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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 "><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 expanded "><a href="stability.html" class="active"><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="stability-attributes"><a class="header" href="#stability-attributes">Stability attributes</a></h1> |
| <ul> |
| <li><a href="#unstable">unstable</a></li> |
| <li><a href="#stable">stable</a></li> |
| <li><a href="#rustc_const_unstable">rustc_const_unstable</a></li> |
| <li><a href="#rustc_const_stable">rustc_const_stable</a></li> |
| <li><a href="#stabilizing-a-library-feature">Stabilizing a library feature</a></li> |
| <li><a href="#allow_internal_unstable">allow_internal_unstable</a></li> |
| <li><a href="#rustc_allow_const_fn_unstable">rustc_allow_const_fn_unstable</a></li> |
| <li><a href="#staged_api">staged_api</a></li> |
| <li><a href="#rustc_deprecated">rustc_deprecated</a></li> |
| </ul> |
| <p>This section is about the stability attributes and schemes that allow stable |
| APIs to use unstable APIs internally in the rustc standard library.</p> |
| <p>For instructions on stabilizing a language feature see <a href="./stabilization_guide.html">Stabilizing |
| Features</a>.</p> |
| <h2 id="unstable"><a class="header" href="#unstable">unstable</a></h2> |
| <p>The <code>#[unstable(feature = "foo", issue = "1234", reason = "lorem ipsum")]</code> |
| attribute explicitly marks an item as unstable. Items that are marked as |
| "unstable" cannot be used without a corresponding <code>#![feature]</code> attribute on |
| the crate, even on a nightly compiler. This restriction only applies across |
| crate boundaries, unstable items may be used within the crate that defines |
| them.</p> |
| <p>The <code>issue</code> field specifies the associated GitHub <a href="https://github.com/rust-lang/rust/issues">issue number</a>. This field is |
| required and all unstable features should have an associated tracking issue. In |
| rare cases where there is no sensible value <code>issue = "none"</code> is used.</p> |
| <p>The <code>unstable</code> attribute infects all sub-items, where the attribute doesn't |
| have to be reapplied. So if you apply this to a module, all items in the module |
| will be unstable.</p> |
| <p>You can make specific sub-items stable by using the <code>#[stable]</code> attribute on |
| them. The stability scheme works similarly to how <code>pub</code> works. You can have |
| public functions of nonpublic modules and you can have stable functions in |
| unstable modules or vice versa.</p> |
| <p>Note, however, that due to a <a href="https://github.com/rust-lang/rust/issues/15702">rustc bug</a>, stable items inside unstable modules |
| <em>are</em> available to stable code in that location! So, for example, stable code |
| can import <code>core::intrinsics::transmute</code> even though <code>intrinsics</code> is an |
| unstable module. Thus, this kind of nesting should be avoided when possible.</p> |
| <p>The <code>unstable</code> attribute may also have the <code>soft</code> value, which makes it a |
| future-incompatible deny-by-default lint instead of a hard error. This is used |
| by the <code>bench</code> attribute which was accidentally accepted in the past. This |
| prevents breaking dependencies by leveraging Cargo's lint capping.</p> |
| <h2 id="stable"><a class="header" href="#stable">stable</a></h2> |
| <p>The <code>#[stable(feature = "foo", since = "1.420.69")]</code> attribute explicitly |
| marks an item as stabilized. Note that stable functions may use unstable things in their body.</p> |
| <h2 id="rustc_const_unstable"><a class="header" href="#rustc_const_unstable">rustc_const_unstable</a></h2> |
| <p>The <code>#[rustc_const_unstable(feature = "foo", issue = "1234", reason = "lorem ipsum")]</code> |
| has the same interface as the <code>unstable</code> attribute. It is used to mark |
| <code>const fn</code> as having their constness be unstable. This allows you to make a |
| function stable without stabilizing its constness or even just marking an existing |
| stable function as <code>const fn</code> without instantly stabilizing the <code>const fn</code>ness.</p> |
| <p>Furthermore this attribute is needed to mark an intrinsic as <code>const fn</code>, because |
| there's no way to add <code>const</code> to functions in <code>extern</code> blocks for now.</p> |
| <h2 id="rustc_const_stable"><a class="header" href="#rustc_const_stable">rustc_const_stable</a></h2> |
| <p>The <code>#[rustc_const_stable(feature = "foo", since = "1.420.69")]</code> attribute explicitly marks |
| a <code>const fn</code> as having its constness be <code>stable</code>. This attribute can make sense |
| even on an <code>unstable</code> function, if that function is called from another |
| <code>rustc_const_stable</code> function.</p> |
| <p>Furthermore this attribute is needed to mark an intrinsic as callable from |
| <code>rustc_const_stable</code> functions.</p> |
| <h2 id="stabilizing-a-library-feature"><a class="header" href="#stabilizing-a-library-feature">Stabilizing a library feature</a></h2> |
| <p>To stabilize a feature, follow these steps:</p> |
| <ol start="0"> |
| <li>Ask a <strong>@T-libs-api</strong> member to start an FCP on the tracking issue and wait for |
