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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 expanded "><a href="query.html" class="active"><strong aria-hidden="true">23.</strong> Queries: demand-driven compilation</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="queries/query-evaluation-model-in-detail.html"><strong aria-hidden="true">23.1.</strong> The Query Evaluation Model in Detail</a></li><li class="chapter-item "><a href="queries/incremental-compilation.html"><strong aria-hidden="true">23.2.</strong> Incremental compilation</a></li><li class="chapter-item "><a href="queries/incremental-compilation-in-detail.html"><strong aria-hidden="true">23.3.</strong> Incremental compilation In Detail</a></li><li class="chapter-item "><a href="incrcomp-debugging.html"><strong aria-hidden="true">23.4.</strong> Debugging and Testing</a></li><li class="chapter-item "><a href="salsa.html"><strong aria-hidden="true">23.5.</strong> Salsa</a></li></ol></li><li class="chapter-item "><a href="memory.html"><strong aria-hidden="true">24.</strong> Memory Management in Rustc</a></li><li class="chapter-item "><a href="serialization.html"><strong aria-hidden="true">25.</strong> Serialization in Rustc</a></li><li class="chapter-item "><a href="parallel-rustc.html"><strong aria-hidden="true">26.</strong> Parallel Compilation</a></li><li class="chapter-item "><a href="rustdoc-internals.html"><strong aria-hidden="true">27.</strong> Rustdoc internals</a></li><li class="chapter-item affix "><li class="part-title">Source Code Representation</li><li class="chapter-item "><a href="part-3-intro.html"><strong aria-hidden="true">28.</strong> Prologue</a></li><li class="chapter-item "><a href="cli.html"><strong aria-hidden="true">29.</strong> Command-line arguments</a></li><li class="chapter-item "><a href="rustc-driver.html"><strong aria-hidden="true">30.</strong> The Rustc Driver and Interface</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="rustc-driver-interacting-with-the-ast.html"><strong aria-hidden="true">30.1.</strong> Ex: Type checking through rustc_interface</a></li><li class="chapter-item "><a href="rustc-driver-getting-diagnostics.html"><strong aria-hidden="true">30.2.</strong> Ex: Getting diagnostics through rustc_interface</a></li></ol></li><li class="chapter-item "><a href="syntax-intro.html"><strong aria-hidden="true">31.</strong> Syntax and the AST</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="the-parser.html"><strong aria-hidden="true">31.1.</strong> Lexing and Parsing</a></li><li class="chapter-item "><a href="macro-expansion.html"><strong aria-hidden="true">31.2.</strong> Macro expansion</a></li><li class="chapter-item "><a href="name-resolution.html"><strong aria-hidden="true">31.3.</strong> Name resolution</a></li><li class="chapter-item "><a href="test-implementation.html"><strong aria-hidden="true">31.4.</strong> #[test] Implementation</a></li><li class="chapter-item "><a href="panic-implementation.html"><strong aria-hidden="true">31.5.</strong> Panic Implementation</a></li><li class="chapter-item "><a href="ast-validation.html"><strong aria-hidden="true">31.6.</strong> AST Validation</a></li><li class="chapter-item "><a href="feature-gate-ck.html"><strong aria-hidden="true">31.7.</strong> Feature Gate Checking</a></li><li class="chapter-item "><a href="lang-items.html"><strong aria-hidden="true">31.8.</strong> Lang Items</a></li></ol></li><li class="chapter-item "><a href="hir.html"><strong aria-hidden="true">32.</strong> The HIR (High-level IR)</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="lowering.html"><strong aria-hidden="true">32.1.</strong> Lowering AST to HIR</a></li><li class="chapter-item "><a href="hir-debugging.html"><strong aria-hidden="true">32.2.</strong> Debugging</a></li></ol></li><li class="chapter-item "><a href="thir.html"><strong aria-hidden="true">33.</strong> The THIR (Typed High-level IR)</a></li><li class="chapter-item "><a href="mir/index.html"><strong aria-hidden="true">34.