Google Git
Sign in
rust/rustc-guide/refs/heads/gh-pages/./backend/implicit-caller-location.html
blob: 4df5074529f284be3ed2210c0b480a387d143854 [file] [edit]
<!DOCTYPE HTML>
<html lang="en" class="sidebar-visible no-js light">
<head>
<!-- Book generated using mdBook -->
<meta charset="UTF-8">
<title>Implicit Caller Location - Guide to Rustc Development</title>
<!-- Custom HTML head -->
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
<meta name="description" content="A guide to developing rustc">
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta name="theme-color" content="#ffffff" />
<link rel="icon" href="../favicon.svg">
<link rel="shortcut icon" href="../favicon.png">
<link rel="stylesheet" href="../css/variables.css">
<link rel="stylesheet" href="../css/general.css">
<link rel="stylesheet" href="../css/chrome.css">
<link rel="stylesheet" href="../css/print.css" media="print">
<!-- Fonts -->
<link rel="stylesheet" href="../FontAwesome/css/font-awesome.css">
<link rel="stylesheet" href="../fonts/fonts.css">
<!-- Highlight.js Stylesheets -->
<link rel="stylesheet" href="../highlight.css">
<link rel="stylesheet" href="../tomorrow-night.css">
<link rel="stylesheet" href="../ayu-highlight.css">
<!-- Custom theme stylesheets -->
</head>
<body>
<!-- Provide site root to javascript -->
<script type="text/javascript">
var path_to_root = "../";
var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "navy" : "light";
</script>
<!-- Work around some values being stored in localStorage wrapped in quotes -->
<script type="text/javascript">
try {
var theme = localStorage.getItem('mdbook-theme');
var sidebar = localStorage.getItem('mdbook-sidebar');
if (theme.startsWith('"') && theme.endsWith('"')) {
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
}
if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
}
} catch (e) { }
</script>
<!-- Set the theme before any content is loaded, prevents flash -->
<script type="text/javascript">
var theme;
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
if (theme === null || theme === undefined) { theme = default_theme; }
var html = document.querySelector('html');
html.classList.remove('no-js')
html.classList.remove('light')
html.classList.add(theme);
html.classList.add('js');
</script>
<!-- Hide / unhide sidebar before it is displayed -->
<script type="text/javascript">
var html = document.querySelector('html');
var sidebar = 'hidden';
if (document.body.clientWidth >= 1080) {
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
sidebar = sidebar || 'visible';
}
html.classList.remove('sidebar-visible');
html.classList.add("sidebar-" + sidebar);
</script>
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
<div class="sidebar-scrollbox">
<ol class="chapter"><li class="chapter-item affix "><a href="../about-this-guide.html">About this guide</a></li><li class="chapter-item affix "><a href="../getting-started.html">Getting Started</a></li><li class="spacer"></li><li class="chapter-item affix "><li class="part-title">Building and debugging rustc</li><li class="chapter-item "><a href="../building/how-to-build-and-run.html"><strong aria-hidden="true">1.</strong> How to Build and Run the Compiler</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../building/prerequisites.html"><strong aria-hidden="true">1.1.</strong> Prerequisites</a></li><li class="chapter-item "><a href="../building/suggested.html"><strong aria-hidden="true">1.2.</strong> Suggested Workflows</a></li><li class="chapter-item "><a href="../building/build-install-distribution-artifacts.html"><strong aria-hidden="true">1.3.</strong> Distribution artifacts</a></li><li class="chapter-item "><a href="../building/compiler-documenting.html"><strong aria-hidden="true">1.4.</strong> Documenting Compiler</a></li><li class="chapter-item "><a href="../rustdoc.html"><strong aria-hidden="true">1.5.</strong> Rustdoc overview</a></li><li class="chapter-item "><a href="../building/new-target.html"><strong aria-hidden="true">1.6.</strong> Adding a new target</a></li></ol></li><li class="chapter-item "><a href="../tests/intro.html"><strong aria-hidden="true">2.