Add direct-recursion Lint

[felix91gr]

Checklist:

• Followed [lint naming conventions][lint_naming]
• Added passing UI tests (including committed .stderr file)
• cargo test passes locally
• Executed cargo dev update_lints
• Added lint documentation
• Run cargo dev fmt

---

changelog: new lint: [direct_recursion]

Fixes #428

After we're done with this, I'll open a follow-up issue for the indirect recursion lint

[felix91gr]

@PLeVasseur WIP.

I think the implementation is correct. I'll complete the documentation later, hopefully this weekend ^^

[PLeVasseur]

Nice! Thanks for taking on the implementation.

[PLeVasseur]

Hey @felix91gr -- thanks for writing this up.
One thing I was unclear on: should this lint also check for recursion that's due to macros or only functions?

[blyxyas]

Could you change this from an allow to an expect? It gives us some future proofing.

[felix91gr]

If I understood correctly, expect makes it so Clippy checks that this "allow" is being used, right? So if I, for example, put an expect line for this lint in a file where this lint isn't being used, Clippy would issue an error because I'm telling it that I expect this lint to be active and it's not.

[felix91gr]

(I did add it to the documentation, btw! Thanks ❤️ )

[felix91gr]

Hey @felix91gr -- thanks for writing this up. One thing I was unclear on: should this lint also check for recursion that's due to macros or only functions?

Well... since it's a LateLintPass, then I believe it should already be linting macro-generated code by default. I wonder if there's a straightforward way of testing it 🤔 I'll give it a spin.

[felix91gr]

Sorry for the CI churn. It took me a while to understand why it was failing the changelog check.

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[felix91gr]

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  * [30beb95](https://github.com/rust-lang/rust-clippy/commit/30beb9536f0d8a8c027e5ceecf207224522aa836)

I feel like this should be in the quick CI checks (like the changelog check), so that CI may do an early exit and avoid running the heavier test suites.

[felix91gr]

Something that I'd like to add in a later PR, is a link to the Safety Critical Coding Guidelines where recursion is addressed.

Currently the functions section is empty, but I hope to get working on filling it as soon as I can https://coding-guidelines.arewesafetycriticalyet.org/coding-guidelines/functions.html