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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 expanded "><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 expanded "><a href="tracing.html" class="active"><strong aria-hidden="true">3.1.</strong> Using the tracing/logging instrumentation</a></li></ol></li><li class="chapter-item "><a href="profiling.html"><strong aria-hidden="true">4.</strong> Profiling the compiler</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="profiling/with_perf.html"><strong aria-hidden="true">4.1.</strong> with the linux perf tool</a></li><li class="chapter-item "><a href="profiling/wpa_profiling.html"><strong aria-hidden="true">4.2.</strong> with Windows Performance Analyzer</a></li></ol></li><li class="chapter-item "><a href="crates-io.html"><strong aria-hidden="true">5.</strong> crates.io Dependencies</a></li><li class="chapter-item affix "><li class="part-title">Contributing to Rust</li><li class="chapter-item "><a href="contributing.html"><strong aria-hidden="true">6.</strong> Introduction</a></li><li class="chapter-item "><a href="compiler-team.html"><strong aria-hidden="true">7.</strong> About the compiler team</a></li><li class="chapter-item "><a href="git.html"><strong aria-hidden="true">8.</strong> Using Git</a></li><li class="chapter-item "><a href="rustbot.html"><strong aria-hidden="true">9.</strong> Mastering @rustbot</a></li><li class="chapter-item "><a href="walkthrough.html"><strong aria-hidden="true">10.</strong> Walkthrough: a typical contribution</a></li><li class="chapter-item "><a href="bug-fix-procedure.html"><strong aria-hidden="true">11.</strong> Bug Fix Procedure</a></li><li class="chapter-item "><a href="implementing_new_features.html"><strong aria-hidden="true">12.</strong> Implementing new features</a></li><li class="chapter-item "><a href="stability.html"><strong aria-hidden="true">13.</strong> Stability attributes</a></li><li class="chapter-item "><a href="stabilization_guide.html"><strong aria-hidden="true">14.</strong> Stabilizing Features</a></li><li class="chapter-item "><a href="feature-gates.html"><strong aria-hidden="true">15.</strong> Feature Gates</a></li><li class="chapter-item "><a href="conventions.html"><strong aria-hidden="true">16.</strong> Coding conventions</a></li><li class="chapter-item "><a href="notification-groups/about.html"><strong aria-hidden="true">17.</strong> Notification groups</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="notification-groups/arm.html"><strong aria-hidden="true">17.1.</strong> ARM</a></li><li class="chapter-item "><a href="notification-groups/cleanup-crew.html"><strong aria-hidden="true">17.2.</strong> Cleanup Crew</a></li><li class="chapter-item "><a href="notification-groups/llvm.html"><strong aria-hidden="true">17.3.</strong> LLVM</a></li><li class="chapter-item "><a href="notification-groups/risc-v.html"><strong aria-hidden="true">17.4.</strong> RISC-V</a></li><li class="chapter-item "><a href="notification-groups/windows.html"><strong aria-hidden="true">17.5.</strong> Windows</a></li></ol></li><li class="chapter-item "><a href="licenses.html"><strong aria-hidden="true">18.</strong> Licenses</a></li><li class="chapter-item affix "><li class="part-title">High-level Compiler Architecture</li><li class="chapter-item "><a href="part-2-intro.html"><strong aria-hidden="true">19.</strong> Prologue</a></li><li class="chapter-item "><a href="overview.html"><strong aria-hidden="true">20.</strong> Overview of the Compiler</a></li><li class="chapter-item "><a href="compiler-src.html"><strong aria-hidden="true">21.</strong> The compiler source code</a></li><li class="chapter-item "><a href="building/bootstrapping.html"><strong aria-hidden="true">22.</strong> Bootstrapping</a></li><li class="chapter-item "><a href="query.html"><strong aria-hidden="true">23.</strong> Queries: demand-driven compilation</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="queries/query-evaluation-model-in-detail.html"><strong aria-hidden="true">23.1.</strong> The Query Evaluation Model in Detail</a></li><li class="chapter-item "><a href="queries/incremental-compilation.html"><strong aria-hidden="true">23.2.</strong> Incremental compilation</a></li><li class="chapter-item "><a href="queries/incremental-compilation-in-detail.html"><strong aria-hidden="true">23.3.