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<h2 id="附录-e版本"><a class="header" href="#附录-e版本">附录 E：版本</a></h2>
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<blockquote>
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<p><a href="https://github.com/rust-lang/book/blob/main/src/appendix-05-editions.md">appendix-05-editions.md</a>
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<br />
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commit 8cf0496bb8e56b683ea3f015871c8631684decf4</p>
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</blockquote>
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<p>早在第一章，我们见过 <code>cargo new</code> 在 <em>Cargo.toml</em> 中增加了一些有关 <code>edition</code> 的元数据。本附录将解释其意义！</p>
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<p>Rust 语言和编译器有一个为期 6 周的发布循环。这意味着用户会稳定得到新功能的更新。其他编程语言发布大更新但不甚频繁；Rust 选择更为频繁的发布小更新。一段时间之后，所有这些小更新会日积月累。不过随着小更新逐次的发布，或许很难回过头来感叹：“哇，从 Rust 1.10 到 Rust 1.31，Rust 的变化真大！”</p>
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<p>每两到三年，Rust 团队会生成一个新的 Rust <strong>版本</strong>（<em>edition</em>）。每一个版本会结合已经落地的功能，并提供一个清晰的带有完整更新文档和工具的功能包。新版本会作为常规的 6 周发布过程的一部分发布。</p>
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<p>这为不同的人群提供了不同的功能：</p>
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<ul>
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<li>对于活跃的 Rust 用户，其将增量的修改与易于理解的功能包相结合。</li>
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<li>对于非用户，它表明发布了一些重大进展，这意味着 Rust 可能变得值得一试。</li>
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<li>对于 Rust 自身开发者，其提供了项目整体的集合点。</li>
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</ul>
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<p>在本文档编写时，Rust 有三个可用版本：Rust 2015、Rust 2018 和 Rust 2021。本书基于 Rust 2021 edition 风格编写。</p>
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<p><em>Cargo.toml</em> 中的 <code>edition</code> 字段表明代码应该使用哪个版本编译。如果该字段不存在，其默认为 <code>2015</code> 以提供后向兼容性。</p>
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<p>每个项目都可以选择不同于默认的 2015 edition 的版本。这样，版本可能会包含不兼容的修改，比如新增关键字可能会与代码中的标识符冲突并导致错误。不过除非选择兼容这些修改，（旧）代码仍将能够编译，即便升级了 Rust 编译器的版本。</p>
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<p>所有 Rust 编译器都支持任何之前存在的编译器版本，并可以链接任何支持版本的 crate。编译器修改只影响最初的解析代码的过程。因此，如果你使用 Rust 2015 而某个依赖使用 Rust 2018，你的项目仍旧能够编译并使用该依赖。反之，若项目使用 Rust 2018 而依赖使用 Rust 2015 亦可工作。</p>
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<p>有一点需要明确：大部分功能在所有版本中都能使用。开发者使用任何 Rust 版本将能继续接收最新稳定版的改进。然而在一些情况，主要是增加了新关键字的时候，则可能出现了只能用于新版本的功能。只需切换版本即可利用新版本的功能。</p>
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<p>请查看 <a href="https://rust-lang-nursery.github.io/edition-guide/">Edition Guide</a> 了解更多细节，这是一个完全介绍版本的书籍，包括如何通过 <code>cargo fix</code> 自动将代码迁移到新版本。</p>
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