feat: support no_std consumers with embedded CI coverage

[robjtede]

Summary

Document support for no_std consumers and verify it with a separate ensure-no-alloc binary, following the approach in robjtede/impl-more. The generated contracts already work without std or an allocator, so no code generator changes are required.

Add just build-no-alloc and a stable CI build for thumbv6m-none-eabi. The fixture covers combined contracts, impl invariants, old(), trait contracts, and debug and test attributes. A second build enables cfg(test) without the standard test harness to check test-only assertions. Logging and MIRAI integrations remain outside the documented guarantee.

Closes #160.

Validation

• just toolchain=+stable build-no-alloc passed both embedded builds.
• nix develop -c just test-no-coverage passed 35 tests, 12 doctests, and documentation checks.
• nix develop -c just clippy passed.
• Clippy for the embedded fixture passed with warnings denied.
• Rust, Just, Markdown, YAML, and TOML formatting checks passed after formatting the README.

The embedded fixture was compiled and linked, not run on hardware.

The fixture deliberately has no global allocator. As a negative control, temporarily adding extern crate alloc made the embedded build fail with a missing-global-allocator error. Both builds passed again after removing it.

[coderabbitai]

Important

Draft PR not reviewed

Draft PRs are not automatically reviewed by default.

• Trigger a manual review

To automatically review draft PRs, update your CodeRabbit configuration:

reviews:
  auto_review:
    drafts: true

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}