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| <ol class="chapter"><li class="chapter-item affix "><a href="about-this-guide.html">About this guide</a></li><li class="chapter-item affix "><a href="getting-started.html">Getting Started</a></li><li class="spacer"></li><li class="chapter-item affix "><li class="part-title">Building and debugging rustc</li><li class="chapter-item "><a href="building/how-to-build-and-run.html"><strong aria-hidden="true">1.</strong> How to Build and Run the Compiler</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="building/prerequisites.html"><strong aria-hidden="true">1.1.</strong> Prerequisites</a></li><li class="chapter-item "><a href="building/suggested.html"><strong aria-hidden="true">1.2.</strong> Suggested Workflows</a></li><li class="chapter-item "><a href="building/build-install-distribution-artifacts.html"><strong aria-hidden="true">1.3.</strong> Distribution artifacts</a></li><li class="chapter-item "><a href="building/compiler-documenting.html"><strong aria-hidden="true">1.4.</strong> Documenting Compiler</a></li><li class="chapter-item "><a href="rustdoc.html"><strong aria-hidden="true">1.5.</strong> Rustdoc overview</a></li><li class="chapter-item "><a href="building/new-target.html"><strong aria-hidden="true">1.6.</strong> Adding a new target</a></li></ol></li><li class="chapter-item "><a href="tests/intro.html"><strong aria-hidden="true">2.</strong> The compiler testing framework</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="tests/running.html"><strong aria-hidden="true">2.1.</strong> Running tests</a></li><li class="chapter-item "><a href="tests/adding.html"><strong aria-hidden="true">2.2.</strong> Adding new tests</a></li><li class="chapter-item "><a href="compiletest.html"><strong aria-hidden="true">2.3.</strong> Using compiletest commands to control test execution</a></li></ol></li><li class="chapter-item "><a href="compiler-debugging.html"><strong aria-hidden="true">3.</strong> Debugging the Compiler</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="tracing.html"><strong aria-hidden="true">3.1.</strong> Using the tracing/logging instrumentation</a></li></ol></li><li class="chapter-item "><a href="profiling.html"><strong aria-hidden="true">4.</strong> Profiling the compiler</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="profiling/with_perf.html"><strong aria-hidden="true">4.1.</strong> with the linux perf tool</a></li><li class="chapter-item "><a href="profiling/wpa_profiling.html"><strong aria-hidden="true">4.2.</strong> with Windows Performance Analyzer</a></li></ol></li><li class="chapter-item "><a href="crates-io.html"><strong aria-hidden="true">5.</strong> crates.io Dependencies</a></li><li class="chapter-item affix "><li class="part-title">Contributing to Rust</li><li class="chapter-item "><a href="contributing.html"><strong aria-hidden="true">6.</strong> Introduction</a></li><li class="chapter-item "><a href="compiler-team.html"><strong aria-hidden="true">7.</strong> About the compiler team</a></li><li class="chapter-item "><a href="git.html"><strong aria-hidden="true">8.</strong> Using Git</a></li><li class="chapter-item "><a href="rustbot.html"><strong aria-hidden="true">9.</strong> Mastering @rustbot</a></li><li class="chapter-item expanded "><a href="walkthrough.html" class="active"><strong aria-hidden="true">10.</strong> Walkthrough: a typical contribution</a></li><li class="chapter-item "><a href="bug-fix-procedure.html"><strong aria-hidden="true">11.</strong> Bug Fix Procedure</a></li><li class="chapter-item "><a href="implementing_new_features.html"><strong aria-hidden="true">12.</strong> Implementing new features</a></li><li class="chapter-item "><a href="stability.html"><strong aria-hidden="true">13.</strong> Stability attributes</a></li><li class="chapter-item "><a href="stabilization_guide.html"><strong aria-hidden="true">14.</strong> Stabilizing Features</a></li><li class="chapter-item "><a href="feature-gates.html"><strong aria-hidden="true">15.</strong> Feature Gates</a></li><li class="chapter-item "><a href="conventions.html"><strong aria-hidden="true">16.</strong> Coding conventions</a></li><li class="chapter-item "><a href="notification-groups/about.html"><strong aria-hidden="true">17.