 read_line 调用的类似更改的道路.

Option 1 "just works". While option 2 leads us down a path of making similar changes closer and closer to the read_line call.

决定选项 1 中的修复后，如果您发现它们过于嘈杂，您可以删除为解决此问题而添加的手动类型注释.

Once you've decided on the fix in option 1, you can remove the manual type annotations that were added to work out the fix, if you find them overly noisy.

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