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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 "><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 expanded "><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 expanded "><a href="../borrow_check/region_inference.html" class="active"><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 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| <h1 id="region-inference-nll"><a class="header" href="#region-inference-nll">Region inference (NLL)</a></h1> |
| <ul> |
| <li><a href="#universal-regions">Universal regions</a></li> |
| <li><a href="#region-variables">Region variables</a></li> |
| <li><a href="#constraints">Constraints</a></li> |
| <li><a href="#inference-overview">Inference Overview</a> |
| <ul> |
| <li><a href="#example">Example</a></li> |
| <li><a href="#some-details">Some details</a></li> |
| </ul> |
| </li> |
| </ul> |
| <p>The MIR-based region checking code is located in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/index.html">the <code>rustc_mir::borrow_check</code> |
| module</a>.</p> |
| <p>The MIR-based region analysis consists of two major functions:</p> |
| <ul> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/nll/fn.replace_regions_in_mir.html"><code>replace_regions_in_mir</code></a>, invoked first, has two jobs: |
| <ul> |
| <li>First, it finds the set of regions that appear within the |
| signature of the function (e.g., <code>'a</code> in <code>fn foo<'a>(&'a u32) { ... }</code>). These are called the "universal" or "free" regions – in |
| particular, they are the regions that <a href="../appendix/background.html#free-vs-bound">appear free</a> in the |
| function body.</li> |
| <li>Second, it replaces all the regions from the function body with |
| fresh inference variables. This is because (presently) those |
| regions are the results of lexical region inference and hence are |
| not of much interest. The intention is that – eventually – they |
| will be "erased regions" (i.e., no information at all), since we |
| won't be doing lexical region inference at all.</li> |
| </ul> |
| </li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/nll/fn.compute_regions.html"><code>compute_regions</code></a>, invoked second: this is given as argument the |
| results of move analysis. It has the job of computing values for all |
| the inference variables that <code>replace_regions_in_mir</code> introduced. |
| <ul> |
| <li>To do that, it first runs the <a href="./type_check.html">MIR type checker</a>. This is |
| basically a normal type-checker but specialized to MIR, which is |
| much simpler than full Rust, of course. Running the MIR type |
| checker will however create various <a href="./region_inference/constraint_propagation.html">constraints</a> between region |
| variables, indicating their potential values and relationships to |
| one another.</li> |
| <li>After this, we perform <a href="./region_inference/constraint_propagation.html">constraint propagation</a> by creating a |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/region_infer/struct.RegionInferenceContext.html"><code>RegionInferenceContext</code></a> and invoking its <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/region_infer/struct.RegionInferenceContext.html#method.solve"><code>solve</code></a> |
| method.</li> |
| <li>The <a href="https://rust-lang.github.io/rfcs/2094-nll.html">NLL RFC</a> also includes fairly thorough (and hopefully readable) |
| coverage.</li> |
| </ul> |
| </li> |
| </ul> |
| <h2 id="universal-regions"><a class="header" href="#universal-regions">Universal regions</a></h2> |
| <p>The <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/universal_regions/struct.UniversalRegions.html"><code>UniversalRegions</code></a> type represents a collection of <em>universal</em> regions |
| corresponding to some MIR <code>DefId</code>. It is constructed in |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/nll/fn.replace_regions_in_mir.html"><code>replace_regions_in_mir</code></a> when we replace all regions with fresh inference |
| variables. <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/universal_regions/struct.UniversalRegions.html"><code>UniversalRegions</code></a> contains indices for all the free regions in |
| the given MIR along with any relationships that are <em>known</em> to hold between |
| them (e.g. implied bounds, where clauses, etc.).</p> |
| <p>For example, given the MIR for the following function:</p> |
| <pre><pre class="playground"><code class="language-rust"> |
| <span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>fn foo<'a>(x: &'a u32) { |
| // ... |
| } |
| <span class="boring">} |
| </span></code></pre></pre> |
| <p>we would create a universal region for <code>'a</code> and one for <code>'static</code>. There may |
| also be some complications for handling closures, but we will ignore those for |
| the moment.</p> |
| <p>TODO: write about <em>how</em> these regions are computed.</p> |
| <p><a name="region-variables"></a></p> |
| <h2 id="region-variables"><a class="header" href="#region-variables">Region variables</a></h2> |
| <p>The value of a region can be thought of as a <strong>set</strong>. This set contains all |
