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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 expanded "><a href="closure.html" class="active"><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 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| <h1 id="closure-expansion-in-rustc"><a class="header" href="#closure-expansion-in-rustc">Closure Expansion in rustc</a></h1> |
| <p>This section describes how rustc handles closures. Closures in Rust are |
| effectively "desugared" into structs that contain the values they use (or |
| references to the values they use) from their creator's stack frame. rustc has |
| the job of figuring out which values a closure uses and how, so it can decide |
| whether to capture a given variable by shared reference, mutable reference, or |
| by move. rustc also has to figure out which of the closure traits (<a href="https://doc.rust-lang.org/std/ops/trait.Fn.html"><code>Fn</code></a>, |
| <a href="https://doc.rust-lang.org/std/ops/trait.FnMut.html"><code>FnMut</code></a>, or <a href="https://doc.rust-lang.org/std/ops/trait.FnOnce.html"><code>FnOnce</code></a>) a closure is capable of |
| implementing.</p> |
| <p>Let's start with a few examples:</p> |
| <h3 id="example-1"><a class="header" href="#example-1">Example 1</a></h3> |
| <p>To start, let's take a look at how the closure in the following example is desugared:</p> |
| <pre><pre class="playground"><code class="language-rust">fn closure(f: impl Fn()) { |
| f(); |
| } |
| |
| fn main() { |
| let x: i32 = 10; |
| closure(|| println!("Hi {}", x)); // The closure just reads x. |
| println!("Value of x after return {}", x); |
| } |
| </code></pre></pre> |
| <p>Let's say the above is the content of a file called <code>immut.rs</code>. If we compile |
| <code>immut.rs</code> using the following command. The <a href="./mir/passes.html"><code>-Z dump-mir=all</code></a> flag will cause |
| <code>rustc</code> to generate and dump the <a href="./mir/index.html">MIR</a> to a directory called <code>mir_dump</code>.</p> |
| <pre><code class="language-console">> rustc +stage1 immut.rs -Z dump-mir=all |
| </code></pre> |
| <p>After we run this command, we will see a newly generated directory in our |
| current working directory called <code>mir_dump</code>, which will contain several files. |
| If we look at file <code>rustc.main.-------.mir_map.0.mir</code>, we will find, among |
| other things, it also contains this line:</p> |
| <pre><code class="language-rust ignore">_4 = &_1; |
| _3 = [closure@immut.rs:7:13: 7:36] { x: move _4 }; |
| </code></pre> |
| <p>Note that in the MIR examples in this chapter, <code>_1</code> is <code>x</code>.</p> |
| <p>Here in first line <code>_4 = &_1;</code>, the <code>mir_dump</code> tells us that <code>x</code> was borrowed |
| as an immutable reference. This is what we would hope as our closure just |
| reads <code>x</code>.</p> |
| <h3 id="example-2"><a class="header" href="#example-2">Example 2</a></h3> |
| <p>Here is another example:</p> |
| <pre><pre class="playground"><code class="language-rust">fn closure(mut f: impl FnMut()) { |
| f(); |
| } |
| |
| fn main() { |
| let mut x: i32 = 10; |
| closure(|| { |
| x += 10; // The closure mutates the value of x |
| println!("Hi {}", x) |
| }); |
| println!("Value of x after return {}", x); |
| } |
| </code></pre></pre> |
| <pre><code class="language-rust ignore">_4 = &mut _1; |
| _3 = [closure@mut.rs:7:13: 10:6] { x: move _4 }; |
| </code></pre> |
| <p>This time along, in the line <code>_4 = &mut _1;</code>, we see that the borrow is changed to mutable borrow. |
| Fair enough! The closure increments <code>x</code> by 10.</p> |
| <h3 id="example-3"><a class="header" href="#example-3">Example 3</a></h3> |
| <p>One more example:</p> |
| <pre><pre class="playground"><code class="language-rust">fn closure(f: impl FnOnce()) { |
| f(); |
| } |
| |
| fn main() { |
| let x = vec![21]; |
| closure(|| { |
| drop(x); // Makes x unusable after the fact. |
| }); |
| // println!("Value of x after return {:?}", x); |
| } |
| </code></pre></pre> |
| <pre><code class="language-rust ignore">_6 = [closure@move.rs:7:13: 9:6] { x: move _1 }; // bb16[3]: scope 1 at move.rs:7:13: 9:6 |
| </code></pre> |
| <p>Here, <code>x</code> is directly moved into the closure and the access to it will not be permitted after the |
| closure.</p> |
| <h2 id="inferences-in-the-compiler"><a class="header" href="#inferences-in-the-compiler">Inferences in the compiler</a></h2> |
| <p>Now let's dive into rustc code and see how all these inferences are done by the compiler.</p> |
| <p>Let's start with defining a term that we will be using quite a bit in the rest of the discussion - |
| <em>upvar</em>. An <strong>upvar</strong> is a variable that is local to the function where the closure is defined. So, |
| in the above examples, <strong>x</strong> will be an upvar to the closure. They are also sometimes referred to as |
| the <em>free variables</em> meaning they are not bound to the context of the closure. |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_query_impl/queries/struct.upvars_mentioned.html"><code>compiler/rustc_middle/src/ty/query/mod.rs</code></a> defines a query called <em>upvars_mentioned</em> |
| for this purpose.</p> |
| <p>Other than lazy invocation, one other thing that distinguishes a closure from a |
| normal function is that it can use the upvars. It borrows these upvars from its surrounding |
| context; therefore the compiler has to determine the upvar's borrow type. The compiler starts with |
