You can migrate from Rust 2018 to 2021 by updating the edition field in your Cargo.toml files to 2021 and then running cargo fix --edition to automatically resolve most syntax changes. While the process is largely automated, you must manually review compiler warnings regarding new default behaviors, such as the stricter handling of async blocks and the removal of the #[must_use] attribute on Result and Option methods.
First, update your Cargo.toml to specify the new edition. If you have a workspace, update the root Cargo.toml and any member crates that override the edition setting.
[package]
name = "my-project"
version = "0.1.0"
edition = "2021"
Once the configuration is updated, run the following command in your project root to apply automatic fixes. This tool scans your code for 2018-specific syntax and rewrites it to match 2021 standards, such as updating async function syntax or adjusting lifetime elision rules.
cargo fix --edition
After running the fixer, you must address any remaining compiler errors or warnings. The most common manual adjustments involve async blocks and impl Trait. In 2021, async blocks are now allowed in more contexts, but you may need to explicitly specify types if the compiler can no longer infer them due to stricter variance rules. Additionally, the #[must_use] attribute is no longer automatically applied to Result and Option methods like unwrap() or map(), which might silence warnings you previously relied on for safety checks.
If you encounter errors related to async functions, ensure you are using the latest stable Rust toolchain, as some 2021 features require specific compiler versions. You can check your version with rustc --version. If the migration introduces breaking changes in your public API, verify that your documentation and examples reflect the new behavior, particularly regarding how async traits are handled if you are using the async-trait crate.
Finally, run your full test suite with cargo test to ensure that the migration hasn't introduced subtle logic errors. The 2021 edition is backward compatible with 2018 code in most cases, but the stricter type checking and new defaults can expose latent bugs in complex generic code. Once all tests pass and warnings are resolved, commit the changes to your repository.