Please recommend package.rust-version in Cargo.toml

[tiran]

Please recommend that package maintainers define a MSRV (package.rust-version) in their Cargo.toml. The lack of MSRV makes the lives of downstream / distro packagers harder.

Several Python packages with maturin or setuptools-rust-based build don't specify a MSRV. Their latest release have inadvertently broken backwards compatibility with Rust < 1.78. The problem is not in code. Their Rust code and dependencies build fine with older Rust versions. The incompatibility is caused by a recent change in Cargo.lock. Rust 1.83's Cargo changed the default Cargo.lock version from v3 to v4. Rust < 1.78 does recognize v4 lock files.

Cargo's changelog recommends:

Cargo 1.83 (2024-11-28):
Lockfile format v4 is now the default for creating/updating a lockfile. Rust toolchains 1.78+ support lockfile v4. For compatibility with earlier MSRV, consdier setting the package.rust-version to 1.82 or earlier.

Implementation:

• recommend rust-version in your documentation https://www.maturin.rs
• maybe print a warning when Cargo.toml is missing MSRV?

Related issues:

• blake3 sdists 1.0.1 / 1.0.2 are not compatible with Rust < 1.78 (Cargo.lock v4) oconnor663/blake3-py#56 (upstream has accepted and landed my fix)
• Cargo.lock version=4 breaks backward compatibility with Rust < 1.78 huggingface/hf_transfer#55
• Restore compatibility with Rust 1.74 huggingface/safetensors#563

[tiran]

PS: The blake3 maintainer has accepted my PR and released a new version of blake3 with MSRV.