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<ol class="chapter"><li class="expanded affix "><a href="title-page.html">The Rust Programming Language</a></li><li class="expanded affix "><a href="foreword.html">Foreword</a></li><li class="expanded affix "><a href="ch00-00-introduction.html">Introduction</a></li><li class="expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li class="expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li class="expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li class="expanded "><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li class="expanded "><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li class="expanded "><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li class="expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> Functions</a></li><li class="expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li class="expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li class="expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li class="expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li class="expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References and Borrowing</a></li><li class="expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> The Slice Type</a></li></ol></li><li class="expanded "><a href="ch05-00-structs.html"><strong aria-hidden="true">5.</strong> Using Structs to Structure Related Data</a></li><li><ol class="section"><li class="expanded "><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li class="expanded "><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li class="expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li class="expanded "><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li class="expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li class="expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li class="expanded "><a href="ch06-03-if-let.html"><strong aria-hidden="true">6.3.</strong> Concise Control Flow with if let</a></li></ol></li><li class="expanded "><a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html"><strong aria-hidden="true">7.</strong> Managing Growing Projects with Packages, Crates, and Modules</a></li><li><ol class="section"><li class="expanded "><a href="ch07-01-packages-and-crates.html" class="active"><strong aria-hidden="true">7.1.</strong> Packages and Crates</a></li><li class="expanded "><a href="ch07-02-defining-modules-to-control-scope-and-privacy.html"><strong aria-hidden="true">7.2.</strong> Defining Modules to Control Scope and Privacy</a></li><li class="expanded "><a href="ch07-03-paths-for-referring-to-an-item-in-the-modu
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<main>
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<h2><a class="header" href="#packages-and-crates" id="packages-and-crates">Packages and Crates</a></h2>
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<p>The first parts of the module system we’ll cover are packages and crates. A
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crate is a binary or library. The <em>crate root</em> is a source file that the Rust
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compiler starts from and makes up the root module of your crate (we’ll explain
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modules in depth in the <a href="ch07-02-defining-modules-to-control-scope-and-privacy.html">“Defining Modules to Control Scope and
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Privacy”</a><!-- ignore -->) section. A <em>package</em> is one or more crates
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that provide a set of functionality. A package contains a <em>Cargo.toml</em> file
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that describes how to build those crates.</p>
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<p>Several rules determine what a package can contain. A package <em>must</em> contain
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zero or one library crates, and no more. It can contain as many binary crates
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as you’d like, but it must contain at least one crate (either library or
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binary).</p>
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<p>Let’s walk through what happens when we create a package. First, we enter the
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command <code>cargo new</code>:</p>
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<pre><code class="language-text">$ 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>When we entered the command, Cargo created a <em>Cargo.toml</em> file, giving us a
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package. Looking at the contents of <em>Cargo.toml</em>, there’s no mention of
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<em>src/main.rs</em> because Cargo follows a convention that <em>src/main.rs</em> is the
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crate root of a binary crate with the same name as the package. Likewise, Cargo
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knows that if the package directory contains <em>src/lib.rs</em>, the package contains
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a library crate with the same name as the package, and <em>src/lib.rs</em> is its
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crate root. Cargo passes the crate root files to <code>rustc</code> to build the library
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or binary.</p>
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<p>Here, we have a package that only contains <em>src/main.rs</em>, meaning it only
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contains a binary crate named <code>my-project</code>. If a package contains <em>src/main.rs</em>
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and <em>src/lib.rs</em>, it has two crates: a library and a binary, both with the same
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name as the package. A package can have multiple binary crates by placing files
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in the <em>src/bin</em> directory: each file will be a separate binary crate.</p>
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<p>A crate will group related functionality together in a scope so the
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functionality is easy to share between multiple projects. For example, the
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<code>rand</code> crate we used in <a href="ch02-00-guessing-game-tutorial.html#generating-a-random-number">Chapter 2</a><!-- ignore --> provides functionality
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that generates random numbers. We can use that functionality in our own
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projects by bringing the <code>rand</code> crate into our project’s scope. All the
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functionality provided by the <code>rand</code> crate is accessible through the crate’s
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name, <code>rand</code>.</p>
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<p>Keeping a crate’s functionality in its own scope clarifies whether particular
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functionality is defined in our crate or the <code>rand</code> crate and prevents
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potential conflicts. For example, the <code>rand</code> crate provides a trait named
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<code>Rng</code>. We can also define a <code>struct</code> named <code>Rng</code> in our own crate. Because a
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crate’s functionality is namespaced in its own scope, when we add <code>rand</code> as a
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dependency, the compiler isn’t confused about what the name <code>Rng</code> refers to. In
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our crate, it refers to the <code>struct Rng</code> that we defined. We would access the
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<code>Rng</code> trait from the <code>rand</code> crate as <code>rand::Rng</code>.</p>
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<p>Let’s move on and talk about the module system!</p>
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<script src="searcher.js" type="text/javascript" charset="utf-8"></script>
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<script src="clipboard.min.js" type="text/javascript" charset="utf-8"></script>
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<script src="highlight.js" type="text/javascript" charset="utf-8"></script>
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<script src="book.js" type="text/javascript" charset="utf-8"></script>
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<!-- Custom JS scripts -->
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<script type="text/javascript" src="ferris.js"></script>
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</body>
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</html>
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