</strong> Notification groups</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="notification-groups/arm.html"><strong aria-hidden="true">17.1.</strong> ARM</a></li><li class="chapter-item "><a href="notification-groups/cleanup-crew.html"><strong aria-hidden="true">17.2.</strong> Cleanup Crew</a></li><li class="chapter-item "><a href="notification-groups/llvm.html"><strong aria-hidden="true">17.3.</strong> LLVM</a></li><li class="chapter-item "><a href="notification-groups/risc-v.html"><strong aria-hidden="true">17.4.</strong> RISC-V</a></li><li class="chapter-item "><a href="notification-groups/windows.html"><strong aria-hidden="true">17.5.</strong> Windows</a></li></ol></li><li class="chapter-item "><a href="licenses.html"><strong aria-hidden="true">18.</strong> Licenses</a></li><li class="chapter-item affix "><li class="part-title">High-level Compiler Architecture</li><li class="chapter-item "><a href="part-2-intro.html"><strong aria-hidden="true">19.</strong> Prologue</a></li><li class="chapter-item "><a href="overview.html"><strong aria-hidden="true">20.</strong> Overview of the Compiler</a></li><li class="chapter-item "><a href="compiler-src.html"><strong aria-hidden="true">21.</strong> The compiler source code</a></li><li class="chapter-item "><a href="building/bootstrapping.html"><strong aria-hidden="true">22.</strong> Bootstrapping</a></li><li class="chapter-item "><a href="query.html"><strong aria-hidden="true">23.</strong> Queries: demand-driven compilation</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="queries/query-evaluation-model-in-detail.html"><strong aria-hidden="true">23.1.</strong> The Query Evaluation Model in Detail</a></li><li class="chapter-item "><a href="queries/incremental-compilation.html"><strong aria-hidden="true">23.2.</strong> Incremental compilation</a></li><li class="chapter-item "><a href="queries/incremental-compilation-in-detail.html"><strong aria-hidden="true">23.3.</strong> Incremental compilation In Detail</a></li><li class="chapter-item "><a href="incrcomp-debugging.html"><strong aria-hidden="true">23.4.</strong> Debugging and Testing</a></li><li class="chapter-item "><a href="salsa.html"><strong aria-hidden="true">23.5.</strong> Salsa</a></li></ol></li><li class="chapter-item "><a href="memory.html"><strong aria-hidden="true">24.</strong> Memory Management in Rustc</a></li><li class="chapter-item "><a href="serialization.html"><strong aria-hidden="true">25.</strong> Serialization in Rustc</a></li><li class="chapter-item "><a href="parallel-rustc.html"><strong aria-hidden="true">26.</strong> Parallel Compilation</a></li><li class="chapter-item "><a href="rustdoc-internals.html"><strong aria-hidden="true">27.</strong> Rustdoc internals</a></li><li class="chapter-item affix "><li class="part-title">Source Code Representation</li><li class="chapter-item "><a href="part-3-intro.html"><strong aria-hidden="true">28.</strong> Prologue</a></li><li class="chapter-item "><a href="cli.html"><strong aria-hidden="true">29.</strong> Command-line arguments</a></li><li class="chapter-item "><a href="rustc-driver.html"><strong aria-hidden="true">30.</strong> The Rustc Driver and Interface</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="rustc-driver-interacting-with-the-ast.html"><strong aria-hidden="true">30.1.</strong> Ex: Type checking through rustc_interface</a></li><li class="chapter-item "><a href="rustc-driver-getting-diagnostics.html"><strong aria-hidden="true">30.2.</strong> Ex: Getting diagnostics through rustc_interface</a></li></ol></li><li class="chapter-item "><a href="syntax-intro.html"><strong aria-hidden="true">31.</strong> Syntax and the AST</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="the-parser.html"><strong aria-hidden="true">31.1.</strong> Lexing and Parsing</a></li><li class="chapter-item "><a href="macro-expansion.html"><strong aria-hidden="true">31.2.</strong> Macro expansion</a></li><li class="chapter-item "><a href="name-resolution.html"><strong aria-hidden="true">31.3.</strong> Name resolution</a></li><li class="chapter-item "><a href="test-implementation.html"><strong aria-hidden="true">31.4.</strong> #[test] Implementation</a></li><li class="chapter-item "><a href="panic-implementation.html"><strong aria-hidden="true">31.5.