</strong> The compiler testing framework</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../tests/running.html"><strong aria-hidden="true">2.1.</strong> Running tests</a></li><li class="chapter-item "><a href="../tests/adding.html"><strong aria-hidden="true">2.2.</strong> Adding new tests</a></li><li class="chapter-item "><a href="../compiletest.html"><strong aria-hidden="true">2.3.</strong> Using compiletest commands to control test execution</a></li></ol></li><li class="chapter-item "><a href="../compiler-debugging.html"><strong aria-hidden="true">3.</strong> Debugging the Compiler</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../tracing.html"><strong aria-hidden="true">3.1.</strong> Using the tracing/logging instrumentation</a></li></ol></li><li class="chapter-item "><a href="../profiling.html"><strong aria-hidden="true">4.</strong> Profiling the compiler</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../profiling/with_perf.html"><strong aria-hidden="true">4.1.</strong> with the linux perf tool</a></li><li class="chapter-item "><a href="../profiling/wpa_profiling.html"><strong aria-hidden="true">4.2.</strong> with Windows Performance Analyzer</a></li></ol></li><li class="chapter-item "><a href="../crates-io.html"><strong aria-hidden="true">5.</strong> crates.io Dependencies</a></li><li class="chapter-item affix "><li class="part-title">Contributing to Rust</li><li class="chapter-item "><a href="../contributing.html"><strong aria-hidden="true">6.</strong> Introduction</a></li><li class="chapter-item "><a href="../compiler-team.html"><strong aria-hidden="true">7.</strong> About the compiler team</a></li><li class="chapter-item "><a href="../git.html"><strong aria-hidden="true">8.</strong> Using Git</a></li><li class="chapter-item "><a href="../rustbot.html"><strong aria-hidden="true">9.</strong> Mastering @rustbot</a></li><li class="chapter-item "><a href="../walkthrough.html"><strong aria-hidden="true">10.</strong> Walkthrough: a typical contribution</a></li><li class="chapter-item "><a href="../bug-fix-procedure.html"><strong aria-hidden="true">11.</strong> Bug Fix Procedure</a></li><li class="chapter-item "><a href="../implementing_new_features.html"><strong aria-hidden="true">12.</strong> Implementing new features</a></li><li class="chapter-item "><a href="../stability.html"><strong aria-hidden="true">13.</strong> Stability attributes</a></li><li class="chapter-item "><a href="../stabilization_guide.html"><strong aria-hidden="true">14.</strong> Stabilizing Features</a></li><li class="chapter-item "><a href="../feature-gates.html"><strong aria-hidden="true">15.</strong> Feature Gates</a></li><li class="chapter-item "><a href="../conventions.html"><strong aria-hidden="true">16.</strong> Coding conventions</a></li><li class="chapter-item "><a href="../notification-groups/about.html"><strong aria-hidden="true">17.</strong> Notification groups</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../notification-groups/arm.html"><strong aria-hidden="true">17.1.</strong> ARM</a></li><li class="chapter-item "><a href="../notification-groups/cleanup-crew.html"><strong aria-hidden="true">17.2.</strong> Cleanup Crew</a></li><li class="chapter-item "><a href="../notification-groups/llvm.html"><strong aria-hidden="true">17.3.</strong> LLVM</a></li><li class="chapter-item "><a href="../notification-groups/risc-v.html"><strong aria-hidden="true">17.4.</strong> RISC-V</a></li><li class="chapter-item "><a href="../notification-groups/windows.html"><strong aria-hidden="true">17.5.</strong> Windows</a></li></ol></li><li class="chapter-item "><a href="../licenses.html"><strong aria-hidden="true">18.</strong> Licenses</a></li><li class="chapter-item affix "><li class="part-title">High-level Compiler Architecture</li><li class="chapter-item "><a href="../part-2-intro.html"><strong aria-hidden="true">19.</strong> Prologue</a></li><li class="chapter-item "><a href="../overview.html"><strong aria-hidden="true">20.