Rust代码有一个问题，它说:类型`std::result::Result<Config，query`上没有字段‘query`

Question

我是Rust的新手，当我尝试在“Rust编程语言”中运行“清单12-9:从Config：：new返回结果”的代码时，出现了3个错误，但我不知道如何修复它们。

下面是信息：

{
    "resource": "/d:/Document/Code/Rust/minigrep/src/main.rs",
    "owner": "rust-file:///d%3A/Document/Code/Rust/minigrep",
    "code": "E0609",
    "severity": 8,
    "message": "no field query on type std::result::Result<Config, &str>",
    "source": "rustc",
    "startLineNumber": 10,
    "startColumn": 41,
    "endLineNumber": 10,
    "endColumn": 46
}
{
    "resource": "/d:/Document/Code/Rust/minigrep/src/main.rs",
    "owner": "rust-file:///d%3A/Document/Code/Rust/minigrep",
    "code": "E0609",
    "severity": 8,
    "message": "no field filename on type std::result::Result<Config, &str>",
    "source": "rustc",
    "startLineNumber": 11,
    "startColumn": 35,
    "endLineNumber": 11,
    "endColumn": 43
}
{
    "resource": "/d:/Document/Code/Rust/minigrep/src/main.rs",
    "owner": "rust-file:///d%3A/Document/Code/Rust/minigrep",
    "code": "E0609",
    "severity": 8,
    "message": "no field filename on type std::result::Result<Config, &str>",
    "source": "rustc",
    "startLineNumber": 15,
    "startColumn": 46,
    "endLineNumber": 15,
    "endColumn": 54
}

下面是代码：

use std::env;
use std::fs;

fn main() {
    let args: Vec<String> = env::args().collect();

    let config = Config::new(&args);

    println!("Searching for {}", config.query);
    println!("In file {}", config.filename);
    // --snip--
    

    let contents = fs::read_to_string(config.filename)
        .expect("Something went wrong reading the file");

    println!("With text:\n{}", contents);
}

struct Config {
    query: String,
    filename: String,
}

impl Config {
    fn new(args: &[String]) -> Result<Config, &'static str> {
        if args.len() < 3 {
            return Err("not enough arguments");
        }

        let query = args[1].clone();
        let filename = args[2].clone();
        
        Ok(Config{query,filename})
    }
}

Answer 1

Config::new看起来像这样：

fn new(args: &[String]) -> Result<Config, &'static str>

这意味着它可能会失败。因此，let config = Config::new(&args);中的config可以是Err或值。

Rust强制您处理枚举的所有可能变体，如Result。例如，您可以在程序遇到Err值时使其崩溃：

let config = Config::new(&args).expect("Oh no, got an Err!");

或者手动处理Err值：

let config = match(Config::new(&args)) {
    Ok(cfg) => cfg,
    Err(msg) => {
        println!("Error: {}", msg);
        return; // return from main
    }
};

Answer 2

Config::new返回一个Result<Config, &'static str>，而不是Config，所以您需要处理Result。您可以通过更改以下行将其解开来执行此操作：

let config = Config::new(&args);

要这样做：

let config = Config::new(&args).unwrap();

如果出现错误，这将导致程序崩溃。或者，您可以将错误传播到main函数，如下所示：

fn main() -> Result<(), &'static str> { // Result return type added here
    let args: Vec<String> = env::args().collect();

    let config = Config::new(&args)?; // ? operator added here

    println!("Searching for {}", config.query);
    println!("In file {}", config.filename);
    // --snip--

    let contents =
        fs::read_to_string(config.filename).expect("Something went wrong reading the file");

    println!("With text:\n{}", contents);
    
    Ok(()) // Ok(()) added here
}

后一个示例更符合习惯用法。