Niko Matsakis
GitHub
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# Walkthrough: a typical contribution
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There are _a lot_ of ways to contribute to the rust compiler, including fixing
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bugs, improving performance, helping design features, providing feedback on
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existing features, etc. This chapter does not claim to scratch the surface.
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Instead, it walks through the design and implementation of a new feature. Not
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all of the steps and processes described here are needed for every
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contribution, and I will try to point those out as they arise.
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In general, if you are interested in making a contribution and aren't sure
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where to start, please feel free to ask!
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## Overview
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The feature I will discuss in this chapter is the `?` Kleene operator for
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macros. Basically, we want to be able to write something like this:
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```rust,ignore
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macro_rules! foo {
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($arg:ident $(, $optional_arg:ident)?) => {
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println!("{}", $arg);
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$(
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println!("{}", $optional_arg);
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)?
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}
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}
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fn main() {
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let x = 0;
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foo!(x); // ok! prints "0"
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foo!(x, x); // ok! prints "0 0"
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}
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```
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So basically, the `$(pat)?` matcher in the macro means "this pattern can occur
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0 or 1 times", similar to other regex syntaxes.
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There were a number of steps to go from an idea to stable rust feature. Here is
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a quick list. We will go through each of these in order below. As I mentioned
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before, not all of these are needed for every type of contribution.
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- **Idea discussion/Pre-RFC** A Pre-RFC is an early draft or design discussion
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of a feature. This stage is intended to flesh out the design space a bit and
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get a grasp on the different merits and problems with an idea. It's a great
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way to get early feedback on your idea before presenting it the wider
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audience. You can find the original discussion [here][prerfc].
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- **RFC** This is when you formally present your idea to the community for
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consideration. You can find the RFC [here][rfc].
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- **Implementation** Implement your idea unstabley in the compiler. You can
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find the original implementation [here][impl1].
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- **Possibly iterate/refine** As the community gets experience with your
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feature on the nightly compiler and in `libstd`, there may be additional
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feedback about design choice that might be adjusted. This particular feature
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went [through][impl2] a [number][impl3] of [iterations][impl4].
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- **Stabilization** When your feature has baked enough, a rust team member may
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[propose to stabilize it][merge]. If there is consensus, this is done.
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- **Relax** Your feature is now a stable rust feature!
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[prerfc]: https://internals.rust-lang.org/t/pre-rfc-at-most-one-repetition-macro-patterns/6557
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[rfc]: https://github.com/rust-lang/rfcs/pull/2298
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[impl1]: https://github.com/rust-lang/rust/pull/47752
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[impl2]: https://github.com/rust-lang/rust/pull/49719
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[impl3]: https://github.com/rust-lang/rust/pull/51336
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[impl4]: https://github.com/rust-lang/rust/pull/51587
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[merge]: https://github.com/rust-lang/rust/issues/48075#issuecomment-433177613
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## Pre-RFC and RFC
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> NOTE: In general, if you are not proposing a _new_ feature or substantial
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> change to rust or the ecosystem, you don't need to follow the RFC process.
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> Instead, you can just jump to [implementation](#impl).
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>
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> You can find the official guidelines for when to open an RFC [here][rfcwhen].
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[rfcwhen]: https://github.com/rust-lang/rfcs#when-you-need-to-follow-this-process
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An RFC is a document that describes the feature or change you are proposing in
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detail. Anyone can write an RFC; the process is the same for everyone,
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including rust team members.
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To open an RFC, open a PR on the
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[rust-lang/rfcs](https://github.com/rust-lang/rfcs) repo on GitHub. You can
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find detailed instructions in the
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[README](https://github.com/rust-lang/rfcs#what-the-process-is).
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Before opening an RFC, you should do the research to "flesh out" your idea.
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Hastily-proposed RFCs tend not to be accepted. You should generally have a good
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description of the motivation, impact, disadvantages, and potential
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interactions with other features.
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If that sounds like a lot of work, it's because it is. But no fear! Even if
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you're not a compiler hacker, you can get great feedback by doing a _pre-RFC_.
