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| <h1 id="implementing-new-language-features"><a class="header" href="#implementing-new-language-features">Implementing new language features</a></h1> |
| <p>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.</p> |
| <p><strong>NOTE: This section is for <em>language</em> features, not <em>library</em> features, |
| which use <a href="./stability.html">a different process</a>.</strong></p> |
| <p>See also <a href="https://lang-team.rust-lang.org/how_to/propose.html">the Rust Language Design Team’s procedures</a> for proposing changes to the language.</p> |
| <h2 id="the-rfcbot-fcp-process"><a class="header" href="#the-rfcbot-fcp-process">The @rfcbot FCP process</a></h2> |
| <p>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).</p> |
| <p>For changes that need the consensus of a team, |
| we use 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.</p> |
| <p>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 <code>rustc_</code> 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 (<a href="https://forge.rust-lang.org/compiler/proposals-and-stabilization.html#how-do-i-submit-an-mcp">MCP</a>) |
| as a lightweight way to garner support and feedback without requiring full consensus.</p> |
| <p>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.</p> |
| <p>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.</p> |
| <h2 id="the-logistics-of-writing-features"><a class="header" href="#the-logistics-of-writing-features">The logistics of writing features</a></h2> |
| <p>There are a few “logistical” hoops you might need to go through |
| in order to implement a feature in a working way.</p> |
| <h3 id="warning-cycles"><a class="header" href="#warning-cycles">Warning Cycles</a></h3> |
| <p>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 <a href="diagnostics.html#edition-gated-lints">edition-gated lints</a>.</p> |
| <h3 id="stability"><a class="header" href="#stability">Stability</a></h3> |
| <p>We <a href="https://github.com/rust-lang/rfcs/blob/master/text/1122-language-semver.md">value the stability of Rust</a>. |
| 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.</p> |
| <p>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.</p> |
| <p>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 (<code>#[feature(foo)]</code>), |
| which can’t be done in a stable/beta compiler. |
| See the <a href="#stability-in-code">stability in code</a> section for the technical details.</p> |
| <p>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 <a href="./stabilization-guide.html">here</a>. |
| 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.</p> |
| <h3 id="tracking-issues"><a class="header" href="#tracking-issues">Tracking Issues</a></h3> |
| <p>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.</p> |
| <p>For features that are part of an accept RFC or approved lang experiment, |
| use the tracking issue for that.</p> |
| <p>For other features, create a tracking issue for that feature. |
| The issue title should be “Tracking issue for YOUR FEATURE”. |
| Use the <a href="https://github.com/rust-lang/rust/issues/new?template=tracking_issue.md">“Tracking Issue” issue template</a>.</p> |
| <h3 id="lang-experiments"><a class="header" href="#lang-experiments">Lang experiments</a></h3> |
| <p>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 <a href="https://lang-team.rust-lang.org/how_to/experiment.html">lang experiment</a>.</p> |
| <p>To propose a new lang experiment, |
| open an issue in <code>rust-lang/rust</code> 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 <a href="https://github.com/rust-lang/rust/issues/new?template=tracking_issue.md">template</a> while also including these other details. |
| Nominate the issue for the lang team and CC <code>@rust-lang/lang</code> and <code>@rust-lang/lang-advisors</code>. |
| When the experiment is approved, the tracking issue will be marked as <code>B-experimental</code>.</p> |
| <p>Feature flags related to a lang experiment must be marked as <code>incomplete</code> |
| until an RFC is accepted for the feature.</p> |
| <h2 id="stability-in-code"><a class="header" href="#stability-in-code">Stability in code</a></h2> |
| <p>The below steps needs to be followed in order to implement a new unstable feature:</p> |
| <ol> |
| <li> |
| <p>Open or identify the <a href="#tracking-issues">tracking issue</a>. |
| For features that are part of an accept RFC or approved lang experiment, |
| use the tracking issue for that.</p> |
| <p>Label the tracking issue with <code>C-tracking-issue</code> and the relevant <code>F-feature_name</code> label |
| (adding that label if needed).</p> |
| </li> |
| <li> |
| <p>Pick a name for the feature gate (for RFCs, use the name in the RFC).</p> |
| </li> |
| <li> |
