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| <main> |
| <h1 id="writing-llm-created-code"><a class="header" href="#writing-llm-created-code">Writing LLM-created code</a></h1> |
| <p>LLMs are a valuable tool, but one that is easy to misuse. |
| The main risks are <strong>overwhelming volume</strong> and <strong>lack of understanding</strong>. |
| When writing your PR, keep in mind that there is a person on the other end who needs to review and understand your change, |
| and that other people in the project will need to read your code for years to come. |
| Help us out by making your change small, targeted, and high-quality.</p> |
| <p>Keep in mind this quote:</p> |
| <blockquote> |
| <p>Programs must be written for people to read, and only incidentally for machines to execute. |
| — Harold Abelson and Gerald Jay Sussman, <a href="https://web.mit.edu/6.001/6.037/sicp.pdf"><em>Structure and Interpretation of Computer Programs</em></a></p> |
| </blockquote> |
| <h2 id="rules"><a class="header" href="#rules">Rules</a></h2> |
| <h3 id="before-you-write-code"><a class="header" href="#before-you-write-code">Before you write code</a></h3> |
| <p>Before anything else, find a reviewer who volunteers to review your PR. |
| If you do not know where to find a reviewer, ask in <a href="https://rust-lang.zulipchat.com/join/rlfvpemsaacs3pfi6kwqnqjb/">#llm-mentoring</a> on Zulip. |
| Your first message should say:</p> |
| <ul> |
| <li>your relevant experience, so we can find an issue that’s suitable for you</li> |
| <li>which problem (or kind of problem) you want to work on</li> |
| <li>(optional) ideas you have so far for a solution</li> |
| <li>(optional) how you expect to test your solution</li> |
| </ul> |
| <p>Mentors are here to help. |
| Talking to them early helps you avoid wasted work.</p> |
| <h3 id="while-working"><a class="header" href="#while-working">While working</a></h3> |
| <p>Write your own doc-comments, <code>// SAFETY</code> comments, diagnostic wording, and soundness-critical code. |
| As before, you can use an LLM to review your work, but not to write it from scratch. |
| If you don’t know what counts as soundness-critical, discuss it with your reviewer.</p> |
| <h3 id="when-opening-a-pr"><a class="header" href="#when-opening-a-pr">When opening a PR</a></h3> |
| <p>Disclose your use of LLMs, following the disclosure guidelines below. |
| Write the disclosure yourself. |
| You may use an LLM to privately review a disclosure you have written, but not to draft or rewrite it.</p> |
| <p><strong>Write your own PR description and comments</strong>. |
| We want to hear from you, not from your agent. |
| LLM-created PR descriptions are banned. |
| LLM-created GitHub comments are banned.</p> |
| <h2 id="guidelines"><a class="header" href="#guidelines">Guidelines</a></h2> |
| <h3 id="before-you-write-code-1"><a class="header" href="#before-you-write-code-1">Before you write code</a></h3> |
| <p>Start with one PR at a time. |
| Keep your changes small enough that you and your reviewer can understand every part of them. |
| Go slow.</p> |
| <p>Do not use an LLM for <code>E-easy</code> issues; those are meant for you to write the code yourself. |
| Ask first before working on an <code>E-mentor</code> issue; mentors may not want to work with LLM-generated code.</p> |
| <p>Determine whether this is a <em>useful</em> and <em>well-scoped</em> change. |
| For example:</p> |
| <ul> |
| <li>Search for related issues and PRs.</li> |
| <li>Find relevant code, tests, git history, and Zulip discussion.</li> |
| <li>If this is a cross-cutting change, consult the <a href="../getting-started.html#pull-requests">pull request guidance</a>.</li> |
| <li><a href="https://google.github.io/eng-practices/review/developer/small-cls.html">Make the smallest change that fixes the problem</a>. |
| Do not combine it with unrelated refactors or cleanups.</li> |
| </ul> |
| <h3 id="while-working-1"><a class="header" href="#while-working-1">While working</a></h3> |
| <p>When fixing a bug, verify that your test fails before and succeeds after your change. |
| Consult <a href="../tests/adding.html">adding new tests</a> and <a href="../tests/best-practices.html">best practices</a> for test procedures. |
| Tests are absolutely required; either existing tests or new tests you write. |
| Untested LLM PRs will not be merged.</p> |
| <p>Mass renames or rewrites should <em>strongly</em> prefer using a proper syntax rewrite tool, such as <a href="https://astgrep.com/"><code>ast-grep</code></a>. |
| You may use an LLM for generating the instructions for that tool, but you should be very cautious about performing the rewrite directly with an LLM.</p> |
| <p>Consider <a href="../tests/perf.html#performance-considerations">performance</a> as you write.</p> |
| <p>Think before adding dependencies; |