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This is an _informal_ discussion of the idea. The best place to do this is
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internals.rust-lang.org. Your post doesn't have to follow any particular
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structure. It doesn't even need to be a cohesive idea. Generally, you will get
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tons of feedback that you can integrate back to produce a good RFC.
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(Another pro-tip: try searching the RFCs repo and internals for prior related
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ideas. A lot of times an idea has already been considered and was either
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rejected or postponed to be tried again later. This can save you and everybody
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else some time)
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In the case of our example, a participant in the pre-RFC thread pointed out a
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syntax ambiguity and a potential resolution. Also, the overall feedback seemed
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positive. In this case, the discussion converged pretty quickly, but for some
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ideas, a lot more discussion can happen (e.g. see [this RFC][nonascii] which
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received a whopping 684 comments!). If that happens, don't be discouraged; it
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means the community is interested in your idea, but it perhaps needs some
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adjustments.
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[nonascii]: https://github.com/rust-lang/rfcs/pull/2457
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The RFC for our `?` macro feature did receive some discussion on the RFC thread
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too. As with most RFCs, there were a few questions that we couldn't answer by
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discussion: we needed experience using the feature to decide. Such questions
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are listed in the "Unresolved Questions" section of the RFC. Also, over the
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course of the RFC discussion, you will probably want to update the RFC document
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itself to reflect the course of the discussion (e.g. new alternatives or prior
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work may be added or you may decide to change parts of the proposal itself).
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In the end, when the discussion seems to reach a consensus and die down a bit,
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a rust team member may propose to move to FCP with one of three possible dispositions.
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This means that they want the other members of the appropriate teams to review
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and comment on the RFC. More discussion may ensue, which may result in more changes
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or unresolved questions being added. At some point, when everyone is
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satisfied, the RFC enters the "final comment period" (FCP), which is the last
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chance for people to bring up objections. When the FCP is over, the disposition is
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adopted. Here are the three possible dispositions:
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- _Merge_: accept the feature. Here is the proposal to merge for our [`?` macro
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feature][rfcmerge].
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- _Close_: this feature in its current form is not a good fit for rust. Don't
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be discouraged if this happens to your RFC, and don't take it personally.
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This is not a reflection on you, but rather a community decision that rust
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will go a different direction.
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- _Postpone_: there is interest in going this direction but not at the moment.
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This happens most often because the appropriate rust team doesn't have the
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bandwidth to shepherd the feature through the process to stabilization. Often
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this is the case when the feature doesn't fit into the team's roadmap.
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Postponed ideas may be revisited later.
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[rfcmerge]: https://github.com/rust-lang/rfcs/pull/2298#issuecomment-360582667
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When an RFC is merged, the PR is merged into the RFCs repo. A new _tracking
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issue_ is created in the [rust-lang/rust] repo to track progress on the feature
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and discuss unresolved questions, implementation progress and blockers, etc.
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Here is the tracking issue on for our [`?` macro feature][tracking].
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[tracking]: https://github.com/rust-lang/rust/issues/48075
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<a name="impl"></a>
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## Implementation
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To make a change to the compiler, open a PR against the [rust-lang/rust] repo.
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[rust-lang/rust]: https://github.com/rust-lang/rust
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Depending on the feature/change/bug fix/improvement, implementation may be
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relatively-straightforward or it may be a major undertaking. You can always ask
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for help or mentorship from more experienced compiler devs. Also, you don't
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have to be the one to implement your feature; but keep in mind that if you
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don't it might be a while before someone else does.
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For the `?` macro feature, I needed to go understand the relevant parts of
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macro expansion in the compiler. Personally, I find that [improving the
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comments][comments] in the code is a helpful way of making sure I understand
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it, but you don't have to do that if you don't want to.
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[comments]: https://github.com/rust-lang/rust/pull/47732
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I then [implemented][impl1] the original feature, as described in the RFC. When
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a new feature is implemented, it goes behind a _feature gate_, which means that
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you have to use `#![feature(my_feature_name)]` to use the feature. The feature
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gate is removed when the feature is stabilized.
