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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 "><a href="walkthrough.html"><strong aria-hidden="true">10.</strong> Walkthrough: a typical contribution</a></li><li class="chapter-item expanded "><a href="bug-fix-procedure.html" class="active"><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="rustc-bug-fix-procedure"><a class="header" href="#rustc-bug-fix-procedure">Rustc Bug Fix Procedure</a></h1> |
| <ul> |
| <li><a href="#motivation">Motivation</a> |
| <ul> |
| <li><a href="#what-qualifies-as-a-bug-fix">What qualifies as a bug fix?</a></li> |
| </ul> |
| </li> |
| <li><a href="#detailed-design">Detailed design</a> |
| <ul> |
| <li><a href="#tracking-issue">Tracking issue</a></li> |
| <li><a href="#tracking-issue-template">Tracking issue template</a></li> |
| <li><a href="#issuing-future-compatibility-warnings">Issuing future compatibility warnings</a></li> |
| <li><a href="#helpful-techniques">Helpful techniques</a></li> |
| <li><a href="#crater-and-cratesio">Crater and crates.io</a></li> |
| <li><a href="#is-it-ever-acceptable-to-go-directly-to-issuing-errors">Is it ever acceptable to go directly to issuing errors?</a></li> |
| <li><a href="#stabilization">Stabilization</a></li> |
| <li><a href="#removing-a-lint">Removing a lint</a></li> |
| <li><a href="#remove-the-lint">Remove the lint.</a></li> |
| <li><a href="#add-the-lint-to-the-list-of-removed-lists">Add the lint to the list of removed lists.</a></li> |
| <li><a href="#update-the-places-that-issue-the-lint">Update the places that issue the lint</a></li> |
| <li><a href="#update-tests">Update tests</a></li> |
| <li><a href="#all-done">All done!</a></li> |
| </ul> |
| </li> |
| </ul> |
| <p>This page defines the best practices procedure for making bug fixes or soundness |
| corrections in the compiler that can cause existing code to stop compiling. This |
| text is based on |
| <a href="https://github.com/rust-lang/rfcs/blob/master/text/1589-rustc-bug-fix-procedure.md">RFC 1589</a>.</p> |
| <h1 id="motivation"><a class="header" href="#motivation">Motivation</a></h1> |
| <p>From time to time, we encounter the need to make a bug fix, soundness |
| correction, or other change in the compiler which will cause existing code to |
| stop compiling. When this happens, it is important that we handle the change in |
| a way that gives users of Rust a smooth transition. What we want to avoid is |
| that existing programs suddenly stop compiling with opaque error messages: we |
| would prefer to have a gradual period of warnings, with clear guidance as to |
| what the problem is, how to fix it, and why the change was made. This RFC |
| describes the procedure that we have been developing for handling breaking |
| changes that aims to achieve that kind of smooth transition.</p> |
| <p>One of the key points of this policy is that (a) warnings should be issued |
| initially rather than hard errors if at all possible and (b) every change that |
| causes existing code to stop compiling will have an associated tracking issue. |
| This issue provides a point to collect feedback on the results of that change. |
| Sometimes changes have unexpectedly large consequences or there may be a way to |
| avoid the change that was not considered. In those cases, we may decide to |
| change course and roll back the change, or find another solution (if warnings |
| are being used, this is particularly easy to do).</p> |
| <h3 id="what-qualifies-as-a-bug-fix"><a class="header" href="#what-qualifies-as-a-bug-fix">What qualifies as a bug fix?</a></h3> |
| <p>Note that this RFC does not try to define when a breaking change is permitted. |
| That is already covered under <a href="https://github.com/rust-lang/rfcs/blob/master/text/1122-language-semver.md">RFC 1122</a>. This document assumes that the |
| change being made is in accordance with those policies. Here is a summary of the |
| conditions from RFC 1122:</p> |
| <ul> |
| <li><strong>Soundness changes:</strong> Fixes to holes uncovered in the type system.</li> |
| <li><strong>Compiler bugs:</strong> Places where the compiler is not implementing the specified |
| semantics found in an RFC or lang-team decision.</li> |
| <li><strong>Underspecified language semantics:</strong> Clarifications to grey areas where the |
| compiler behaves inconsistently and no formal behavior had been previously |
| decided.</li> |
| </ul> |
| <p>Please see <a href="https://github.com/rust-lang/rfcs/blob/master/text/1122-language-semver.md">the RFC</a> for full details!</p> |
| <h1 id="detailed-design"><a class="header" href="#detailed-design">Detailed design</a></h1> |
| <p>The procedure for making a breaking change is as follows (each of these steps is |
| described in more detail below):</p> |
| <ol start="0"> |
| <li>Do a <strong>crater run</strong> to assess the impact of the change.</li> |
