Rust中如何解决返回持有局部字符串引用的Future编译错误？

问题 #

我尝试创建一个Vec<Future>配合try_join_all使用，代码如下：

use futures::future::try_join_all;

struct Image {
    pub file_name: String,
}

struct Storage;

impl Storage {
    async fn upload(&self, filename: &str) -> Result<String, ()> {
        Ok(filename.to_owned())
    }
}

struct ImagesRepository {
    storage: Storage,
}

impl ImagesRepository {
    async fn upload_images(&self, images: &[Image]) -> Result<Vec<String>, ()> {
        let futures = images
            .iter()
            .map(|image| {
                let path_name = format!("somedirectory/{}", &image.file_name);

                let future = self.storage.upload(path_name.as_str());

                future
            })
            .collect::<Vec<_>>();

        let results = try_join_all(futures).await?;

        Ok(results)
    }
}

但出现编译错误：

Compiling playground v0.0.1 (/playground)
error[E0515]: cannot return value referencing local variable `path_name`
  --> src/lib.rs:30:17
   |
26 |                 let future = self.storage.upload(path_name.as_str());
   |                                                  ------------------ `path_name` is borrowed here
...
30 |                 future
   |                 ^^^^^^ returns a value referencing data owned by the current function

For more information about this error, try `rustc --explain E0515`.
error: could not compile `playground` due to previous error

错误原因是传递给storage.upload的path_name引用仅存在于闭包内部。我知道可以将storage.upload改为接收String类型，但这会破坏其他调用该函数的代码。请问有没有办法修复这个编译错误？

解决方法 #

核心问题是path_name是闭包内的局部变量，当闭包返回future时，path_name会被销毁，导致future持有悬垂引用。要解决这个问题，需要让path_name的生命周期和future绑定在一起，具体可以通过async块实现：

修改map闭包的逻辑，把path_name的创建和upload调用放在同一个async块里，这样path_name会被async块捕获，其生命周期会延续到future执行完毕：

use futures::future::try_join_all;

struct Image {
    pub file_name: String,
}

struct Storage;

impl Storage {
    async fn upload(&self, filename: &str) -> Result<String, ()> {
        Ok(filename.to_owned())
    }
}

struct ImagesRepository {
    storage: Storage,
}

impl ImagesRepository {
    async fn upload_images(&self, images: &[Image]) -> Result<Vec<String>, ()> {
        let futures = images
            .iter()
            .map(|image| async {
                let path_name = format!("somedirectory/{}", &image.file_name);
                self.storage.upload(path_name.as_str()).await
            })
            .collect::<Vec<_>>();

        let results = try_join_all(futures).await?;

        Ok(results)
    }
}

原理说明 #

• 原写法中，future是upload方法返回的未执行异步任务，它持有对path_name的引用，但path_name在闭包结束后就会被释放，导致悬垂引用。
• 使用async块后，path_name会被包含在async块生成的future内部，async块会捕获path_name的所有权并维持其有效性直到future执行完毕，彻底解决引用生命周期不匹配的问题。

这种修改不会改变Storage::upload的原有签名，完全兼容其他调用该函数的代码。

内容的提问来源于stack exchange，提问作者Steven