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RFCs for changes to Rust

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Quick Overview

The rust-lang/rfcs repository is the official place for proposing, discussing, and deciding on new features and major changes to the Rust programming language. It contains a collection of Request for Comments (RFCs) documents that outline proposed changes, their rationale, and implementation details.

Pros

  • Provides a transparent and structured process for evolving the Rust language
  • Encourages community participation and feedback in language development
  • Maintains a historical record of language design decisions
  • Ensures thorough consideration of new features before implementation

Cons

  • Can be a slow process, potentially delaying important language improvements
  • May be intimidating for newcomers to contribute due to the formal nature of RFCs
  • Requires significant effort to write and review RFCs, which can be time-consuming
  • Some proposals may get stuck in lengthy discussions without reaching a conclusion

Competitor Comparisons

rust-lang logo

rust

96,644

Empowering everyone to build reliable and efficient software.

Pros of rust

  • Contains the actual Rust compiler and standard library implementation
  • Allows direct contributions to the Rust language and core tools
  • Provides a comprehensive test suite for Rust's functionality

Cons of rust

  • Larger codebase with higher complexity, making it harder to contribute
  • Requires more technical expertise to make meaningful contributions
  • Longer review and merge process due to the critical nature of the code

Code comparison

rust:

pub fn compile_crate(sess: &Session, crate_name: &str) -> Result<(), ErrorReported> {
    // Actual compiler implementation
}

rfcs:

# Feature Name: (fill me in with a unique ident, `my_awesome_feature`)
# Start Date: (fill me in with today's date, YYYY-MM-DD)
# RFC PR: (leave this empty)
# Rust Issue: (leave this empty)

# Summary
[summary]: #summary

One paragraph explanation of the feature.

Summary

The rust repository contains the actual Rust compiler and standard library implementation, allowing direct contributions to the language and core tools. It provides a comprehensive test suite but has a larger, more complex codebase requiring more technical expertise to contribute.

In contrast, the rfcs repository is focused on proposing and discussing new features and changes to the Rust language. It has a simpler structure, making it easier for a wider range of contributors to participate in the language's evolution through RFC submissions and discussions.

rust-lang logo

cargo

12,551

The Rust package manager

Pros of Cargo

  • Practical implementation of Rust's package manager and build system
  • Directly impacts developers' daily workflow and project management
  • Actively maintained with frequent updates and bug fixes

Cons of Cargo

  • More focused scope compared to the broader impact of RFCs
  • May require more technical expertise to contribute effectively
  • Changes can have immediate consequences on the Rust ecosystem

Code Comparison

Cargo (Rust code):

pub fn build_config(
    ws: &Workspace<'_>,
    mode: CompileMode,
    target: Option<&str>,
    options: &CompileOptions,
) -> CargoResult<BuildConfig> {
    // ...
}

RFCs (Markdown):

# Feature Name: (fill me in with a unique ident, `my_awesome_feature`)
# Start Date: (fill me in with today's date, YYYY-MM-DD)
# RFC PR: (leave this empty)
# Rust Issue: (leave this empty)

# Summary
[summary]: #summary

One paragraph explanation of the feature.

Summary

Cargo is the practical implementation of Rust's package management system, while RFCs focus on the language's design and evolution. Cargo directly impacts developers' daily work, offering frequent updates and improvements. However, it has a narrower scope compared to RFCs and may require more technical expertise for contributions. RFCs, on the other hand, have a broader impact on the Rust ecosystem but may take longer to implement. The code comparison shows Cargo's Rust implementation versus the RFC template in Markdown format.

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book

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The Rust Programming Language

Pros of book

  • More accessible for beginners and those learning Rust
  • Provides comprehensive, structured learning material
  • Includes practical examples and exercises

Cons of book

  • Less frequently updated compared to RFCs
  • May not cover the latest Rust features or changes as quickly
  • Focuses on established concepts rather than proposed changes

Code comparison

book:

fn main() {
    println!("Hello, world!");
}

rfcs:

pub struct RFC {
    number: u32,
    title: String,
    status: RFCStatus,
}

Summary

The book repository contains the source for "The Rust Programming Language" book, offering a comprehensive guide for learning Rust. It's ideal for beginners and those seeking structured learning material. However, it may not always reflect the latest changes in the language.

