Implementing new language features

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.

NOTE: This section is for language features, not library features, which use a different process.

See also the Rust Language Design Team's procedures for proposing changes to the language.

The @rfcbot FCP process

When the change is small, uncontroversial, non-breaking, and does not affect the stable language in any user-observable ways or add any new unstable features, then it can be done with just writing a PR and getting an r+ from someone who knows that part of the code. However, if not, more must be done. Even for compiler-internal work, it would be a bad idea to push a controversial change 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).

For changes that need the consensus of a team, we us the process of proposing a final comment period (FCP). If you're not on the relevant team (and thus don't have @rfcbot permissions), ask someone who is to start one; unless they have a concern themselves, they should.

The FCP process is only needed if you need consensus – if no processes require consensus for your change and you don't think anyone would have a problem with it, it's OK to rely on only an r+. For example, it is OK to add or modify unstable command-line flags or attributes in the reserved compiler-internal rustc_ namespace without an FCP for compiler development or standard library use, as long as you don't expect them to be in wide use in the nightly ecosystem. Some teams have lighter weight processes that they use in scenarios like this; for example, the compiler team recommends filing a Major Change Proposal (MCP) as a lightweight way to garner support and feedback without requiring full consensus.

You don't need to have the implementation fully ready for r+ to propose an FCP, 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.

When an FCP is proposed, it requires all members of the team to sign off on the FCP. After they all do so, there's a 10-day-long "final comment period" (hence the name) where everybody can comment, and if no concerns are raised, the PR/issue gets FCP approval.

The logistics of writing features

There are a few "logistical" 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'll 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 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 do not gain tenure by being unstable and unchanged for long periods of time.

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. When creating issues and PRs related to the feature, reference this tracking issue, and when there are updates about the feature's progress, post those to the tracking issue.

For features that are part of an accept RFC or approved lang experiment, use the tracking issue for that.

For other features, create a tracking issue for that feature. The issue title should be "Tracking issue for YOUR FEATURE". Use the "Tracking Issue" issue template.

Lang experiments

To land in the compiler, features that have user-visible effects on the language (even unstable ones) must either be part of an accepted RFC or an approved lang experiment.

To propose a new lang experiment, open an issue in rust-lang/rust that describes the motivation and the intended solution. If it's accepted, this issue will become the tracking issue for the experiment, so use the tracking issue template while also including these other details. Nominate the issue for the lang team and CC @rust-lang/lang and @rust-lang/lang-advisors. When the experiment is approved, the tracking issue will be marked as B-experimental.

Feature flags related to a lang experiment must be marked as incomplete until an RFC is accepted for the feature.

Stability in code

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

Open or identify the tracking issue. For features that are part of an accept RFC or approved lang experiment, use the tracking issue for that.

Label the tracking issue with C-tracking-issue and the relevant F-feature_name label (adding that label if needed).
Pick a name for the feature gate (for RFCs, use the name in the RFC).
Add the feature name to rustc_span/src/symbol.rs in the Symbols {...} block.

Note that this block must be in alphabetical order.
Add a feature gate declaration to rustc_feature/src/unstable.rs in the unstable declare_features block.
```
/// description of feature
(unstable, $feature_name, "CURRENT_RUSTC_VERSION", Some($tracking_issue_number))
```
If you haven't yet opened a tracking issue (e.g. because you want initial feedback on whether the feature is likely to be accepted), you can temporarily use None - but make sure to update it before the PR is merged!

For example:
```
/// Allows defining identifiers beyond ASCII.
(unstable, non_ascii_idents, "CURRENT_RUSTC_VERSION", Some(55467), None),
```
Features can be marked as incomplete, and trigger the warn-by-default incomplete_features lint by setting their type to incomplete:
```
/// Allows deref patterns.
(incomplete, deref_patterns, "CURRENT_RUSTC_VERSION", Some(87121), None),
```
Feature flags related to a lang experiment must be marked as incomplete until an RFC is accepted for the feature.

To avoid semantic merge conflicts, use CURRENT_RUSTC_VERSION instead of 1.70 or another explicit version number.
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().

If the feature gate is not set, you should either maintain the pre-feature behavior or raise an error, depending on what makes sense. Errors should generally use rustc_session::parse::feature_err. For an example of adding an error, see #81015.

For features introducing new syntax, pre-expansion gating should be used instead. During parsing, when the new syntax is parsed, the symbol must be inserted to the current crate's GatedSpans via self.sess.gated_span.gate(sym::my_feature, span).

After being inserted to the gated spans, the span must be checked in the rustc_ast_passes::feature_gate::check_crate function, which actually denies features. Exactly how it is gated depends on the exact type of feature, but most likely will use the gate_all!() macro.
Add a test to ensure the feature cannot be used without a feature gate, by creating tests/ui/feature-gates/feature-gate-$feature_name.rs. You can generate the corresponding .stderr file by running ./x test tests/ui/feature-gates/ --bless.
Add a section to the unstable book, in src/doc/unstable-book/src/language-features/$feature_name.md.
Write a lot of tests for the new feature, preferably in tests/ui/$feature_name/. PRs without tests will not be accepted!
Get your PR reviewed and land it. You have now successfully implemented a feature in Rust!

Call for testing

Once the implementation is complete, the feature will be available to nightly users but not yet part of stable Rust. This is a good time to write a blog post on the main Rust blog and issue a "call for testing".

Some earlier such blog posts include:

Alternatively, This Week in Rust has a section for this. One example of this having been used is:

Call for testing on boolean literals as cfg predicates

Which option to choose might depend on how significant the language change is, though note that the This Week in Rust section might be less visible than a dedicated post on the main Rust blog.

Polishing

Giving users a polished experience means more than just implementing the feature in rustc. We need to think about all of the tools and resources that we ship. This work includes:

Documenting the language feature in the Rust Reference.
Extending rustfmt to format any new syntax (if applicable).
Extending rust-analyzer (if applicable). The extent of this work can depend on the nature of the language feature, as some features don't need to be blocked on full support.
- When a language feature degrades the user experience simply by existing before support is implemented in rust-analyzer, that may lead the lang team to raise a blocking concern.
- Examples of such might include new syntax that rust-analyzer can't parse or type inference changes it doesn't understand when those lead to bogus diagnostics.

Stabilization

The final step in the feature lifecycle is stabilization, which is when the feature becomes available to all Rust users. At this point, backward incompatible changes are generally no longer permitted (see the lang team's defined semver policies for details). To learn more about stabilization, see the stabilization guide.

Rust Compiler Development Guide