| assigning an immutable borrow type and lowers the restriction (that is, changes it from |
| <strong>immutable</strong> to <strong>mutable</strong> to <strong>move</strong>) as needed, based on the usage. In the Example 1 above, the |
| closure only uses the variable for printing but does not modify it in any way and therefore, in the |
| <code>mir_dump</code>, we find the borrow type for the upvar <code>x</code> to be immutable. In example 2, however, the |
| closure modifies <code>x</code> and increments it by some value. Because of this mutation, the compiler, which |
| started off assigning <code>x</code> as an immutable reference type, has to adjust it as a mutable reference. |
| Likewise in the third example, the closure drops the vector and therefore this requires the variable |
| <code>x</code> to be moved into the closure. Depending on the borrow kind, the closure has to implement the |
| appropriate trait: <code>Fn</code> trait for immutable borrow, <code>FnMut</code> for mutable borrow, |
| and <code>FnOnce</code> for move semantics.</p> |
| <p>Most of the code related to the closure is in the |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_typeck/check/upvar/index.html"><code>compiler/rustc_typeck/src/check/upvar.rs</code></a> file and the data structures are |
| declared in the file <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/index.html"><code>compiler/rustc_middle/src/ty/mod.rs</code></a>.</p> |
| <p>Before we go any further, let's discuss how we can examine the flow of control through the rustc |
| codebase. For closures specifically, set the <code>RUST_LOG</code> env variable as below and collect the |
| output in a file:</p> |
| <pre><code class="language-console">> RUST_LOG=rustc_typeck::check::upvar rustc +stage1 -Z dump-mir=all \ |
| <.rs file to compile> 2> <file where the output will be dumped> |
| </code></pre> |
| <p>This uses the stage1 compiler and enables <code>debug!</code> logging for the |
| <code>rustc_typeck::check::upvar</code> module.</p> |
| <p>The other option is to step through the code using lldb or gdb.</p> |
| <ol> |
| <li><code>rust-lldb build/x86_64-apple-darwin/stage1/bin/rustc test.rs</code></li> |
| <li>In lldb: |
| <ol> |
| <li><code>b upvar.rs:134</code> // Setting the breakpoint on a certain line in the upvar.rs file`</li> |
| <li><code>r</code> // Run the program until it hits the breakpoint</li> |
| </ol> |
| </li> |
| </ol> |
| <p>Let's start with <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_typeck/check/upvar/index.html"><code>upvar.rs</code></a>. This file has something called |
| the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_typeck/expr_use_visitor/struct.ExprUseVisitor.html"><code>euv::ExprUseVisitor</code></a> which walks the source of the closure and |
| invokes a callback for each upvar that is borrowed, mutated, or moved.</p> |
| <pre><pre class="playground"><code class="language-rust">fn main() { |
| let mut x = vec![21]; |
| let _cl = || { |
| let y = x[0]; // 1. |
| x[0] += 1; // 2. |
| }; |
| } |
| </code></pre></pre> |
| <p>In the above example, our visitor will be called twice, for the lines marked 1 and 2, once for a |
| shared borrow and another one for a mutable borrow. It will also tell us what was borrowed.</p> |
| <p>The callbacks are defined by implementing the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_typeck/expr_use_visitor/trait.Delegate.html"><code>Delegate</code></a> trait. The |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_typeck/check/upvar/struct.InferBorrowKind.html"><code>InferBorrowKind</code></a> type implements <code>Delegate</code> and keeps a map that |
| records for each upvar which mode of capture was required. The modes of capture |
| can be <code>ByValue</code> (moved) or <code>ByRef</code> (borrowed). For <code>ByRef</code> borrows, the possible |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.BorrowKind.html"><code>BorrowKind</code></a>s are <code>ImmBorrow</code>, <code>UniqueImmBorrow</code>, <code>MutBorrow</code> as defined in the |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/index.html"><code>compiler/rustc_middle/src/ty/mod.rs</code></a>.</p> |
| <p><code>Delegate</code> defines a few different methods (the different callbacks): |
| <strong>consume</strong> for <em>move</em> of a variable, <strong>borrow</strong> for a <em>borrow</em> of some kind |
| (shared or mutable), and <strong>mutate</strong> when we see an <em>assignment</em> of something.</p> |
| <p>All of these callbacks have a common argument <em>cmt</em> which stands for Category, |
| Mutability and Type and is defined in |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_typeck/mem_categorization/index.html"><code>compiler/rustc_middle/src/middle/mem_categorization.rs</code></a>. Borrowing from the code |
| comments, "<code>cmt</code> is a complete categorization of a value indicating where it |
| originated and how it is located, as well as the mutability of the memory in |
| which the value is stored". Based on the callback (consume, borrow etc.), we |
| will call the relevant <code>adjust_upvar_borrow_kind_for_<something></code> and pass the |
| <code>cmt</code> along. Once the borrow type is adjusted, we store it in the table, which |
| basically says what borrows were made for each closure.</p> |
| <pre><code class="language-rust ignore">self.tables |
| .borrow_mut() |
| .upvar_capture_map |
| .extend(delegate.adjust_upvar_captures); |
| </code></pre> |
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