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| <h1 id="walkthrough-a-typical-contribution"><a class="header" href="#walkthrough-a-typical-contribution">Walkthrough: a typical contribution</a></h1> |
| <p>There are <em>a lot</em> of ways to contribute to the Rust compiler, including fixing |
| bugs, improving performance, helping design features, providing feedback on existing features, etc. |
| This chapter does not claim to scratch the surface. |
| Instead, it walks through the design and implementation of a new feature. |
| Not all of the steps and processes described here are needed for every |
| contribution, and I will try to point those out as they arise.</p> |
| <p>In general, if you are interested in making a contribution and aren’t sure |
| where to start, please feel free to ask!</p> |
| <h2 id="overview"><a class="header" href="#overview">Overview</a></h2> |
| <p>The feature I will discuss in this chapter is the <code>?</code> Kleene operator for macros. |
| Basically, we want to be able to write something like this:</p> |
| <pre><code class="language-rust ignore">macro_rules! foo { |
| ($arg:ident $(, $optional_arg:ident)?) => { |
| println!("{}", $arg); |
| |
| $( |
| println!("{}", $optional_arg); |
| )? |
| } |
| } |
| |
| fn main() { |
| let x = 0; |
| foo!(x); // ok! prints "0" |
| foo!(x, x); // ok! prints "0 0" |
| }</code></pre> |
| <p>So basically, the <code>$(pat)?</code> matcher in the macro means “this pattern can occur |
| 0 or 1 times”, similar to other regex syntaxes.</p> |
| <p>There were a number of steps to go from an idea to stable Rust feature. |
| Here is a quick list. |
| We will go through each of these in order below. |
| As I mentioned before, not all of these are needed for every type of contribution.</p> |
| <ul> |
| <li><strong>Idea discussion/Pre-RFC</strong> A Pre-RFC is an early draft or design discussion |
| of a feature. |
| This stage is intended to flesh out the design space a bit and |
| get a grasp on the different merits and problems with an idea. |
| It’s a great way to get early feedback on your idea before presenting it to the wider |
| audience. |
| You can find the original discussion <a href="https://internals.rust-lang.org/t/pre-rfc-at-most-one-repetition-macro-patterns/6557">here</a>.</li> |
| <li><strong>RFC</strong> This is when you formally present your idea to the community for |
| consideration. |
| You can find the RFC <a href="https://github.com/rust-lang/rfcs/pull/2298">here</a>.</li> |
| <li><strong>Implementation</strong> Implement your idea unstably in the compiler. You can |
| find the original implementation <a href="https://github.com/rust-lang/rust/pull/47752">here</a>.</li> |
| <li><strong>Possibly iterate/refine</strong> As the community gets experience with your |
| feature on the nightly compiler and in <code>std</code>, there may be additional |
| feedback about design choice that might be adjusted. |
| This particular feature went <a href="https://github.com/rust-lang/rust/pull/49719">through</a> a <a href="https://github.com/rust-lang/rust/pull/51336">number</a> of <a href="https://github.com/rust-lang/rust/pull/51587">iterations</a>.</li> |
| <li><strong>Stabilization</strong> When your feature has baked enough, a Rust team member may |
| <a href="https://github.com/rust-lang/rust/issues/48075#issuecomment-433177613">propose to stabilize it</a>. |
| If there is consensus, this is done.</li> |
| <li><strong>Relax</strong> Your feature is now a stable Rust feature!</li> |
| </ul> |
| <h2 id="pre-rfc-and-rfc"><a class="header" href="#pre-rfc-and-rfc">Pre-RFC and RFC</a></h2> |
| <blockquote> |
| <p>NOTE: In general, if you are not proposing a <em>new</em> feature or substantial |
| change to Rust or the ecosystem, you don’t need to follow the RFC process. |
| Instead, you can just jump to <a href="#impl">implementation</a>.</p> |
| <p>You can find the official guidelines for when to open an RFC <a href="https://github.com/rust-lang/rfcs#when-you-need-to-follow-this-process">here</a>.</p> |
| </blockquote> |
| <p>An RFC is a document that describes the feature or change you are proposing in detail. |
| Anyone can write an RFC; |
| the process is the same for everyone, including Rust team members.</p> |
| <p>To open an RFC, open a PR on the <a href="https://github.com/rust-lang/rfcs">rust-lang/rfcs</a> repo on GitHub. |
| You can find detailed instructions in the |
| <a href="https://github.com/rust-lang/rfcs#what-the-process-is">README</a>.</p> |
| <p>Before opening an RFC, you should do the research to “flesh out” your idea. |
| Hastily-proposed RFCs tend not to be accepted. |
| You should generally have a good description of the motivation, impact, disadvantages, and potential |
| interactions with other features.</p> |
| <p>If that sounds like a lot of work, it’s because it is. |
| But no fear! |
| Even if you’re not a compiler hacker, you can get great feedback by doing a <em>pre-RFC</em>. |
| This is an <em>informal</em> discussion of the idea. |
| The best place to do this is internals.rust-lang.org. |
| Your post doesn’t have to follow any particular structure. |
| It doesn’t even need to be a cohesive idea. |
| Generally, you will get tons of feedback that you can integrate back to produce a good RFC.</p> |
| <p>(Another pro-tip: try searching the RFCs repo and internals for prior related ideas. |
