2021-05-10 00:19:46 +08:00
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## Appendix E - Editions
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In Chapter 1, you saw that `cargo new` adds a bit of metadata to your
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*Cargo.toml* file about an edition. This appendix talks about what that means!
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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!”
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Every two or three years, the Rust team produces a new Rust *edition*. 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.
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Editions serve different purposes for different people:
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* For active Rust users, a new edition brings together incremental changes into
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an easy-to-understand package.
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* For non-users, a new edition signals that some major advancements have
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landed, which might make Rust worth another look.
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* For those developing Rust, a new edition provides a rallying point for the
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project as a whole.
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2022-02-13 03:25:29 +08:00
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At the time of this writing, three Rust editions are available: Rust 2015, Rust
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2018, and Rust 2021. This book is written using Rust 2021 edition idioms.
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2021-05-10 00:19:46 +08:00
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The `edition` key in *Cargo.toml* indicates which edition the compiler should
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use for your code. If the key doesn’t exist, Rust uses `2015` as the edition
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value for backward compatibility reasons.
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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.
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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.
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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.
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For more details, the [*Edition
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Guide*](https://doc.rust-lang.org/stable/edition-guide/) 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 `cargo fix`.
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