</strong> Panic Implementation</a></li><li class="chapter-item "><a href="ast-validation.html"><strong aria-hidden="true">31.6.</strong> AST Validation</a></li><li class="chapter-item "><a href="feature-gate-ck.html"><strong aria-hidden="true">31.7.</strong> Feature Gate Checking</a></li><li class="chapter-item "><a href="lang-items.html"><strong aria-hidden="true">31.8.</strong> Lang Items</a></li></ol></li><li class="chapter-item "><a href="hir.html"><strong aria-hidden="true">32.</strong> The HIR (High-level IR)</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="lowering.html"><strong aria-hidden="true">32.1.</strong> Lowering AST to HIR</a></li><li class="chapter-item "><a href="hir-debugging.html"><strong aria-hidden="true">32.2.</strong> Debugging</a></li></ol></li><li class="chapter-item "><a href="thir.html"><strong aria-hidden="true">33.</strong> The THIR (Typed High-level IR)</a></li><li class="chapter-item "><a href="mir/index.html"><strong aria-hidden="true">34.</strong> The MIR (Mid-level IR)</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="mir/construction.html"><strong aria-hidden="true">34.1.</strong> MIR construction</a></li><li class="chapter-item "><a href="mir/visitor.html"><strong aria-hidden="true">34.2.</strong> MIR visitor and traversal</a></li><li class="chapter-item "><a href="mir/passes.html"><strong aria-hidden="true">34.3.</strong> MIR passes: getting the MIR for a function</a></li></ol></li><li class="chapter-item "><a href="identifiers.html"><strong aria-hidden="true">35.</strong> Identifiers in the Compiler</a></li><li class="chapter-item "><a href="closure.html"><strong aria-hidden="true">36.</strong> Closure expansion</a></li><li class="chapter-item affix "><li class="part-title">Analysis</li><li class="chapter-item "><a href="part-4-intro.html"><strong aria-hidden="true">37.</strong> Prologue</a></li><li class="chapter-item "><a href="ty.html"><strong aria-hidden="true">38.</strong> The ty module: representing types</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="generics.html"><strong aria-hidden="true">38.1.</strong> Generics and substitutions</a></li><li class="chapter-item "><a href="ty-fold.html"><strong aria-hidden="true">38.2.</strong> TypeFolder and TypeFoldable</a></li><li class="chapter-item "><a href="generic_arguments.html"><strong aria-hidden="true">38.3.</strong> Generic arguments</a></li><li class="chapter-item "><a href="constants.html"><strong aria-hidden="true">38.4.</strong> Constants in the type system</a></li></ol></li><li class="chapter-item "><a href="type-inference.html"><strong aria-hidden="true">39.</strong> Type inference</a></li><li class="chapter-item "><a href="traits/resolution.html"><strong aria-hidden="true">40.</strong> Trait solving</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="early-late-bound.html"><strong aria-hidden="true">40.1.</strong> Early and Late Bound Parameters</a></li><li class="chapter-item "><a href="traits/hrtb.html"><strong aria-hidden="true">40.2.</strong> Higher-ranked trait bounds</a></li><li class="chapter-item "><a href="traits/caching.html"><strong aria-hidden="true">40.3.</strong> Caching subtleties</a></li><li class="chapter-item "><a href="traits/specialization.html"><strong aria-hidden="true">40.4.</strong> Specialization</a></li><li class="chapter-item "><a href="traits/chalk.html"><strong aria-hidden="true">40.5.</strong> Chalk-based trait solving</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="traits/lowering-to-logic.html"><strong aria-hidden="true">40.5.1.</strong> Lowering to logic</a></li><li class="chapter-item "><a href="traits/goals-and-clauses.html"><strong aria-hidden="true">40.5.2.</strong> Goals and clauses</a></li><li class="chapter-item "><a href="traits/canonical-queries.html"><strong aria-hidden="true">40.5.3.</strong> Canonical queries</a></li></ol></li></ol></li><li class="chapter-item "><a href="type-checking.html"><strong aria-hidden="true">41.</strong> Type checking</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="method-lookup.html"><strong aria-hidden="true">41.1.</strong> Method Lookup</a></li><li class="chapter-item "><a href="variance.html"><strong aria-hidden="true">41.2.</strong> Variance</a></li><li class="chapter-item "><a href="opaque-types-type-alias-impl-trait.html"><strong aria-hidden="true">41.3.