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Summary

This RFC proposes a process for deciding detailed guidelines for code formatting, and default settings for Rustfmt. The outcome of the process should be an approved formatting style defined by a style guide and enforced by Rustfmt.

This RFC proposes creating a new repository under the rust-lang organisation called fmt-rfcs. It will be operated in a similar manner to the RFCs repository, but restricted to formatting issues. A new sub-team will be created to deal with those RFCs. Both the team and repository are expected to be temporary. Once the style guide is complete, the team can be disbanded and the repository frozen.

Motivation

There is a need to decide on detailed guidelines for the format of Rust code. A uniform, language-wide formatting style makes comprehending new code-bases easier and forestalls bikeshedding arguments in teams of Rust users. The utility of such guidelines has been proven by Go, amongst other languages.

The Rustfmt tool is reaching maturity and currently enforces a somewhat arbitrary, lightly discussed style, with many configurable options.

If Rustfmt is to become a widely accepted tool, there needs to be a process for the Rust community to decide on the default style, and how configurable that style should be.

These discussions should happen in the open and be highly visible. It is important that the Rust community has significant input to the process. The RFC repository would be an ideal place to have this discussion because it exists to satisfy these goals, and is tried and tested. However, the discussion is likely to be a high-bandwidth one (code style is a contentious and often subjective topic, and syntactic RFCs tend to be the highest traffic ones). Therefore, having the discussion on the RFCs repository could easily overwhelm it and make it less useful for other important discussions.

There currently exists a style guide as part of the Rust documentation. This is far more wide-reaching than just formatting style, but also not detailed enough to specify Rustfmt. This was originally developed in its own repository, but is now part of the main Rust repository. That seems like a poor venue for discussion of these guidelines due to visibility.

Detailed design

Process

The process for style RFCs will mostly follow the process for other RFCs. Anyone may submit an RFC. An overview of the process is:

  • If there is no single, obvious style, then open a GitHub issue on the fmt-rfcs repo for initial discussion. This initial discussion should identify which Rustfmt options are required to enforce the guideline.
  • Implement the style in rustfmt (behind an option if it is not the current default). In exceptional circumstances (such as where the implementation would require very deep changes to rustfmt), this step may be skipped.
  • Write an RFC formalising the formatting convention and referencing the implementation, submit as a PR to fmt-rfcs. The RFC should include the default values for options to enforce the guideline and which non-default options should be kept.
  • The RFC PR will be triaged by the style team and either assigned to a team member for shepherding, or closed.
  • When discussion has reached a fixed point, the RFC PR will be put into a final comment period (FCP).
  • After FCP, the RFC will either be accepted and merged or closed.
  • Implementation in Rustfmt can then be finished (including any changes due to discussion of the RFC), and defaults are set.

Scope of the process

This process is specifically limited to formatting style guidelines which can be enforced by Rustfmt with its current architecture. Guidelines that cannot be enforced by Rustfmt without a large amount of work are out of scope, even if they only pertain to formatting.

Note whether Rustfmt should be configurable at all, and if so how configurable is a decision that should be dealt with using the formatting RFC process. That will be a rather exceptional RFC.

Size of RFCs

RFCs should be self-contained and coherent, whilst being as small as possible to keep discussion focused. For example, an RFC on 'arithmetic and logic expressions' is about the right size; 'expressions' would be too big, and 'addition' would be too small.

When is a guideline ready for RFC?

The purpose of the style RFC process is to foster an open discussion about style guidelines. Therefore, RFC PRs should be made early rather than late. It is expected that there may be more discussion and changes to style RFCs than is typical for Rust RFCs. However, at submission, RFC PRs should be completely developed and explained to the level where they can be used as a specification.

A guideline should usually be implemented in Rustfmt before an RFC PR is submitted. The RFC should be used to select an option to be the default behaviour, rather than to identify a range of options. An RFC can propose a combination of options (rather than a single one) as default behaviour. An RFC may propose some reorganisation of options.

Usually a style should be widely used in the community before it is submitted as an RFC. Where multiple styles are used, they should be covered as alternatives in the RFC, rather than being submitted as multiple RFCs. In some cases, a style may be proposed without wide use (we don't want to discourage innovation), however, it should have been used in some real code, rather than just being sketched out.

Triage

RFC PRs are triaged by the style team. An RFC may be closed during triage (with feedback for the author) if the style team think it is not specified in enough detail, has too narrow or broad scope, or is not appropriate in some way (e.g., applies to more than just formatting). Otherwise, the PR will be assigned a shepherd as for other RFCs.

FCP

FCP will last for two weeks (assuming the team decide to meet every two weeks) and will be announced in the style team sub-team report.

Decision and post-decision process

The style team will make the ultimate decision on accepting or closing a style RFC PR. Decisions should be by consensus. Most discussion should take place on the PR comment thread, a decision should ideally be made when consensus is reached on the thread. Any additional discussion amongst the style team will be summarised on the thread.

If an RFC PR is accepted, it will be merged. An issue for implementation will be filed in the appropriate place (usually the Rustfmt repository) referencing the RFC. If the style guide needs to be updated, then an issue for that should be filed on the Rust repository.

