# Appendix E - Editions

Way back in Chapter 1, we saw that `cargo new` adds a bit of metadata to your Cargo.toml about an `edition`. This appendix talks about what that means!

The Rust language and compiler have a six-week release cycle. This means users get a constant stream of new features. Other programming languages release larger changes less often; Rust chooses to release smaller updates more frequently. After a while, all of those tiny changes add up. But from release to release, it can be hard to look back and say “Wow, between Rust 1.10 and Rust 1.31, Rust has changed a lot!”

Every two or three years, the Rust team produces a new edition of Rust. Each edition brings together the features that have landed into a clear package with fully updated documentation and tooling. New editions ship as part of the usual six-week release process.

This serves different purposes for different people:

- For active Rust users, it brings together incremental changes into an easy-to-understand package.
- For non-users, it signals that some major advancements have landed, which might make Rust worth another look.
- For those developing Rust itself, it provides a rallying point for the project as a whole.

At the time of writing, there are two editions: Rust 2015 and Rust 2018. This book is written using Rust 2018 edition idioms.

The `edition` key in Cargo.toml indicates which edition your code should be compiled under. If the key does not exist, it defaults to `2015` for backwards compatibility reasons.

Each project can choose to opt in to an edition other than the default 2015 edition. By doing so, editions can contain incompatible changes, such as adding a new keyword that might conflict with identifiers in code or turning warnings into errors. But unless you opt in to those changes, your code will continue to compile even as you upgrade the version of the Rust compiler that you use. All Rust compiler versions support any edition that existed prior to that compiler’s release, and they can link crates of any supported editions together. Edition changes only affect the way the compiler initially parses code. Therefore, if you’re using Rust 2015 and one of your dependencies uses Rust 2018, your project will compile and be able to use that dependency. The opposite situation, where your project uses Rust 2018 and a dependency uses Rust 2015, works as well.

To be clear: most features will be available on all editions. Developers using any edition of Rust will continue to see improvements as new stable releases are made. In some cases, however, mainly when new keywords are added, there may be new features that are only available in later editions. You only need to switch editions if you want to take advantage of such features.

For more details, the Edition Guide is a complete book about editions, including how to automatically upgrade your code to a new edition via `cargo fix`.