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Cargo: Rust-version aware resolver

Summary

• edition = "2024" implies resolver = "3" in Cargo.toml which enables a Rust-version aware dependency resolver.

Details

Since Rust 1.84.0, Cargo has opt-in support for compatibility with package.rust-version to be considered when selecting dependency versions by setting resolver.incompatible-rust-version = "fallback" in .cargo/config.toml.

Starting in Rust 2024, this will be the default. That is, writing edition = "2024" in Cargo.toml will imply resolver = "3" which will imply resolver.incompatible-rust-version = "fallback".

The resolver is a global setting for a workspace, and the setting is ignored in dependencies. The setting is only honored for the top-level package of the workspace. If you are using a virtual workspace, you will still need to explicitly set the resolver field in the [workspace] definition if you want to opt-in to the new resolver.

For more details on how Rust-version aware dependency resolution works, see the Cargo book.

Migration

There are no automated migration tools for updating for the new resolver.

We recommend projects verify against the latest dependencies in CI to catch bugs in dependencies as soon as possible.