</strong> Opaque Types</a></li></ol></li><li class="chapter-item "><a href="pat-exhaustive-checking.html"><strong aria-hidden="true">42.</strong> Pattern and Exhaustiveness Checking</a></li><li class="chapter-item "><a href="mir/dataflow.html"><strong aria-hidden="true">43.</strong> MIR dataflow</a></li><li class="chapter-item "><a href="borrow_check.html"><strong aria-hidden="true">44.</strong> The borrow checker</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="borrow_check/moves_and_initialization.html"><strong aria-hidden="true">44.1.</strong> Tracking moves and initialization</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="borrow_check/moves_and_initialization/move_paths.html"><strong aria-hidden="true">44.1.1.</strong> Move paths</a></li></ol></li><li class="chapter-item "><a href="borrow_check/type_check.html"><strong aria-hidden="true">44.2.</strong> MIR type checker</a></li><li class="chapter-item "><a href="borrow_check/region_inference.html"><strong aria-hidden="true">44.3.</strong> Region inference</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="borrow_check/region_inference/constraint_propagation.html"><strong aria-hidden="true">44.3.1.</strong> Constraint propagation</a></li><li class="chapter-item "><a href="borrow_check/region_inference/lifetime_parameters.html"><strong aria-hidden="true">44.3.2.</strong> Lifetime parameters</a></li><li class="chapter-item "><a href="borrow_check/region_inference/member_constraints.html"><strong aria-hidden="true">44.3.3.</strong> Member constraints</a></li><li class="chapter-item "><a href="borrow_check/region_inference/placeholders_and_universes.html"><strong aria-hidden="true">44.3.4.</strong> Placeholders and universes</a></li><li class="chapter-item "><a href="borrow_check/region_inference/closure_constraints.html"><strong aria-hidden="true">44.3.5.</strong> Closure constraints</a></li><li class="chapter-item "><a href="borrow_check/region_inference/error_reporting.html"><strong aria-hidden="true">44.3.6.</strong> Error reporting</a></li></ol></li><li class="chapter-item "><a href="borrow_check/two_phase_borrows.html"><strong aria-hidden="true">44.4.</strong> Two-phase-borrows</a></li></ol></li><li class="chapter-item "><a href="param_env.html"><strong aria-hidden="true">45.</strong> Parameter Environments</a></li><li class="chapter-item "><a href="diagnostics.html"><strong aria-hidden="true">46.</strong> Errors and Lints</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="diagnostics/sessiondiagnostic.html"><strong aria-hidden="true">46.1.</strong> Creating Errors With SessionDiagnostic</a></li><li class="chapter-item "><a href="diagnostics/lintstore.html"><strong aria-hidden="true">46.2.</strong> LintStore</a></li><li class="chapter-item "><a href="diagnostics/diagnostic-codes.html"><strong aria-hidden="true">46.3.</strong> Diagnostic Codes</a></li><li class="chapter-item "><a href="diagnostics/diagnostic-items.html"><strong aria-hidden="true">46.4.</strong> Diagnostic Items</a></li></ol></li><li class="chapter-item "><li class="part-title">MIR to Binaries</li><li class="chapter-item "><a href="part-5-intro.html"><strong aria-hidden="true">47.</strong> Prologue</a></li><li class="chapter-item "><a href="mir/optimizations.html"><strong aria-hidden="true">48.</strong> MIR optimizations</a></li><li class="chapter-item "><a href="mir/debugging.html"><strong aria-hidden="true">49.</strong> Debugging</a></li><li class="chapter-item "><a href="const-eval.html"><strong aria-hidden="true">50.</strong> Constant evaluation</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="miri.html"><strong aria-hidden="true">50.1.</strong> miri const evaluator</a></li></ol></li><li class="chapter-item "><a href="backend/monomorph.html"><strong aria-hidden="true">51.</strong> Monomorphization</a></li><li class="chapter-item "><a href="backend/lowering-mir.html"><strong aria-hidden="true">52.</strong> Lowering MIR</a></li><li class="chapter-item "><a href="backend/codegen.html"><strong aria-hidden="true">53.</strong> Code Generation</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="backend/updating-llvm.html"><strong aria-hidden="true">53.1.</strong> Updating LLVM</a></li><li class="chapter-item "><a href="backend/debugging.html"><strong aria-hidden="true">53.2.