</strong> Overview of the Compiler</a></li><li class="chapter-item "><a href="../compiler-src.html"><strong aria-hidden="true">21.</strong> The compiler source code</a></li><li class="chapter-item "><a href="../building/bootstrapping.html"><strong aria-hidden="true">22.</strong> Bootstrapping</a></li><li class="chapter-item "><a href="../query.html"><strong aria-hidden="true">23.</strong> Queries: demand-driven compilation</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../queries/query-evaluation-model-in-detail.html"><strong aria-hidden="true">23.1.</strong> The Query Evaluation Model in Detail</a></li><li class="chapter-item "><a href="../queries/incremental-compilation.html"><strong aria-hidden="true">23.2.</strong> Incremental compilation</a></li><li class="chapter-item "><a href="../queries/incremental-compilation-in-detail.html"><strong aria-hidden="true">23.3.</strong> Incremental compilation In Detail</a></li><li class="chapter-item "><a href="../incrcomp-debugging.html"><strong aria-hidden="true">23.4.</strong> Debugging and Testing</a></li><li class="chapter-item "><a href="../salsa.html"><strong aria-hidden="true">23.5.</strong> Salsa</a></li></ol></li><li class="chapter-item "><a href="../memory.html"><strong aria-hidden="true">24.</strong> Memory Management in Rustc</a></li><li class="chapter-item "><a href="../serialization.html"><strong aria-hidden="true">25.</strong> Serialization in Rustc</a></li><li class="chapter-item "><a href="../parallel-rustc.html"><strong aria-hidden="true">26.</strong> Parallel Compilation</a></li><li class="chapter-item "><a href="../rustdoc-internals.html"><strong aria-hidden="true">27.</strong> Rustdoc internals</a></li><li class="chapter-item affix "><li class="part-title">Source Code Representation</li><li class="chapter-item "><a href="../part-3-intro.html"><strong aria-hidden="true">28.</strong> Prologue</a></li><li class="chapter-item "><a href="../cli.html"><strong aria-hidden="true">29.</strong> Command-line arguments</a></li><li class="chapter-item "><a href="../rustc-driver.html"><strong aria-hidden="true">30.</strong> The Rustc Driver and Interface</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../rustc-driver-interacting-with-the-ast.html"><strong aria-hidden="true">30.1.</strong> Ex: Type checking through rustc_interface</a></li><li class="chapter-item "><a href="../rustc-driver-getting-diagnostics.html"><strong aria-hidden="true">30.2.</strong> Ex: Getting diagnostics through rustc_interface</a></li></ol></li><li class="chapter-item "><a href="../syntax-intro.html"><strong aria-hidden="true">31.</strong> Syntax and the AST</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../the-parser.html"><strong aria-hidden="true">31.1.</strong> Lexing and Parsing</a></li><li class="chapter-item "><a href="../macro-expansion.html"><strong aria-hidden="true">31.2.</strong> Macro expansion</a></li><li class="chapter-item "><a href="../name-resolution.html"><strong aria-hidden="true">31.3.</strong> Name resolution</a></li><li class="chapter-item "><a href="../test-implementation.html"><strong aria-hidden="true">31.4.</strong> #[test] Implementation</a></li><li class="chapter-item "><a href="../panic-implementation.html"><strong aria-hidden="true">31.5.</strong> Panic Implementation</a></li><li class="chapter-item "><a href="../ast-validation.html"><strong aria-hidden="true">31.6.</strong> AST Validation</a></li><li class="chapter-item "><a href="../feature-gate-ck.html"><strong aria-hidden="true">31.7.</strong> Feature Gate Checking</a></li><li class="chapter-item "><a href="../lang-items.html"><strong aria-hidden="true">31.8.</strong> Lang Items</a></li></ol></li><li class="chapter-item "><a href="../hir.html"><strong aria-hidden="true">32.</strong> The HIR (High-level IR)</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../lowering.html"><strong aria-hidden="true">32.1.</strong> Lowering AST to HIR</a></li><li class="chapter-item "><a href="../hir-debugging.html"><strong aria-hidden="true">32.2.</strong> Debugging</a></li></ol></li><li class="chapter-item "><a href="../thir.html"><strong aria-hidden="true">33.