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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 expanded "><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 expanded "><a href="generics.html" class="active"><strong aria-hidden="true">38.1.</strong> Generics and substitutions</a></li><li class="chapter-item "><a href="ty-fold.html"><strong aria-hidden="true">38.2.</strong> TypeFolder and TypeFoldable</a></li><li class="chapter-item "><a href="generic_arguments.html"><strong aria-hidden="true">38.3.</strong> Generic arguments</a></li><li class="chapter-item "><a href="constants.html"><strong aria-hidden="true">38.4.</strong> Constants in the type system</a></li></ol></li><li class="chapter-item "><a href="type-inference.html"><strong aria-hidden="true">39.</strong> Type inference</a></li><li class="chapter-item "><a href="traits/resolution.html"><strong aria-hidden="true">40.</strong> Trait solving</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="early-late-bound.html"><strong aria-hidden="true">40.1.</strong> Early and Late Bound Parameters</a></li><li class="chapter-item "><a href="traits/hrtb.html"><strong aria-hidden="true">40.2.</strong> Higher-ranked trait bounds</a></li><li class="chapter-item "><a href="traits/caching.html"><strong aria-hidden="true">40.3.</strong> Caching subtleties</a></li><li class="chapter-item "><a href="traits/specialization.html"><strong aria-hidden="true">40.4.</strong> Specialization</a></li><li class="chapter-item "><a href="traits/chalk.html"><strong aria-hidden="true">40.5.</strong> Chalk-based trait solving</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="traits/lowering-to-logic.html"><strong aria-hidden="true">40.5.1.</strong> Lowering to logic</a></li><li class="chapter-item "><a href="traits/goals-and-clauses.html"><strong aria-hidden="true">40.5.2.</strong> Goals and clauses</a></li><li class="chapter-item "><a href="traits/canonical-queries.html"><strong aria-hidden="true">40.5.3.</strong> Canonical queries</a></li></ol></li></ol></li><li class="chapter-item "><a href="type-checking.html"><strong aria-hidden="true">41.</strong> Type checking</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="method-lookup.html"><strong aria-hidden="true">41.1.</strong> Method Lookup</a></li><li class="chapter-item "><a href="variance.html"><strong aria-hidden="true">41.2.</strong> Variance</a></li><li class="chapter-item "><a href="opaque-types-type-alias-impl-trait.html"><strong aria-hidden="true">41.3.</strong> Opaque Types</a></li></ol></li><li class="chapter-item "><a href="pat-exhaustive-checking.html"><strong aria-hidden="true">42.</strong> Pattern and Exhaustiveness Checking</a></li><li class="chapter-item "><a href="mir/dataflow.html"><strong aria-hidden="true">43.</strong> MIR dataflow</a></li><li class="chapter-item "><a href="borrow_check.html"><strong aria-hidden="true">44.</strong> The borrow checker</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="borrow_check/moves_and_initialization.html"><strong aria-hidden="true">44.1.</strong> Tracking moves and initialization</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="borrow_check/moves_and_initialization/move_paths.html"><strong aria-hidden="true">44.1.1.</strong> Move paths</a></li></ol></li><li class="chapter-item "><a href="borrow_check/type_check.html"><strong aria-hidden="true">44.2.</strong> MIR type checker</a></li><li class="chapter-item "><a href="borrow_check/region_inference.html"><strong aria-hidden="true">44.3.</strong> Region inference</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="borrow_check/region_inference/constraint_propagation.html"><strong aria-hidden="true">44.3.1.</strong> Constraint propagation</a></li><li class="chapter-item "><a href="borrow_check/region_inference/lifetime_parameters.html"><strong aria-hidden="true">44.3.2.</strong> Lifetime parameters</a></li><li class="chapter-item "><a href="borrow_check/region_inference/member_constraints.html"><strong aria-hidden="true">44.3.3.</strong> Member constraints</a></li><li class="chapter-item "><a href="borrow_check/region_inference/placeholders_and_universes.html"><strong aria-hidden="true">44.3.4.</strong> Placeholders and universes</a></li><li class="chapter-item "><a href="borrow_check/region_inference/closure_constraints.html"><strong aria-hidden="true">44.3.5.</strong> Closure constraints</a></li><li class="chapter-item "><a href="borrow_check/region_inference/error_reporting.html"><strong aria-hidden="true">44.3.6.