| the FCP to complete (with <code>disposition-merge</code>).</li> |
| <li>Change <code>#[unstable(...)]</code> to <code>#[stable(since = "version")]</code>. |
| <code>version</code> should be the <em>current nightly</em>, i.e. stable+2. You can see which version is |
| the current nightly <a href="https://forge.rust-lang.org/#current-release-versions">on Forge</a>.</li> |
| <li>Remove <code>#![feature(...)]</code> from any test or doc-test for this API. If the feature is used in the |
| compiler or tools, remove it from there as well.</li> |
| <li>If applicable, change <code>#[rustc_const_unstable(...)]</code> to |
| <code>#[rustc_const_stable(since = "version")]</code>.</li> |
| <li>Open a PR against <code>rust-lang/rust</code>. |
| <ul> |
| <li>Add the appropriate labels: <code>@rustbot modify labels: +T-libs-api</code>.</li> |
| <li>Link to the tracking issue and say "Closes #XXXXX".</li> |
| </ul> |
| </li> |
| </ol> |
| <p>You can see an example of stabilizing a feature with |
| <a href="https://github.com/rust-lang/rust/issues/81656">tracking issue #81656 with FCP</a> |
| and the associated |
| <a href="https://github.com/rust-lang/rust/pull/84642">implementation PR #84642</a>.</p> |
| <h2 id="allow_internal_unstable"><a class="header" href="#allow_internal_unstable">allow_internal_unstable</a></h2> |
| <p>Macros and compiler desugarings expose their bodies to the call |
| site. To work around not being able to use unstable things in the standard |
| library's macros, there's the <code>#[allow_internal_unstable(feature1, feature2)]</code> |
| attribute that allows the given features to be used in stable macros.</p> |
| <h2 id="rustc_allow_const_fn_unstable"><a class="header" href="#rustc_allow_const_fn_unstable">rustc_allow_const_fn_unstable</a></h2> |
| <p><code>const fn</code>, while not directly exposing their body to the world, are going to get |
| evaluated at compile time in stable crates. If their body does something const-unstable, |
| that could lock us into certain features indefinitely by accident. Thus no unstable const |
| features are allowed inside stable <code>const fn</code>.</p> |
| <p>However, sometimes we do know that a feature will get |
| stabilized, just not when, or there is a stable (but e.g. runtime-slow) workaround, so we |
| could always fall back to some stable version if we scrapped the unstable feature. |
| In those cases, the rustc_allow_const_fn_unstable attribute can be used to allow some |
| unstable features in the body of a stable <code>const fn</code>.</p> |
| <p>You also need to take care to uphold the <code>const fn</code> invariant that calling it at runtime and |
| compile-time needs to behave the same (see also <a href="https://www.ralfj.de/blog/2018/07/19/const.html">this blog post</a>). This means that you |
| may not create a <code>const fn</code> that e.g. transmutes a memory address to an integer, |
| because the addresses of things are nondeterministic and often unknown at |
| compile-time.</p> |
| <p>Always ping @rust-lang/wg-const-eval if you are adding more |
| <code>rustc_allow_const_fn_unstable</code> attributes to any <code>const fn</code>.</p> |
| <h2 id="staged_api"><a class="header" href="#staged_api">staged_api</a></h2> |
| <p>Any crate that uses the <code>stable</code>, <code>unstable</code>, or <code>rustc_deprecated</code> attributes |
| must include the <code>#![feature(staged_api)]</code> attribute on the crate.</p> |
| <h2 id="rustc_deprecated"><a class="header" href="#rustc_deprecated">rustc_deprecated</a></h2> |
| <p>The deprecation system shares the same infrastructure as the stable/unstable |
| attributes. The <code>rustc_deprecated</code> attribute is similar to the <a href="https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-deprecated-attribute"><code>deprecated</code> |
| attribute</a>. It was previously called <code>deprecated</code>, but was split off when |
| <code>deprecated</code> was stabilized. The <code>deprecated</code> attribute cannot be used in a |
| <code>staged_api</code> crate, <code>rustc_deprecated</code> must be used instead. The deprecated |
| item must also have a <code>stable</code> or <code>unstable</code> attribute.</p> |
| <p><code>rustc_deprecated</code> has the following form:</p> |
| <pre><code class="language-rust ignore">#[rustc_deprecated( |
| since = "1.38.0", |
| reason = "explanation for deprecation", |
| suggestion = "other_function" |
| )] |
| </code></pre> |
| <p>The <code>suggestion</code> field is optional. If given, it should be a string that can be |
| used as a machine-applicable suggestion to correct the warning. This is |
| typically used when the identifier is renamed, but no other significant changes |
| are necessary.</p> |
| <p>Another difference from the <code>deprecated</code> attribute is that the <code>since</code> field is |
| actually checked against the current version of <code>rustc</code>. If <code>since</code> is in a |
| future version, then the <code>deprecated_in_future</code> lint is triggered which is |
| default <code>allow</code>, but most of the standard library raises it to a warning with |
| <code>#![warn(deprecated_in_future)]</code>.</p> |
| |
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