</strong> The THIR (Typed High-level IR)</a></li><li class="chapter-item "><a href="../mir/index.html"><strong aria-hidden="true">34.</strong> The MIR (Mid-level IR)</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../mir/construction.html"><strong aria-hidden="true">34.1.</strong> MIR construction</a></li><li class="chapter-item "><a href="../mir/visitor.html"><strong aria-hidden="true">34.2.</strong> MIR visitor and traversal</a></li><li class="chapter-item "><a href="../mir/passes.html"><strong aria-hidden="true">34.3.</strong> MIR passes: getting the MIR for a function</a></li></ol></li><li class="chapter-item "><a href="../identifiers.html"><strong aria-hidden="true">35.</strong> Identifiers in the Compiler</a></li><li class="chapter-item "><a href="../closure.html"><strong aria-hidden="true">36.</strong> Closure expansion</a></li><li class="chapter-item affix "><li class="part-title">Analysis</li><li class="chapter-item "><a href="../part-4-intro.html"><strong aria-hidden="true">37.</strong> Prologue</a></li><li class="chapter-item "><a href="../ty.html"><strong aria-hidden="true">38.</strong> The ty module: representing types</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../generics.html"><strong aria-hidden="true">38.1.</strong> Generics and substitutions</a></li><li class="chapter-item "><a href="../ty-fold.html"><strong aria-hidden="true">38.2.</strong> TypeFolder and TypeFoldable</a></li><li class="chapter-item "><a href="../generic_arguments.html"><strong aria-hidden="true">38.3.</strong> Generic arguments</a></li><li class="chapter-item "><a href="../constants.html"><strong aria-hidden="true">38.4.</strong> Constants in the type system</a></li></ol></li><li class="chapter-item "><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 expanded "><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 expanded "><a href="../backend/implicit-caller-location.html" class="active"><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>
</div>
<div id="sidebar-resize-handle" class="sidebar-resize-handle"></div>
</nav>
<div id="page-wrapper" class="page-wrapper">
<div class="page">
<div id="menu-bar-hover-placeholder"></div>
<div id="menu-bar" class="menu-bar sticky bordered">
<div class="left-buttons">
<button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
<i class="fa fa-bars"></i>
</button>
<button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
<i class="fa fa-paint-brush"></i>
</button>
<ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
<li role="none"><button role="menuitem" class="theme" id="light">Light (default)</button></li>
<li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
<li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
<li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
<li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
</ul>
<button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">
<i class="fa fa-search"></i>
</button>
</div>
<h1 class="menu-title">Guide to Rustc Development</h1>
<div class="right-buttons">
<a href="../print.html" title="Print this book" aria-label="Print this book">
<i id="print-button" class="fa fa-print"></i>
</a>
<a href="https://github.com/rust-lang/rustc-dev-guide" title="Git repository" aria-label="Git repository">
<i id="git-repository-button" class="fa fa-github"></i>
</a>
<a href="https://github.com/rust-lang/rustc-dev-guide/tree/master/src/backend/implicit-caller-location.md?mode&#x3D;edit" title="Suggest an edit" aria-label="Suggest an edit">
<i id="git-edit-button" class="fa fa-edit"></i>
</a>
</div>
</div>
<div id="search-wrapper" class="hidden">
<form id="searchbar-outer" class="searchbar-outer">
<input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header">
</form>
<div id="searchresults-outer" class="searchresults-outer hidden">
<div id="searchresults-header" class="searchresults-header"></div>
<ul id="searchresults">
</ul>
</div>
</div>
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
<script type="text/javascript">
document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