</strong> The MIR (Mid-level IR)</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="mir/construction.html"><strong aria-hidden="true">34.1.</strong> MIR construction</a></li><li class="chapter-item "><a href="mir/visitor.html"><strong aria-hidden="true">34.2.</strong> MIR visitor and traversal</a></li><li class="chapter-item "><a href="mir/passes.html"><strong aria-hidden="true">34.3.</strong> MIR passes: getting the MIR for a function</a></li></ol></li><li class="chapter-item "><a href="identifiers.html"><strong aria-hidden="true">35.</strong> Identifiers in the Compiler</a></li><li class="chapter-item "><a href="closure.html"><strong aria-hidden="true">36.</strong> Closure expansion</a></li><li class="chapter-item affix "><li class="part-title">Analysis</li><li class="chapter-item "><a href="part-4-intro.html"><strong aria-hidden="true">37.</strong> Prologue</a></li><li class="chapter-item "><a href="ty.html"><strong aria-hidden="true">38.</strong> The ty module: representing types</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="generics.html"><strong aria-hidden="true">38.1.</strong> Generics and substitutions</a></li><li class="chapter-item "><a href="ty-fold.html"><strong aria-hidden="true">38.2.</strong> TypeFolder and TypeFoldable</a></li><li class="chapter-item "><a href="generic_arguments.html"><strong aria-hidden="true">38.3.</strong> Generic arguments</a></li><li class="chapter-item "><a href="constants.html"><strong aria-hidden="true">38.4.</strong> Constants in the type system</a></li></ol></li><li class="chapter-item "><a href="type-inference.html"><strong aria-hidden="true">39.</strong> Type inference</a></li><li class="chapter-item "><a href="traits/resolution.html"><strong aria-hidden="true">40.</strong> Trait solving</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="early-late-bound.html"><strong aria-hidden="true">40.1.</strong> Early and Late Bound Parameters</a></li><li class="chapter-item "><a href="traits/hrtb.html"><strong aria-hidden="true">40.2.</strong> Higher-ranked trait bounds</a></li><li class="chapter-item "><a href="traits/caching.html"><strong aria-hidden="true">40.3.</strong> Caching subtleties</a></li><li class="chapter-item "><a href="traits/specialization.html"><strong aria-hidden="true">40.4.</strong> Specialization</a></li><li class="chapter-item "><a href="traits/chalk.html"><strong aria-hidden="true">40.5.</strong> Chalk-based trait solving</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="traits/lowering-to-logic.html"><strong aria-hidden="true">40.5.1.</strong> Lowering to logic</a></li><li class="chapter-item "><a href="traits/goals-and-clauses.html"><strong aria-hidden="true">40.5.2.</strong> Goals and clauses</a></li><li class="chapter-item "><a href="traits/canonical-queries.html"><strong aria-hidden="true">40.5.3.</strong> Canonical queries</a></li></ol></li></ol></li><li class="chapter-item "><a href="type-checking.html"><strong aria-hidden="true">41.</strong> Type checking</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="method-lookup.html"><strong aria-hidden="true">41.1.</strong> Method Lookup</a></li><li class="chapter-item "><a href="variance.html"><strong aria-hidden="true">41.2.</strong> Variance</a></li><li class="chapter-item "><a href="opaque-types-type-alias-impl-trait.html"><strong aria-hidden="true">41.3.</strong> Opaque Types</a></li></ol></li><li class="chapter-item "><a href="pat-exhaustive-checking.html"><strong aria-hidden="true">42.</strong> Pattern and Exhaustiveness Checking</a></li><li class="chapter-item "><a href="mir/dataflow.html"><strong aria-hidden="true">43.</strong> MIR dataflow</a></li><li class="chapter-item "><a href="borrow_check.html"><strong aria-hidden="true">44.</strong> The borrow checker</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="borrow_check/moves_and_initialization.html"><strong aria-hidden="true">44.1.</strong> Tracking moves and initialization</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="borrow_check/moves_and_initialization/move_paths.html"><strong aria-hidden="true">44.1.1.