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<main>
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<h1 id="coding-conventions"><a class="header" href="#coding-conventions">Coding conventions</a></h1>
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<p>This file offers some tips on the coding conventions for rustc. This
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chapter covers <a href="#formatting">formatting</a>, <a href="#cc">coding for correctness</a>,
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<a href="#cio">using crates from crates.io</a>, and some tips on
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<a href="#er">structuring your PR for easy review</a>.</p>
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<p><a id="formatting"></a></p>
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<h2 id="formatting-and-the-tidy-script"><a class="header" href="#formatting-and-the-tidy-script">Formatting and the tidy script</a></h2>
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<p>rustc is moving towards the <a href="https://github.com/rust-dev-tools/fmt-rfcs">Rust standard coding style</a>.</p>
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<p>However, for now we don't use stable <code>rustfmt</code>; we use a pinned version with a
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special config, so this may result in different style from normal <a href="https://github.com/rust-lang/rustfmt"><code>rustfmt</code></a>.
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Therefore, formatting this repository using <code>cargo fmt</code> is not recommended.</p>
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<p>Instead, formatting should be done using <code>./x fmt</code>. It's a good habit to run
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<code>./x fmt</code> before every commit, as this reduces conflicts later.</p>
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<p>Formatting is checked by the <code>tidy</code> script. It runs automatically when you do
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<code>./x test</code> and can be run in isolation with <code>./x fmt --check</code>.</p>
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<p>If you want to use format-on-save in your editor, the pinned version of
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<code>rustfmt</code> is built under <code>build/<target>/stage0/bin/rustfmt</code>.</p>
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<h3 id="formatting-c-code"><a class="header" href="#formatting-c-code">Formatting C++ code</a></h3>
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<p>The compiler contains some C++ code for interfacing with parts of LLVM that
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don't have a stable C API.
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When modifying that code, use this command to format it:</p>
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<pre><code class="language-console">./x test tidy --extra-checks cpp:fmt --bless
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</code></pre>
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<p>This uses a pinned version of <code>clang-format</code>, to avoid relying on the local
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environment.</p>
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<h3 id="formatting-and-linting-python-code"><a class="header" href="#formatting-and-linting-python-code">Formatting and linting Python code</a></h3>
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<p>The Rust repository contains quite a lot of Python code. We try to keep
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it both linted and formatted by the <a href="https://github.com/astral-sh/ruff">ruff</a> tool.</p>
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<p>When modifying Python code, use this command to format it:</p>
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<pre><code class="language-console">./x test tidy --extra-checks py:fmt --bless
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</code></pre>
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<p>And, the following command to run lints:</p>
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<pre><code class="language-console">./x test tidy --extra-checks py:lint
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</code></pre>
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<p>These use a pinned version of <code>ruff</code>, to avoid relying on the local environment.</p>
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<p><a id="copyright"></a></p>
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<!-- REUSE-IgnoreStart -->
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<!-- Prevent REUSE from interpreting the heading as a copyright notice -->
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<h3 id="copyright-notice"><a class="header" href="#copyright-notice">Copyright notice</a></h3>
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<!-- REUSE-IgnoreEnd -->
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<p>In the past, files began with a copyright and license notice. Please <strong>omit</strong>
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this notice for new files licensed under the standard terms (dual
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MIT/Apache-2.0).</p>
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<p>All of the copyright notices should be gone by now, but if you come across one
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in the rust-lang/rust repo, feel free to open a PR to remove it.</p>
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<h3 id="line-length"><a class="header" href="#line-length">Line length</a></h3>
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<p>Lines should be at most 100 characters. It's even better if you can
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keep things to 80.</p>
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<p>Sometimes, and particularly for tests, it can be necessary to exempt yourself from this limit.
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In that case, you can add a comment towards the top of the file like so:</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>// ignore-tidy-linelength
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<span class="boring">}</span></code></pre></pre>
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<h3 id="tabs-vs-spaces"><a class="header" href="#tabs-vs-spaces">Tabs vs spaces</a></h3>
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<p>Prefer 4-space indents.</p>
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<p><a id="cc"></a></p>
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<h2 id="coding-for-correctness"><a class="header" href="#coding-for-correctness">Coding for correctness</a></h2>
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<p>Beyond formatting, there are a few other tips that are worth
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following.</p>
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<h3 id="prefer-exhaustive-matches"><a class="header" href="#prefer-exhaustive-matches">Prefer exhaustive matches</a></h3>
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<p>Using <code>_</code> in a match is convenient, but it means that when new
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variants are added to the enum, they may not get handled correctly.