});
</script>
<div id="content" class="content">
<main>
<h1 id="implicit-caller-location"><a class="header" href="#implicit-caller-location">Implicit Caller Location</a></h1>
<ul>
<li><a href="#motivating-example">Motivating Example</a></li>
<li><a href="#reading-caller-location">Reading Caller Location</a></li>
<li><a href="#caller-location-in-const">Caller Location in <code>const</code></a>
<ul>
<li><a href="#finding-the-right-location">Finding the right <code>Location</code></a></li>
<li><a href="#allocating-a-static-location">Allocating a static <code>Location</code></a></li>
</ul>
</li>
<li><a href="#generating-code-for-track_caller-callees">Generating code for <code>#[track_caller]</code> callees</a>
<ul>
<li><a href="#codegen-examples">Codegen examples</a></li>
<li><a href="#dynamic-dispatch">Dynamic Dispatch</a></li>
</ul>
</li>
<li><a href="#the-attribute">The Attribute</a>
<ul>
<li><a href="#traits">Traits</a></li>
</ul>
</li>
<li><a href="#backgroundhistory">Background/History</a></li>
</ul>
<p>Approved in <a href="https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md">RFC 2091</a>, this feature enables the accurate reporting of caller location during panics
initiated from functions like <code>Option::unwrap</code>, <code>Result::expect</code>, and <code>Index::index</code>. This feature
adds the <a href="https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-track_caller-attribute"><code>#[track_caller]</code></a> attribute for functions, the
<a href="https://doc.rust-lang.org/nightly/core/intrinsics/fn.caller_location.html"><code>caller_location</code></a> intrinsic, and the stabilization-friendly
<a href="https://doc.rust-lang.org/nightly/core/panic/struct.Location.html#method.caller"><code>core::panic::Location::caller</code></a> wrapper.</p>
<h2 id="motivating-example"><a class="header" href="#motivating-example">Motivating Example</a></h2>
<p>Take this example program:</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
let foo: Option&lt;()&gt; = None;
foo.unwrap(); // this should produce a useful panic message!
}
</code></pre></pre>
<p>Prior to Rust 1.42, panics like this <code>unwrap()</code> printed a location in core:</p>
<pre><code>$ rustc +1.41.0 example.rs; example.exe
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value',...core\macros\mod.rs:15:40
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
</code></pre>
<p>As of 1.42, we get a much more helpful message:</p>
<pre><code>$ rustc +1.42.0 example.rs; example.exe
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', example.rs:3:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
</code></pre>
<p>These error messages are achieved through a combination of changes to <code>panic!</code> internals to make use
of <code>core::panic::Location::caller</code> and a number of <code>#[track_caller]</code> annotations in the standard
library which propagate caller information.</p>
<h2 id="reading-caller-location"><a class="header" href="#reading-caller-location">Reading Caller Location</a></h2>
<p>Previously, <code>panic!</code> made use of the <code>file!()</code>, <code>line!()</code>, and <code>column!()</code> macros to construct a
<a href="https://doc.rust-lang.org/core/panic/struct.Location.html"><code>Location</code></a> pointing to where the panic occurred. These macros couldn't be given an overridden
location, so functions which intentionally invoked <code>panic!</code> couldn't provide their own location,
hiding the actual source of error.</p>
<p>Internally, <code>panic!()</code> now calls <a href="https://doc.rust-lang.org/nightly/core/panic/struct.Location.html#method.caller"><code>core::panic::Location::caller()</code></a> to find out where it
was expanded. This function is itself annotated with <code>#[track_caller]</code> and wraps the
<a href="https://doc.rust-lang.org/nightly/core/intrinsics/fn.caller_location.html"><code>caller_location</code></a> compiler intrinsic implemented by rustc. This intrinsic is easiest
explained in terms of how it works in a <code>const</code> context.</p>