</strong> Move paths</a></li></ol></li><li class="chapter-item "><a href="borrow_check/type_check.html"><strong aria-hidden="true">44.2.</strong> MIR type checker</a></li><li class="chapter-item "><a href="borrow_check/region_inference.html"><strong aria-hidden="true">44.3.</strong> Region inference</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="borrow_check/region_inference/constraint_propagation.html"><strong aria-hidden="true">44.3.1.</strong> Constraint propagation</a></li><li class="chapter-item "><a href="borrow_check/region_inference/lifetime_parameters.html"><strong aria-hidden="true">44.3.2.</strong> Lifetime parameters</a></li><li class="chapter-item "><a href="borrow_check/region_inference/member_constraints.html"><strong aria-hidden="true">44.3.3.</strong> Member constraints</a></li><li class="chapter-item "><a href="borrow_check/region_inference/placeholders_and_universes.html"><strong aria-hidden="true">44.3.4.</strong> Placeholders and universes</a></li><li class="chapter-item "><a href="borrow_check/region_inference/closure_constraints.html"><strong aria-hidden="true">44.3.5.</strong> Closure constraints</a></li><li class="chapter-item "><a href="borrow_check/region_inference/error_reporting.html"><strong aria-hidden="true">44.3.6.</strong> Error reporting</a></li></ol></li><li class="chapter-item "><a href="borrow_check/two_phase_borrows.html"><strong aria-hidden="true">44.4.</strong> Two-phase-borrows</a></li></ol></li><li class="chapter-item "><a href="param_env.html"><strong aria-hidden="true">45.</strong> Parameter Environments</a></li><li class="chapter-item "><a href="diagnostics.html"><strong aria-hidden="true">46.</strong> Errors and Lints</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="diagnostics/sessiondiagnostic.html"><strong aria-hidden="true">46.1.</strong> Creating Errors With SessionDiagnostic</a></li><li class="chapter-item "><a href="diagnostics/lintstore.html"><strong aria-hidden="true">46.2.</strong> LintStore</a></li><li class="chapter-item "><a href="diagnostics/diagnostic-codes.html"><strong aria-hidden="true">46.3.</strong> Diagnostic Codes</a></li><li class="chapter-item "><a href="diagnostics/diagnostic-items.html"><strong aria-hidden="true">46.4.</strong> Diagnostic Items</a></li></ol></li><li class="chapter-item "><li class="part-title">MIR to Binaries</li><li class="chapter-item "><a href="part-5-intro.html"><strong aria-hidden="true">47.</strong> Prologue</a></li><li class="chapter-item "><a href="mir/optimizations.html"><strong aria-hidden="true">48.</strong> MIR optimizations</a></li><li class="chapter-item "><a href="mir/debugging.html"><strong aria-hidden="true">49.</strong> Debugging</a></li><li class="chapter-item "><a href="const-eval.html"><strong aria-hidden="true">50.</strong> Constant evaluation</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="miri.html"><strong aria-hidden="true">50.1.</strong> miri const evaluator</a></li></ol></li><li class="chapter-item "><a href="backend/monomorph.html"><strong aria-hidden="true">51.</strong> Monomorphization</a></li><li class="chapter-item "><a href="backend/lowering-mir.html"><strong aria-hidden="true">52.</strong> Lowering MIR</a></li><li class="chapter-item "><a href="backend/codegen.html"><strong aria-hidden="true">53.</strong> Code Generation</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="backend/updating-llvm.html"><strong aria-hidden="true">53.1.</strong> Updating LLVM</a></li><li class="chapter-item "><a href="backend/debugging.html"><strong aria-hidden="true">53.2.</strong> Debugging LLVM</a></li><li class="chapter-item "><a href="backend/backend-agnostic.html"><strong aria-hidden="true">53.3.</strong> Backend Agnostic Codegen</a></li><li class="chapter-item "><a href="backend/implicit-caller-location.html"><strong aria-hidden="true">53.4.</strong> Implicit Caller Location</a></li></ol></li><li class="chapter-item "><a href="backend/libs-and-metadata.html"><strong aria-hidden="true">54.