</strong> Debugging LLVM</a></li><li class="chapter-item "><a href="backend/backend-agnostic.html"><strong aria-hidden="true">53.3.</strong> Backend Agnostic Codegen</a></li><li class="chapter-item "><a href="backend/implicit-caller-location.html"><strong aria-hidden="true">53.4.</strong> Implicit Caller Location</a></li></ol></li><li class="chapter-item "><a href="backend/libs-and-metadata.html"><strong aria-hidden="true">54.</strong> Libraries and Metadata</a></li><li class="chapter-item "><a href="profile-guided-optimization.html"><strong aria-hidden="true">55.</strong> Profile-guided Optimization</a></li><li class="chapter-item "><a href="llvm-coverage-instrumentation.html"><strong aria-hidden="true">56.</strong> LLVM Source-Based Code Coverage</a></li><li class="chapter-item "><a href="sanitizers.html"><strong aria-hidden="true">57.</strong> Sanitizers Support</a></li><li class="chapter-item "><a href="debugging-support-in-rustc.html"><strong aria-hidden="true">58.</strong> Debugging Support in the Rust Compiler</a></li><li class="spacer"></li><li class="chapter-item affix "><a href="appendix/background.html">Appendix A: Background topics</a></li><li class="chapter-item affix "><a href="appendix/glossary.html">Appendix B: Glossary</a></li><li class="chapter-item affix "><a href="appendix/code-index.html">Appendix C: Code Index</a></li><li class="chapter-item affix "><a href="appendix/compiler-lecture.html">Appendix D: Compiler Lecture Series</a></li><li class="chapter-item affix "><a href="appendix/bibliography.html">Appendix E: Bibliography</a></li><li class="chapter-item affix "><a href="appendix/humorust.html">Appendix Z: HumorRust</a></li><li class="spacer"></li></ol> |
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| <h1 id="walkthrough-a-typical-contribution"><a class="header" href="#walkthrough-a-typical-contribution">Walkthrough: a typical contribution</a></h1> |
| <ul> |
| <li><a href="#overview">Overview</a></li> |
| <li><a href="#pre-rfc-and-rfc">Pre-RFC and RFC</a></li> |
| <li><a href="#implementation">Implementation</a></li> |
| <li><a href="#refining-your-implementation">Refining your implementation</a></li> |
| <li><a href="#stabilization">Stabilization</a></li> |
| </ul> |
| <p>There are <em>a lot</em> of ways to contribute to the rust compiler, including fixing |
| bugs, improving performance, helping design features, providing feedback on |
| existing features, etc. This chapter does not claim to scratch the surface. |
| Instead, it walks through the design and implementation of a new feature. Not |
| all of the steps and processes described here are needed for every |
| contribution, and I will try to point those out as they arise.</p> |
| <p>In general, if you are interested in making a contribution and aren't sure |
| where to start, please feel free to ask!</p> |
| <h2 id="overview"><a class="header" href="#overview">Overview</a></h2> |
| <p>The feature I will discuss in this chapter is the <code>?</code> Kleene operator for |
| macros. Basically, we want to be able to write something like this:</p> |
| <pre><code class="language-rust ignore">macro_rules! foo { |
| ($arg:ident $(, $optional_arg:ident)?) => { |
| println!("{}", $arg); |
| |
| $( |
| println!("{}", $optional_arg); |
| )? |
| } |
| } |
| |
| fn main() { |
| let x = 0; |
| foo!(x); // ok! prints "0" |
| foo!(x, x); // ok! prints "0 0" |
| } |
| </code></pre> |
| <p>So basically, the <code>$(pat)?</code> matcher in the macro means "this pattern can occur |
| 0 or 1 times", similar to other regex syntaxes.</p> |
| <p>There were a number of steps to go from an idea to stable rust feature. Here is |
| a quick list. We will go through each of these in order below. As I mentioned |
| before, not all of these are needed for every type of contribution.</p> |
| <ul> |
| <li><strong>Idea discussion/Pre-RFC</strong> A Pre-RFC is an early draft or design discussion |
| of a feature. This stage is intended to flesh out the design space a bit and |
| get a grasp on the different merits and problems with an idea. It's a great |
| way to get early feedback on your idea before presenting it the wider |
| audience. You can find the original discussion <a href="https://internals.rust-lang.org/t/pre-rfc-at-most-one-repetition-macro-patterns/6557">here</a>.</li> |
