[harehare]

Summary

A module now errors when it binds the same name through def, or through a def and a let/var. Redeclaring module m keeps extending the existing module, but a member already bound by an earlier declaration in the same scope is rejected instead of silently replaced. Document the rule in modules_and_imports.md.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• ♻️ Refactor
• 📝 Documentation
• ⚡ Performance
• ✅ Test
• 📦 Build / dependencies
• 👷 CI

Checklist

• I ran cargo fmt and cargo clippy and addressed any warnings
• I ran just test-all and all tests pass
• I added or updated tests covering this change
• I updated relevant documentation (/docs, crate README.md) if needed

Additional Context

[codspeed]

Merging this PR will not alter performance

✅ 43 untouched benchmarks
⏩ 10 skipped benchmarks¹

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_{Comparing fix/lang-module-duplicate-def (08a2721) with main (bd7b48c)²}

Footnotes

1. 10 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (6563581) during the generation of this report, so bd7b48c was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