</strong> Libraries and Metadata</a></li><li class="chapter-item "><a href="profile-guided-optimization.html"><strong aria-hidden="true">55.</strong> Profile-guided Optimization</a></li><li class="chapter-item "><a href="llvm-coverage-instrumentation.html"><strong aria-hidden="true">56.</strong> LLVM Source-Based Code Coverage</a></li><li class="chapter-item "><a href="sanitizers.html"><strong aria-hidden="true">57.</strong> Sanitizers Support</a></li><li class="chapter-item "><a href="debugging-support-in-rustc.html"><strong aria-hidden="true">58.</strong> Debugging Support in the Rust Compiler</a></li><li class="spacer"></li><li class="chapter-item affix "><a href="appendix/background.html">Appendix A: Background topics</a></li><li class="chapter-item affix "><a href="appendix/glossary.html">Appendix B: Glossary</a></li><li class="chapter-item affix "><a href="appendix/code-index.html">Appendix C: Code Index</a></li><li class="chapter-item affix "><a href="appendix/compiler-lecture.html">Appendix D: Compiler Lecture Series</a></li><li class="chapter-item affix "><a href="appendix/bibliography.html">Appendix E: Bibliography</a></li><li class="chapter-item affix "><a href="appendix/humorust.html">Appendix Z: HumorRust</a></li><li 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| <h1 id="queries-demand-driven-compilation"><a class="header" href="#queries-demand-driven-compilation">Queries: demand-driven compilation</a></h1> |
| <p>As described in <a href="./compiler-src.html">the high-level overview of the compiler</a>, the Rust compiler |
| is still (as of <!-- date: 2021-07 --> July 2021) transitioning from a |
| traditional "pass-based" setup to a "demand-driven" system. <strong>The Compiler Query |
| System is the key to our new demand-driven organization.</strong> The idea is pretty |
| simple. You have various queries that compute things about the input – for |
| example, there is a query called <code>type_of(def_id)</code> that, given the <a href="appendix/glossary.html#def-id">def-id</a> of |
| some item, will compute the type of that item and return it to you.</p> |
| <p>Query execution is <strong>memoized</strong> – so the first time you invoke a |
| query, it will go do the computation, but the next time, the result is |
| returned from a hashtable. Moreover, query execution fits nicely into |
| <strong>incremental computation</strong>; the idea is roughly that, when you do a |
| query, the result <strong>may</strong> be returned to you by loading stored data |
| from disk (but that's a separate topic we won't discuss further here).</p> |
| <p>The overall vision is that, eventually, the entire compiler |
| control-flow will be query driven. There will effectively be one |
| top-level query ("compile") that will run compilation on a crate; this |
| will in turn demand information about that crate, starting from the |
| <em>end</em>. For example:</p> |
| <ul> |
| <li>This "compile" query might demand to get a list of codegen-units |
| (i.e. modules that need to be compiled by LLVM).</li> |
| <li>But computing the list of codegen-units would invoke some subquery |
| that returns the list of all modules defined in the Rust source.</li> |
| <li>That query in turn would invoke something asking for the HIR.</li> |
| <li>This keeps going further and further back until we wind up doing the |
| actual parsing.</li> |
| </ul> |
| <p>However, that vision is not fully realized. Still, big chunks of the |
| compiler (for example, generating MIR) work exactly like this.</p> |
| <h3 id="incremental-compilation-in-detail"><a class="header" href="#incremental-compilation-in-detail">Incremental Compilation in Detail</a></h3> |
| <p>The <a href="queries/incremental-compilation-in-detail.html">Incremental Compilation in Detail</a> chapter gives a more |
| in-depth description of what queries are and how they work. |
| If you intend to write a query of your own, this is a good read.</p> |
| <h3 id="invoking-queries"><a class="header" href="#invoking-queries">Invoking queries</a></h3> |