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**Most bug fixes and improvements** don't require a feature gate. You can just
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make your changes/improvements.
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When you open a PR on the [rust-lang/rust], a bot will assign your PR to a
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review. If there is a particular rust team member you are working with, you can
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request that reviewer by leaving a comment on the thread with `r?
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@reviewer-github-id` (e.g. `r? @eddyb`). If you don't know who to request,
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don't request anyone; the bot will assign someone automatically.
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The reviewer may request changes before they approve your PR. Feel free to ask
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questions or discuss things you don't understand or disagree with. However,
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recognize that the PR won't be merged unless someone on the rust team approves
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it.
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When your review approves the PR, it will go into a queue for yet another bot
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called `@bors`. `@bors` manages the CI build/merge queue. When your PR reaches
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the head of the `@bors` queue, `@bors` will test out the merge by running all
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tests against your PR on Travis CI. This takes about 2 hours as of this
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writing. If all tests pass, the PR is merged and becomes part of the next
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nightly compiler!
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There are a couple of things that may happen for some PRs during the review process
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- If the change is substantial enough, the reviewer may request an FCP on
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the PR. This gives all members of the appropriate team a chance to review the
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changes.
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- If the change may cause breakage, the reviewer may request a [crater] run.
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This compiles the compiler with your changes and then attempts to compile all
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crates on crates.io with your modified compiler. This is a great smoke test
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to check if you introduced a change to compiler behavior that affects a large
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portion of the ecosystem.
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- If the diff of your PR is large or the reviewer is busy, your PR may have
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some merge conflicts with other PRs that happen to get merged first. You
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should fix these merge conflicts using the normal git procedures.
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[crater]: ./tests/intro.html#crater
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If you are not doing a new feature or something like that (e.g. if you are
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fixing a bug), then that's it! Thanks for your contribution :)
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## Refining your implementation
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As people get experience with your new feature on nightly, slight changes may
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be proposed and unresolved questions may become resolved. Updates/changes go
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through the same process for implementing any other changes, as described
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above (i.e. submit a PR, go through review, wait for `@bors`, etc).
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Some changes may be major enough to require an FCP and some review by rust team
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members.
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For the `?` macro feature, we went through a few different iterations after the
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original implementation: [1][impl2], [2][impl3], [3][impl4].
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Along the way, we decided that `?` should not take a separator, which was
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previously an unresolved question listed in the RFC. We also changed the
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disambiguation strategy: we decided to remove the ability to use `?` as a
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separator token for other repetition operators (e.g. `+` or `*`). However,
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since this was a breaking change, we decided to do it over an edition boundary.
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Thus, the new feature can be enabled only in edition 2018. These deviations
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from the original RFC required [another
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FCP](https://github.com/rust-lang/rust/issues/51934).
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## Stabilization
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Finally, after the feature had baked for a while on nightly, a language team member
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[moved to stabilize it][stabilizefcp].
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[stabilizefcp]: https://github.com/rust-lang/rust/issues/48075#issuecomment-433177613
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A _stabilization report_ needs to be written that includes
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- brief description of the behavior and any deviations from the RFC
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- which edition(s) are affected and how
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- links to a few tests to show the interesting aspects
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The stabilization report for our feature is [here][stabrep].
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[stabrep]: https://github.com/rust-lang/rust/issues/48075#issuecomment-433243048
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After this, [a PR is made][stab] to remove the feature gate, enabling the feature by
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default (on the 2018 edition). A note is added to the [Release notes][relnotes]
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about the feature.
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[stab]: https://github.com/rust-lang/rust/pull/56245
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TODO: currently, we have a [forge article][feature-stab] about stabilization, but
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we really ought to move that to the guide (in fact, we probably should have a whole
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chapter about feature gates and stabilization).
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[feature-stab]: https://forge.rust-lang.org/stabilization-guide.html
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[relnotes]: https://github.com/rust-lang/rust/blob/master/RELEASES.md
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