這個Rust功能怎麼不起作用？

Question

我想了解如何在Rust中做一些簡單的事情，但我一直在努力結束與借用檢查器的對抗，我不知道爲什麼。我寫了一個簡單的函數，在其中傳遞一個文件名，併爲您提供主目錄中該文件的路徑。

這裏的程序：

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") 
    }; 
} 

當我嘗試構建它，我得到這個錯誤：

$ 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`. 

我不明白爲什麼不工作。如果env::home_dir()失敗，則函數返回None。如果成功，則可變變量homedir將獲得其值（這是一個std::path::PathBuf）。此時，homedir變量應屬於filename_in_homedir範圍。下一行修改homedir以將文件名添加到最後，並且此工作正常。最後一行，呼籲.to_str(),returns和Option<&str>本身。

由於我最終返回該記錄指向內部homedir某處&str，也許當filename_in_homedir範圍結束和homedir被刪除，即&str確實太少，這就是爲什麼它拋出這個錯誤？

如何修改此功能以正常工作，我做錯了什麼？

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?

這是正確的：你想返回一個指針由filename_in_homedir的堆棧幀所擁有的價值，一旦它返回時，指針無效，將被丟棄。您需要退回String，而不是&str。這裏有一種方法：

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") 
    }; 
}