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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"><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-module-tree.html"><
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<main>
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<h2><a class="header" href="#appendix-e---editions" id="appendix-e---editions">Appendix E - Editions</a></h2>
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<p>In Chapter 1, you saw that <code>cargo new</code> adds a bit of metadata to your
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<em>Cargo.toml</em> file about an edition. This appendix talks about what that means!</p>
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<p>The Rust language and compiler have a six-week release cycle, meaning users get
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a constant stream of new features. Other programming languages release larger
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changes less often; Rust releases smaller updates more frequently. After a
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while, all of these tiny changes add up. But from release to release, it can be
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difficult to look back and say, “Wow, between Rust 1.10 and Rust 1.31, Rust has
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changed a lot!”</p>
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<p>Every two or three years, the Rust team produces a new Rust <em>edition</em>. Each
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edition brings together the features that have landed into a clear package with
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fully updated documentation and tooling. New editions ship as part of the usual
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six-week release process.</p>
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<p>Editions serve different purposes for different people:</p>
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<ul>
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<li>For active Rust users, a new edition brings together incremental changes into
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an easy-to-understand package.</li>
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<li>For non-users, a new edition signals that some major advancements have
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landed, which might make Rust worth another look.</li>
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<li>For those developing Rust, a new edition provides a rallying point for the
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project as a whole.</li>
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</ul>
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<p>At the time of this writing, two Rust editions are available: Rust 2015 and
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Rust 2018. This book is written using Rust 2018 edition idioms.</p>
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<p>The <code>edition</code> key in <em>Cargo.toml</em> indicates which edition the compiler should
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use for your code. If the key doesn’t exist, Rust uses <code>2015</code> as the edition
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value for backward compatibility reasons.</p>
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<p>Each project can opt in to an edition other than the default 2015 edition.
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Editions can contain incompatible changes, such as including a new keyword that
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conflicts with identifiers in code. However, unless you opt in to those
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changes, your code will continue to compile even as you upgrade the Rust
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compiler version you use.</p>
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<p>All Rust compiler versions support any edition that existed prior to that
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compiler’s release, and they can link crates of any supported editions
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together. Edition changes only affect the way the compiler initially parses
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code. Therefore, if you’re using Rust 2015 and one of your dependencies uses
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Rust 2018, your project will compile and be able to use that dependency. The
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opposite situation, where your project uses Rust 2018 and a dependency uses
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Rust 2015, works as well.</p>
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<p>To be clear: most features will be available on all editions. Developers using
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any Rust edition will continue to see improvements as new stable releases are
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made. However, in some cases, mainly when new keywords are added, some new
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features might only be available in later editions. You will need to switch
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editions if you want to take advantage of such features.</p>
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<p>For more details, the <a href="https://doc.rust-lang.org/stable/edition-guide/"><em>Edition
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Guide</em></a> is a complete book
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about editions that enumerates the differences between editions and explains
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how to automatically upgrade your code to a new edition via <code>cargo fix</code>.</p>
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