| <p>Add the feature name to <code>rustc_span/src/symbol.rs</code> in the <code>Symbols {...}</code> block.</p> |
| <p>Note that this block must be in alphabetical order.</p> |
| </li> |
| <li> |
| <p>Add a feature gate declaration to <code>rustc_feature/src/unstable.rs</code> |
| in the unstable <code>declare_features</code> block.</p> |
| <pre><code class="language-rust ignore">/// description of feature |
| (unstable, $feature_name, "CURRENT_RUSTC_VERSION", Some($tracking_issue_number))</code></pre> |
| <p>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 <code>None</code> - but make sure to update it before the PR is merged!</p> |
| <p>For example:</p> |
| <pre><code class="language-rust ignore">/// Allows defining identifiers beyond ASCII. |
| (unstable, non_ascii_idents, "CURRENT_RUSTC_VERSION", Some(55467)),</code></pre> |
| <p>Features can be marked as incomplete, |
| and trigger the warn-by-default <a href="https://doc.rust-lang.org/rustc/lints/listing/warn-by-default.html#incomplete-features"><code>incomplete_features</code> lint</a> |
| by setting their type to <code>incomplete</code>:</p> |
| <pre><code class="language-rust ignore">/// Allows deref patterns. |
| (incomplete, deref_patterns, "CURRENT_RUSTC_VERSION", Some(87121)),</code></pre> |
| <p>Feature flags related to a lang experiment must be marked as <code>incomplete</code> |
| until an RFC is accepted for the feature.</p> |
| <p>To avoid <a href="https://bors.tech/essay/2017/02/02/pitch/">semantic merge conflicts</a>, |
| use <code>CURRENT_RUSTC_VERSION</code> instead of <code>1.70</code> or another explicit version number.</p> |
| </li> |
| <li> |
| <p>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 <code>tcx.features().$feature_name()</code>.</p> |
| <p>If the feature gate is not set, |
| you should either maintain the pre-feature behavior or raise an error, |
| depending on what makes sense. |
| Errors should generally use <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_session/errors/fn.feature_err.html"><code>rustc_session::errors::feature_err</code></a>. |
| For an example of adding an error, see <a href="https://github.com/rust-lang/rust/pull/81015">#81015</a>.</p> |
| <p>For features introducing new syntax, pre-expansion gating should be used instead. |
| During parsing, when the new syntax is parsed, |
| the symbol must be inserted to the current crate’s <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_session/parse/struct.GatedSpans.html"><code>GatedSpans</code></a> |
| via <code>self.psess.gated_spans.gate(sym::my_feature, span)</code>.</p> |
| <p>After being inserted to the gated spans, |
| the span must be checked in the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast_passes/feature_gate/fn.check_crate.html"><code>rustc_ast_passes::feature_gate::check_crate</code></a> function, |
| which actually denies features. |
| Exactly how it is gated depends on the exact type of feature, |
| but most likely will use the <code>gate_all!()</code> macro.</p> |
| </li> |
| <li> |
| <p>Add a test to ensure the feature cannot be used without a feature gate, |
| by creating <code>tests/ui/feature-gates/feature-gate-$feature_name.rs</code>. |
| You can generate the corresponding <code>.stderr</code> file |
| by running <code>./x test tests/ui/feature-gates/ --bless</code>.</p> |
| </li> |
| <li> |
| <p>Add a section to the unstable book, |
| in <code>src/doc/unstable-book/src/language-features/$feature_name.md</code>.</p> |
| </li> |
| <li> |
| <p>Write a lot of tests for the new feature, preferably in <code>tests/ui/$feature_name/</code>. |
| PRs without tests will not be accepted!</p> |
| </li> |
| <li> |
| <p>Get your PR reviewed and land it. |
| You have now successfully implemented a feature in Rust!</p> |
| </li> |
| </ol> |
| <h2 id="adding-unstable-compiler-flags"><a class="header" href="#adding-unstable-compiler-flags">Adding unstable compiler flags</a></h2> |
| <p>For compiler-internal experiments, diagnostics, debugging aids, and temporary implementation |
| controls, a <code>-Z</code> flag can be more appropriate than a language feature gate. |
| Feature gates should still be preferred for user-facing language features, |
| especially when crates need to opt in from source code.</p> |
| <p>Before adding a new flag, check the compiler team’s <a href="https://forge.rust-lang.org/compiler/proposals-and-stabilization.html?highlight=unstable%20flag#compiler-flags">guidance for compiler flags</a>. |
| Some flags, especially those intended for direct nightly user workflows or broader |
| ecosystem use, may need additional sign-off beyond an r+ on the implementation PR.</p> |
| <p>When adding a new <code>-Z</code> flag:</p> |
| <ol> |
| <li> |
| <p>Add the option to <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_session/options/struct.UnstableOptions.html"><code>UnstableOptions</code></a> in |
