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## Appendix E: Editions

In Chapter 1, you saw that `cargo new` adds a bit of metadata to your
*Cargo.toml* file about an edition. This appendix talks about what that means!

The Rust language and compiler have a six-week release cycle, meaning users get
a constant stream of new features. Other programming languages release larger
changes less often; Rust releases smaller updates more frequently. After a
while, all of these tiny changes add up. But from release to release, it can be
difficult 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 Rust *edition*. 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.

Editions serve different purposes for different people:

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

At the time of this writing, three Rust editions are available: Rust 2015, Rust
2018, and Rust 2021. This book is written using Rust 2021 edition idioms.

The `edition` key in *Cargo.toml* indicates which edition the compiler should
use for your code. If the key doesn’t exist, Rust uses `2015` as the edition
value for backward compatibility reasons.

Each project can opt in to an edition other than the default 2015 edition.
Editions can contain incompatible changes, such as including a new keyword that
conflicts with identifiers in code. However, unless you opt in to those
changes, your code will continue to compile even as you upgrade the Rust
compiler version 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 Rust edition will continue to see improvements as new stable releases are
made. However, in some cases, mainly when new keywords are added, some new
features might only be available in later editions. You will need to switch
editions if you want to take advantage of such features.

For more details, *The* *Edition Guide* at
*https://doc.rust-lang.org/stable/edition-guide* is a complete book about
editions that enumerates the differences between editions and explains how to
automatically upgrade your code to a new edition via `cargo fix`.