| <li><strong>RFC</strong> This is when you formally present your idea to the community for |
| consideration. You can find the RFC <a href="https://github.com/rust-lang/rfcs/pull/2298">here</a>.</li> |
| <li><strong>Implementation</strong> Implement your idea unstably in the compiler. You can |
| find the original implementation <a href="https://github.com/rust-lang/rust/pull/47752">here</a>.</li> |
| <li><strong>Possibly iterate/refine</strong> As the community gets experience with your |
| feature on the nightly compiler and in <code>std</code>, there may be additional |
| feedback about design choice that might be adjusted. This particular feature |
| went <a href="https://github.com/rust-lang/rust/pull/49719">through</a> a <a href="https://github.com/rust-lang/rust/pull/51336">number</a> of <a href="https://github.com/rust-lang/rust/pull/51587">iterations</a>.</li> |
| <li><strong>Stabilization</strong> When your feature has baked enough, a rust team member may |
| <a href="https://github.com/rust-lang/rust/issues/48075#issuecomment-433177613">propose to stabilize it</a>. If there is consensus, this is done.</li> |
| <li><strong>Relax</strong> Your feature is now a stable rust feature!</li> |
| </ul> |
| <h2 id="pre-rfc-and-rfc"><a class="header" href="#pre-rfc-and-rfc">Pre-RFC and RFC</a></h2> |
| <blockquote> |
| <p>NOTE: In general, if you are not proposing a <em>new</em> feature or substantial |
| change to rust or the ecosystem, you don't need to follow the RFC process. |
| Instead, you can just jump to <a href="#impl">implementation</a>.</p> |
| <p>You can find the official guidelines for when to open an RFC <a href="https://github.com/rust-lang/rfcs#when-you-need-to-follow-this-process">here</a>.</p> |
| </blockquote> |
| <p>An RFC is a document that describes the feature or change you are proposing in |
| detail. Anyone can write an RFC; the process is the same for everyone, |
| including rust team members.</p> |
| <p>To open an RFC, open a PR on the |
| <a href="https://github.com/rust-lang/rfcs">rust-lang/rfcs</a> repo on GitHub. You can |
| find detailed instructions in the |
| <a href="https://github.com/rust-lang/rfcs#what-the-process-is">README</a>.</p> |
| <p>Before opening an RFC, you should do the research to "flesh out" your idea. |
| Hastily-proposed RFCs tend not to be accepted. You should generally have a good |
| description of the motivation, impact, disadvantages, and potential |
| interactions with other features.</p> |
| <p>If that sounds like a lot of work, it's because it is. But no fear! Even if |
| you're not a compiler hacker, you can get great feedback by doing a <em>pre-RFC</em>. |
| This is an <em>informal</em> discussion of the idea. The best place to do this is |
| internals.rust-lang.org. Your post doesn't have to follow any particular |
| structure. It doesn't even need to be a cohesive idea. Generally, you will get |
| tons of feedback that you can integrate back to produce a good RFC.</p> |
| <p>(Another pro-tip: try searching the RFCs repo and internals for prior related |
| ideas. A lot of times an idea has already been considered and was either |
| rejected or postponed to be tried again later. This can save you and everybody |
| else some time)</p> |
| <p>In the case of our example, a participant in the pre-RFC thread pointed out a |
| syntax ambiguity and a potential resolution. Also, the overall feedback seemed |
| positive. In this case, the discussion converged pretty quickly, but for some |
| ideas, a lot more discussion can happen (e.g. see <a href="https://github.com/rust-lang/rfcs/pull/2457">this RFC</a> which |
| received a whopping 684 comments!). If that happens, don't be discouraged; it |
| means the community is interested in your idea, but it perhaps needs some |
| adjustments.</p> |
| <p>The RFC for our <code>?</code> macro feature did receive some discussion on the RFC thread |
| too. As with most RFCs, there were a few questions that we couldn't answer by |
| discussion: we needed experience using the feature to decide. Such questions |
| are listed in the "Unresolved Questions" section of the RFC. Also, over the |
| course of the RFC discussion, you will probably want to update the RFC document |
| itself to reflect the course of the discussion (e.g. new alternatives or prior |