</strong> Incremental compilation In Detail</a></li><li class="chapter-item "><a href="incrcomp-debugging.html"><strong aria-hidden="true">23.4.</strong> Debugging and Testing</a></li><li class="chapter-item "><a href="salsa.html"><strong aria-hidden="true">23.5.</strong> Salsa</a></li></ol></li><li class="chapter-item "><a href="memory.html"><strong aria-hidden="true">24.</strong> Memory Management in Rustc</a></li><li class="chapter-item "><a href="serialization.html"><strong aria-hidden="true">25.</strong> Serialization in Rustc</a></li><li class="chapter-item "><a href="parallel-rustc.html"><strong aria-hidden="true">26.</strong> Parallel Compilation</a></li><li class="chapter-item "><a href="rustdoc-internals.html"><strong aria-hidden="true">27.</strong> Rustdoc internals</a></li><li class="chapter-item affix "><li class="part-title">Source Code Representation</li><li class="chapter-item "><a href="part-3-intro.html"><strong aria-hidden="true">28.</strong> Prologue</a></li><li class="chapter-item "><a href="cli.html"><strong aria-hidden="true">29.</strong> Command-line arguments</a></li><li class="chapter-item "><a href="rustc-driver.html"><strong aria-hidden="true">30.</strong> The Rustc Driver and Interface</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="rustc-driver-interacting-with-the-ast.html"><strong aria-hidden="true">30.1.</strong> Ex: Type checking through rustc_interface</a></li><li class="chapter-item "><a href="rustc-driver-getting-diagnostics.html"><strong aria-hidden="true">30.2.</strong> Ex: Getting diagnostics through rustc_interface</a></li></ol></li><li class="chapter-item "><a href="syntax-intro.html"><strong aria-hidden="true">31.</strong> Syntax and the AST</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="the-parser.html"><strong aria-hidden="true">31.1.</strong> Lexing and Parsing</a></li><li class="chapter-item "><a href="macro-expansion.html"><strong aria-hidden="true">31.2.</strong> Macro expansion</a></li><li class="chapter-item "><a href="name-resolution.html"><strong aria-hidden="true">31.3.</strong> Name resolution</a></li><li class="chapter-item "><a href="test-implementation.html"><strong aria-hidden="true">31.4.</strong> #[test] Implementation</a></li><li class="chapter-item "><a href="panic-implementation.html"><strong aria-hidden="true">31.5.</strong> Panic Implementation</a></li><li class="chapter-item "><a href="ast-validation.html"><strong aria-hidden="true">31.6.</strong> AST Validation</a></li><li class="chapter-item "><a href="feature-gate-ck.html"><strong aria-hidden="true">31.7.</strong> Feature Gate Checking</a></li><li class="chapter-item "><a href="lang-items.html"><strong aria-hidden="true">31.8.</strong> Lang Items</a></li></ol></li><li class="chapter-item "><a href="hir.html"><strong aria-hidden="true">32.</strong> The HIR (High-level IR)</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="lowering.html"><strong aria-hidden="true">32.1.</strong> Lowering AST to HIR</a></li><li class="chapter-item "><a href="hir-debugging.html"><strong aria-hidden="true">32.2.</strong> Debugging</a></li></ol></li><li class="chapter-item "><a href="thir.html"><strong aria-hidden="true">33.</strong> The THIR (Typed High-level IR)</a></li><li class="chapter-item "><a href="mir/index.html"><strong aria-hidden="true">34.</strong> The MIR (Mid-level IR)</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="mir/construction.html"><strong aria-hidden="true">34.1.</strong> MIR construction</a></li><li class="chapter-item "><a href="mir/visitor.html"><strong aria-hidden="true">34.2.</strong> MIR visitor and traversal</a></li><li class="chapter-item "><a href="mir/passes.html"><strong aria-hidden="true">34.3.</strong> MIR passes: getting the MIR for a function</a></li></ol></li><li class="chapter-item "><a href="identifiers.html"><strong aria-hidden="true">35.</strong> Identifiers in the Compiler</a></li><li class="chapter-item "><a href="closure.html"><strong aria-hidden="true">36.</strong> Closure expansion</a></li><li class="chapter-item affix "><li class="part-title">Analysis</li><li class="chapter-item "><a href="part-4-intro.html"><strong aria-hidden="true">37.</strong> Prologue</a></li><li class="chapter-item "><a href="ty.html"><strong aria-hidden="true">38.