| <p>To invoke a query is simple. The tcx ("type context") offers a method |
| for each defined query. So, for example, to invoke the <code>type_of</code> |
| query, you would just do this:</p> |
| <pre><code class="language-rust ignore">let ty = tcx.type_of(some_def_id); |
| </code></pre> |
| <h3 id="how-the-compiler-executes-a-query"><a class="header" href="#how-the-compiler-executes-a-query">How the compiler executes a query</a></h3> |
| <p>So you may be wondering what happens when you invoke a query |
| method. The answer is that, for each query, the compiler maintains a |
| cache – if your query has already been executed, then, the answer is |
| simple: we clone the return value out of the cache and return it |
| (therefore, you should try to ensure that the return types of queries |
| are cheaply cloneable; insert an <code>Rc</code> if necessary).</p> |
| <h4 id="providers"><a class="header" href="#providers">Providers</a></h4> |
| <p>If, however, the query is <em>not</em> in the cache, then the compiler will |
| try to find a suitable <strong>provider</strong>. A provider is a function that has |
| been defined and linked into the compiler somewhere that contains the |
| code to compute the result of the query.</p> |
| <p><strong>Providers are defined per-crate.</strong> The compiler maintains, |
| internally, a table of providers for every crate, at least |
| conceptually. Right now, there are really two sets: the providers for |
| queries about the <strong>local crate</strong> (that is, the one being compiled) |
| and providers for queries about <strong>external crates</strong> (that is, |
| dependencies of the local crate). Note that what determines the crate |
| that a query is targeting is not the <em>kind</em> of query, but the <em>key</em>. |
| For example, when you invoke <code>tcx.type_of(def_id)</code>, that could be a |
| local query or an external query, depending on what crate the <code>def_id</code> |
| is referring to (see the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_query_impl/keys/trait.Key.html"><code>self::keys::Key</code></a> trait for more |
| information on how that works).</p> |
| <p>Providers always have the same signature:</p> |
| <pre><code class="language-rust ignore">fn provider<'tcx>( |
| tcx: TyCtxt<'tcx>, |
| key: QUERY_KEY, |
| ) -> QUERY_RESULT { |
| ... |
| } |
| </code></pre> |
| <p>Providers take two arguments: the <code>tcx</code> and the query key. |
| They return the result of the query.</p> |
| <h4 id="how-providers-are-setup"><a class="header" href="#how-providers-are-setup">How providers are setup</a></h4> |
| <p>When the tcx is created, it is given the providers by its creator using |
| the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/query/struct.Providers.html"><code>Providers</code></a> struct. This struct is generated by |
| the macros here, but it is basically a big list of function pointers:</p> |
| <pre><code class="language-rust ignore">struct Providers { |
| type_of: for<'tcx> fn(TyCtxt<'tcx>, DefId) -> Ty<'tcx>, |
| ... |
| } |
| </code></pre> |
| <p>At present, we have one copy of the struct for local crates, and one |
| for external crates, though the plan is that we may eventually have |
| one per crate.</p> |
| <p>These <code>Providers</code> structs are ultimately created and populated by |
| <code>rustc_driver</code>, but it does this by distributing the work |
| throughout the other <code>rustc_*</code> crates. This is done by invoking |
| various <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/hir/fn.provide.html"><code>provide</code></a> functions. These functions tend to look |
| something like this:</p> |
| <pre><code class="language-rust ignore">pub fn provide(providers: &mut Providers) { |
| *providers = Providers { |
| type_of, |
| ..*providers |
| }; |
| } |
| </code></pre> |
| <p>That is, they take an <code>&mut Providers</code> and mutate it in place. Usually |
| we use the formulation above just because it looks nice, but you could |
