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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"><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 expanded "><a href="salsa.html" class="active"><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 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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="how-salsa-works"><a class="header" href="#how-salsa-works">How Salsa works</a></h1> |
| <ul> |
| <li><a href="#what-is-salsa">What is Salsa?</a></li> |
| <li><a href="#how-does-it-work">How does it work?</a></li> |
| <li><a href="#key-salsa-concepts">Key Salsa concepts</a> |
| <ul> |
| <li><a href="#query">Query</a></li> |
| <li><a href="#database">Database</a></li> |
| <li><a href="#query-groups">Query Groups</a></li> |
| </ul> |
| </li> |
| </ul> |
| <p>This chapter is based on the explanation given by Niko Matsakis in this |
| <a href="https://www.youtube.com/watch?v=_muY4HjSqVw">video</a> about |
| <a href="https://github.com/salsa-rs/salsa">Salsa</a>. To find out more you may |
| want to watch <a href="https://www.youtube.com/watch?v=i_IhACacPRY">Salsa In More |
| Depth</a>, also by Niko |
| Matsakis.</p> |
| <blockquote> |
| <p>As of <!-- date: 2021-07 --> July 2021, although Salsa is inspired by |
| (among other things) rustc's query system, it is not used directly in rustc. |
| It <em>is</em> used in chalk and extensively in <code>rust-analyzer</code>, but there are no |
| medium or long-term concrete plans to integrate it into the compiler.</p> |
| </blockquote> |
| <h2 id="what-is-salsa"><a class="header" href="#what-is-salsa">What is Salsa?</a></h2> |
| <p>Salsa is a library for incremental recomputation. This means it allows reusing |
| computations that were already done in the past to increase the efficiency |
| of future computations.</p> |
| <p>The objectives of Salsa are:</p> |
| <ul> |
| <li>Provide that functionality in an automatic way, so reusing old computations |
| is done automatically by the library</li> |
| <li>Doing so in a "sound", or "correct", way, therefore leading to the same |
| results as if it had been done from scratch</li> |
| </ul> |
| <p>Salsa's actual model is much richer, allowing many kinds of inputs and many |
| different outputs. |
| For example, integrating Salsa with an IDE could mean that the inputs could be |
| the manifest (<code>Cargo.toml</code>), entire source files (<code>foo.rs</code>), snippets and so |
| on; the outputs of such an integration could range from a binary executable, to |
| lints, types (for example, if a user selects a certain variable and wishes to |
| see its type), completions, etc.</p> |
| <h2 id="how-does-it-work"><a class="header" href="#how-does-it-work">How does it work?</a></h2> |
| <p>The first thing that Salsa has to do is identify the "base inputs" that |
| are not something computed but given as input.</p> |
| <p>Then Salsa has to also identify intermediate, "derived" values, which are |
| something that the library produces, but, for each derived value there's a |
| "pure" function that computes the derived value.</p> |
| <p>For example, there might be a function <code>ast(x: Path) -> AST</code>. The produced |
| <code>AST</code> isn't a final value, it's an intermediate value that the library would |
| use for the computation.</p> |
| <p>This means that when you try to compute with the library, Salsa is going to |
| compute various derived values, and eventually read the input and produce the |
| result for the asked computation.</p> |
| <p>In the course of computing, Salsa tracks which inputs were accessed and which |
| values are derived. This information is used to determine what's going to |
| happen when the inputs change: are the derived values still valid?</p> |
| <p>This doesn't necessarily mean that each computation downstream from the input |
| is going to be checked, which could be costly. Salsa only needs to check each |
| downstream computation until it finds one that isn't changed. At that point, it |
| won't check other derived computations since they wouldn't need to change.</p> |
| <p>It's is helpful to think about this as a graph with nodes. Each derived value |
| has a dependency on other values, which could themselves be either base or |
| derived. Base values don't have a dependency.</p> |
| <pre><code class="language-ignore">I <- A <- C ... |
| | |
| J <- B <--+ |
| </code></pre> |
| <p>When an input <code>I</code> changes, the derived value <code>A</code> could change. The derived |
| value <code>B</code> , which does not depend on <code>I</code>, <code>A</code>, or any value derived from <code>A</code> or |
| <code>I</code>, is not subject to change. Therefore, Salsa can reuse the computation done |
| for <code>B</code> in the past, without having to compute it again.</p> |
| <p>The computation could also terminate early. Keeping the same graph as before, |
| say that input <code>I</code> has changed in some way (and input <code>J</code> hasn't) but, when |
| computing <code>A</code> again, it's found that <code>A</code> hasn't changed from the previous |
| computation. This leads to an "early termination", because there's no need to |
| check if <code>C</code> needs to change, since both <code>C</code> direct inputs, <code>A</code> and <code>B</code>, |
| haven't changed.</p> |
| <h2 id="key-salsa-concepts"><a class="header" href="#key-salsa-concepts">Key Salsa concepts</a></h2> |
| <h3 id="query"><a class="header" href="#query">Query</a></h3> |
| <p>A query is some value that Salsa can access in the course of computation. Each |
| query can have a number of keys (from 0 to many), and all queries have a |
