dschatzberg / rfcs

RFCs for changes to Rust

Rust RFCs

(jump forward to: Table of Contents, Active RFC List)

Many changes, including bug fixes and documentation improvements can be implemented and reviewed via the normal GitHub pull request workflow.

Some changes though are "substantial", and we ask that these be put through a bit of a design process and produce a consensus among the Rust community and the core team.

The "RFC" (request for comments) process is intended to provide a consistent and controlled path for new features to enter the language and standard libraries, so that all stakeholders can be confident about the direction the language is evolving in.

Active RFC List

• 0008-new-intrinsics.md
• 0016-more-attributes.md
• 0019-opt-in-builtin-traits.md
• 0066-better-temporary-lifetimes.md
• 0090-lexical-syntax-simplification.md
• 0107-pattern-guards-with-bind-by-move.md
• 0132-ufcs.md
• 0135-where.md
• 0141-lifetime-elision.md
• 0195-associated-items.md
• 0213-defaulted-type-params.md
• 0218-empty-struct-with-braces.md
• 0320-nonzeroing-dynamic-drop.md
• 0339-statically-sized-literals.md
• 0385-module-system-cleanup.md
• 0401-coercions.md
• 0447-no-unused-impl-parameters.md
• 0495-array-pattern-changes.md
• 0501-consistent_no_prelude_attributes.md
• 0509-collections-reform-part-2.md
• 0517-io-os-reform.md
• 0560-integer-overflow.md
• 0639-discriminant-intrinsic.md
• 0769-sound-generic-drop.md
• 0771-std-iter-once.md
• 0803-type-ascription.md
• 0809-box-and-in-for-stdlib.md
• 0888-compiler-fence-intrinsics.md
• 0909-move-thread-local-to-std-thread.md
• 0911-const-fn.md
• 0968-closure-return-type-syntax.md
• 0979-align-splitn-with-other-languages.md
• 1011-process.exit.md
• 1023-rebalancing-coherence.md
• 1040-duration-reform.md
• 1044-io-fs-2.1.md

Table of Contents

• Opening
• Active RFC List
• Table of Contents
• When you need to follow this process
• Before creating an RFC
• What the process is
• The role of the shepherd
• The RFC life-cycle
• Reviewing RFC's
• Implementing an RFC
• RFC Postponement
• Help this is all too informal!

When you need to follow this process

You need to follow this process if you intend to make "substantial" changes to Rust, Cargo, Crates.io, or the RFC process itself. What constitutes a "substantial" change is evolving based on community norms, but may include the following.

• Any semantic or syntactic change to the language that is not a bugfix.
• Removing language features, including those that are feature-gated.
• Changes to the interface between the compiler and libraries, including lang items and intrinsics.
• Additions to std

Some changes do not require an RFC:

• Rephrasing, reorganizing, refactoring, or otherwise "changing shape does not change meaning".
• Additions that strictly improve objective, numerical quality criteria (warning removal, speedup, better platform coverage, more parallelism, trap more errors, etc.)
• Additions only likely to be noticed by other developers-of-rust, invisible to users-of-rust.

If you submit a pull request to implement a new feature without going through the RFC process, it may be closed with a polite request to submit an RFC first.

Before creating an RFC

A hastily-proposed RFC can hurt its chances of acceptance. Low quality proposals, proposals for previously-rejected features, or those that don't fit into the near-term roadmap, may be quickly rejected, which can be demotivating for the unprepared contributor. Laying some groundwork ahead of the RFC can make the process smoother.

Although there is no single way to prepare for submitting an RFC, it is generally a good idea to pursue feedback from other project developers beforehand, to ascertain that the RFC may be desirable: having a consistent impact on the project requires concerted effort toward consensus-building.

The most common preparations for writing and submitting an RFC include talking the idea over on #rust-internals, filing and discusssing ideas on the RFC issue tracker, and occasionally posting 'pre-RFCs' on the developer discussion forum for early review.

As a rule of thumb, receiving encouraging feedback from long-standing project developers, and particularly members of the core team is a good indication that the RFC is worth pursuing.

What the process is

In short, to get a major feature added to Rust, one must first get the RFC merged into the RFC repo as a markdown file. At that point the RFC is 'active' and may be implemented with the goal of eventual inclusion into Rust.