| points in the MIR where the region is valid along with any regions that are |
| outlived by this region (e.g. if <code>'a: 'b</code>, then <code>end('b)</code> is in the set for |
| <code>'a</code>); we call the domain of this set a <code>RegionElement</code>. In the code, the value |
| for all regions is maintained in <a href="https://github.com/rust-lang/rust/tree/master/compiler/rustc_mir/src/borrow_check/region_infer/">the |
| <code>rustc_mir::borrow_check::nll::region_infer</code> module</a>. For each region we |
| maintain a set storing what elements are present in its value (to make this |
| efficient, we give each kind of element an index, the <code>RegionElementIndex</code>, and |
| use sparse bitsets).</p> |
| <p>The kinds of region elements are as follows:</p> |
| <ul> |
| <li>Each <strong><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/struct.Location.html"><code>location</code></a></strong> in the MIR control-flow graph: a location is just |
| the pair of a basic block and an index. This identifies the point |
| <strong>on entry</strong> to the statement with that index (or the terminator, if |
| the index is equal to <code>statements.len()</code>).</li> |
| <li>There is an element <code>end('a)</code> for each universal region <code>'a</code>, |
| corresponding to some portion of the caller's (or caller's caller, |
| etc) control-flow graph.</li> |
| <li>Similarly, there is an element denoted <code>end('static)</code> corresponding |
| to the remainder of program execution after this function returns.</li> |
| <li>There is an element <code>!1</code> for each placeholder region <code>!1</code>. This |
| corresponds (intuitively) to some unknown set of other elements – |
| for details on placeholders, see the section |
| <a href="region_inference/placeholders_and_universes.html">placeholders and universes</a>.</li> |
| </ul> |
| <h2 id="constraints"><a class="header" href="#constraints">Constraints</a></h2> |
| <p>Before we can infer the value of regions, we need to collect |
| constraints on the regions. The full set of constraints is described |
| in <a href="./region_inference/constraint_propagation.html">the section on constraint propagation</a>, but the two most |
| common sorts of constraints are:</p> |
| <ol> |
| <li>Outlives constraints. These are constraints that one region outlives another |
| (e.g. <code>'a: 'b</code>). Outlives constraints are generated by the <a href="./type_check.html">MIR type |
| checker</a>.</li> |
| <li>Liveness constraints. Each region needs to be live at points where it can be |
| used. These constraints are collected by <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/constraint_generation/fn.generate_constraints.html"><code>generate_constraints</code></a>.</li> |
| </ol> |
| <h2 id="inference-overview"><a class="header" href="#inference-overview">Inference Overview</a></h2> |
| <p>So how do we compute the contents of a region? This process is called <em>region |
| inference</em>. The high-level idea is pretty simple, but there are some details we |
| need to take care of.</p> |
| <p>Here is the high-level idea: we start off each region with the MIR locations we |
| know must be in it from the liveness constraints. From there, we use all of the |
| outlives constraints computed from the type checker to <em>propagate</em> the |
| constraints: for each region <code>'a</code>, if <code>'a: 'b</code>, then we add all elements of |
| <code>'b</code> to <code>'a</code>, including <code>end('b)</code>. This all happens in |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/region_infer/struct.RegionInferenceContext.html#method.propagate_constraints"><code>propagate_constraints</code></a>.</p> |
| <p>Then, we will check for errors. We first check that type tests are satisfied by |
| calling <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/region_infer/struct.RegionInferenceContext.html#method.check_type_tests"><code>check_type_tests</code></a>. This checks constraints like <code>T: 'a</code>. Second, we |
| check that universal regions are not "too big". This is done by calling |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/region_infer/struct.RegionInferenceContext.html#method.check_universal_regions"><code>check_universal_regions</code></a>. This checks that for each region <code>'a</code> if <code>'a</code> |
| contains the element <code>end('b)</code>, then we must already know that <code>'a: 'b</code> holds |
| (e.g. from a where clause). If we don't already know this, that is an error... |
| well, almost. There is some special handling for closures that we will discuss |
| later.</p> |
| <h3 id="example"><a class="header" href="#example">Example</a></h3> |
| <p>Consider the following example:</p> |
| <pre><code class="language-rust ignore">fn foo<'a, 'b>(x: &'a usize) -> &'b usize { |
| x |
| } |
| </code></pre> |
| <p>Clearly, this should not compile because we don't know if <code>'a</code> outlives <code>'b</code> |
| (if it doesn't then the return value could be a dangling reference).</p> |
| <p>Let's back up a bit. We need to introduce some free inference variables (as is |