| <li>Make a <strong>special tracking issue</strong> dedicated to the change.</li> |
| <li>Do not report an error right away. Instead, <strong>issue forwards-compatibility |
| lint warnings</strong>. |
| <ul> |
| <li>Sometimes this is not straightforward. See the text below for suggestions |
| on different techniques we have employed in the past.</li> |
| <li>For cases where warnings are infeasible: |
| <ul> |
| <li>Report errors, but make every effort to give a targeted error message |
| that directs users to the tracking issue</li> |
| <li>Submit PRs to all known affected crates that fix the issue |
| <ul> |
| <li>or, at minimum, alert the owners of those crates to the problem and |
| direct them to the tracking issue</li> |
| </ul> |
| </li> |
| </ul> |
| </li> |
| </ul> |
| </li> |
| <li>Once the change has been in the wild for at least one cycle, we can |
| <strong>stabilize the change</strong>, converting those warnings into errors.</li> |
| </ol> |
| <p>Finally, for changes to <code>rustc_ast</code> that will affect plugins, the general policy |
| is to batch these changes. That is discussed below in more detail.</p> |
| <h3 id="tracking-issue"><a class="header" href="#tracking-issue">Tracking issue</a></h3> |
| <p>Every breaking change should be accompanied by a <strong>dedicated tracking issue</strong> |
| for that change. The main text of this issue should describe the change being |
| made, with a focus on what users must do to fix their code. The issue should be |
| approachable and practical; it may make sense to direct users to an RFC or some |
| other issue for the full details. The issue also serves as a place where users |
| can comment with questions or other concerns.</p> |
| <p>A template for these breaking-change tracking issues can be found below. An |
| example of how such an issue should look can be <a href="https://gist.github.com/nikomatsakis/631ec8b4af9a18b5d062d9d9b7d3d967">found |
| here</a>.</p> |
| <p>The issue should be tagged with (at least) <code>B-unstable</code> and <code>T-compiler</code>.</p> |
| <h3 id="tracking-issue-template"><a class="header" href="#tracking-issue-template">Tracking issue template</a></h3> |
| <p>This is a template to use for tracking issues:</p> |
| <pre><code>This is the **summary issue** for the `YOUR_LINT_NAME_HERE` |
| future-compatibility warning and other related errors. The goal of |
| this page is describe why this change was made and how you can fix |
| code that is affected by it. It also provides a place to ask questions |
| or register a complaint if you feel the change should not be made. For |
| more information on the policy around future-compatibility warnings, |
| see our [breaking change policy guidelines][guidelines]. |
| |
| [guidelines]: LINK_TO_THIS_RFC |
| |
| #### What is the warning for? |
| |
| *Describe the conditions that trigger the warning and how they can be |
| fixed. Also explain why the change was made.** |
| |
| #### When will this warning become a hard error? |
| |
| At the beginning of each 6-week release cycle, the Rust compiler team |
| will review the set of outstanding future compatibility warnings and |
| nominate some of them for **Final Comment Period**. Toward the end of |
| the cycle, we will review any comments and make a final determination |
| whether to convert the warning into a hard error or remove it |
| entirely. |
| </code></pre> |
| <h3 id="issuing-future-compatibility-warnings"><a class="header" href="#issuing-future-compatibility-warnings">Issuing future compatibility warnings</a></h3> |
| <p>The best way to handle a breaking change is to begin by issuing |
| future-compatibility warnings. These are a special category of lint warning. |
| Adding a new future-compatibility warning can be done as follows.</p> |
| <pre><pre class="playground"><code class="language-rust"> |
| <span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>// 1. Define the lint in `compiler/rustc_middle/src/lint/builtin.rs`: |
| declare_lint! { |
| pub YOUR_ERROR_HERE, |
| Warn, |
| "illegal use of foo bar baz" |
| } |
| |
| // 2. Add to the list of HardwiredLints in the same file: |
| impl LintPass for HardwiredLints { |
| fn get_lints(&self) -> LintArray { |
| lint_array!( |
| .., |
| YOUR_ERROR_HERE |
| ) |
| } |
| } |
| |
| // 3. Register the lint in `compiler/rustc_lint/src/lib.rs`: |
| store.register_future_incompatible(sess, vec![ |
| ..., |
| FutureIncompatibleInfo { |
| id: LintId::of(YOUR_ERROR_HERE), |
| reference: "issue #1234", // your tracking issue here! |
| }, |
| ]); |
| |
| // 4. Report the lint: |
| tcx.lint_node( |
| lint::builtin::YOUR_ERROR_HERE, |
| path_id, |
| binding.span, |
| format!("some helper message here")); |
| <span class="boring">} |
| </span></code></pre></pre> |
| <h4 id="helpful-techniques"><a class="header" href="#helpful-techniques">Helpful techniques</a></h4> |