The author of an RFC is not required to implement the guideline. If you are interested in working on the implementation for an 'active' RFC, but cannot determine if someone else is already working on it, feel free to ask (e.g. by leaving a comment on the associated issue).

The fmt-rfcs repository

The form of the fmt-rfcs repository will follow the rfcs repository. Accepted RFCs will live in a text directory, the README.md will include information taken from this RFC, there will be an RFC template in the root of the repository. Issues on the repository can be used for placeholders for future RFCs and for preliminary discussion.

The RFC format will be illustrated by the RFC template. It will have the following sections:

  • summary
  • details
  • implementation
  • rationale
  • alternatives
  • unresolved questions

The 'details' section should contain examples of both what should and shouldn't be done, cover simple and complex cases, and the interaction with other style guidelines.

The 'implementation' section should specify how options must be set to enforce the guideline, and what further changes (including additional options) are required. It should specify any renaming, reorganisation, or removal of options.

The 'rationale' section should motivate the choices behind the RFC. It should reference existing code bases which use the proposed style. 'Alternatives' should cover alternative possible guidelines, if appropriate.

Guidelines may include more than one acceptable rule, but should offer guidance for when to use each rule (which should be formal enough to be used by a tool).

For example:

A struct literal must be formatted either on a single line (with spaces after the opening brace and before the closing brace, and with fields separated by commas and spaces), or on multiple lines (with one field per line and newlines after the opening brace and before the closing brace). The former approach should be used for short struct literals, the latter for longer struct literals. For tools, the first approach should be used when the width of the fields (excluding commas and braces) is 16 characters. E.g.,

let x = Foo { a: 42, b: 34 };
let y = Foo {
    a: 42,
    b: 34,
    c: 1000
};

(Note this is just an example, not a proposed guideline).

The repository in embryonic form lives at nrc/fmt-rfcs. It illustrates what issues and PRs might look like, as well as including the RFC template. Note that typically there should be more discussion on an issue before submitting an RFC PR.

The repository should be updated as this RFC develops, and moved to the rust-lang GitHub organisation if this RFC is accepted.

The style team

The style sub-team will be responsible for handling style RFCs and making decisions related to code style and formatting.

Per the governance RFC, the core team would pick a leader who would then pick the rest of the team. I propose that the team should include members representative of the following areas:

  • Rustfmt,
  • the language, tools, and libraries sub-teams (since each has a stake in code style),
  • large Rust projects.

Because activity such as this hasn't been done before in the RUst community, it is hard to identify suitable candidates for the team ahead of time. The team will probably start small and consist of core members of the Rust community. I expect that once the process gets underway the team can be rapidly expanded with community members who are active in the fmt-rfcs repository (i.e., submitting and constructively commenting on RFCs).

There will be a dedicated irc channel for discussion on formatting issues: #rust-style.

Style guide

The existing style guide will be split into two guides: one dealing with API design and similar issues which will be managed by the libs team, and one dealing with formatting issues which will be managed by the style team. Note that the formatting part of the guide may include guidelines which are not enforced by Rustfmt. Those are outside the scope of the process defined in this RFC, but still belong in that part of the style guide.

When RFCs are accepted the style guide may need to be updated. Towards the end of the process, the style team should audit and edit the guide to ensure it is a coherent document.

Material goals

Hopefully, the style guideline process will have limited duration, one year seems reasonable. After that time, style guidelines for new syntax could be included with regular RFCs, or the fmt-rfcs repository could be maintained in a less active fashion.

At the end of the process, the fmt-rfcs repository should be a fairly complete guide for formatting Rust code, and useful as a specification for Rustfmt and tools with similar goals, such as IDEs. In particular, there should be a decision made on how configurable Rustfmt should be, and an agreed set of default options. The formatting style guide in the Rust repository should be a more human-friendly source of formatting guidelines, and should be in sync with the fmt-rfcs repo.

Drawbacks

This RFC introduces more process and bureaucracy, and requires more meetings for some core Rust contributors. Precious time and energy will need to be devoted to discussions.

Alternatives

Benevolent dictator - a single person dictates style rules which will be followed without question by the community. This seems to work for Go, I suspect it will not work for Rust.

Parliamentary 'democracy' - the community 'elects' a style team (via the usual RFC consensus process, rather than actual voting). The style team decides on style issues without an open process. This would be more efficient, but doesn't fit very well with the open ethos of the Rust community.

Use the RFCs repo, rather than a new repo. This would have the benefit that style RFCs would get more visibility, and it is one less place to keep track of for Rust community members. However, it risks overwhelming the RFC repo with style debate.

Use issues on Rustfmt. I feel that the discussions would not have enough visibility in this fashion, but perhaps that can be addressed by wide and regular announcement.

Use a book format for the style repo, rather than a collection of RFCs. This would make it easier to see how the 'final product' style guide would look. However, I expect there will be many issues that are important to be aware of while discussing an RFC, that are not important to include in a final guide.

Have an existing team handle the process, rather than create a new style team. Saves on a little bureaucracy. Candidate teams would be language and tools. However, the language team has very little free bandwidth, and the tools team is probably not broad enough to effectively handle the style decisions.

Unresolved questions