| work may be added or you may decide to change parts of the proposal itself).</p> |
| <p>In the end, when the discussion seems to reach a consensus and die down a bit, |
| a rust team member may propose to move to "final comment period" (FCP) with one |
| of three possible dispositions. This means that they want the other members of |
| the appropriate teams to review and comment on the RFC. More discussion may |
| ensue, which may result in more changes or unresolved questions being added. At |
| some point, when everyone is satisfied, the RFC enters the FCP, which is the |
| last chance for people to bring up objections. When the FCP is over, the |
| disposition is adopted. Here are the three possible dispositions:</p> |
| <ul> |
| <li><em>Merge</em>: accept the feature. Here is the proposal to merge for our <a href="https://github.com/rust-lang/rfcs/pull/2298#issuecomment-360582667"><code>?</code> macro |
| feature</a>.</li> |
| <li><em>Close</em>: this feature in its current form is not a good fit for rust. Don't |
| be discouraged if this happens to your RFC, and don't take it personally. |
| This is not a reflection on you, but rather a community decision that rust |
| will go a different direction.</li> |
| <li><em>Postpone</em>: there is interest in going this direction but not at the moment. |
| This happens most often because the appropriate rust team doesn't have the |
| bandwidth to shepherd the feature through the process to stabilization. Often |
| this is the case when the feature doesn't fit into the team's roadmap. |
| Postponed ideas may be revisited later.</li> |
| </ul> |
| <p>When an RFC is merged, the PR is merged into the RFCs repo. A new <em>tracking |
| issue</em> is created in the <a href="https://github.com/rust-lang/rust">rust-lang/rust</a> repo to track progress on the feature |
| and discuss unresolved questions, implementation progress and blockers, etc. |
| Here is the tracking issue on for our <a href="https://github.com/rust-lang/rust/issues/48075"><code>?</code> macro feature</a>.</p> |
| <p><a name="impl"></a></p> |
| <h2 id="implementation"><a class="header" href="#implementation">Implementation</a></h2> |
| <p>To make a change to the compiler, open a PR against the <a href="https://github.com/rust-lang/rust">rust-lang/rust</a> repo.</p> |
| <p>Depending on the feature/change/bug fix/improvement, implementation may be |
| relatively-straightforward or it may be a major undertaking. You can always ask |
| for help or mentorship from more experienced compiler devs. Also, you don't |
| have to be the one to implement your feature; but keep in mind that if you |
| don't it might be a while before someone else does.</p> |
| <p>For the <code>?</code> macro feature, I needed to go understand the relevant parts of |
| macro expansion in the compiler. Personally, I find that <a href="https://github.com/rust-lang/rust/pull/47732">improving the |
| comments</a> in the code is a helpful way of making sure I understand |
| it, but you don't have to do that if you don't want to.</p> |
| <p>I then <a href="https://github.com/rust-lang/rust/pull/47752">implemented</a> the original feature, as described in the RFC. When |
| a new feature is implemented, it goes behind a <em>feature gate</em>, which means that |
| you have to use <code>#![feature(my_feature_name)]</code> to use the feature. The feature |
| gate is removed when the feature is stabilized.</p> |
| <p><strong>Most bug fixes and improvements</strong> don't require a feature gate. You can just |
| make your changes/improvements.</p> |
| <p>When you open a PR on the <a href="https://github.com/rust-lang/rust">rust-lang/rust</a>, a bot will assign your PR to a |
| review. If there is a particular rust team member you are working with, you can |
| request that reviewer by leaving a comment on the thread with <code>r? @reviewer-github-id</code> (e.g. <code>r? @eddyb</code>). If you don't know who to request, |
| don't request anyone; the bot will assign someone automatically.</p> |
| <p>The reviewer may request changes before they approve your PR. Feel free to ask |
| questions or discuss things you don't understand or disagree with. However, |
| recognize that the PR won't be merged unless someone on the Rust team approves |
| it.</p> |
