internal: Define rules for LLMs

[Wilfred]

rust-analyzer allows AI usage (see #21314), but requires contributors to declare usage. This adds a rule file that improves LLM output quality and instructs the LLM to declare usage in commit messages.

I've written the rules in CLAUDE.md, but also symlinked it to AGENTS.md so other LLM tools pick it up.

Rules file contents

(1) Instructions for both humans and AIs to declare AI usage.

(2) Relevant commands for testing, linting and codegen.

Note that I deliberately didn't include an overview of the project structure on a folder-by-folder basis. This can go stale, and there's some evidence that project structure can hurt LLM output quality overall.

See the following paper:

Evaluating AGENTS.md:
Are Repository-Level Context Files Helpful for Coding Agents?
https://arxiv.org/pdf/2602.11988

Testing

I exercised this change with the following contrived prompt. Note that in practice rust-analyzer is hitting review scaling limits for new code actions, but it was easy to test end-to-end.

Add a new code action that replaces the content of a string literal
with the text "banana".
...
commit it

This produced a functional code action with both Codex and Claude, and in both cases the commit message mentioned that it was AI generated. Example commit message:

Add "Replace string with banana" code action

Add a new assist that replaces a string literal's content with "banana"
when the cursor is on a STRING token.

AI: Generated with Claude Code (claude-opus-4-6).

I confirmed that the code action worked by testing a rust-analyzer build in Emacs, and also confirmed that the generated tests looked sensible.

AI Usage Disclosures

I wrote the first draft of the rules file with Opus 4.6, manually reviewed everything.

[Veykril]

For any readers of this PR, given the PR used an assist as an example. Assists are very very unlikely to get merged these days. that goes even more so for ai generated ones

View changes since this review

[Veykril]

This can likely be improved more (in the future), its not really much stuff

[rustbot]

⚠️ Warning ⚠️

• There are issue links (such as #123) in the commit messages of the following commits.
  Please move them to the PR description, to avoid spamming the issues with references to the commit, and so this bot can automatically canonicalize them to avoid issues with subtree.
  • ef3ab9a

[lnicola]

@Wilfred the bot can rewrite issue links in the PR descriptions, but not the commit messages, which kind of defeats the purpose of rewriting.

[Wilfred]

@lnicola sorry about this. All my PRs since #t-compiler/rust-analyzer > Warning: Issue links in the commit message should have proper fully qualified issue references, I hadn't understood before.