• Fork the RFC repo http://github.com/rust-lang/rfcs
• Copy 0000-template.md to text/0000-my-feature.md (where 'my-feature' is descriptive. don't assign an RFC number yet).
• Fill in the RFC. Put care into the details: RFCs that do not present convincing motivation, demonstrate understanding of the impact of the design, or are disingenuous about the drawbacks or alternatives tend to be poorly-received.
• Submit a pull request. As a pull request the RFC will receive design feedback from the larger community, and the author should be prepared to revise it in response.
• During Rust triage, the pull request will either be closed (for RFCs that clearly will not be accepted) or assigned a shepherd. The shepherd is a trusted developer who is familiar with the process, who will help to move the RFC forward, and ensure that the right people see and review it.
• Build consensus and integrate feedback. RFCs that have broad support are much more likely to make progress than those that don't receive any comments. The shepherd assigned to your RFC should help you get feedback from Rust developers as well.
• The shepherd may schedule meetings with the author and/or relevant stakeholders to discuss the issues in greater detail, and in some cases the topic may be discussed at the larger weekly meeting. In either case a summary from the meeting will be posted back to the RFC pull request.
• Once both proponents and opponents have clarified and defended positions and the conversation has settled, the shepherd will take it to the core team for a final decision.
• Eventually, someone from the core team will either accept the RFC by merging the pull request, assigning the RFC a number (corresponding to the pull request number), at which point the RFC is 'active', or reject it by closing the pull request.

The role of the shepherd

During triage, every RFC will either be closed or assigned a shepherd. The role of the shepherd is to move the RFC through the process. This starts with simply reading the RFC in detail and providing initial feedback. The shepherd should also solicit feedback from people who are likely to have strong opinions about the RFC. Finally, when this feedback has been incorporated and the RFC seems to be in a steady state, the shepherd will bring it to the meeting. In general, the idea here is to "front-load" as much of the feedback as possible before the point where we actually reach a decision.

The RFC life-cycle

Once an RFC becomes active then authors may implement it and submit the feature as a pull request to the Rust repo. Being 'active' is not a rubber stamp, and in particular still does not mean the feature will ultimately be merged; it does mean that in principle all the major stakeholders have agreed to the feature and are amenable to merging it.

Furthermore, the fact that a given RFC has been accepted and is 'active' implies nothing about what priority is assigned to its implementation, nor does it imply anything about whether a Rust developer has been assigned the task of implementing the feature. While it is not necessary that the author of the RFC also write the implementation, it is by far the most effective way to see an RFC through to completion: authors should not expect that other project developers will take on responsibility for implementing their accepted feature.

Modifications to active RFC's can be done in followup PR's. We strive to write each RFC in a manner that it will reflect the final design of the feature; but the nature of the process means that we cannot expect every merged RFC to actually reflect what the end result will be at the time of the next major release; therefore we try to keep each RFC document somewhat in sync with the language feature as planned, tracking such changes via followup pull requests to the document.

An RFC that makes it through the entire process to implementation is considered 'complete' and is moved to the 'complete' folder; an RFC that fails after becoming active is 'inactive' and moves to the 'inactive' folder.

Reviewing RFC's

While the RFC PR is up, the shepherd may schedule meetings with the author and/or relevant stakeholders to discuss the issues in greater detail, and in some cases the topic may be discussed at the larger weekly meeting. In either case a summary from the meeting will be posted back to the RFC pull request.

The core team makes final decisions about RFCs after the benefits and drawbacks are well understood. These decisions can be made at any time, but the core team will regularly issue decisions on at least a weekly basis. When a decision is made, the RFC PR will either be merged or closed, in either case with a comment describing the rationale for the decision. The comment should largely be a summary of discussion already on the comment thread.

Implementing an RFC

Some accepted RFC's represent vital features that need to be implemented right away. Other accepted RFC's can represent features that can wait until some arbitrary developer feels like doing the work. Every accepted RFC has an associated issue tracking its implementation in the Rust repository; thus that associated issue can be assigned a priority via the triage process that the team uses for all issues in the Rust repository.

The author of an RFC is not obligated to implement it. Of course, the RFC author (like any other developer) is welcome to post an implementation for review after the RFC has been accepted.

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).

RFC Postponement

Some RFC pull requests are tagged with the 'postponed' label when they are closed (as part of the rejection process). An RFC closed with “postponed” is marked as such because we want neither to think about evaluating the proposal nor about implementing the described feature until after the next major release, and we believe that we can afford to wait until then to do so.

Usually an RFC pull request marked as “postponed” has already passed an informal first round of evaluation, namely the round of “do we think we would ever possibly consider making this change, as outlined in the RFC pull request, or some semi-obvious variation of it.” (When the answer to the latter question is “no”, then the appropriate response is to close the RFC, not postpone it.)

Help this is all too informal!

The process is intended to be as lightweight as reasonable for the present circumstances. As usual, we are trying to let the process be driven by consensus and community norms, not impose more structure than necessary.