为什么这个 Rust 函数不起作用？

Question

我试图了解如何在 Rust 中做一些简单的事情，但我一直在与借用检查器作斗争，我不确定为什么。我编写了一个简单的函数，您可以将文件名传递给它，它会为您提供主目录中该文件的路径。

程序如下：

use std::env;

fn filename_in_homedir(filename: &str) -> Option<&str> {
    let mut homedir = match env::home_dir() {
        None => return None,
        Some(p) => p
    };  
    homedir.push(filename);
    homedir.to_str()
}

fn main() {
    match filename_in_homedir(".ssh/id_rsa.pub") {
        Some(s) => println!("{}", s), 
        None => println!("Oops can't get it")
    };  
}

当我尝试构建它时出现此错误：

$ cargo build
   Compiling homedir-test v0.1.0 (file:///home/user/code/homedir-test)
src/main.rs:9:5: 9:12 error: `homedir` does not live long enough
src/main.rs:9     homedir.to_str()
                  ^~~~~~~
src/main.rs:3:56: 10:2 note: reference must be valid for the anonymous lifetime #1 defined on the block at 3:55...
src/main.rs:3 fn filename_in_homedir(filename: &str) -> Option<&str> {
src/main.rs:4     let mut homedir = match env::home_dir() {
src/main.rs:5         None => return None,
src/main.rs:6         Some(p) => p
src/main.rs:7     };
src/main.rs:8     homedir.push(filename);
              ...
src/main.rs:7:7: 10:2 note: ...but borrowed value is only valid for the block suffix following statement 0 at 7:6
src/main.rs: 7     };
src/main.rs: 8     homedir.push(filename);
src/main.rs: 9     homedir.to_str()
src/main.rs:10 }
error: aborting due to previous error
Could not compile `homedir-test`.

我不明白为什么不起作用。如果 env::home_dir() 失败，函数 returns None。如果成功，则可变变量 homedir 将获得其值（std::path::PathBuf）。此时，homedir 变量应该属于 filename_in_homedir 范围。下一行修改 homedir 以将文件名添加到末尾，这工作正常。最后一行，将 .to_str()、returns 称为 Option<&str> 本身。

因为我最终要返回指向 homedir 内部某处的 &str，也许当 filename_in_homedir 范围结束并且 homedir 被删除时，&str 也是，这就是它抛出此错误的原因？

如何修改此功能才能正常工作，我做错了什么？

Answer 1

Since I'm ultimately returning a &str that's pointing somewhere inside homedir, maybe when the filename_in_homedir scope ends and homedir gets deleted, that &str does too, which is why it's throwing this error?

正确：您正在尝试 return 一个指向 filename_in_homedir 的堆栈帧所拥有的值的指针，一旦它 returns，它将被删除，使指针。你需要 return 一个 String，而不是一个 &str。这是一种方法：

use std::env;
use std::borrow::ToOwned;

fn filename_in_homedir(filename: &str) -> Option<String> {
    let mut homedir = match env::home_dir() {
        None => return None,
        Some(p) => p
    };  
    homedir.push(filename);
    homedir.to_str().map(ToOwned::to_owned)
}

fn main() {
    match filename_in_homedir(".ssh/id_rsa.pub") {
        Some(s) => println!("{}", s), 
        None => println!("Oops can't get it")
    };
}

为什么这个 Rust 函数不起作用？

How come this Rust function doesn't work?

rust

borrow-checker