| <p>It can often be challenging to filter out new warnings from older, pre-existing |
| errors. One technique that has been used in the past is to run the older code |
| unchanged and collect the errors it would have reported. You can then issue |
| warnings for any errors you would give which do not appear in that original set. |
| Another option is to abort compilation after the original code completes if |
| errors are reported: then you know that your new code will only execute when |
| there were no errors before.</p> |
| <h4 id="crater-and-cratesio"><a class="header" href="#crater-and-cratesio">Crater and crates.io</a></h4> |
| <p>We should always do a crater run to assess impact. It is polite and considerate |
| to at least notify the authors of affected crates the breaking change. If we can |
| submit PRs to fix the problem, so much the better.</p> |
| <h4 id="is-it-ever-acceptable-to-go-directly-to-issuing-errors"><a class="header" href="#is-it-ever-acceptable-to-go-directly-to-issuing-errors">Is it ever acceptable to go directly to issuing errors?</a></h4> |
| <p>Changes that are believed to have negligible impact can go directly to issuing |
| an error. One rule of thumb would be to check against <code>crates.io</code>: if fewer than |
| 10 <strong>total</strong> affected projects are found (<strong>not</strong> root errors), we can move |
| straight to an error. In such cases, we should still make the "breaking change" |
| page as before, and we should ensure that the error directs users to this page. |
| In other words, everything should be the same except that users are getting an |
| error, and not a warning. Moreover, we should submit PRs to the affected |
| projects (ideally before the PR implementing the change lands in rustc).</p> |
| <p>If the impact is not believed to be negligible (e.g., more than 10 crates are |
| affected), then warnings are required (unless the compiler team agrees to grant |
| a special exemption in some particular case). If implementing warnings is not |
| feasible, then we should make an aggressive strategy of migrating crates before |
| we land the change so as to lower the number of affected crates. Here are some |
| techniques for approaching this scenario:</p> |
| <ol> |
| <li>Issue warnings for subparts of the problem, and reserve the new errors for |
| the smallest set of cases you can.</li> |
| <li>Try to give a very precise error message that suggests how to fix the problem |
| and directs users to the tracking issue.</li> |
| <li>It may also make sense to layer the fix: |
| <ul> |
| <li>First, add warnings where possible and let those land before proceeding to |
| issue errors.</li> |
| <li>Work with authors of affected crates to ensure that corrected versions are |
| available <em>before</em> the fix lands, so that downstream users can use them.</li> |
| </ul> |
| </li> |
| </ol> |
| <h3 id="stabilization"><a class="header" href="#stabilization">Stabilization</a></h3> |
| <p>After a change is made, we will <strong>stabilize</strong> the change using the same process |
| that we use for unstable features:</p> |
| <ul> |
| <li> |
| <p>After a new release is made, we will go through the outstanding tracking |
| issues corresponding to breaking changes and nominate some of them for <strong>final |
| comment period</strong> (FCP).</p> |
| </li> |
| <li> |
| <p>The FCP for such issues lasts for one cycle. In the final week or two of the |
| cycle, we will review comments and make a final determination:</p> |
| <ul> |
| <li>Convert to error: the change should be made into a hard error.</li> |
| <li>Revert: we should remove the warning and continue to allow the older code to |
| compile.</li> |
| <li>Defer: can't decide yet, wait longer, or try other strategies.</li> |
| </ul> |
| </li> |
| </ul> |
| <p>Ideally, breaking changes should have landed on the <strong>stable branch</strong> of the |
| compiler before they are finalized.</p> |
| <a name="guide"> |
| <h3 id="removing-a-lint"><a class="header" href="#removing-a-lint">Removing a lint</a></h3> |
| <p>Once we have decided to make a "future warning" into a hard error, we need a PR |
| that removes the custom lint. As an example, here are the steps required to |
| remove the <code>overlapping_inherent_impls</code> compatibility lint. First, convert the |
| name of the lint to uppercase (<code>OVERLAPPING_INHERENT_IMPLS</code>) ripgrep through the |
| source for that string. We will basically by converting each place where this |
| lint name is mentioned (in the compiler, we use the upper-case name, and a macro |
| automatically generates the lower-case string; so searching for |
| <code>overlapping_inherent_impls</code> would not find much).</p> |
| <blockquote> |
| <p>NOTE: these exact files don't exist anymore, but the procedure is still the same.</p> |
| </blockquote> |
| <h4 id="remove-the-lint"><a class="header" href="#remove-the-lint">Remove the lint.</a></h4> |