<h2 id="caller-location-in-const"><a class="header" href="#caller-location-in-const">Caller Location in <code>const</code></a></h2>
<p>There are two main phases to returning the caller location in a const context: walking up the stack
to find the right location and allocating a const value to return.</p>
<h3 id="finding-the-right-location"><a class="header" href="#finding-the-right-location">Finding the right <code>Location</code></a></h3>
<p>In a const context we &quot;walk up the stack&quot; from where the intrinsic is invoked, stopping when we
reach the first function call in the stack which does <em>not</em> have the attribute. This walk is in
<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.InterpCx.html#method.find_closest_untracked_caller_location"><code>InterpCx::find_closest_untracked_caller_location()</code></a>.</p>
<p>Starting at the bottom, we iterate up over stack <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.Frame.html"><code>Frame</code></a>s in the
<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.InterpCx.html#structfield.stack"><code>InterpCx::stack</code></a>, calling
<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.InstanceDef.html#method.requires_caller_location"><code>InstanceDef::requires_caller_location</code></a> on the
<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.Frame.html#structfield.instance"><code>Instance</code>s from each <code>Frame</code></a>. We stop once we find one that returns <code>false</code> and
return the span of the <em>previous</em> frame which was the &quot;topmost&quot; tracked function.</p>
<h3 id="allocating-a-static-location"><a class="header" href="#allocating-a-static-location">Allocating a static <code>Location</code></a></h3>
<p>Once we have a <code>Span</code>, we need to allocate static memory for the <code>Location</code>, which is performed by
the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html#method.const_caller_location"><code>TyCtxt::const_caller_location()</code></a> query. Internally this calls
<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.InterpCx.html#method.alloc_caller_location"><code>InterpCx::alloc_caller_location()</code></a> and results in a unique
<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/enum.MemoryKind.html#variant.CallerLocation">memory kind</a> (<code>MemoryKind::CallerLocation</code>). The SSA codegen backend is able
to emit code for these same values, and we use this code there as well.</p>
<p>Once our <code>Location</code> has been allocated in static memory, our intrinsic returns a reference to it.</p>
<h2 id="generating-code-for-track_caller-callees"><a class="header" href="#generating-code-for-track_caller-callees">Generating code for <code>#[track_caller]</code> callees</a></h2>
<p>To generate efficient code for a tracked function and its callers, we need to provide the same
behavior from the intrinsic's point of view without having a stack to walk up at runtime. We invert
the approach: as we grow the stack down we pass an additional argument to calls of tracked functions
rather than walking up the stack when the intrinsic is called. That additional argument can be
returned wherever the caller location is queried.</p>
<p>The argument we append is of type <code>&amp;'static core::panic::Location&lt;'static&gt;</code>. A reference was chosen
to avoid unnecessary copying because a pointer is a third the size of
<code>std::mem::size_of::&lt;core::panic::Location&gt;() == 24</code> at time of writing.</p>
<p>When generating a call to a function which is tracked, we pass the location argument the value of
<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/struct.FunctionCx.html#method.get_caller_location"><code>FunctionCx::get_caller_location</code></a>.</p>
<p>If the calling function is tracked, <code>get_caller_location</code> returns the local in
<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/struct.FunctionCx.html#structfield.caller_location"><code>FunctionCx::caller_location</code></a> which was populated by the current caller's caller.