| as well do <code>providers.type_of = type_of</code>, which would be equivalent. |
| (Here, <code>type_of</code> would be a top-level function, defined as we saw |
| before.) So, if we want to add a provider for some other query, |
| let's call it <code>fubar</code>, into the crate above, we might modify the <code>provide()</code> |
| function like so:</p> |
| <pre><code class="language-rust ignore">pub fn provide(providers: &mut Providers) { |
| *providers = Providers { |
| type_of, |
| fubar, |
| ..*providers |
| }; |
| } |
| |
| fn fubar<'tcx>(tcx: TyCtxt<'tcx>, key: DefId) -> Fubar<'tcx> { ... } |
| </code></pre> |
| <p>N.B. Most of the <code>rustc_*</code> crates only provide <strong>local |
| providers</strong>. Almost all <strong>extern providers</strong> wind up going through the |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/index.html"><code>rustc_metadata</code> crate</a>, which loads the information |
| from the crate metadata. But in some cases there are crates that |
| provide queries for <em>both</em> local and external crates, in which case |
| they define both a <code>provide</code> and a <code>provide_extern</code> function, through |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/back/symbol_export/fn.wasm_import_module_map.html"><code>wasm_import_module_map</code></a>, that <code>rustc_driver</code> can invoke.</p> |
| <h3 id="adding-a-new-kind-of-query"><a class="header" href="#adding-a-new-kind-of-query">Adding a new kind of query</a></h3> |
| <p>So suppose you want to add a new kind of query, how do you do so? |
| Well, defining a query takes place in two steps:</p> |
| <ol> |
| <li>first, you have to specify the query name and arguments; and then,</li> |
| <li>you have to supply query providers where needed.</li> |
| </ol> |
| <p>To specify the query name and arguments, you simply add an entry to |
| the big macro invocation in |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/query/index.html"><code>compiler/rustc_middle/src/query/mod.rs</code></a>, which looks something like:</p> |
| <pre><code class="language-rust ignore">rustc_queries! { |
| Other { |
| /// Records the type of every item. |
| query type_of(key: DefId) -> Ty<'tcx> { |
| cache { key.is_local() } |
| } |
| } |
| |
| ... |
| } |
| </code></pre> |
| <p>Queries are grouped into categories (<code>Other</code>, <code>Codegen</code>, <code>TypeChecking</code>, etc.). |
| Each group contains one or more queries. Each query definition is broken up like |
| this:</p> |
| <pre><code class="language-rust ignore">query type_of(key: DefId) -> Ty<'tcx> { ... } |
| ^^ ^^^^^^^ ^^^^^ ^^^^^^^^ ^^^ |
| | | | | | |
| | | | | query modifiers |
| | | | result type of query |
| | | query key type |
| | name of query |
| query keyword |
| </code></pre> |
| <p>Let's go over them one by one:</p> |
| <ul> |
| <li><strong>Query keyword:</strong> indicates a start of a query definition.</li> |
| <li><strong>Name of query:</strong> the name of the query method |
| (<code>tcx.type_of(..)</code>). Also used as the name of a struct |
| (<code>ty::queries::type_of</code>) that will be generated to represent |
| this query.</li> |
| <li><strong>Query key type:</strong> the type of the argument to this query. |
| This type must implement the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_query_impl/keys/trait.Key.html"><code>ty::query::keys::Key</code></a> trait, which |
| defines (for example) how to map it to a crate, and so forth.</li> |
| <li><strong>Result type of query:</strong> the type produced by this query. This type |
| should (a) not use <code>RefCell</code> or other interior mutability and (b) be |
| cheaply cloneable. Interning or using <code>Rc</code> or <code>Arc</code> is recommended for |
| non-trivial data types. |
| <ul> |
| <li>The one exception to those rules is the <code>ty::steal::Steal</code> type, |
| which is used to cheaply modify MIR in place. See the definition |
| of <code>Steal</code> for more details. New uses of <code>Steal</code> should <strong>not</strong> be |