| result, akin to functions. 0-key queries are called "input" queries.</p> |
| <h3 id="database"><a class="header" href="#database">Database</a></h3> |
| <p>The database is basically the context for the entire computation, it's meant to |
| store Salsa's internal state, all intermediate values for each query, and |
| anything else that the computation might need. The database must know all the |
| queries that the library is going to do before it can be built, but they don't |
| need to be specified in the same place.</p> |
| <p>After the database is formed, it can be accessed with queries that are very |
| similar to functions. Since each query's result is stored in the database, |
| when a query is invoked N times, it will return N <strong>cloned</strong> results, without |
| having to recompute the query (unless the input has changed in such a way that |
| it warrants recomputation).</p> |
| <p>For each input query (0-key), a "set" method is generated, allowing the user to |
| change the output of such query, and trigger previous memoized values to be |
| potentially invalidated.</p> |
| <h3 id="query-groups"><a class="header" href="#query-groups">Query Groups</a></h3> |
| <p>A query group is a set of queries which have been defined together as a unit. |
| The database is formed by combining query groups. Query groups are akin to |
| "Salsa modules".</p> |
| <p>A set of queries in a query group are just a set of methods in a trait.</p> |
| <p>To create a query group a trait annotated with a specific attribute |
| (<code>#[salsa::query_group(...)]</code>) has to be created.</p> |
| <p>An argument must also be provided to said attribute as it will be used by Salsa |
| to create a struct to be used later when the database is created.</p> |
| <p>Example input query group:</p> |
| <pre><code class="language-rust ignore">/// This attribute will process this tree, produce this tree as output, and produce |
| /// a bunch of intermediate stuff that Salsa also uses. One of these things is a |
| /// "StorageStruct", whose name we have specified in the attribute. |
| /// |
| /// This query group is a bunch of **input** queries, that do not rely on any |
| /// derived input. |
| #[salsa::query_group(InputsStorage)] |
| pub trait Inputs { |
| /// This attribute (`#[salsa::input]`) indicates that this query is a base |
| /// input, therefore `set_manifest` is going to be auto-generated |
| #[salsa::input] |
| fn manifest(&self) -> Manifest; |
| |
| #[salsa::input] |
| fn source_text(&self, name: String) -> String; |
| } |
| </code></pre> |
| <p>To create a <strong>derived</strong> query group, one must specify which other query groups |
| this one depends on by specifying them as supertraits, as seen in the following |
| example:</p> |
| <pre><code class="language-rust ignore">/// This query group is going to contain queries that depend on derived values a |
| /// query group can access another query group's queries by specifying the |
| /// dependency as a super trait query groups can be stacked as much as needed using |
| /// that pattern. |
| #[salsa::query_group(ParserStorage)] |
| pub trait Parser: Inputs { |
| /// This query `ast` is not an input query, it's a derived query this means |
| /// that a definition is necessary. |
| fn ast(&self, name: String) -> String; |
| } |
| </code></pre> |
| <p>When creating a derived query the implementation of said query must be defined |
| outside the trait. The definition must take a database parameter as an <code>impl Trait</code> (or <code>dyn Trait</code>), where <code>Trait</code> is the query group that the definition |
| belongs to, in addition to the other keys.</p> |
| <pre><code class="language-rust ignore">///This is going to be the definition of the `ast` query in the `Parser` trait. |
| ///So, when the query `ast` is invoked, and it needs to be recomputed, Salsa is going to call this function |
| ///and it's is going to give it the database as `impl Parser`. |
| ///The function doesn't need to be aware of all the queries of all the query groups |
| fn ast(db: &impl Parser, name: String) -> String { |
| //! Note, `impl Parser` is used here but `dyn Parser` works just as well |
| /* code */ |
| ///By passing an `impl Parser`, this is allowed |
| let source_text = db.input_file(name); |
| /* do the actual parsing */ |
| return ast; |
| } |
| </code></pre> |
| <p>Eventually, after all the query groups have been defined, the database can be |
| created by declaring a struct.</p> |
| <p>To specify which query groups are going to be part of the database an attribute |
| (<code>#[salsa::database(...)]</code>) must be added. The argument of said attribute is a |
| list of identifiers, specifying the query groups <strong>storages</strong>.</p> |
| <pre><code class="language-rust ignore">///This attribute specifies which query groups are going to be in the database |
| #[salsa::database(InputsStorage, ParserStorage)] |
| #[derive(Default)] //optional! |
| struct MyDatabase { |
| ///You also need this one field |
| runtime : salsa::Runtime<MyDatabase>, |
| } |
| ///And this trait has to be implemented |
| impl salsa::Databse for MyDatabase { |
| fn salsa_runtime(&self) -> &salsa::Runtime<MyDatabase> { |
| &self.runtime |
| } |
| } |
| </code></pre> |
| <p>Example usage:</p> |
| <pre><code class="language-rust ignore">fn main() { |
| let db = MyDatabase::default(); |
| db.set_manifest(...); |
| db.set_source_text(...); |
| loop { |
| db.ast(...); //will reuse results |
| db.set_source_text(...); |
| } |
| } |
| </code></pre> |
| |
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