In these cases the intrinsic &quot;returns&quot; a reference which was actually provided in an argument to its
caller.</p>
<p>If the calling function is not tracked, <code>get_caller_location</code> allocates a <code>Location</code> static from
the current <code>Span</code> and returns a reference to that.</p>
<p>We more efficiently achieve the same behavior as a loop starting from the bottom by passing a single
<code>&amp;Location</code> value through the <code>caller_location</code> fields of multiple <code>FunctionCx</code>s as we grow the
stack downward.</p>
<h3 id="codegen-examples"><a class="header" href="#codegen-examples">Codegen examples</a></h3>
<p>What does this transformation look like in practice? Take this example which uses the new feature:</p>
<pre><pre class="playground"><code class="language-rust">#![feature(track_caller)]
use std::panic::Location;
#[track_caller]
fn print_caller() {
println!(&quot;called from {}&quot;, Location::caller());
}
fn main() {
print_caller();
}
</code></pre></pre>
<p>Here <code>print_caller()</code> appears to take no arguments, but we compile it to something like this:</p>
<pre><pre class="playground"><code class="language-rust">#![feature(panic_internals)]
use std::panic::Location;
fn print_caller(caller: &amp;Location) {
println!(&quot;called from {}&quot;, caller);
}
fn main() {
print_caller(&amp;Location::internal_constructor(file!(), line!(), column!()));
}
</code></pre></pre>
<h3 id="dynamic-dispatch"><a class="header" href="#dynamic-dispatch">Dynamic Dispatch</a></h3>
<p>In codegen contexts we have to modify the callee ABI to pass this information down the stack, but
the attribute expressly does <em>not</em> modify the type of the function. The ABI change must be
transparent to type checking and remain sound in all uses.</p>
<p>Direct calls to tracked functions will always know the full codegen flags for the callee and can
generate appropriate code. Indirect callers won't have this information and it's not encoded in
the type of the function pointer they call, so we generate a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.InstanceDef.html#variant.ReifyShim"><code>ReifyShim</code></a> around the function
whenever taking a pointer to it. This shim isn't able to report the actual location of the indirect
call (the function's definition site is reported instead), but it prevents miscompilation and is
probably the best we can do without modifying fully-stabilized type signatures.</p>
<blockquote>
<p><em>Note:</em> We always emit a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.InstanceDef.html#variant.ReifyShim"><code>ReifyShim</code></a> when taking a pointer to a tracked function. While the
constraint here is imposed by codegen contexts, we don't know during MIR construction of the shim
whether we'll be called in a const context (safe to ignore shim) or in a codegen context (unsafe
to ignore shim). Even if we did know, the results from const and codegen contexts must agree.</p>
</blockquote>
<h2 id="the-attribute"><a class="header" href="#the-attribute">The Attribute</a></h2>
<p>The <code>#[track_caller]</code> attribute is checked alongside other codegen attributes to ensure the
function:</p>
<ul>
<li>has the <code>&quot;Rust&quot;</code> ABI (as opposed to e.g., <code>&quot;C&quot;</code>)</li>
<li>is not a closure</li>
<li>is not <code>#[naked]</code></li>
</ul>
<p>If the use is valid, we set <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/middle/codegen_fn_attrs/struct.CodegenFnAttrFlags.html#associatedconstant.TRACK_CALLER"><code>CodegenFnAttrsFlags::TRACK_CALLER</code></a>. This flag influences
the return value of <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.InstanceDef.html#method.requires_caller_location"><code>InstanceDef::requires_caller_location</code></a> which is in turn
used in both const and codegen contexts to ensure correct propagation.</p>
<h3 id="traits"><a class="header" href="#traits">Traits</a></h3>
<p>When applied to trait method implementations, the attribute works as it does for regular functions.</p>
<p>When applied to a trait method prototype, the attribute applies to all implementations of the
method. When applied to a default trait method implementation, the attribute takes effect on
that implementation <em>and</em> any overrides.</p>
<p>Examples:</p>
<pre><pre class="playground"><code class="language-rust">#![feature(track_caller)]
macro_rules! assert_tracked {
() =&gt; {{
let location = std::panic::Location::caller();
assert_eq!(location.file(), file!());
assert_ne!(location.line(), line!(), &quot;line should be outside this fn&quot;);
println!(&quot;called at {}&quot;, location);
}};
}
trait TrackedFourWays {
/// All implementations inherit `#[track_caller]`.
