rfcs/README.md

# Rust RFCs
[Rust RFCs]: #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 [sub-team]s.

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
[Active RFC List]: #active-rfc-list

* [0016-more-attributes.md](text/0016-more-attributes.md)
* [0019-opt-in-builtin-traits.md](text/0019-opt-in-builtin-traits.md)
* [0066-better-temporary-lifetimes.md](text/0066-better-temporary-lifetimes.md)
* [0090-lexical-syntax-simplification.md](text/0090-lexical-syntax-simplification.md)
* [0107-pattern-guards-with-bind-by-move.md](text/0107-pattern-guards-with-bind-by-move.md)
* [0132-ufcs.md](text/0132-ufcs.md)
* [0135-where.md](text/0135-where.md)
* [0141-lifetime-elision.md](text/0141-lifetime-elision.md)
* [0195-associated-items.md](text/0195-associated-items.md)
* [0213-defaulted-type-params.md](text/0213-defaulted-type-params.md)
* [0218-empty-struct-with-braces.md](text/0218-empty-struct-with-braces.md)
* [0320-nonzeroing-dynamic-drop.md](text/0320-nonzeroing-dynamic-drop.md)
* [0339-statically-sized-literals.md](text/0339-statically-sized-literals.md)
* [0385-module-system-cleanup.md](text/0385-module-system-cleanup.md)
* [0401-coercions.md](text/0401-coercions.md)
* [0447-no-unused-impl-parameters.md](text/0447-no-unused-impl-parameters.md)
* [0495-array-pattern-changes.md](text/0495-array-pattern-changes.md)
* [0501-consistent_no_prelude_attributes.md](text/0501-consistent_no_prelude_attributes.md)
* [0509-collections-reform-part-2.md](text/0509-collections-reform-part-2.md)
* [0517-io-os-reform.md](text/0517-io-os-reform.md)
* [0560-integer-overflow.md](text/0560-integer-overflow.md)
* [0639-discriminant-intrinsic.md](text/0639-discriminant-intrinsic.md)
* [0769-sound-generic-drop.md](text/0769-sound-generic-drop.md)
* [0771-std-iter-once.md](text/0771-std-iter-once.md)
* [0803-type-ascription.md](text/0803-type-ascription.md)
* [0809-box-and-in-for-stdlib.md](text/0809-box-and-in-for-stdlib.md)
* [0873-type-macros.md](text/0873-type-macros.md)
* [0888-compiler-fence-intrinsics.md](text/0888-compiler-fence-intrinsics.md)
* [0909-move-thread-local-to-std-thread.md](text/0909-move-thread-local-to-std-thread.md)
* [0911-const-fn.md](text/0911-const-fn.md)
* [0968-closure-return-type-syntax.md](text/0968-closure-return-type-syntax.md)
* [0980-read-exact.md](text/0980-read-exact.md)
* [0982-dst-coercion.md](text/0982-dst-coercion.md)
* [0979-align-splitn-with-other-languages.md](text/0979-align-splitn-with-other-languages.md)
* [1011-process.exit.md](text/1011-process.exit.md)
* [1023-rebalancing-coherence.md](text/1023-rebalancing-coherence.md)
* [1040-duration-reform.md](text/1040-duration-reform.md)
* [1044-io-fs-2.1.md](text/1044-io-fs-2.1.md)
* [1066-safe-mem-forget.md](text/1066-safe-mem-forget.md)
* [1096-remove-static-assert.md](text/1096-remove-static-assert.md)
* [1122-language-semver.md](text/1122-language-semver.md)
* [1131-likely-intrinsic.md](text/1131-likely-intrinsic.md)
* [1156-adjust-default-object-bounds.md](text/1156-adjust-default-object-bounds.md)
* [1184-stabilize-no_std.md](text/1184-stabilize-no_std.md)
* [1214-projections-lifetimes-and-wf.md](text/1214-projections-lifetimes-and-wf.md)
* [1219-use-group-as.md](text/1219-use-group-as.md)

## Table of Contents
[Table of Contents]: #table-of-contents
* [Opening](#rust-rfcs)
* [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
[When you need to follow this process]: #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.

For more details on when an RFC is required, please see the following specific
guidelines, these correspond with some of the Rust community's
[sub-teams](http://www.rust-lang.org/team.html):

* [language changes](lang_changes.md),
* [library changes](libs_changes.md),
* [compiler changes](compiler_changes.md).


## Before creating an RFC
[Before creating an RFC]: #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][issues], and occasionally posting
'pre-RFCs' on [the developer discussion forum][discuss] for early
review.

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.

[issues]: https://github.com/rust-lang/rfcs/issues
[discuss]: http://discuss.rust-lang.org/


## What the process is
[What the process is]: #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.
* Each pull request will be labeled with the most relevant [sub-team].
* Each sub-team triages its RFC PRs. The sub-team will will either close the PR
(for RFCs that clearly will not be accepted) or assign it a *shepherd*. The
shepherd is a trusted developer who is familiar with the RFC 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.
* The sub-team will discuss the RFC PR, as much as possible in the comment
thread of the PR itself. Offline discussion will be summarized on the PR comment
thread.
* Once both proponents and opponents have clarified and defended positions and
the conversation has settled, the RFC will enter its *final comment period*
(FCP). This is a final opportunity for the community to comment on the PR and is
a reminder for all members of the sub-team to be aware of the RFC.
* The FCP lasts one week. It may be extended if consensus between sub-team
members cannot be reached. At the end of the FCP,  the [sub-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. How exactly the sub-team decide on an
RFC is up to the sub-team.


## The role of the shepherd
[The role of the shepherd]: #the-role-of-the-shepherd

During triage, every RFC will either be closed or assigned a shepherd from the
relevant sub-team. 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. When this feedback has been incorporated
and the RFC seems to be in a steady state, the shepherd and/or sub-team leader
will announce an FCP. 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 -
by the end of the FCP, the decision on whether or not to accept the RFC should
usually be obvious from the RFC discussion thread. On occasion, there may not be
consensus but discussion has stalled. In this case, the relevant team will make
a decision.


## The RFC life-cycle
[The RFC life-cycle]: #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 follow-up 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.

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. There are some more
specific guidelines in the sub-team RFC guidelines for the [language](lang_changes.md),
[libraries](libs_changes.md), and [compiler](compiler_changes.md).


## Reviewing RFC's
[Reviewing RFC's]: #reviewing-rfcs

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 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 PR 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
[Implementing an RFC]: #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
[RFC Postponement]: #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 PRs 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!
[Help this is all too informal!]: #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.

[sub-team]: http://www.rust-lang.org/team.html