| <p>The first reference you will likely find is the lint definition <a href="https://github.com/rust-lang/rust/blob/085d71c3efe453863739c1fb68fd9bd1beff214f/src/librustc/lint/builtin.rs#L171-L175">in |
| <code>rustc_session/src/lint/builtin.rs</code> that resembles this</a>:</p> |
| <pre><pre class="playground"><code class="language-rust"> |
| <span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>declare_lint! { |
| pub OVERLAPPING_INHERENT_IMPLS, |
| Deny, // this may also say Warning |
| "two overlapping inherent impls define an item with the same name were erroneously allowed" |
| } |
| <span class="boring">} |
| </span></code></pre></pre> |
| <p>This <code>declare_lint!</code> macro creates the relevant data structures. Remove it. You |
| will also find that there is a mention of <code>OVERLAPPING_INHERENT_IMPLS</code> later in |
| the file as <a href="https://github.com/rust-lang/rust/blob/085d71c3efe453863739c1fb68fd9bd1beff214f/src/librustc/lint/builtin.rs#L252-L290">part of a <code>lint_array!</code></a>; remove it too.</p> |
| <p>Next, you see <a href="https://github.com/rust-lang/rust/blob/085d71c3efe453863739c1fb68fd9bd1beff214f/src/librustc_lint/lib.rs#L202-L205">a reference to <code>OVERLAPPING_INHERENT_IMPLS</code> in |
| <code>rustc_lint/src/lib.rs</code></a>. This is defining the lint as a "future |
| compatibility lint":</p> |
| <pre><pre class="playground"><code class="language-rust"> |
| <span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>FutureIncompatibleInfo { |
| id: LintId::of(OVERLAPPING_INHERENT_IMPLS), |
| reference: "issue #36889 <https://github.com/rust-lang/rust/issues/36889>", |
| }, |
| <span class="boring">} |
| </span></code></pre></pre> |
| <p>Remove this too.</p> |
| <h4 id="add-the-lint-to-the-list-of-removed-lists"><a class="header" href="#add-the-lint-to-the-list-of-removed-lists">Add the lint to the list of removed lists.</a></h4> |
| <p>In <code>compiler/rustc_lint/src/lib.rs</code> there is a list of "renamed and removed lints". |
| You can add this lint to the list:</p> |
| <pre><pre class="playground"><code class="language-rust"> |
| <span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>store.register_removed("overlapping_inherent_impls", "converted into hard error, see #36889"); |
| <span class="boring">} |
| </span></code></pre></pre> |
| <p>where <code>#36889</code> is the tracking issue for your lint.</p> |
| <h4 id="update-the-places-that-issue-the-lint"><a class="header" href="#update-the-places-that-issue-the-lint">Update the places that issue the lint</a></h4> |
| <p>Finally, the last class of references you will see are the places that actually |
| <strong>trigger</strong> the lint itself (i.e., what causes the warnings to appear). These |
| you do not want to delete. Instead, you want to convert them into errors. In |
| this case, the <a href="https://github.com/rust-lang/rust/blob/085d71c3efe453863739c1fb68fd9bd1beff214f/src/librustc_typeck/coherence/inherent.rs#L300-L303"><code>add_lint</code> call</a> looks like this:</p> |
| <pre><pre class="playground"><code class="language-rust"> |
| <span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>self.tcx.sess.add_lint(lint::builtin::OVERLAPPING_INHERENT_IMPLS, |
| node_id, |
| self.tcx.span_of_impl(item1).unwrap(), |
| msg); |
| <span class="boring">} |
| </span></code></pre></pre> |
| <p>We want to convert this into an error. In some cases, there may be an |
| existing error for this scenario. In others, we will need to allocate a |
| fresh diagnostic code. <a href="./diagnostics/diagnostic-codes.html">Instructions for allocating a fresh diagnostic |
| code can be found here.</a> You may want |
| to mention in the extended description that the compiler behavior |
| changed on this point, and include a reference to the tracking issue for |
| the change.</p> |
| <p>Let's say that we've adopted <code>E0592</code> as our code. Then we can change the |
| <code>add_lint()</code> call above to something like:</p> |
| <pre><pre class="playground"><code class="language-rust"> |
| <span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>struct_span_err!(self.tcx.sess, self.tcx.span_of_impl(item1).unwrap(), msg) |
| .emit(); |
| <span class="boring">} |
| </span></code></pre></pre> |
| <h4 id="update-tests"><a class="header" href="#update-tests">Update tests</a></h4> |
| <p>Finally, run the test suite. These should be some tests that used to reference |
| the <code>overlapping_inherent_impls</code> lint, those will need to be updated. In |
| general, if the test used to have <code>#[deny(overlapping_inherent_impls)]</code>, that |
| can just be removed.</p> |
| <pre><code>./x.py test |
| </code></pre> |
| <h4 id="all-done"><a class="header" href="#all-done">All done!</a></h4> |
| <p>Open a PR. =)</p> |
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