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<h2 id="包和-crate"><a class="header" href="#包和-crate">包和 Crate</a></h2>
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<!-- https://github.com/rust-lang/book/blob/main/src/ch07-01-packages-and-crates.md -->
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<!-- commit 02e053cdbbb3bf9edd9ad32ed49eb533404350a9 -->
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<p>模块系统的第一部分，我们将介绍包和 crate。</p>
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<p>crate 是 Rust 在编译时最小的代码单位。即使你用 <code>rustc</code> 而不是 <code>cargo</code> 来编译一个单独的源代码文件（正如我们在第 1 章“编写并运行 Rust 程序”中所做的那样），编译器还是会将那个文件视为一个 crate。crate 可以包含模块，模块可以定义在其他文件，然后和 crate 一起编译，我们会在接下来的章节中遇到。</p>
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<p>crate 有两种形式：二进制 crate 和库 crate。<strong>二进制 crate</strong>（<em>Binary crates</em>）可以被编译为可执行程序，比如命令行程序或者服务端。它们必须有一个名为 <code>main</code> 函数来定义当程序被执行的时候所需要做的事情。目前我们所创建的 crate 都是二进制 crate。</p>
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<p><strong>库 crate</strong>（<em>Library crates</em>）并没有 <code>main</code> 函数，它们也不会编译为可执行程序。相反它们定义了可供多个项目复用的功能模块。比如 <a href="ch02-00-guessing-game-tutorial.html#%E7%94%9F%E6%88%90%E4%B8%80%E4%B8%AA%E9%9A%8F%E6%9C%BA%E6%95%B0">第二章</a> 的 <code>rand</code> crate 就提供了生成随机数的功能。大多数时间 <code>Rustaceans</code> 说的 “crate” 指的都是库 crate，这与其他编程语言中 “library” 概念一致。</p>
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<p><em>crate root</em> 是一个源文件，Rust 编译器以它为起始点，并构成你的 crate 的根模块（我们将在 <a href="ch07-02-defining-modules-to-control-scope-and-privacy.html">“定义模块来控制作用域与私有性”</a> 一节深入解读）。</p>
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<p><em>包</em>（<em>package</em>）是提供一系列功能的一个或者多个 crate 的捆绑。一个包会包含一个 <em>Cargo.toml</em> 文件，阐述如何去构建这些 crate。Cargo 实际上就是一个包，它包含了用于构建你代码的命令行工具的二进制 crate。其他项目也依赖 Cargo 库来实现与 Cargo 命令行程序一样的逻辑。</p>
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<p>包中可以包含至多一个库 crate(library crate)。包中可以包含任意多个二进制 crate(binary crate)，但是必须至少包含一个 crate（无论是库的还是二进制的）。</p>
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<p>让我们来看看创建包的时候会发生什么。首先，我们输入命令 <code>cargo new my-project</code>：</p>
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<pre><code class="language-console">$ cargo new my-project
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Created binary (application) `my-project` package
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$ ls my-project
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Cargo.toml
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src
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$ ls my-project/src
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main.rs
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</code></pre>
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<p>运行了这条命令后，我们先用 <code>ls</code> （译者注：此命令为 Linux 平台的指令，Windows 下可用 dir）来看看 Cargo 给我们创建了什么，Cargo 会给我们的包创建一个 <em>Cargo.toml</em> 文件。查看 <em>Cargo.toml</em> 的内容，会发现并没有提到 <em>src/main.rs</em>，因为 Cargo 遵循的一个约定：<em>src/main.rs</em> 就是一个与包同名的二进制 crate 的 crate 根。同样的，Cargo 知道如果包目录中包含 <em>src/lib.rs</em>，则包带有与其同名的库 crate，且 <em>src/lib.rs</em> 是 crate 根。crate 根文件将由 Cargo 传递给 <code>rustc</code> 来实际构建库或者二进制项目。</p>
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<p>在此，我们有了一个只包含 <em>src/main.rs</em> 的包，意味着它只含有一个名为 <code>my-project</code> 的二进制 crate。如果一个包同时含有 <em>src/main.rs</em> 和 <em>src/lib.rs</em>，则它有两个 crate：一个二进制的和一个库的，且名字都与包相同。通过将文件放在 <em>src/bin</em> 目录下，一个包可以拥有多个二进制 crate：每个 <em>src/bin</em> 下的文件都会被编译成一个独立的二进制 crate。</p>
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