| added without alerting <code>@rust-lang/compiler</code>.</li> |
| </ul> |
| </li> |
| <li><strong>Query modifiers:</strong> various flags and options that customize how the |
| query is processed (mostly with respect to <a href="queries/incremental-compilation-in-detail.html#query-modifiers">incremental compilation</a>).</li> |
| </ul> |
| <p>So, to add a query:</p> |
| <ul> |
| <li>Add an entry to <code>rustc_queries!</code> using the format above.</li> |
| <li>Link the provider by modifying the appropriate <code>provide</code> method; |
| or add a new one if needed and ensure that <code>rustc_driver</code> is invoking it.</li> |
| </ul> |
| <h4 id="query-structs-and-descriptions"><a class="header" href="#query-structs-and-descriptions">Query structs and descriptions</a></h4> |
| <p>For each kind, the <code>rustc_queries</code> macro will generate a "query struct" |
| named after the query. This struct is a kind of a place-holder |
| describing the query. Each such struct implements the |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_query_system/query/config/trait.QueryConfig.html"><code>self::config::QueryConfig</code></a> trait, which has associated types for the |
| key/value of that particular query. Basically the code generated looks something |
| like this:</p> |
| <pre><code class="language-rust ignore">// Dummy struct representing a particular kind of query: |
| pub struct type_of<'tcx> { data: PhantomData<&'tcx ()> } |
| |
| impl<'tcx> QueryConfig for type_of<'tcx> { |
| type Key = DefId; |
| type Value = Ty<'tcx>; |
| |
| const NAME: QueryName = QueryName::type_of; |
| const CATEGORY: ProfileCategory = ProfileCategory::Other; |
| } |
| </code></pre> |
| <p>There is an additional trait that you may wish to implement called |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_query_system/query/config/trait.QueryDescription.html"><code>self::config::QueryDescription</code></a>. This trait is |
| used during cycle errors to give a "human readable" name for the query, |
| so that we can summarize what was happening when the cycle occurred. |
| Implementing this trait is optional if the query key is <code>DefId</code>, but |
| if you <em>don't</em> implement it, you get a pretty generic error ("processing <code>foo</code>..."). |
| You can put new impls into the <code>config</code> module. They look something like this:</p> |
| <pre><code class="language-rust ignore">impl<'tcx> QueryDescription for queries::type_of<'tcx> { |
| fn describe(tcx: TyCtxt, key: DefId) -> String { |
| format!("computing the type of `{}`", tcx.def_path_str(key)) |
| } |
| } |
| </code></pre> |
| <p>Another option is to add <code>desc</code> modifier:</p> |
| <pre><code class="language-rust ignore">rustc_queries! { |
| Other { |
| /// Records the type of every item. |
| query type_of(key: DefId) -> Ty<'tcx> { |
| desc { |tcx| "computing the type of `{}`", tcx.def_path_str(key) } |
| } |
| } |
| } |
| </code></pre> |
| <p><code>rustc_queries</code> macro will generate an appropriate <code>impl</code> automatically.</p> |
| <h2 id="external-links"><a class="header" href="#external-links">External Links</a></h2> |
| <p>Related design ideas, and tracking issues:</p> |
| <ul> |
| <li>Design document: <a href="https://github.com/nikomatsakis/rustc-on-demand-incremental-design-doc/blob/master/0000-rustc-on-demand-and-incremental.md">On-demand Rustc incremental design doc</a></li> |
| <li>Tracking Issue: <a href="https://github.com/rust-lang/rust/issues/42293">"Red/Green" dependency tracking in compiler</a></li> |
| </ul> |
| <p>More discussion and issues:</p> |
| <ul> |
| <li><a href="https://github.com/rust-lang/rust/issues/42633">GitHub issue #42633</a></li> |
| <li><a href="https://internals.rust-lang.org/t/incremental-compilation-beta/4721">Incremental Compilation Beta</a></li> |
| <li><a href="https://blog.rust-lang.org/2016/09/08/incremental.html">Incremental Compilation Announcement</a></li> |
| </ul> |
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