Implement New Feature

When you want to implement a new significant feature in the compiler, you need to go through this process to make sure everything goes smoothly.

The @rfcbot (p)FCP process

When the change is small and uncontroversial, then it can be done with just writing a PR and getting r+ from someone who knows that part of the code. However, if the change is potentially controversial, it would be a bad idea to push it without consensus from the rest of the team (both in the "distributed system" sense to make sure you don't break anything you don't know about, and in the social sense to avoid PR fights).

If such a change seems to be too small to require a full formal RFC process (e.g. a big refactoring of the code, or a "technically-breaking" change, or a "big bugfix" that basically amounts to a small feature) but is still too controversial or big to get by with a single r+, you can start a pFCP (or, if you don't have r+ rights, ask someone who has them to start one - and unless they have a concern themselves, they should).

Again, the pFCP process is only needed if you need consensus - if you don't think anyone would have a problem with your change, it's ok to get by with only an r+. For example, it is OK to add or modify unstable command-line flags or attributes without an pFCP for compiler development or standard library use, as long as you don't expect them to be in wide use in the nightly ecosystem.

You don't need to have the implementation fully ready for r+ to ask for a pFCP, but it is generally a good idea to have at least a proof of concept so that people can see what you are talking about.

That starts a "proposed final comment period" (pFCP), which requires all members of the team to sign off the FCP. After they all do so, there's a 10 day long "final comment period" where everybody can comment, and if no new concerns are raised, the PR/issue gets FCP approval.

The logistics of writing features

There are a few "logistic" hoops you might need to go through in order to implement a feature in a working way.

Warning Cycles

In some cases, a feature or bugfix might break some existing programs in some edge cases. In that case, you might want to do a crater run to assess the impact and possibly add a future-compatibility lint, similar to those used for edition-gated lints.

Stability

We value the stability of Rust. Code that works and runs on stable should (mostly) not break. Because of that, we don't want to release a feature to the world with only team consensus and code review - we want to gain real-world experience on using that feature on nightly, and we might want to change the feature based on that experience.

To allow for that, we must make sure users don't accidentally depend on that new feature - otherwise, especially if experimentation takes time or is delayed and the feature takes the trains to stable, it would end up de facto stable and we'll not be able to make changes in it without breaking people's code.

The way we do that is that we make sure all new features are feature gated - they can't be used without a enabling a feature gate (#[feature(foo)]), which can't be done in a stable/beta compiler. See the stability in code section for the technical details.

Eventually, after we gain enough experience using the feature, make the necessary changes, and are satisfied, we expose it to the world using the stabilization process described here. Until then, the feature is not set in stone: every part of the feature can be changed, or the feature might be completely rewritten or removed. Features are not supposed to gain tenure by being unstable and unchanged for a year.

Tracking Issues

To keep track of the status of an unstable feature, the experience we get while using it on nightly, and of the concerns that block its stabilization, every feature-gate needs a tracking issue.

General discussions about the feature should be done on the tracking issue.

For features that have an RFC, you should use the RFC's tracking issue for the feature.

For other features, you'll have to make a tracking issue for that feature. The issue title should be "Tracking issue for YOUR FEATURE".

For tracking issues for features (as opposed to future-compat warnings), I don't think the description has to contain anything specific. Generally we put the list of items required for stabilization using a github list, e.g.

    **Steps:**

    - [ ] Implement the RFC (cc @rust-lang/compiler -- can anyone write
          up mentoring instructions?)
    - [ ] Adjust documentation ([see instructions on forge][doc-guide])
    - Note: no stabilization step here.

Stability in code

The below steps needs to be followed in order to implement a new unstable feature:

  1. Open a tracking issue - if you have an RFC, you can use the tracking issue for the RFC.

    The tracking issue should be labeled with at least C-tracking-issue. For a language feature, a label F-feature_name should be added as well.

  2. Pick a name for the feature gate (for RFCs, use the name in the RFC).

  3. Add a feature gate declaration to libsyntax/active.rs in the active declare_features block:

    // description of feature
    (active, $feature_name, "$current_nightly_version", Some($tracking_issue_number), $edition)

where $edition has the type Option<Edition>, and is typically just None.

For example:

    // allow '|' at beginning of match arms (RFC 1925)
(   active, match_beginning_vert, "1.21.0", Some(44101), None),

When added, the current version should be the one for the current nightly. Once the feature is moved to accepted.rs, the version is changed to that nightly version.

  1. Prevent usage of the new feature unless the feature gate is set. You can check it in most places in the compiler using the expression tcx.features().$feature_name (or sess.features_untracked().$feature_name if the tcx is unavailable)

    If the feature gate is not set, you should either maintain the pre-feature behavior or raise an error, depending on what makes sense.

    For features introducing new syntax, pre-expansion gating should be used instead. To do so, extend the GatedSpans struct, add spans to it during parsing, and then finally feature-gate all the spans in feature_gate::check::check_crate.

  2. Add a test to ensure the feature cannot be used without a feature gate, by creating feature-gate-$feature_name.rs and feature-gate-$feature_name.stderr files under the directory where the other tests for your feature reside.

  3. Add a section to the unstable book, in src/doc/unstable-book/src/language-features/$feature_name.md.

  4. Write a lots of tests for the new feature. PRs without tests will not be accepted!

  5. Get your PR reviewed and land it. You have now successfully implemented a feature in Rust!