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Ask yourself: if a new variant were added to this enum, what's the
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chance that it would want to use the <code>_</code> code, versus having some
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other treatment? Unless the answer is "low", then prefer an
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exhaustive match.</p>
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<p>The same advice applies to <code>if let</code> and <code>while let</code>,
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which are effectively tests for a single variant.</p>
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<h3 id="use-todo-comments-for-things-you-dont-want-to-forget"><a class="header" href="#use-todo-comments-for-things-you-dont-want-to-forget">Use "TODO" comments for things you don't want to forget</a></h3>
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<p>As a useful tool to yourself, you can insert a <code>// TODO</code> comment
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for something that you want to get back to before you land your PR:</p>
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<pre><code class="language-rust ignore">fn do_something() {
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if something_else {
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unimplemented!(); // TODO write this
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}
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}</code></pre>
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<p>The tidy script will report an error for a <code>// TODO</code> comment, so this
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code would not be able to land until the TODO is fixed (or removed).</p>
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<p>This can also be useful in a PR as a way to signal from one commit that you are
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leaving a bug that a later commit will fix:</p>
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<pre><code class="language-rust ignore">if foo {
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return true; // TODO wrong, but will be fixed in a later commit
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}</code></pre>
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<p><a id="cio"></a></p>
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<h2 id="using-crates-from-cratesio"><a class="header" href="#using-crates-from-cratesio">Using crates from crates.io</a></h2>
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<p>See the <a href="./crates-io.html">crates.io dependencies</a> section.</p>
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<p><a id="er"></a></p>
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<h2 id="how-to-structure-your-pr"><a class="header" href="#how-to-structure-your-pr">How to structure your PR</a></h2>
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<p>How you prepare the commits in your PR can make a big difference for the
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reviewer. Here are some tips.</p>
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<p><strong>Isolate "pure refactorings" into their own commit.</strong> For example, if
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you rename a method, then put that rename into its own commit, along
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with the renames of all the uses.</p>
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<p><strong>More commits is usually better.</strong> If you are doing a large change,
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it's almost always better to break it up into smaller steps that can
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be independently understood. The one thing to be aware of is that if
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you introduce some code following one strategy, then change it
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dramatically (versus adding to it) in a later commit, that
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'back-and-forth' can be confusing.</p>
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<p><strong>Format liberally.</strong> While only the final commit of a PR must be correctly
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formatted, it is both easier to review and less noisy to format each commit
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individually using <code>./x fmt</code>.</p>
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<p><strong>No merges.</strong> We do not allow merge commits into our history, other
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than those by bors. If you get a merge conflict, rebase instead via a
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command like <code>git rebase -i rust-lang/master</code> (presuming you use the
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name <code>rust-lang</code> for your remote).</p>
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<p><strong>Individual commits do not have to build (but it's nice).</strong> We do not
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require that every intermediate commit successfully builds – we only
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expect to be able to bisect at a PR level. However, if you <em>can</em> make
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individual commits build, that is always helpful.</p>
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<h2 id="naming-conventions"><a class="header" href="#naming-conventions">Naming conventions</a></h2>
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<p>Apart from normal Rust style/naming conventions, there are also some specific
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to the compiler.</p>
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<ul>
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<li>
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<p><code>cx</code> tends to be short for "context" and is often used as a suffix. For
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example, <code>tcx</code> is a common name for the <a href="./ty.html">Typing Context</a>.</p>
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</li>
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<li>
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<p><a href="./ty.html"><code>'tcx</code></a> is used as the lifetime name for the Typing Context.</p>
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</li>
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<li>
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<p>Because <code>crate</code> is a keyword, if you need a variable to represent something
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crate-related, often the spelling is changed to <code>krate</code>.</p>
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</li>
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</ul>
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