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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 expanded "><a href="type-inference.html" class="active"><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="type-inference"><a class="header" href="#type-inference">Type inference</a></h1> |
| <ul> |
| <li><a href="#a-note-on-terminology">A note on terminology</a></li> |
| <li><a href="#creating-an-inference-context">Creating an inference context</a></li> |
| <li><a href="#inference-variables">Inference variables</a></li> |
| <li><a href="#enforcing-equality--subtyping">Enforcing equality / subtyping</a></li> |
| <li><a href="#trying-equality">"Trying" equality</a></li> |
| <li><a href="#snapshots">Snapshots</a></li> |
| <li><a href="#subtyping-obligations">Subtyping obligations</a></li> |
| <li><a href="#region-constraints">Region constraints</a></li> |
| <li><a href="#extracting-region-constraints">Extracting region constraints</a></li> |
| <li><a href="#lexical-region-resolution">Lexical region resolution</a></li> |
| </ul> |
| <p>Type inference is the process of automatic detection of the type of an |
| expression.</p> |
| <p>It is what allows Rust to work with fewer or no type annotations, |
| making things easier for users:</p> |
| <pre><pre class="playground"><code class="language-rust">fn main() { |
| let mut things = vec![]; |
| things.push("thing"); |
| } |
| </code></pre></pre> |
| <p>Here, the type of <code>things</code> is <em>inferred</em> to be <code>Vec<&str></code> because of the value |
| we push into <code>things</code>.</p> |
| <p>The type inference is based on the standard Hindley-Milner (HM) type inference |
| algorithm, but extended in various way to accommodate subtyping, region |
| inference, and higher-ranked types.</p> |
| <h2 id="a-note-on-terminology"><a class="header" href="#a-note-on-terminology">A note on terminology</a></h2> |
| <p>We use the notation <code>?T</code> to refer to inference variables, also called |
| existential variables.</p> |
| <p>We use the terms "region" and "lifetime" interchangeably. Both refer to |
| the <code>'a</code> in <code>&'a T</code>.</p> |
| <p>The term "bound region" refers to a region that is bound in a function |
| signature, such as the <code>'a</code> in <code>for<'a> fn(&'a u32)</code>. A region is |
| "free" if it is not bound.</p> |
| <h2 id="creating-an-inference-context"><a class="header" href="#creating-an-inference-context">Creating an inference context</a></h2> |
| <p>You create and "enter" an inference context by doing something like |
| the following:</p> |
| <pre><code class="language-rust ignore">tcx.infer_ctxt().enter(|infcx| { |
| // Use the inference context `infcx` here. |
| }) |
| </code></pre> |
| <p>Within the closure, <code>infcx</code> has the type <code>InferCtxt<'cx, 'tcx></code> for some |
| fresh <code>'cx</code>, while <code>'tcx</code> is the same as outside the inference context. |
| (Again, see the <a href="ty.html"><code>ty</code> chapter</a> for more details on this setup.)</p> |
| <p>The <code>tcx.infer_ctxt</code> method actually returns a builder, which means |
| there are some kinds of configuration you can do before the <code>infcx</code> is |
| created. See <code>InferCtxtBuilder</code> for more information.</p> |
| <p><a name="vars"></a></p> |
| <h2 id="inference-variables"><a class="header" href="#inference-variables">Inference variables</a></h2> |
| <p>The main purpose of the inference context is to house a bunch of |
| <strong>inference variables</strong> – these represent types or regions whose precise |
| value is not yet known, but will be uncovered as we perform type-checking.</p> |
| <p>If you're familiar with the basic ideas of unification from H-M type |
| systems, or logic languages like Prolog, this is the same concept. If |
| you're not, you might want to read a tutorial on how H-M type |
| inference works, or perhaps this blog post on |
| <a href="http://smallcultfollowing.com/babysteps/blog/2017/03/25/unification-in-chalk-part-1/">unification in the Chalk project</a>.</p> |
| <p>All told, the inference context stores five kinds of inference variables |
| (as of <!-- date: 2021-06 --> June 2021):</p> |
| <ul> |
| <li>Type variables, which come in three varieties: |
| <ul> |
| <li>General type variables (the most common). These can be unified with any |
| type.</li> |
| <li>Integral type variables, which can only be unified with an integral type, |
| and arise from an integer literal expression like <code>22</code>.</li> |
| <li>Float type variables, which can only be unified with a float type, and |
| arise from a float literal expression like <code>22.0</code>.</li> |
| </ul> |
| </li> |
| <li>Region variables, which represent lifetimes, and arise all over the place.</li> |
