Restore compatibility with Rust 1.74

[tiran]

What does this PR do?

PR #544 added support for no_std feature. The PR changed std::error::Error to core::error::Error. The core::error trait was stabilized in Rust 1.81, so the change bumped MSRV to 1.81. Before the Python package built with Rust 1.66 and the safetensors create with all features built with 1.74.

This commit restores compatibility with Rust 1.74 for std builds:

• mixed_integer_ops feature requires 1.66
• half v2.4.1 requires 1.70
• clap_lex v0.7.4 requires 1.74

I'm also adding rust-version to Cargo.toml, so cargo creates a backwards compatible Cargo.lock. By default, Cargo >= 1.83 creates a v4 lock file, which is not compatible with Cargo < 1.78.

[Narsil]

Do you mind explaining a bit more the where and what of the issue (what distro, build system etc ?) ?

I'm curious why cargo new doesn't define rust-version as a default in Cargo.toml directly.
Thanks for the fix.

[Narsil]

I updated the github action to unify both locations for testing.

[tiran]

@Narsil Thanks for merging my PR!

Sure, I'm happy to tell you a bit more about our work. I'm a member of Red Hat's AI platform core components team. We are providing AI stacks for several AI accelerators (CUDA, Gaudi, ROCm, ...) for other teams in Red Hat. As part of our workflow, we are rebuilding all Python wheels from sources and with system packages. This is done for security and compliance reasons (e.g. SBOM), but also to build packages with special settings, optimizations, or to support more hardware.

Python wheels are built in our Fromager pipeline on RHEL 9. We have created Fromager to automate re-building a dependency tree from sdists. The tool also does some extra steps to create self-contained source distributions. For Rust-based packages (maturin, setuptools-rust), it cargo vendors all crates into an sdist, too.

We ran into problems with Cargo.lock v4 when Rust 1.83 made v4 the default for projects without lower MSRV. For compatibility with hardware vendors, we have to support even RHEL y-stream releases with extended update support (e.g. 9.4) and cannot update to 9.5. RHEL 9.4 has with Rust 1.75, which does not understand Cargo.lock v4. I honestly don't know why cargo new doesn't add MSRV automatically...

Our build system has the capability to apply patches and we could just fix our build problem in a downstream patch. However we prefer to contribute our fixes to upstream, so upstream and other downstream rebuilders benefit from our work, too. Safetensors is the third project I fixed. The blake3 maintainer has merged my MSRV change, too. My hf_transfer PR is waiting for review.

Feel free to ping me if you have more questions. I'm planning to write a longer explanation around downstream packaging and MSRV for Python packages with Rust extensions, too.