</strong> Error reporting</a></li></ol></li><li class="chapter-item "><a href="borrow_check/two_phase_borrows.html"><strong aria-hidden="true">44.4.</strong> Two-phase-borrows</a></li></ol></li><li class="chapter-item "><a href="param_env.html"><strong aria-hidden="true">45.</strong> Parameter Environments</a></li><li class="chapter-item "><a href="diagnostics.html"><strong aria-hidden="true">46.</strong> Errors and Lints</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="diagnostics/sessiondiagnostic.html"><strong aria-hidden="true">46.1.</strong> Creating Errors With SessionDiagnostic</a></li><li class="chapter-item "><a href="diagnostics/lintstore.html"><strong aria-hidden="true">46.2.</strong> LintStore</a></li><li class="chapter-item "><a href="diagnostics/diagnostic-codes.html"><strong aria-hidden="true">46.3.</strong> Diagnostic Codes</a></li><li class="chapter-item "><a href="diagnostics/diagnostic-items.html"><strong aria-hidden="true">46.4.</strong> Diagnostic Items</a></li></ol></li><li class="chapter-item "><li class="part-title">MIR to Binaries</li><li class="chapter-item "><a href="part-5-intro.html"><strong aria-hidden="true">47.</strong> Prologue</a></li><li class="chapter-item "><a href="mir/optimizations.html"><strong aria-hidden="true">48.</strong> MIR optimizations</a></li><li class="chapter-item "><a href="mir/debugging.html"><strong aria-hidden="true">49.</strong> Debugging</a></li><li class="chapter-item "><a href="const-eval.html"><strong aria-hidden="true">50.</strong> Constant evaluation</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="miri.html"><strong aria-hidden="true">50.1.</strong> miri const evaluator</a></li></ol></li><li class="chapter-item "><a href="backend/monomorph.html"><strong aria-hidden="true">51.</strong> Monomorphization</a></li><li class="chapter-item "><a href="backend/lowering-mir.html"><strong aria-hidden="true">52.</strong> Lowering MIR</a></li><li class="chapter-item "><a href="backend/codegen.html"><strong aria-hidden="true">53.</strong> Code Generation</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="backend/updating-llvm.html"><strong aria-hidden="true">53.1.</strong> Updating LLVM</a></li><li class="chapter-item "><a href="backend/debugging.html"><strong aria-hidden="true">53.2.</strong> Debugging LLVM</a></li><li class="chapter-item "><a href="backend/backend-agnostic.html"><strong aria-hidden="true">53.3.</strong> Backend Agnostic Codegen</a></li><li class="chapter-item "><a href="backend/implicit-caller-location.html"><strong aria-hidden="true">53.4.</strong> Implicit Caller Location</a></li></ol></li><li class="chapter-item "><a href="backend/libs-and-metadata.html"><strong aria-hidden="true">54.</strong> Libraries and Metadata</a></li><li class="chapter-item "><a href="profile-guided-optimization.html"><strong aria-hidden="true">55.</strong> Profile-guided Optimization</a></li><li class="chapter-item "><a href="llvm-coverage-instrumentation.html"><strong aria-hidden="true">56.</strong> LLVM Source-Based Code Coverage</a></li><li class="chapter-item "><a href="sanitizers.html"><strong aria-hidden="true">57.</strong> Sanitizers Support</a></li><li class="chapter-item "><a href="debugging-support-in-rustc.html"><strong aria-hidden="true">58.</strong> Debugging Support in the Rust Compiler</a></li><li class="spacer"></li><li class="chapter-item affix "><a href="appendix/background.html">Appendix A: Background topics</a></li><li class="chapter-item affix "><a href="appendix/glossary.html">Appendix B: Glossary</a></li><li class="chapter-item affix "><a href="appendix/code-index.html">Appendix C: Code Index</a></li><li class="chapter-item affix "><a href="appendix/compiler-lecture.html">Appendix D: Compiler Lecture Series</a></li><li class="chapter-item affix "><a href="appendix/bibliography.html">Appendix E: Bibliography</a></li><li class="chapter-item affix "><a href="appendix/humorust.html">Appendix Z: HumorRust</a></li><li class="spacer"></li></ol> |
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| <h1 id="generics-and-substitutions"><a class="header" href="#generics-and-substitutions">Generics and substitutions</a></h1> |
| <p>Given a generic type <code>MyType<A, B, …></code>, we may want to swap out the generics <code>A, B, …</code> for some |
| other types (possibly other generics or concrete types). We do this a lot while doing type |
| inference, type checking, and trait solving. Conceptually, during these routines, we may find out |
