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:

1. Run cargo fix --edition
2. Edit Cargo.toml and set the edition field to the next edition, for example edition = "2021"
3. Run cargo build or cargo test to verify the fixes worked.

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. 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 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!

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.

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