| consult our <a href="../crates-io.html">guidance for new dependencies</a>.</p> |
| <p>Verify your understanding against the existing code, documentation, and tests. |
| You can get better advice from your LLM by telling <em>it</em> to read the relevant materials. |
| Do not rely on the LLM as a source of truth.</p> |
| <h3 id="write-maintainable-code"><a class="header" href="#write-maintainable-code">Write maintainable code</a></h3> |
| <p>Treat generated code as a <em>draft</em>, not a final product. |
| Follow our <a href="../conventions.html#cc">correctness and maintainability conventions</a>.</p> |
| <p>Avoid unnecessary abstractions and compatibility layers. |
| Rustc does not have a stable API; you do not need to preserve backwards compatibility for internal compiler APIs.</p> |
| <h3 id="commit-structure"><a class="header" href="#commit-structure">Commit structure</a></h3> |
| <p>See <a href="../contributing.html#er">“How to structure your PR”</a>. |
| Commit messages must be authored by you, not your LLM.</p> |
| <h3 id="before-opening-a-pr"><a class="header" href="#before-opening-a-pr">Before opening a PR</a></h3> |
| <p>Review your own PR before opening it: Does it make sense? |
| Can you tell what the goal of the PR is? |
| Does it achieve that goal?</p> |
| <p>Remove outdated or prototyping code and debugging.</p> |
| <p>Re-read the whole diff, <em>not</em> just your conversation with the agent. |
| Your reviewer is going to see your code, not your conversation.</p> |
| <p><a href="../tests/running.html">Run tests</a> to verify your change works. |
| Do NOT report which UI tests you ran in the PR description; |
| that’s noise, since CI will run them anyway. |
| If you did manual testing or benchmarking, do report that, |
| but note that all LLM PRs must have automated tests.</p> |
| <p><a href="../tests/adding.html#step-4-review-the-output">Review diagnostic snapshots</a>; |
| don’t simply <code>--bless</code> them away.</p> |
| <p>We recommend using a different model for adversarial local review before publishing your changes. |
| You’re still responsible for reviewing your own changes yourself.</p> |
| <h3 id="understand-your-own-change"><a class="header" href="#understand-your-own-change">Understand your own change</a></h3> |
| <p>We want you to understand and be able to explain your own change and its edge cases. |
| Asking the LLM can be a starting point but it’s not the same as explaining it yourself.</p> |
| <p>Try explaining your change to yourself before opening the PR. |
| For example, ask yourself:</p> |
| <ul> |
| <li>What is the original bug? |
| When does it happen? |
| How severe is it? |
| What causes it?</li> |
| <li>Why is this the right fix? |
| Are there other fixes possible? |
| What are their advantages or disadvantages?</li> |
| <li>Are there any edge cases? |
| Does your code handle them?</li> |
| <li>Does the code have existing <a href="https://brooker.co.za/blog/2023/07/28/ds-testing.html">invariants</a>? |
| Did you preserve those invariants?</li> |
| <li>What behavior is <em>unchanged</em>? |
| What test establishes that?</li> |
| <li>Why does your test trigger the bug?</li> |
| <li>What are you still not certain about?</li> |
| </ul> |
| <p>It’s ok to be uncertain and to ask for help. |
| We would much rather help you because you’re not sure than have you guess wrong and then have to reverse-engineer where you went wrong.</p> |
| <h3 id="disclosure-guidelines"><a class="header" href="#disclosure-guidelines">Disclosure guidelines</a></h3> |
| <p>Disclose the <em>extent</em> and <em>purpose</em> of your LLM use. |
| We don’t care which model you used, but we do care whether you used the LLM to implement the idea or to come up with it.</p> |
| <p><strong>Good</strong> examples:</p> |
| <blockquote> |
| <p>LLM disclosure: I wrote the three commits by hand after viewing profiling data. I used an LLM to review the commits before submitting. The LLM identified that <code>ImplString::is_negative</code> was no longer used, so I removed that field by hand.</p> |
| </blockquote> |
| <blockquote> |
| <p>Created with the help of an LLM, which:</p> |
| <ul> |
| <li>traced the missing cache hits to the unconditional <code>return(pass)</code> by inspecting Fastly vs CloudFront headers,</li> |
| <li>reviewed the git history to understand why the snippet was added, and</li> |
| <li>made the VCL change.</li> |
| </ul> |
| </blockquote> |
| <p><strong>Bad</strong> examples:</p> |
| <blockquote> |
| <p>🤖 Generated with Claude Code</p> |
| </blockquote> |
| <blockquote> |
| <p><code>Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com></code></p> |
| </blockquote> |
| <h2 id="llm-policy"><a class="header" href="#llm-policy">LLM policy</a></h2> |
| <p>See <a href="https://forge.rust-lang.org/policies/llm-usage.html">Forge</a>.</p> |
| |
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