Transitioning an existing project to a new edition

Rust includes tooling to automatically transition a project from one edition to the next. It will update your source code so that it is compatible with the next edition. Briefly, the steps to update to the next edition are:

Run cargo update to update your dependencies to the latest versions.
Run cargo fix --edition
Edit Cargo.toml and set the edition field to the next edition, for example edition = "2024"
Run cargo build or cargo test to verify the fixes worked.
Run cargo fmt to reformat your project.

The following sections dig into the details of these steps, and some of the issues you may encounter along the way.

It's our intention that the migration to new editions is as smooth an experience as possible. If it's difficult for you to upgrade to the latest edition, we consider that a bug. If you run into problems with this process, please file a bug report. Thank you!

Starting the migration

As an example, let's take a look at transitioning from the 2015 edition to the 2018 edition. The steps are essentially the same when transitioning to other editions like 2021.

Imagine we have a crate that has this code in src/lib.rs:


#![allow(unused)]
fn main() {
trait Foo {
    fn foo(&self, i32);
}
}

This code uses an anonymous parameter, that i32. This is not supported in Rust 2018, and so this would fail to compile. Let's get this code up to date!

Updating your dependencies

Before we get started, it is recommended to update your dependencies. Some dependencies, particularly some proc-macros or dependencies that do build-time code generation, may have compatibility issues with newer editions. New releases may have been made since you last updated which may fix these issues. Run the following:


cargo update

After updating, you may want to run your tests to verify everything is working. If you are using a source control tool such as git, you may want to commit these changes separately to keep a logical separation of commits.

Updating your code to be compatible with the new edition

Your code may or may not use features that are incompatible with the new edition. In order to help transition to the next edition, Cargo includes the cargo fix subcommand to automatically update your source code. To start, let's run it:


cargo fix --edition

This will check your code, and automatically fix any issues that it can. Let's look at src/lib.rs again:


#![allow(unused)]
fn main() {
trait Foo {
    fn foo(&self, _: i32);
}
}

It's re-written our code to introduce a parameter name for that i32 value. In this case, since it had no name, cargo fix will replace it with _, which is conventional for unused variables.

cargo fix can't always fix your code automatically. If cargo fix can't fix something, it will print the warning that it cannot fix to the console. If you see one of these warnings, you'll have to update your code manually. See the Advanced migration strategies chapter for more on working with the migration process, and read the chapters in this guide which explain which changes are needed. If you have problems, please seek help at the user's forums.

Enabling the new edition to use new features

In order to use some new features, you must explicitly opt in to the new edition. Once you're ready to continue, change your Cargo.toml to add the new edition key/value pair. For example:


[package]
name = "foo"
version = "0.1.0"
edition = "2018"

If there's no edition key, Cargo will default to Rust 2015. But in this case, we've chosen 2018, and so our code will compile with Rust 2018!

Testing your code in the new edition

The next step is to test your project on the new edition. Run your project tests to verify that everything still works, such as running cargo test. If new warnings are issued, you may want to consider running cargo fix again (without the --edition flag) to apply any suggestions given by the compiler.

At this point, you may still need to do some manual changes. For example, the automatic migration does not update doctests, and build-time code generation or macros may need manual updating. See the advanced migrations chapter for more information.

Congrats! Your code is now valid in both Rust 2015 and Rust 2018!

Reformatting with rustfmt

If you use rustfmt to automatically maintain formatting within your project, then you should consider reformatting using the new formatting rules of the new edition.

Before reformatting, if you are using a source control tool such as git, you may want to commit all the changes you have made up to this point before taking this step. It can be useful to put formatting changes in a separate commit, because then you can see which changes are just formatting versus other code changes, and also possibly ignore the formatting changes in git blame.


cargo fmt

See the style editions chapter for more information.

Migrating to an unstable edition

After an edition is released, there is roughly a three year window before the next edition. During that window, new features may be added to the next edition, which will only be available on the nightly channel. If you want to help test those new features before they are stabilized, you can use the nightly channel to try them out.

The steps are roughly similar to the stable channel:

Install the most recent nightly: rustup update nightly.
Run cargo +nightly fix --edition.
Edit Cargo.toml and place cargo-features = ["edition20xx"] at the top (above [package]), and change the edition field to say edition = "20xx" where 20xx is the edition you are upgrading to.
Run cargo +nightly check to verify it now works in the new edition.

⚠ Caution: Features implemented in the next edition may not have automatic migrations implemented with cargo fix, and the features themselves may not be finished. When possible, this guide should contain information about which features are implemented on nightly along with more information about their status. A few months before the edition is stabilized, all of the new features should be fully implemented, and the Rust Blog will announce a call for testing.

The Rust Edition Guide