| <p>When your reviewer approves the PR, it will go into a queue for yet another bot |
| called <code>@bors</code>. <code>@bors</code> manages the CI build/merge queue. When your PR reaches |
| the head of the <code>@bors</code> queue, <code>@bors</code> will test out the merge by running all |
| tests against your PR on GitHub Actions. This takes a lot of time to |
| finish. If all tests pass, the PR is merged and becomes part of the next |
| nightly compiler!</p> |
| <p>There are a couple of things that may happen for some PRs during the review process</p> |
| <ul> |
| <li>If the change is substantial enough, the reviewer may request an FCP on |
| the PR. This gives all members of the appropriate team a chance to review the |
| changes.</li> |
| <li>If the change may cause breakage, the reviewer may request a <a href="./tests/intro.html#crater">crater</a> run. |
| This compiles the compiler with your changes and then attempts to compile all |
| crates on crates.io with your modified compiler. This is a great smoke test |
| to check if you introduced a change to compiler behavior that affects a large |
| portion of the ecosystem.</li> |
| <li>If the diff of your PR is large or the reviewer is busy, your PR may have |
| some merge conflicts with other PRs that happen to get merged first. You |
| should fix these merge conflicts using the normal git procedures.</li> |
| </ul> |
| <p>If you are not doing a new feature or something like that (e.g. if you are |
| fixing a bug), then that's it! Thanks for your contribution :)</p> |
| <h2 id="refining-your-implementation"><a class="header" href="#refining-your-implementation">Refining your implementation</a></h2> |
| <p>As people get experience with your new feature on nightly, slight changes may |
| be proposed and unresolved questions may become resolved. Updates/changes go |
| through the same process for implementing any other changes, as described |
| above (i.e. submit a PR, go through review, wait for <code>@bors</code>, etc).</p> |
| <p>Some changes may be major enough to require an FCP and some review by rust team |
| members.</p> |
| <p>For the <code>?</code> macro feature, we went through a few different iterations after the |
| original implementation: <a href="https://github.com/rust-lang/rust/pull/49719">1</a>, <a href="https://github.com/rust-lang/rust/pull/51336">2</a>, <a href="https://github.com/rust-lang/rust/pull/51587">3</a>.</p> |
| <p>Along the way, we decided that <code>?</code> should not take a separator, which was |
| previously an unresolved question listed in the RFC. We also changed the |
| disambiguation strategy: we decided to remove the ability to use <code>?</code> as a |
| separator token for other repetition operators (e.g. <code>+</code> or <code>*</code>). However, |
| since this was a breaking change, we decided to do it over an edition boundary. |
| Thus, the new feature can be enabled only in edition 2018. These deviations |
| from the original RFC required <a href="https://github.com/rust-lang/rust/issues/51934">another |
| FCP</a>.</p> |
| <h2 id="stabilization"><a class="header" href="#stabilization">Stabilization</a></h2> |
| <p>Finally, after the feature had baked for a while on nightly, a language team member |
| <a href="https://github.com/rust-lang/rust/issues/48075#issuecomment-433177613">moved to stabilize it</a>.</p> |
| <p>A <em>stabilization report</em> needs to be written that includes</p> |
| <ul> |
| <li>brief description of the behavior and any deviations from the RFC</li> |
| <li>which edition(s) are affected and how</li> |
| <li>links to a few tests to show the interesting aspects</li> |
| </ul> |
| <p>The stabilization report for our feature is <a href="https://github.com/rust-lang/rust/issues/48075#issuecomment-433243048">here</a>.</p> |
| <p>After this, <a href="https://github.com/rust-lang/rust/pull/56245">a PR is made</a> to remove the feature gate, enabling the feature by |
| default (on the 2018 edition). A note is added to the <a href="https://github.com/rust-lang/rust/blob/master/RELEASES.md">Release notes</a> |
| about the feature.</p> |
| <p>Steps to stabilize the feature can be found at <a href="./stabilization_guide.html">Stabilizing Features</a>.</p> |
| |
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