#[track_caller]
fn blanket_tracked();
/// Implementors can annotate themselves.
fn local_tracked();
/// This implementation is tracked (overrides are too).
#[track_caller]
fn default_tracked() {
assert_tracked!();
}
/// Overrides of this implementation are tracked (it is too).
#[track_caller]
fn default_tracked_to_override() {
assert_tracked!();
}
}
/// This impl uses the default impl for `default_tracked` and provides its own for
/// `default_tracked_to_override`.
impl TrackedFourWays for () {
fn blanket_tracked() {
assert_tracked!();
}
#[track_caller]
fn local_tracked() {
assert_tracked!();
}
fn default_tracked_to_override() {
assert_tracked!();
}
}
fn main() {
&lt;() as TrackedFourWays&gt;::blanket_tracked();
&lt;() as TrackedFourWays&gt;::default_tracked();
&lt;() as TrackedFourWays&gt;::default_tracked_to_override();
&lt;() as TrackedFourWays&gt;::local_tracked();
}
</code></pre></pre>
<h2 id="backgroundhistory"><a class="header" href="#backgroundhistory">Background/History</a></h2>
<p>Broadly speaking, this feature's goal is to improve common Rust error messages without breaking
stability guarantees, requiring modifications to end-user source, relying on platform-specific
debug-info, or preventing user-defined types from having the same error-reporting benefits.</p>
<p>Improving the output of these panics has been a goal of proposals since at least mid-2016 (see
<a href="https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md#non-viable-alternatives">non-viable alternatives</a> in the approved RFC for details). It took two more years until RFC 2091
was approved, much of its <a href="https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md#rationale">rationale</a> for this feature's design having been discovered through the
discussion around several earlier proposals.</p>
<p>The design in the original RFC limited itself to implementations that could be done inside the
compiler at the time without significant refactoring. However in the year and a half between the
approval of the RFC and the actual implementation work, a <a href="https://github.com/rust-lang/rust/issues/47809#issuecomment-443538059">revised design</a> was proposed and written
up on the tracking issue. During the course of implementing that, it was also discovered that an
implementation was possible without modifying the number of arguments in a function's MIR, which
would simplify later stages and unlock use in traits.</p>
<p>Because the RFC's implementation strategy could not readily support traits, the semantics were not
originally specified. They have since been implemented following the path which seemed most correct
to the author and reviewers.</p>
</main>
<nav class="nav-wrapper" aria-label="Page navigation">
<!-- Mobile navigation buttons -->
<a rel="prev" href="../backend/backend-agnostic.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
<i class="fa fa-angle-left"></i>
</a>
<a rel="next" href="../backend/libs-and-metadata.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
<i class="fa fa-angle-right"></i>
</a>
<div style="clear: both"></div>
</nav>
</div>
</div>
<nav class="nav-wide-wrapper" aria-label="Page navigation">
<a rel="prev" href="../backend/backend-agnostic.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
<i class="fa fa-angle-left"></i>
</a>
<a rel="next" href="../backend/libs-and-metadata.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
<i class="fa fa-angle-right"></i>
</a>
</nav>
</div>
<script type="text/javascript">
window.playground_copyable = true;
</script>
<script src="../elasticlunr.min.js" type="text/javascript" charset="utf-8"></script>
<script src="../mark.min.js" type="text/javascript" charset="utf-8"></script>
<script src="../searcher.js" type="text/javascript" charset="utf-8"></script>
<script src="../clipboard.min.js" type="text/javascript" charset="utf-8"></script>
<script src="../highlight.js" type="text/javascript" charset="utf-8"></script>
<script src="../book.js" type="text/javascript" charset="utf-8"></script>
<!-- Custom JS scripts -->
</body>
</html>