| done in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/nll/fn.replace_regions_in_mir.html"><code>replace_regions_in_mir</code></a>). This example doesn't use the exact regions |
| produced, but it (hopefully) is enough to get the idea across.</p> |
| <pre><code class="language-rust ignore">fn foo<'a, 'b>(x: &'a /* '#1 */ usize) -> &'b /* '#3 */ usize { |
| x // '#2, location L1 |
| } |
| </code></pre> |
| <p>Some notation: <code>'#1</code>, <code>'#3</code>, and <code>'#2</code> represent the universal regions for the |
| argument, return value, and the expression <code>x</code>, respectively. Additionally, I |
| will call the location of the expression <code>x</code> <code>L1</code>.</p> |
| <p>So now we can use the liveness constraints to get the following starting points:</p> |
| <table><thead><tr><th>Region</th><th>Contents</th></tr></thead><tbody> |
| <tr><td>'#1</td><td></td></tr> |
| <tr><td>'#2</td><td><code>L1</code></td></tr> |
| <tr><td>'#3</td><td><code>L1</code></td></tr> |
| </tbody></table> |
| <p>Now we use the outlives constraints to expand each region. Specifically, we |
| know that <code>'#2: '#3</code> ...</p> |
| <table><thead><tr><th>Region</th><th>Contents</th></tr></thead><tbody> |
| <tr><td>'#1</td><td><code>L1</code></td></tr> |
| <tr><td>'#2</td><td><code>L1, end('#3) // add contents of '#3 and end('#3)</code></td></tr> |
| <tr><td>'#3</td><td><code>L1</code></td></tr> |
| </tbody></table> |
| <p>... and <code>'#1: '#2</code>, so ...</p> |
| <table><thead><tr><th>Region</th><th>Contents</th></tr></thead><tbody> |
| <tr><td>'#1</td><td><code>L1, end('#2), end('#3) // add contents of '#2 and end('#2)</code></td></tr> |
| <tr><td>'#2</td><td><code>L1, end('#3)</code></td></tr> |
| <tr><td>'#3</td><td><code>L1</code></td></tr> |
| </tbody></table> |
| <p>Now, we need to check that no regions were too big (we don't have any type |
| tests to check in this case). Notice that <code>'#1</code> now contains <code>end('#3)</code>, but |
| we have no <code>where</code> clause or implied bound to say that <code>'a: 'b</code>... that's an |
| error!</p> |
| <h3 id="some-details"><a class="header" href="#some-details">Some details</a></h3> |
| <p>The <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/region_infer/struct.RegionInferenceContext.html"><code>RegionInferenceContext</code></a> type contains all of the information needed to |
| do inference, including the universal regions from <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/nll/fn.replace_regions_in_mir.html"><code>replace_regions_in_mir</code></a> and |
| the constraints computed for each region. It is constructed just after we |
| compute the liveness constraints.</p> |
| <p>Here are some of the fields of the struct:</p> |
| <ul> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/region_infer/struct.RegionInferenceContext.html#structfield.constraints"><code>constraints</code></a>: contains all the outlives constraints.</li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/region_infer/struct.RegionInferenceContext.html#structfield.liveness_constraints"><code>liveness_constraints</code></a>: contains all the liveness constraints.</li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/region_infer/struct.RegionInferenceContext.html#structfield.universal_regions"><code>universal_regions</code></a>: contains the <code>UniversalRegions</code> returned by |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/nll/fn.replace_regions_in_mir.html"><code>replace_regions_in_mir</code></a>.</li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/region_infer/struct.RegionInferenceContext.html#structfield.universal_region_relations"><code>universal_region_relations</code></a>: contains relations known to be true about |
| universal regions. For example, if we have a where clause that <code>'a: 'b</code>, that |
| relation is assumed to be true while borrow checking the implementation (it |
| is checked at the caller), so <code>universal_region_relations</code> would contain <code>'a: 'b</code>.</li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/region_infer/struct.RegionInferenceContext.html#structfield.type_tests"><code>type_tests</code></a>: contains some constraints on types that we must check after |
| inference (e.g. <code>T: 'a</code>).</li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/region_infer/struct.RegionInferenceContext.html#structfield.closure_bounds_mapping"><code>closure_bounds_mapping</code></a>: used for propagating region constraints from |
| closures back out to the creator of the closure.</li> |
| </ul> |
| <p>TODO: should we discuss any of the others fields? What about the SCCs?</p> |
| <p>Ok, now that we have constructed a <code>RegionInferenceContext</code>, we can do |
| inference. This is done by calling the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/region_infer/struct.RegionInferenceContext.html#method.solve"><code>solve</code></a> method on the context. This |
| is where we call <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/region_infer/struct.RegionInferenceContext.html#method.propagate_constraints"><code>propagate_constraints</code></a> and then check the resulting type |
| tests and universal regions, as discussed above.</p> |
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