当T没有实现Copy时，如何返回Arc后面的值<Mutex<T>>？

Question

我一直在尝试使用以下函数启动一个值（这里是 hashbrown 的哈希图）：

use std::collections::HashMap;
use std::sync::{Arc, Mutex};
use std::thread;

#[derive(Clone)]
struct Entity;

fn create_value(center: (f32, f32), length: f32) -> HashMap<(i32, i32), Option<Entity>> {
    let cities: Arc<Mutex<HashMap<(i32, i32), Option<Entity>>>> = Arc::default();
    let mut handles = vec![];

    for i in 0..5 {
        for j in 0..5 {
            let city_thread = Arc::clone(&cities);
            handles.push(thread::spawn(move || {
                // do stuff in parallel
                if let Ok(mut cities) = city_thread.lock() {
                    cities.insert((i, j), None);
                } 
            }));
        }
    }

    for handle in handles {
        handle.join().unwrap();
    }

    // this part is the problem
    if let Ok(city) = cities.lock() {
        let city = city.clone();
        return city;
    } else {
        panic!()
    }
}

我收到以下错误：

`cities` does not live long enough
the temporary is part of an expression at the end of a block;
consider forcing this temporary to be dropped sooner, before the block's local variables are dropped

我知道我做错了什么，但我找不到任何东西或方法来返回该值。

Answer 1

我们需要将

city.clone()

的结果放入在最终 if let 块之前

声明的变量中，并将 

return

 移出 

if let

 块。

use std::collections::HashMap;
use std::sync::{Arc, Mutex};
use std::thread;

#[derive(Clone)]
struct Entity;

fn create_value(center: (f32, f32), length: f32) -> HashMap<(i32, i32), Option<Entity>> {
    let cities: Arc<Mutex<HashMap<(i32, i32), Option<Entity>>>> = Arc::default();
    let mut handles = vec![];

    for i in 0..5 {
        for j in 0..5 {
            let city_thread = Arc::clone(&cities);
            handles.push(thread::spawn(move || {
                // do stuff in parallel
                if let Ok(mut cities) = city_thread.lock() {
                    cities.insert((i, j), None);
                }
            }));
        }
    }

    for handle in handles {
        handle.join().unwrap();
    }

    let result;
    if let Ok(city) = cities.lock() {
        result = city.clone();
    } else {
        panic!()
    }

    result
}

或者，我们可以通过在

if let

 后面添加分号来避免它成为“块末尾的表达式”。这很奇怪，但它有效！

use std::collections::HashMap;
use std::sync::{Arc, Mutex};
use std::thread;

#[derive(Clone)]
struct Entity;

fn create_value(center: (f32, f32), length: f32) -> HashMap<(i32, i32), Option<Entity>> {
    let cities: Arc<Mutex<HashMap<(i32, i32), Option<Entity>>>> = Arc::default();
    let mut handles = vec![];

    for i in 0..5 {
        for j in 0..5 {
            let city_thread = Arc::clone(&cities);
            handles.push(thread::spawn(move || {
                // do stuff in parallel
                if let Ok(mut cities) = city_thread.lock() {
                    cities.insert((i, j), None);
                } 
            }));
        }
    }

    for handle in handles {
        handle.join().unwrap();
    }

    // this part is the problem
    if let Ok(city) = cities.lock() {
        let city = city.clone();
        return city;
    } else {
        panic!()
    }
    
    ;
}

（不过我推荐第一种方法。）