</strong> The ty module: representing types</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="generics.html"><strong aria-hidden="true">38.1.</strong> Generics and substitutions</a></li><li class="chapter-item "><a href="ty-fold.html"><strong aria-hidden="true">38.2.</strong> TypeFolder and TypeFoldable</a></li><li class="chapter-item "><a href="generic_arguments.html"><strong aria-hidden="true">38.3.</strong> Generic arguments</a></li><li class="chapter-item "><a href="constants.html"><strong aria-hidden="true">38.4.</strong> Constants in the type system</a></li></ol></li><li class="chapter-item "><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="using-tracing-to-debug-the-compiler"><a class="header" href="#using-tracing-to-debug-the-compiler">Using tracing to debug the compiler</a></h1> |
| <ul> |
| <li><a href="#function-level-filters">Function level filters</a> |
| <ul> |
| <li><a href="#i-dont-want-everything">I don't want everything</a></li> |
| <li><a href="#i-dont-want-all-calls">I don't want all calls</a></li> |
| </ul> |
| </li> |
| <li><a href="#broad-module-level-filters">Broad module level filters</a></li> |
| <li><a href="#log-colors">Log colors</a></li> |
| <li><a href="#how-to-keep-or-remove-debug-and-trace-calls-from-the-resulting-binary">How to keep or remove <code>debug!</code> and <code>trace!</code> calls from the resulting binary</a></li> |
| <li><a href="#logging-etiquette-and-conventions">Logging etiquette and conventions</a></li> |
| </ul> |
| <p>The compiler has a lot of <a href="https://docs.rs/tracing/0.1/tracing/macro.debug.html"><code>debug!</code></a> (or <code>trace!</code>) calls, which print out logging information |
| at many points. These are very useful to at least narrow down the location of |
| a bug if not to find it entirely, or just to orient yourself as to why the |
| compiler is doing a particular thing.</p> |
| <p>To see the logs, you need to set the <code>RUSTC_LOG</code> environment variable to your |
| log filter. The full syntax of the log filters can be found in the <a href="https://docs.rs/tracing-subscriber/0.2.24/tracing_subscriber/filter/struct.EnvFilter.html#directives">rustdoc |
| of <code>tracing-subscriber</code></a>.</p> |
| <h2 id="function-level-filters"><a class="header" href="#function-level-filters">Function level filters</a></h2> |
| <p>Lots of functions in rustc are annotated with</p> |
| <pre><code>#[instrument(level = "debug", skip(self))] |
| fn foo(&self, bar: Type) {} |
| </code></pre> |
| <p>which allows you to use</p> |
| <pre><code>RUSTC_LOG=[foo] |
| </code></pre> |
| <p>to do the following all at once</p> |
| <ul> |
| <li>log all function calls to <code>foo</code></li> |
| <li>log the arguments (except for those in the <code>skip</code> list)</li> |
| <li>log everything (from anywhere else in the compiler) until the function returns</li> |
| </ul> |
| <h3 id="i-dont-want-everything"><a class="header" href="#i-dont-want-everything">I don't want everything</a></h3> |
| <p>Depending on the scope of the function, you may not want to log everything in its body. |
| As an example: the <code>do_mir_borrowck</code> function will dump hundreds of lines even for trivial |
| code being borrowchecked.</p> |
| <p>Since you can combine all filters, you can add a crate/module path, e.g.</p> |
| <pre><code>RUSTC_LOG=rustc_borrowck[do_mir_borrowck] |
| </code></pre> |
| <h3 id="i-dont-want-all-calls"><a class="header" href="#i-dont-want-all-calls">I don't want all calls</a></h3> |
| <p>If you are compiling libcore, you likely don't want <em>all</em> borrowck dumps, but only one |
| for a specific function. You can filter function calls by their arguments by regexing them.</p> |
| <pre><code>RUSTC_LOG=[do_mir_borrowck{id=\.\*from_utf8_unchecked\.\*}] |
| </code></pre> |
| <p>will only give you the logs of borrowchecking <code>from_utf8_unchecked</code>. Note that you will |
| still get a short message per ignored <code>do_mir_borrowck</code>, but none of the things inside those |
| calls. This helps you in looking through the calls that are happening and helps you adjust |
| your regex if you mistyped it.</p> |
| <h2 id="broad-module-level-filters"><a class="header" href="#broad-module-level-filters">Broad module level filters</a></h2> |