| <code>compiler/rustc_session/src/options.rs</code>. |
| The option name is written as snake_case in the struct and is exposed as |
| kebab-case on the command line. |
| Choose the parser, default value, and dependency tracking marker carefully. |
| Use <code>[UNTRACKED]</code> for flags that only affect diagnostics or debugging output, |
| and use <code>[TRACKED]</code> when changing the flag can change compilation results.</p> |
| </li> |
| <li> |
| <p>Update the option tests in <code>compiler/rustc_interface/src/tests.rs</code>. |
| <code>options.rs</code> also has a short checklist near the <code>UnstableOptions</code> list for |
| files that need to stay in sync.</p> |
| </li> |
| <li> |
| <p>Use the option through <code>sess.opts.unstable_opts.$flag_name</code> or |
| <code>tcx.sess.opts.unstable_opts.$flag_name</code>. |
| If bootstrap or rustdoc needs to pass the flag internally, |
| thread it through the relevant command construction code as part of the same |
| change.</p> |
| </li> |
| <li> |
| <p>Add focused tests for the behavior controlled by the flag. |
| For example, UI tests can use <code>//@ compile-flags: -Zyour-flag</code>, |
| while rustdoc behavior usually belongs in <code>tests/rustdoc</code>.</p> |
| </li> |
| <li> |
| <p>If the flag is meant for users of nightly rustc or rustdoc, |
| document it in the unstable book under <code>src/doc/unstable-book/src/compiler-flags</code>.</p> |
| </li> |
| </ol> |
| <p>Before opening the PR, run <code>./x test tidy</code> and the narrow test suite that exercises the new flag.</p> |
| <h2 id="call-for-testing"><a class="header" href="#call-for-testing">Call for testing</a></h2> |
| <p>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 <a href="https://github.com/rust-lang/blog.rust-lang.org/">the main Rust blog</a> |
| and issue a “call for testing”.</p> |
| <p>Some earlier such blog posts include:</p> |
| <ol> |
| <li><a href="https://blog.rust-lang.org/2021/08/03/GATs-stabilization-push/">The push for GATs stabilization</a></li> |
| <li><a href="https://blog.rust-lang.org/2024/09/05/impl-trait-capture-rules.html">Changes to <code>impl Trait</code> in Rust 2024</a></li> |
| <li><a href="https://blog.rust-lang.org/inside-rust/2024/08/09/async-closures-call-for-testing/">Async Closures MVP: Call for Testing!</a></li> |
| </ol> |
| <p>Alternatively, <a href="https://github.com/rust-lang/this-week-in-rust"><em>This Week in Rust</em></a> has a <a href="https://this-week-in-rust.org/blog/2025/01/22/this-week-in-rust-583/#calls-for-testing">section</a> for this. |
| One example of this having been used is:</p> |
| <ul> |
| <li><a href="https://github.com/rust-lang/rust/issues/131204#issuecomment-2569314526">Call for testing on boolean literals as cfg predicates</a></li> |
| </ul> |
| <p>Which option to choose might depend on how significant the language change is, |
| though note that the <a href="https://github.com/rust-lang/this-week-in-rust"><em>This Week in Rust</em></a> section might be less visible |
| than a dedicated post on the main Rust blog.</p> |
| <h2 id="polishing"><a class="header" href="#polishing">Polishing</a></h2> |
| <p>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:</p> |
| <ul> |
| <li>Documenting the language feature in the <a href="https://github.com/rust-lang/reference">Rust Reference</a>.</li> |
| <li>Extending <a href="https://github.com/rust-lang/rustfmt"><code>rustfmt</code></a> to format any new syntax (if applicable).</li> |
| <li>Extending <a href="https://github.com/rust-lang/rust-analyzer"><code>rust-analyzer</code></a> (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 <em>full</em> support. |
| <ul> |
| <li>When a language feature degrades the user experience |
| simply by existing before support is implemented in <a href="https://github.com/rust-lang/rust-analyzer"><code>rust-analyzer</code></a>, |
| that may lead the lang team to raise a blocking concern.</li> |
| <li>Examples of such might include new syntax that <a href="https://github.com/rust-lang/rust-analyzer"><code>rust-analyzer</code></a> can’t parse |
| or type inference changes it doesn’t understand when those lead to bogus diagnostics.</li> |
| </ul> |
| </li> |
| </ul> |
| <h2 id="stabilization"><a class="header" href="#stabilization">Stabilization</a></h2> |
| <p>The final step in the feature lifecycle is <a href="./stabilization-guide.html">stabilization</a>, |
| 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 <a href="https://rust-lang.github.io/rfcs/1122-language-semver.html">defined semver policies</a> for details). |
| To learn more about stabilization, see the <a href="./stabilization-guide.html">stabilization guide</a>.</p> |
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