| that one type is equal to another type and want to swap one out for the other and then swap that out |
| for another type and so on until we eventually get some concrete types (or an error).</p> |
| <p>In rustc this is done using the <code>SubstsRef</code> that we mentioned above (“substs” = “substitutions”). |
| Conceptually, you can think of <code>SubstsRef</code> as a list of types that are to be substituted for the |
| generic type parameters of the ADT.</p> |
| <p><code>SubstsRef</code> is a type alias of <code>List<GenericArg<'tcx>></code> (see <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.List.html"><code>List</code> rustdocs</a>). |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/subst/struct.GenericArg.html"><code>GenericArg</code></a> is essentially a space-efficient wrapper around <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/subst/enum.GenericArgKind.html"><code>GenericArgKind</code></a>, which is an enum |
| indicating what kind of generic the type parameter is (type, lifetime, or const). Thus, <code>SubstsRef</code> |
| is conceptually like a <code>&'tcx [GenericArgKind<'tcx>]</code> slice (but it is actually a <code>List</code>).</p> |
| <p>So why do we use this <code>List</code> type instead of making it really a slice? It has the length "inline", |
| so <code>&List</code> is only 32 bits. As a consequence, it cannot be "subsliced" (that only works if the |
| length is out of line).</p> |
| <p>This also implies that you can check two <code>List</code>s for equality via <code>==</code> (which would be not be |
| possible for ordinary slices). This is precisely because they never represent a "sub-list", only the |
| complete <code>List</code>, which has been hashed and interned.</p> |
| <p>So pulling it all together, let’s go back to our example above:</p> |
| <pre><code class="language-rust ignore">struct MyStruct<T> |
| </code></pre> |
| <ul> |
| <li>There would be an <code>AdtDef</code> (and corresponding <code>DefId</code>) for <code>MyStruct</code>.</li> |
| <li>There would be a <code>TyKind::Param</code> (and corresponding <code>DefId</code>) for <code>T</code> (more later).</li> |
| <li>There would be a <code>SubstsRef</code> containing the list <code>[GenericArgKind::Type(Ty(T))]</code> |
| <ul> |
| <li>The <code>Ty(T)</code> here is my shorthand for entire other <code>ty::Ty</code> that has <code>TyKind::Param</code>, which we |
| mentioned in the previous point.</li> |
| </ul> |
| </li> |
| <li>This is one <code>TyKind::Adt</code> containing the <code>AdtDef</code> of <code>MyStruct</code> with the <code>SubstsRef</code> above.</li> |
| </ul> |
| <p>Finally, we will quickly mention the |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Generics.html"><code>Generics</code></a> type. It |
| is used to give information about the type parameters of a type.</p> |
| <h3 id="unsubstituted-generics"><a class="header" href="#unsubstituted-generics">Unsubstituted Generics</a></h3> |
| <p>So above, recall that in our example the <code>MyStruct</code> struct had a generic type <code>T</code>. When we are (for |
| example) type checking functions that use <code>MyStruct</code>, we will need to be able to refer to this type |
| <code>T</code> without actually knowing what it is. In general, this is true inside all generic definitions: we |
| need to be able to work with unknown types. This is done via <code>TyKind::Param</code> (which we mentioned in |
| the example above).</p> |
| <p>Each <code>TyKind::Param</code> contains two things: the name and the index. In general, the index fully |
| defines the parameter and is used by most of the code. The name is included for debug print-outs. |
| There are two reasons for this. First, the index is convenient, it allows you to include into the |
| list of generic arguments when substituting. Second, the index is more robust. For example, you |
| could in principle have two distinct type parameters that use the same name, e.g. <code>impl<A> Foo<A> { fn bar<A>() { .. } }</code>, although the rules against shadowing make this difficult (but those language |
| rules could change in the future).</p> |
| <p>The index of the type parameter is an integer indicating its order in the list of the type |
| parameters. Moreover, we consider the list to include all of the type parameters from outer scopes. |
| Consider the following example:</p> |
| <pre><code class="language-rust ignore">struct Foo<A, B> { |
| // A would have index 0 |
| // B would have index 1 |
| |
| .. // some fields |
| } |
| impl<X, Y> Foo<X, Y> { |