| <p>You can also use filters similar to the <code>log</code> crate's filters, which will enable |
| everything within a specific module. This is often too verbose and too unstructured, |
| so it is recommended to use function level filters.</p> |
| <p>Your log filter can be just <code>debug</code> to get all <code>debug!</code> output and |
| higher (e.g., it will also include <code>info!</code>), or <code>path::to::module</code> to get <em>all</em> |
| output (which will include <code>trace!</code>) from a particular module, or |
| <code>path::to::module=debug</code> to get <code>debug!</code> output and higher from a particular |
| module.</p> |
| <p>For example, to get the <code>debug!</code> output and higher for a specific module, you |
| can run the compiler with <code>RUSTC_LOG=path::to::module=debug rustc my-file.rs</code>. |
| All <code>debug!</code> output will then appear in standard error.</p> |
| <p>Note that you can use a partial path and the filter will still work. For |
| example, if you want to see <code>info!</code> output from only |
| <code>rustdoc::passes::collect_intra_doc_links</code>, you could use |
| <code>RUSTDOC_LOG=rustdoc::passes::collect_intra_doc_links=info</code> <em>or</em> you could use |
| <code>RUSTDOC_LOG=rustdoc::passes::collect_intra=info</code>.</p> |
| <p>If you are developing rustdoc, use <code>RUSTDOC_LOG</code> instead. If you are developing |
| Miri, use <code>MIRI_LOG</code> instead. You get the idea :)</p> |
| <p>See the <a href="https://docs.rs/tracing"><code>tracing</code></a> crate's docs, and specifically the docs for <a href="https://docs.rs/tracing/0.1/tracing/macro.debug.html"><code>debug!</code></a> to |
| see the full syntax you can use. (Note: unlike the compiler, the <a href="https://docs.rs/tracing"><code>tracing</code></a> |
| crate and its examples use the <code>RUST_LOG</code> environment variable. rustc, rustdoc, |
| and other tools set custom environment variables.)</p> |
| <p><strong>Note that unless you use a very strict filter, the logger will emit a lot of |
| output, so use the most specific module(s) you can (comma-separated if |
| multiple)</strong>. It's typically a good idea to pipe standard error to a file and |
| look at the log output with a text editor.</p> |
| <p>So, to put it together:</p> |
| <pre><code class="language-bash"># This puts the output of all debug calls in `rustc_middle/src/traits` into |
| # standard error, which might fill your console backscroll. |
| $ RUSTC_LOG=rustc_middle::traits=debug rustc +stage1 my-file.rs |
| |
| # This puts the output of all debug calls in `rustc_middle/src/traits` in |
| # `traits-log`, so you can then see it with a text editor. |
| $ RUSTC_LOG=rustc_middle::traits=debug rustc +stage1 my-file.rs 2>traits-log |
| |
| # Not recommended! This will show the output of all `debug!` calls |
| # in the Rust compiler, and there are a *lot* of them, so it will be |
| # hard to find anything. |
| $ RUSTC_LOG=debug rustc +stage1 my-file.rs 2>all-log |
| |
| # This will show the output of all `info!` calls in `rustc_codegen_ssa`. |
| # |
| # There's an `info!` statement in `codegen_instance` that outputs |
| # every function that is codegen'd. This is useful to find out |
| # which function triggers an LLVM assertion, and this is an `info!` |
| # log rather than a `debug!` log so it will work on the official |
| # compilers. |
| $ RUSTC_LOG=rustc_codegen_ssa=info rustc +stage1 my-file.rs |
| |
| # This will show the output of all `info!` calls made by rustdoc |
| # or any rustc library it calls. |
| $ RUSTDOC_LOG=info rustdoc +stage1 my-file.rs |
| |
| # This will only show `debug!` calls made by rustdoc directly, |
| # not any `rustc*` crate. |
| $ RUSTDOC_LOG=rustdoc=debug rustdoc +stage1 my-file.rs |
| </code></pre> |
| <h2 id="log-colors"><a class="header" href="#log-colors">Log colors</a></h2> |
| <p>By default, rustc (and other tools, like rustdoc and Miri) will be smart about |
| when to use ANSI colors in the log output. If they are outputting to a terminal, |
| they will use colors, and if they are outputting to a file or being piped |
| somewhere else, they will not. However, it's hard to read log output in your |
| terminal unless you have a very strict filter, so you may want to pipe the |