The rfcs repository is where Rust's design process takes place through Request for Comments (RFCs). It's more technical and focuses on proposed changes and new features. While it's frequently updated, it's less accessible for newcomers to the language.

The code examples illustrate the difference in focus: book shows a simple "Hello, world!" program, while rfcs demonstrates a more complex structure related to the RFC process itself.

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rustlings

52,586

:crab: Small exercises to get you used to reading and writing Rust code!

Pros of rustlings

  • Hands-on learning experience with interactive exercises
  • Gradual progression from basic to advanced Rust concepts
  • Immediate feedback through automated tests

Cons of rustlings

  • Limited scope compared to the broader discussions in RFCs
  • May not cover cutting-edge language features or proposals
  • Less comprehensive for understanding Rust's design decisions

Code comparison

rustlings:

fn main() {
    let x = 5;
    println!("x has the value {}", x);
}

RFCs:

# Feature Name: `my_feature`

- Start Date: (fill me in with today's date, YYYY-MM-DD)
- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)

Summary

rustlings is an interactive learning tool for Rust beginners, offering hands-on exercises with immediate feedback. It's excellent for practical learning but may not cover advanced language design topics.

RFCs, on the other hand, is a repository for proposing and discussing new features and changes to the Rust language. It provides in-depth insights into Rust's evolution but lacks the interactive learning aspect of rustlings.

While rustlings focuses on code examples and exercises, RFCs primarily contain detailed textual proposals and discussions about language design and implementation.

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rustfmt

5,953

Format Rust code

Pros of rustfmt

  • Directly impacts code quality and consistency
  • Provides immediate, practical benefits to developers
  • Easier to contribute to for those with formatting expertise

Cons of rustfmt

  • Narrower scope compared to RFCs
  • Less influence on the overall direction of Rust
  • May lead to occasional disagreements about specific formatting rules

Code Comparison

rustfmt:

fn main() {
    println!("Hello, world!");
}

RFCs:

# Feature Name: (fill me in with a unique ident, `my_awesome_feature`)
# Start Date: (fill me in with today's date, YYYY-MM-DD)
# RFC PR: (leave this empty)
# Rust Issue: (leave this empty)

Summary

RFCs is focused on proposing and discussing changes to the Rust language and ecosystem, while rustfmt is a tool for automatically formatting Rust code. RFCs has a broader scope and more significant impact on Rust's evolution, but rustfmt provides immediate practical benefits to developers. RFCs requires more in-depth knowledge of Rust and its ecosystem, while rustfmt is more accessible for contributors interested in code formatting. Both projects play essential roles in the Rust community, with RFCs shaping the language's future and rustfmt ensuring consistent code style across projects.

rust-lang logo

rust-analyzer

14,054

A Rust compiler front-end for IDEs

Pros of rust-analyzer

  • Provides real-time code analysis and IDE support for Rust
  • Offers a more interactive and immediate development experience
  • Continuously updated to support the latest Rust features and syntax

Cons of rust-analyzer

  • More complex codebase, requiring deeper technical knowledge to contribute
  • Focuses on tooling rather than language design and evolution
  • May have a steeper learning curve for new contributors

Code Comparison

rfcs:

# Feature Name: (fill me in with a unique ident, `my_awesome_feature`)
# Start Date: (fill me in with today's date, YYYY-MM-DD)
# RFC PR: (leave this empty)
# Rust Issue: (leave this empty)

rust-analyzer:

pub(crate) fn analyze(
    db: &dyn HirDatabase,
    file_id: FileId,
) -> Arc<InferenceResult> {
    db.infer(file_id)
}

The rfcs repository contains RFC (Request for Comments) documents in Markdown format, while rust-analyzer contains Rust code for implementing language analysis features. The code snippets demonstrate the different focus areas of these projects: language design documentation versus tooling implementation.

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README

Rust RFCs - RFC Book - Active RFC List

The "RFC" (request for comments) process is intended to provide a consistent and controlled path for changes to Rust (such as new features) so that all stakeholders can be confident about the direction of the project.

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 sub-teams.

Table of Contents

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 and varies depending on what part of the ecosystem you are proposing to change, 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.