| A lot of times an idea has already been considered and was either |
| rejected or postponed to be tried again later. |
| This can save you and everybody else some time)</p> |
| <p>In the case of our example, a participant in the pre-RFC thread pointed out a |
| syntax ambiguity and a potential resolution. |
| Also, the overall feedback seemed positive. |
| In this case, the discussion converged pretty quickly, but for some |
| ideas, a lot more discussion can happen (e.g. see <a href="https://github.com/rust-lang/rfcs/pull/2457">this RFC</a> which |
| received a whopping 684 comments!). |
| If that happens, don’t be discouraged; |
| it means the community is interested in your idea, but it perhaps needs some |
| adjustments.</p> |
| <p>The RFC for our <code>?</code> macro feature did receive some discussion on the RFC thread too. |
| As with most RFCs, there were a few questions that we couldn’t answer by |
| discussion: we needed experience using the feature to decide. |
| Such questions are listed in the “Unresolved Questions” section of the RFC. |
| Also, over the course of the RFC discussion, you will probably want to update the RFC document |
| itself to reflect the course of the discussion (e.g. new alternatives or prior |
| work may be added or you may decide to change parts of the proposal itself).</p> |
| <p>In the end, when the discussion seems to reach a consensus and die down a bit, |
| a Rust team member may propose to move to “final comment period” (FCP) with one |
| of three possible dispositions. |
| This means that they want the other members of |
| the appropriate teams to review and comment on the RFC. |
| More discussion may ensue, which may result in more changes or unresolved questions being added. |
| At some point, when everyone is satisfied, the RFC enters the FCP, which is the |
| last chance for people to bring up objections. |
| When the FCP is over, the disposition is adopted. |
| Here are the three possible dispositions:</p> |
| <ul> |
| <li><em>Merge</em>: accept the feature. Here is the proposal to merge for our <a href="https://github.com/rust-lang/rfcs/pull/2298#issuecomment-360582667"><code>?</code> macro |
| feature</a>.</li> |
| <li><em>Close</em>: this feature in its current form is not a good fit for rust. Don’t |
| be discouraged if this happens to your RFC, and don’t take it personally. |
| This is not a reflection on you, but rather a community decision that rust |
| will go a different direction.</li> |
| <li><em>Postpone</em>: there is interest in going this direction but not at the moment. |
| This happens most often because the appropriate Rust team doesn’t have the |
| bandwidth to shepherd the feature through the process to stabilization. |
| Often this is the case when the feature doesn’t fit into the team’s roadmap. |
| Postponed ideas may be revisited later.</li> |
| </ul> |
| <p>When an RFC is merged, the PR is merged into the RFCs repo. |
| A new <em>tracking issue</em> is created in the <a href="https://github.com/rust-lang/rust">rust-lang/rust</a> repo to track progress on the feature |
| and discuss unresolved questions, implementation progress and blockers, etc. |
| Here is the tracking issue on for our <a href="https://github.com/rust-lang/rust/issues/48075"><code>?</code> macro feature</a>.</p> |
| <p><a id="impl"></a></p> |
| <h2 id="implementation"><a class="header" href="#implementation">Implementation</a></h2> |
| <p>To make a change to the compiler, open a PR against the <a href="https://github.com/rust-lang/rust">rust-lang/rust</a> repo.</p> |
| <p>Depending on the feature/change/bug fix/improvement, implementation may be |
| relatively-straightforward or it may be a major undertaking. |
| You can always ask for help or mentorship from more experienced compiler devs. |
| Also, you don’t have to be the one to implement your feature; |
| but keep in mind that if you don’t, it might be a while before someone else does.</p> |
| <p>For the <code>?</code> macro feature, I needed to go understand the relevant parts of |
| macro expansion in the compiler. |
| Personally, I find that <a href="https://github.com/rust-lang/rust/pull/47732">improving the |
| comments</a> in the code is a helpful way of making sure I understand |
| it, but you don’t have to do that if you don’t want to.</p> |
| <p>I then <a href="https://github.com/rust-lang/rust/pull/47752">implemented</a> the original feature, as described in the RFC. |
| When a new feature is implemented, it goes behind a <em>feature gate</em>, which means that |
| you have to use <code>#![feature(my_feature_name)]</code> to use the feature. |
| The feature gate is removed when the feature is stabilized.</p> |
| <p><strong>Most bug fixes and improvements</strong> don’t require a feature gate. You can just |
| make your changes/improvements.</p> |
| <p>When you open a PR on the <a href="https://github.com/rust-lang/rust">rust-lang/rust</a>, a bot will assign your PR to a reviewer. |
| If there is a particular Rust team member you are working with, you can |
| request that reviewer by leaving a comment on the thread with <code>r? @reviewer-github-id</code> (e.g. <code>r? @eddyb</code>). If you don’t know who to request, |
| don’t request anyone; |
| the bot will assign someone automatically based on which files you changed.</p> |
| <p>The reviewer may request changes before they approve your PR, they may mark the PR with label |