| output to a pager like <code>less</code>. But then there won't be any colors, which makes |
| it hard to pick out what you're looking for!</p> |
| <p>You can override whether to have colors in log output with the <code>RUSTC_LOG_COLOR</code> |
| environment variable (or <code>RUSTDOC_LOG_COLOR</code> for rustdoc, or <code>MIRI_LOG_COLOR</code> |
| for Miri, etc.). There are three options: <code>auto</code> (the default), <code>always</code>, and |
| <code>never</code>. So, if you want to enable colors when piping to <code>less</code>, use something |
| similar to this command:</p> |
| <pre><code class="language-bash"># The `-R` switch tells less to print ANSI colors without escaping them. |
| $ RUSTC_LOG=debug RUSTC_LOG_COLOR=always rustc +stage1 ... | less -R |
| </code></pre> |
| <p>Note that <code>MIRI_LOG_COLOR</code> will only color logs that come from Miri, not logs |
| from rustc functions that Miri calls. Use <code>RUSTC_LOG_COLOR</code> to color logs from |
| rustc.</p> |
| <h2 id="how-to-keep-or-remove-debug-and-trace-calls-from-the-resulting-binary"><a class="header" href="#how-to-keep-or-remove-debug-and-trace-calls-from-the-resulting-binary">How to keep or remove <code>debug!</code> and <code>trace!</code> calls from the resulting binary</a></h2> |
| <p>While calls to <code>error!</code>, <code>warn!</code> and <code>info!</code> are included in every build of the compiler, |
| calls to <code>debug!</code> and <code>trace!</code> are only included in the program if |
| <code>debug-logging=true</code> is turned on in config.toml (it is |
| turned off by default), so if you don't see <code>DEBUG</code> logs, especially |
| if you run the compiler with <code>RUSTC_LOG=rustc rustc some.rs</code> and only see |
| <code>INFO</code> logs, make sure that <code>debug-logging=true</code> is turned on in your |
| config.toml.</p> |
| <h2 id="logging-etiquette-and-conventions"><a class="header" href="#logging-etiquette-and-conventions">Logging etiquette and conventions</a></h2> |
| <p>Because calls to <code>debug!</code> are removed by default, in most cases, don't worry |
| about the performance of adding "unnecessary" calls to <code>debug!</code> and leaving them in code you |
| commit - they won't slow down the performance of what we ship.</p> |
| <p>That said, there can also be excessive tracing calls, especially |
| when they are redundant with other calls nearby or in functions called from |
| here. There is no perfect balance to hit here, and is left to the reviewer's |
| discretion to decide whether to let you leave <code>debug!</code> statements in or whether to ask |
| you to remove them before merging.</p> |
| <p>It may be preferable to use <code>trace!</code> over <code>debug!</code> for very noisy logs.</p> |
| <p>A loosely followed convention is to use <code>#[instrument(level = "debug")]</code> |
| (<a href="https://docs.rs/tracing-attributes/0.1.17/tracing_attributes/attr.instrument.html">also see the attribute's documentation</a>) |
| in favour of <code>debug!("foo(...)")</code> at the start of a function <code>foo</code>. |
| Within functions, prefer <code>debug!(?variable.field)</code> over <code>debug!("xyz = {:?}", variable.field)</code> |
| and <code>debug!(bar = ?var.method(arg))</code> over <code>debug!("bar = {:?}", var.method(arg))</code>. |
| The documentation for this syntax can be found <a href="https://docs.rs/tracing/0.1.28/tracing/#recording-fields">here</a>.</p> |
| <p>One thing to be <strong>careful</strong> of is <strong>expensive</strong> operations in logs.</p> |
| <p>If in the module <code>rustc::foo</code> you have a statement</p> |
| <pre><code class="language-Rust">debug!(x = ?random_operation(tcx)); |
| </code></pre> |
| <p>Then if someone runs a debug <code>rustc</code> with <code>RUSTC_LOG=rustc::foo</code>, then |
| <code>random_operation()</code> will run. <code>RUSTC_LOG</code> filters that do not enable this |
| debug statement will not execute <code>random_operation</code>.</p> |
| <p>This means that you should not put anything too expensive or likely to crash |
| there - that would annoy anyone who wants to use logging for that module. |
| No-one will know it until someone tries to use logging to find <em>another</em> bug.</p> |
| |
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