Sub-team specific guidelines

For more details on when an RFC is required for the following areas, please see the Rust community's sub-team specific guidelines for:

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 our official Zulip server, discussing the topic on our developer discussion forum, and occasionally posting "pre-RFCs" on the developer forum. You may file issues on this repo for discussion, but these are not actively looked at by the teams.

As a rule of thumb, receiving encouraging feedback from long-standing project developers, and particularly members of the relevant sub-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 repository 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 RFC repository
  • Copy 0000-template.md to text/0000-my-feature.md (where "my-feature" is descriptive). Don't assign an RFC number yet; This is going to be the PR number and we'll rename the file accordingly if the RFC is accepted.
  • Fill in the RFC. Put care into the details: RFCs that do not present convincing motivation, demonstrate lack of understanding of the design's impact, 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.
  • Now that your RFC has an open pull request, use the issue number of the PR to rename the file: update your 0000- prefix to that number. Also update the "RFC PR" link at the top of the file.
  • Each pull request will be labeled with the most relevant sub-team, which will lead to its being triaged by that team in a future meeting and assigned to a member of the subteam.
  • 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. Feel free to reach out to the RFC assignee in particular to get help identifying stakeholders and obstacles.
  • The sub-team will discuss the RFC pull request, as much as possible in the comment thread of the pull request itself. Offline discussion will be summarized on the pull request comment thread.
  • RFCs rarely go through this process unchanged, especially as alternatives and drawbacks are shown. You can make edits, big and small, to the RFC to clarify or change the design, but make changes as new commits to the pull request, and leave a comment on the pull request explaining your changes. Specifically, do not squash or rebase commits after they are visible on the pull request.
  • At some point, a member of the subteam will propose a "motion for final comment period" (FCP), along with a disposition for the RFC (merge, close, or postpone).
    • This step is taken when enough of the tradeoffs have been discussed that the subteam is in a position to make a decision. That does not require consensus amongst all participants in the RFC thread (which is usually impossible). However, the argument supporting the disposition on the RFC needs to have already been clearly articulated, and there should not be a strong consensus against that position outside of the subteam. Subteam members use their best judgment in taking this step, and the FCP itself ensures there is ample time and notification for stakeholders to push back if it is made prematurely.
    • For RFCs with lengthy discussion, the motion to FCP is usually preceded by a summary comment trying to lay out the current state of the discussion and major tradeoffs/points of disagreement.
    • Before actually entering FCP, all members of the subteam must sign off; this is often the point at which many subteam members first review the RFC in full depth.
  • The FCP lasts ten calendar days, so that it is open for at least 5 business days. It is also advertised widely, e.g. in This Week in Rust. This way all stakeholders have a chance to lodge any final objections before a decision is reached.
  • In most cases, the FCP period is quiet, and the RFC is either merged or closed. However, sometimes substantial new arguments or ideas are raised, the FCP is canceled, and the RFC goes back into development mode.

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" RFCs can be done in follow-up pull requests. 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.

In general, once accepted, RFCs should not be substantially changed. Only very minor changes should be submitted as amendments. More substantial changes should be new RFCs, with a note added to the original RFC. Exactly what counts as a "very minor change" is up to the sub-team to decide; check Sub-team specific guidelines for more details.

Reviewing RFCs

While the RFC pull request is up, the sub-team 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 a sub-team meeting. In either case a summary from the meeting will be posted back to the RFC pull request.

A sub-team makes final decisions about RFCs after the benefits and drawbacks are well understood. These decisions can be made at any time, but the sub-team will regularly issue decisions. When a decision is made, the RFC pull request will either be merged or closed. In either case, if the reasoning is not clear from the discussion in thread, the sub-team will add a comment describing the rationale for the decision.

Implementing an RFC

Some accepted RFCs represent vital features that need to be implemented right away. Other accepted RFCs 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 some time in the future, and we believe that we can afford to wait until then to do so. Historically, "postponed" was used to postpone features until after 1.0. Postponed pull requests may be re-opened when the time is right. We don't have any formal process for that, you should ask members of the relevant sub-team.

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.

License

This repository is currently in the process of being licensed under either of:

at your option. Some parts of the repository are already licensed according to those terms. For more see RFC 2044 and its tracking issue.

Contributions

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.