| “S-waiting-on-author” after leaving comments, this means that the PR is blocked on you to make |
| some requested changes. |
| When you finished iterating on the changes, you can mark the PR as |
| <code>S-waiting-on-review</code> again by leaving a comment with <code>@rustbot ready</code>, this will remove the |
| <code>S-waiting-on-author</code> label and add the <code>S-waiting-on-review</code> label.</p> |
| <p>Feel free to ask questions or discuss things you don’t understand or disagree with. |
| However, recognize that the PR won’t be merged unless someone on the Rust team approves |
| it. |
| If a reviewer leave a comment like <code>r=me after fixing ...</code>, that means they approve the PR and |
| you can merge it with comment with <code>@bors r=reviewer-github-id</code>(e.g. <code>@bors r=eddyb</code>) to merge it |
| after fixing trivial issues. |
| Note that <code>r=someone</code> requires permission and bors could say |
| something like “🔑 Insufficient privileges…” when commenting <code>r=someone</code>. |
| In that case, you have to ask the reviewer to revisit your PR.</p> |
| <p>When your reviewer approves the PR, it will go into a queue for yet another bot called <code>@bors</code>. |
| <code>@bors</code> manages the CI build/merge queue. |
| When your PR reaches the head of the <code>@bors</code> queue, <code>@bors</code> will test out the merge by running all |
| tests against your PR on GitHub Actions. |
| This takes a lot of time to finish. |
| If all tests pass, the PR is merged and becomes part of the next nightly compiler!</p> |
| <p>There are a couple of things that may happen for some PRs during the review process</p> |
| <ul> |
| <li>If the change is substantial enough, the reviewer may request an FCP on |
| the PR. |
| This gives all members of the appropriate team a chance to review the changes.</li> |
| <li>If the change may cause breakage, the reviewer may request a <a href="./tests/crater.html">crater</a> run. |
| This compiles the compiler with your changes and then attempts to compile all |
| crates on crates.io with your modified compiler. |
| This is a great smoke test |
| to check if you introduced a change to compiler behavior that affects a large |
| portion of the ecosystem.</li> |
| <li>If the diff of your PR is large or the reviewer is busy, your PR may have |
| some merge conflicts with other PRs that happen to get merged first. |
| You should fix these merge conflicts using the normal git procedures.</li> |
| </ul> |
| <p>If you are not doing a new feature or something like that (e.g. if you are |
| fixing a bug), then that’s it! |
| Thanks for your contribution :)</p> |
| <h2 id="refining-your-implementation"><a class="header" href="#refining-your-implementation">Refining your implementation</a></h2> |
| <p>As people get experience with your new feature on nightly, slight changes may |
| be proposed and unresolved questions may become resolved. |
| Updates/changes go through the same process for implementing any other changes, as described |
| above (i.e. submit a PR, go through review, wait for <code>@bors</code>, etc).</p> |
| <p>Some changes may be major enough to require an FCP and some review by Rust team members.</p> |
| <p>For the <code>?</code> macro feature, we went through a few different iterations after the |
| original implementation: <a href="https://github.com/rust-lang/rust/pull/49719">1</a>, <a href="https://github.com/rust-lang/rust/pull/51336">2</a>, <a href="https://github.com/rust-lang/rust/pull/51587">3</a>.</p> |
| <p>Along the way, we decided that <code>?</code> should not take a separator, which was |
| previously an unresolved question listed in the RFC. |
| We also changed the disambiguation strategy: we decided to remove the ability to use <code>?</code> as a |
| separator token for other repetition operators (e.g. <code>+</code> or <code>*</code>). However, |
| since this was a breaking change, we decided to do it over an edition boundary. |
| Thus, the new feature can be enabled only in edition 2018. These deviations |
| from the original RFC required <a href="https://github.com/rust-lang/rust/issues/51934">another FCP</a>.</p> |
| <h2 id="stabilization"><a class="header" href="#stabilization">Stabilization</a></h2> |
| <p>Finally, after the feature had baked for a while on nightly, a language team member |
| <a href="https://github.com/rust-lang/rust/issues/48075#issuecomment-433177613">moved to stabilize it</a>.</p> |
| <p>A <em>stabilization report</em> needs to be written that includes</p> |
| <ul> |
| <li>brief description of the behavior and any deviations from the RFC</li> |
| <li>which edition(s) are affected and how</li> |
| <li>links to a few tests to show the interesting aspects</li> |
| </ul> |
| <p>The stabilization report for our feature is <a href="https://github.com/rust-lang/rust/issues/48075#issuecomment-433243048">here</a>.</p> |
| <p>After this, <a href="https://github.com/rust-lang/rust/pull/56245">a PR is made</a> to remove the feature gate, enabling the feature by |
| default (on the 2018 edition). |
| A note is added to the <a href="https://github.com/rust-lang/rust/blob/HEAD/RELEASES.md">Release notes</a> about the feature.</p> |
| <p>Steps to stabilize the feature can be found at <a href="./stabilization_guide.html">Stabilizing Features</a>.</p> |
| |
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