| <li>Const variables, which represent constants.</li> |
| </ul> |
| <p>All the type variables work in much the same way: you can create a new |
| type variable, and what you get is <code>Ty<'tcx></code> representing an |
| unresolved type <code>?T</code>. Then later you can apply the various operations |
| that the inferencer supports, such as equality or subtyping, and it |
| will possibly <strong>instantiate</strong> (or <strong>bind</strong>) that <code>?T</code> to a specific |
| value as a result.</p> |
| <p>The region variables work somewhat differently, and are described |
| below in a separate section.</p> |
| <h2 id="enforcing-equality--subtyping"><a class="header" href="#enforcing-equality--subtyping">Enforcing equality / subtyping</a></h2> |
| <p>The most basic operations you can perform in the type inferencer is |
| <strong>equality</strong>, which forces two types <code>T</code> and <code>U</code> to be the same. The |
| recommended way to add an equality constraint is to use the <code>at</code> |
| method, roughly like so:</p> |
| <pre><code class="language-rust ignore">infcx.at(...).eq(t, u); |
| </code></pre> |
| <p>The first <code>at()</code> call provides a bit of context, i.e. why you are |
| doing this unification, and in what environment, and the <code>eq</code> method |
| performs the actual equality constraint.</p> |
| <p>When you equate things, you force them to be precisely equal. Equating |
| returns an <code>InferResult</code> – if it returns <code>Err(err)</code>, then equating |
| failed, and the enclosing <code>TypeError</code> will tell you what went wrong.</p> |
| <p>The success case is perhaps more interesting. The "primary" return |
| type of <code>eq</code> is <code>()</code> – that is, when it succeeds, it doesn't return a |
| value of any particular interest. Rather, it is executed for its |
| side-effects of constraining type variables and so forth. However, the |
| actual return type is not <code>()</code>, but rather <code>InferOk<()></code>. The |
| <code>InferOk</code> type is used to carry extra trait obligations – your job is |
| to ensure that these are fulfilled (typically by enrolling them in a |
| fulfillment context). See the <a href="traits/resolution.html">trait chapter</a> for more background on that.</p> |
| <p>You can similarly enforce subtyping through <code>infcx.at(..).sub(..)</code>. The same |
| basic concepts as above apply.</p> |
| <h2 id="trying-equality"><a class="header" href="#trying-equality">"Trying" equality</a></h2> |
| <p>Sometimes you would like to know if it is <em>possible</em> to equate two |
| types without error. You can test that with <code>infcx.can_eq</code> (or |
| <code>infcx.can_sub</code> for subtyping). If this returns <code>Ok</code>, then equality |
| is possible – but in all cases, any side-effects are reversed.</p> |
| <p>Be aware, though, that the success or failure of these methods is always |
| <strong>modulo regions</strong>. That is, two types <code>&'a u32</code> and <code>&'b u32</code> will |
| return <code>Ok</code> for <code>can_eq</code>, even if <code>'a != 'b</code>. This falls out from the |
| "two-phase" nature of how we solve region constraints.</p> |
| <h2 id="snapshots"><a class="header" href="#snapshots">Snapshots</a></h2> |
| <p>As described in the previous section on <code>can_eq</code>, often it is useful |
| to be able to do a series of operations and then roll back their |
| side-effects. This is done for various reasons: one of them is to be |
| able to backtrack, trying out multiple possibilities before settling |
| on which path to take. Another is in order to ensure that a series of |
| smaller changes take place atomically or not at all.</p> |
| <p>To allow for this, the inference context supports a <code>snapshot</code> method. |
| When you call it, it will start recording changes that occur from the |
| operations you perform. When you are done, you can either invoke |
| <code>rollback_to</code>, which will undo those changes, or else <code>confirm</code>, which |
| will make them permanent. Snapshots can be nested as long as you follow |
| a stack-like discipline.</p> |
| <p>Rather than use snapshots directly, it is often helpful to use the |
| methods like <code>commit_if_ok</code> or <code>probe</code> that encapsulate higher-level |
| patterns.</p> |
| <h2 id="subtyping-obligations"><a class="header" href="#subtyping-obligations">Subtyping obligations</a></h2> |
| <p>One thing worth discussing is subtyping obligations. When you force |
| two types to be a subtype, like <code>?T <: i32</code>, we can often convert those |
| into equality constraints. This follows from Rust's rather limited notion |
| of subtyping: so, in the above case, <code>?T <: i32</code> is equivalent to <code>?T = i32</code>.</p> |
