“未解决的导入——可能是缺少 extern"；当外部声明存在时 [英] "unresolved import -- maybe a missing extern" When extern declaration exists

问题描述

我有一个小项目，它在一个大的 .rs 文件中构建时没有问题.我想让它更容易使用，所以我把它分解成模块，现在项目的结构是这样的:

I have a small project which built with no issues when it was all in one big .rs file. I wanted to make it easier to work with, so I broke it up into modules, and the project is now structured like this:

├── GameState
│   ├── ballstate.rs
│   ├── collidable.rs
│   ├── gamestate.rs
│   ├── mod.rs
│   └── playerstate.rs
├── lib.rs
└── main.rs

在ballstate.rs 中，我需要使用rand 板条箱.这是该文件的缩写版本:

In ballstate.rs, I need to use the rand crate. Here's an abbreviated version of the file:

extern crate rand;

pub struct BallState {
    dir: Point,         
    frame: BoundingBox  
}                     

impl BallState {
    fn update_dir(&mut self) {
        use rand::*;                                                                                                                                                                    
        let mut rng = rand::thread_rng();                                                                      
        self.dir.x = if rng.gen() { Direction::Forwards.as_float() } else { Direction::Backwards.as_float()  };
        self.dir.y = if rng.gen()  { Direction::Forwards.as_float() } else { Direction::Backwards.as_float() };
    }                                                                                                        
}

但是，当我从顶级目录运行 cargo build 时，出现以下错误:

However, when I run cargo build from the top level directory, I get the following error:

GameState/ballstate.rs:42:9: 42:13 错误:未解析的导入 rand::*.也许缺少 extern crate rand?

GameState/ballstate.rs:42:9: 42:13 error: unresolved import rand::*. Maybe a missing extern crate rand?

当我的 main.rs 文件中有 extern crate 声明时，这有效.现在它在一个单独的模块中有什么变化?

When I just had the extern crate declaration in my main.rs file, this worked. What's changed now that it's in a separate module?

推荐答案

引自 CrateRust 书的和模块章节:

[...] use 声明是绝对路径，从你的 crate 根开始.self 使该路径相对于您在层次结构中的当前位置.

[...] use declarations are absolute paths, starting from your crate root. self makes that path relative to your current place in the hierarchy instead.

编译器正确；没有rand这样的东西，因为你已经把它放在一个模块中，所以它的正确路径应该是GameState::ballstate::rand，或者<GameState::ballstate 模块中的 code>self::rand.

The compiler is correct; there is no such thing as rand, because you've put it inside a module, so the correct path to it would be GameState::ballstate::rand, or self::rand from within the GameState::ballstate module.

您需要将extern crate rand;移动到根模块或在GameState中使用self::rand::ballstate 模块.

You need to either move extern crate rand; to the root module or use self::rand within the GameState::ballstate module.

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