| fn method<Z>() { |
| // inside here, X, Y and Z are all in scope |
| // X has index 0 |
| // Y has index 1 |
| // Z has index 2 |
| } |
| } |
| </code></pre> |
| <p>When we are working inside the generic definition, we will use <code>TyKind::Param</code> just like any other |
| <code>TyKind</code>; it is just a type after all. However, if we want to use the generic type somewhere, then |
| we will need to do substitutions.</p> |
| <p>For example suppose that the <code>Foo<A, B></code> type from the previous example has a field that is a |
| <code>Vec<A></code>. Observe that <code>Vec</code> is also a generic type. We want to tell the compiler that the type |
| parameter of <code>Vec</code> should be replaced with the <code>A</code> type parameter of <code>Foo<A, B></code>. We do that with |
| substitutions:</p> |
| <pre><code class="language-rust ignore">struct Foo<A, B> { // Adt(Foo, &[Param(0), Param(1)]) |
| x: Vec<A>, // Adt(Vec, &[Param(0)]) |
| .. |
| } |
| |
| fn bar(foo: Foo<u32, f32>) { // Adt(Foo, &[u32, f32]) |
| let y = foo.x; // Vec<Param(0)> => Vec<u32> |
| } |
| </code></pre> |
| <p>This example has a few different substitutions:</p> |
| <ul> |
| <li>In the definition of <code>Foo</code>, in the type of the field <code>x</code>, we replace <code>Vec</code>'s type parameter with |
| <code>Param(0)</code>, the first parameter of <code>Foo<A, B></code>, so that the type of <code>x</code> is <code>Vec<A></code>.</li> |
| <li>In the function <code>bar</code>, we specify that we want a <code>Foo<u32, f32></code>. This means that we will |
| substitute <code>Param(0)</code> and <code>Param(1)</code> with <code>u32</code> and <code>f32</code>.</li> |
| <li>In the body of <code>bar</code>, we access <code>foo.x</code>, which has type <code>Vec<Param(0)></code>, but <code>Param(0)</code> has been |
| substituted for <code>u32</code>, so <code>foo.x</code> has type <code>Vec<u32></code>.</li> |
| </ul> |
| <p>Let’s look a bit more closely at that last substitution to see why we use indexes. If we want to |
| find the type of <code>foo.x</code>, we can get generic type of <code>x</code>, which is <code>Vec<Param(0)></code>. Now we can take |
| the index <code>0</code> and use it to find the right type substitution: looking at <code>Foo</code>'s <code>SubstsRef</code>, we |
| have the list <code>[u32, f32]</code> , since we want to replace index <code>0</code>, we take the 0-th index of this |
| list, which is <code>u32</code>. Voila!</p> |
| <p>You may have a couple of followup questions…</p> |
| <p><strong><code>type_of</code></strong> How do we get the “generic type of <code>x</code>"? You can get the type of pretty much anything |
| with the <code>tcx.type_of(def_id)</code> query. In this case, we would pass the <code>DefId</code> of the field <code>x</code>. |
| The <code>type_of</code> query always returns the definition with the generics that are in scope of the |
| definition. For example, <code>tcx.type_of(def_id_of_my_struct)</code> would return the “self-view” of |
| <code>MyStruct</code>: <code>Adt(Foo, &[Param(0), Param(1)])</code>.</p> |
| <p><strong><code>subst</code></strong> How do we actually do the substitutions? There is a function for that too! You use |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/subst/trait.Subst.html"><code>subst</code></a> to |
| replace a <code>SubstRef</code> with another list of types.</p> |
| <p><a href="https://github.com/rust-lang/rust/blob/597f432489f12a3f33419daa039ccef11a12c4fd/src/librustc_typeck/astconv.rs#L942-L953">Here is an example of actually using <code>subst</code> in the compiler</a>. The exact details are not |
| too important, but in this piece of code, we happen to be converting from the <code>rustc_hir::Ty</code> to |
| a real <code>ty::Ty</code>. You can see that we first get some substitutions (<code>substs</code>). Then we call |
| <code>type_of</code> to get a type and call <code>ty.subst(substs)</code> to get a new version of <code>ty</code> with |
| the substitutions made.</p> |
| <p><strong>Note on indices:</strong> It is possible for the indices in <code>Param</code> to not match with what we expect. For |
| example, the index could be out of bounds or it could be the index of a lifetime when we were |
| expecting a type. These sorts of errors would be caught earlier in the compiler when translating |
| from a <code>rustc_hir::Ty</code> to a <code>ty::Ty</code>. If they occur later, that is a compiler bug.</p> |
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