| <p>However, in some cases we have to be more careful. For example, when |
| regions are involved. So if you have <code>?T <: &'a i32</code>, what we would do |
| is to first "generalize" <code>&'a i32</code> into a type with a region variable: |
| <code>&'?b i32</code>, and then unify <code>?T</code> with that (<code>?T = &'?b i32</code>). We then |
| relate this new variable with the original bound:</p> |
| <pre><code class="language-text">&'?b i32 <: &'a i32 |
| </code></pre> |
| <p>This will result in a region constraint (see below) of <code>'?b: 'a</code>.</p> |
| <p>One final interesting case is relating two unbound type variables, |
| like <code>?T <: ?U</code>. In that case, we can't make progress, so we enqueue |
| an obligation <code>Subtype(?T, ?U)</code> and return it via the <code>InferOk</code> |
| mechanism. You'll have to try again when more details about <code>?T</code> or |
| <code>?U</code> are known.</p> |
| <h2 id="region-constraints"><a class="header" href="#region-constraints">Region constraints</a></h2> |
| <p>Regions are inferenced somewhat differently from types. Rather than |
| eagerly unifying things, we simply collect constraints as we go, but |
| make (almost) no attempt to solve regions. These constraints have the |
| form of an "outlives" constraint:</p> |
| <pre><code class="language-text">'a: 'b |
| </code></pre> |
| <p>Actually the code tends to view them as a subregion relation, but it's the same |
| idea:</p> |
| <pre><code class="language-text">'b <= 'a |
| </code></pre> |
| <p>(There are various other kinds of constraints, such as "verifys"; see |
| the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/region_constraints/index.html"><code>region_constraints</code></a> module for details.)</p> |
| <p>There is one case where we do some amount of eager unification. If you have an |
| equality constraint between two regions</p> |
| <pre><code class="language-text">'a = 'b |
| </code></pre> |
| <p>we will record that fact in a unification table. You can then use |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/region_constraints/struct.RegionConstraintCollector.html#method.opportunistic_resolve_var"><code>opportunistic_resolve_var</code></a> to convert <code>'b</code> to <code>'a</code> (or vice |
| versa). This is sometimes needed to ensure termination of fixed-point |
| algorithms.</p> |
| <h2 id="extracting-region-constraints"><a class="header" href="#extracting-region-constraints">Extracting region constraints</a></h2> |
| <p>Ultimately, region constraints are only solved at the very end of |
| type-checking, once all other constraints are known. There are two |
| ways to solve region constraints right now: lexical and |
| non-lexical. Eventually there will only be one.</p> |
| <p>To solve <strong>lexical</strong> region constraints, you invoke |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/struct.InferCtxt.html#method.resolve_regions_and_report_errors"><code>resolve_regions_and_report_errors</code></a>. This "closes" the region |
| constraint process and invokes the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/lexical_region_resolve/index.html"><code>lexical_region_resolve</code></a> code. Once |
| this is done, any further attempt to equate or create a subtyping |
| relationship will yield an ICE.</p> |
| <p>Non-lexical region constraints are not handled within the inference |
| context. Instead, the NLL solver (actually, the MIR type-checker) |
| invokes <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/struct.InferCtxt.html#method.take_and_reset_region_constraints"><code>take_and_reset_region_constraints</code></a> periodically. This |
| extracts all of the outlives constraints from the region solver, but |
| leaves the set of variables intact. This is used to get <em>just</em> the |
| region constraints that resulted from some particular point in the |
| program, since the NLL solver needs to know not just <em>what</em> regions |
| were subregions, but also <em>where</em>. Finally, the NLL solver invokes |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/struct.InferCtxt.html#method.take_region_var_origins"><code>take_region_var_origins</code></a>, which "closes" the region constraint |
| process in the same way as normal solving.</p> |
| <h2 id="lexical-region-resolution"><a class="header" href="#lexical-region-resolution">Lexical region resolution</a></h2> |
| <p>Lexical region resolution is done by initially assigning each region |
| variable to an empty value. We then process each outlives constraint |
| repeatedly, growing region variables until a fixed-point is reached. |
| Region variables can be grown using a least-upper-bound relation on |
| the region lattice in a fairly straightforward fashion.</p> |
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