| <!DOCTYPE HTML> |
| <html lang="en" class="light sidebar-visible" dir="ltr"> |
| <head> |
| <!-- Book generated using mdBook --> |
| <meta charset="UTF-8"> |
| <title>The solver - Rust Compiler Development Guide</title> |
| |
| |
| <!-- Custom HTML head --> |
| |
| <meta name="description" content="A guide to developing the Rust compiler (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" id="highlight-css" href="../highlight.css"> |
| <link rel="stylesheet" id="tomorrow-night-css" href="../tomorrow-night.css"> |
| <link rel="stylesheet" id="ayu-highlight-css" href="../ayu-highlight.css"> |
| |
| <!-- Custom theme stylesheets --> |
| <link rel="stylesheet" href="../pagetoc.css"> |
| |
| |
| <!-- Provide site root and default themes to javascript --> |
| <script> |
| const path_to_root = "../"; |
| const default_light_theme = "light"; |
| const default_dark_theme = "navy"; |
| window.path_to_searchindex_js = "../searchindex.js"; |
| </script> |
| <!-- Start loading toc.js asap --> |
| <script src="../toc.js"></script> |
| </head> |
| <body> |
| <div id="mdbook-help-container"> |
| <div id="mdbook-help-popup"> |
| <h2 class="mdbook-help-title">Keyboard shortcuts</h2> |
| <div> |
| <p>Press <kbd>←</kbd> or <kbd>→</kbd> to navigate between chapters</p> |
| <p>Press <kbd>S</kbd> or <kbd>/</kbd> to search in the book</p> |
| <p>Press <kbd>?</kbd> to show this help</p> |
| <p>Press <kbd>Esc</kbd> to hide this help</p> |
| </div> |
| </div> |
| </div> |
| <div id="body-container"> |
| <!-- Work around some values being stored in localStorage wrapped in quotes --> |
| <script> |
| try { |
| let theme = localStorage.getItem('mdbook-theme'); |
| let 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> |
| const default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? default_dark_theme : default_light_theme; |
| let theme; |
| try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { } |
| if (theme === null || theme === undefined) { theme = default_theme; } |
| const html = document.documentElement; |
| html.classList.remove('light') |
| html.classList.add(theme); |
| html.classList.add("js"); |
| </script> |
| |
| <input type="checkbox" id="sidebar-toggle-anchor" class="hidden"> |
| |
| <!-- Hide / unhide sidebar before it is displayed --> |
| <script> |
| let sidebar = null; |
| const sidebar_toggle = document.getElementById("sidebar-toggle-anchor"); |
| if (document.body.clientWidth >= 1080) { |
| try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { } |
| sidebar = sidebar || 'visible'; |
| } else { |
| sidebar = 'hidden'; |
| sidebar_toggle.checked = false; |
| } |
| if (sidebar === 'visible') { |
| sidebar_toggle.checked = true; |
| } else { |
| html.classList.remove('sidebar-visible'); |
| } |
| </script> |
| |
| <nav id="sidebar" class="sidebar" aria-label="Table of contents"> |
| <!-- populated by js --> |
| <mdbook-sidebar-scrollbox class="sidebar-scrollbox"></mdbook-sidebar-scrollbox> |
| <noscript> |
| <iframe class="sidebar-iframe-outer" src="../toc.html"></iframe> |
| </noscript> |
| <div id="sidebar-resize-handle" class="sidebar-resize-handle"> |
| <div class="sidebar-resize-indicator"></div> |
| </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"> |
| <div class="left-buttons"> |
| <label id="sidebar-toggle" class="icon-button" for="sidebar-toggle-anchor" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar"> |
| <i class="fa fa-bars"></i> |
| </label> |
| <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="default_theme">Auto</button></li> |
| <li role="none"><button role="menuitem" class="theme" id="light">Light</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 (`/`)" 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">Rust Compiler Development Guide</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/edit/main/src/solve/the-solver.md" title="Suggest an edit" aria-label="Suggest an edit" rel="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"> |
| <div class="search-wrapper"> |
| <input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header"> |
| <div class="spinner-wrapper"> |
| <i class="fa fa-spinner fa-spin"></i> |
| </div> |
| </div> |
| </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> |
| 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="the-solver"><a class="header" href="#the-solver">The solver</a></h1> |
| <p>Also consider reading the documentation for <a href="https://rust-lang.github.io/chalk/book/recursive.html">the recursive solver in chalk</a> |
| as it is very similar to this implementation and also talks about limitations of this |
| approach.</p> |
| <h2 id="a-rough-walkthrough"><a class="header" href="#a-rough-walkthrough">A rough walkthrough</a></h2> |
| <p>The entry-point of the solver is <code>InferCtxtEvalExt::evaluate_root_goal</code>. This |
| function sets up the root <code>EvalCtxt</code> and then calls <code>EvalCtxt::evaluate_goal</code>, |
| to actually enter the trait solver.</p> |
| <p><code>EvalCtxt::evaluate_goal</code> handles <a href="./canonicalization.html">canonicalization</a>, caching, |
| overflow, and solver cycles. Once that is done, it creates a nested <code>EvalCtxt</code> with a |
| separate local <code>InferCtxt</code> and calls <code>EvalCtxt::compute_goal</code>, which is responsible for the |
| 'actual solver behavior'. We match on the <code>PredicateKind</code>, delegating to a separate function |
| for each one.</p> |
| <p>For trait goals, such a <code>Vec<T>: Clone</code>, <code>EvalCtxt::compute_trait_goal</code> has |
| to collect all the possible ways this goal can be proven via |
| <code>EvalCtxt::assemble_and_evaluate_candidates</code>. Each candidate is handled in |
| a separate "probe", to not leak inference constraints to the other candidates. |
| We then try to merge the assembled candidates via <code>EvalCtxt::merge_candidates</code>.</p> |
| <h2 id="important-concepts-and-design-patterns"><a class="header" href="#important-concepts-and-design-patterns">Important concepts and design patterns</a></h2> |
| <h3 id="evalctxtadd_goal"><a class="header" href="#evalctxtadd_goal"><code>EvalCtxt::add_goal</code></a></h3> |
| <p>To prove nested goals, we don't directly call <code>EvalCtxt::compute_goal</code>, but instead |
| add the goal to the <code>EvalCtxt</code> with <code>EvalCtxt::all_goal</code>. We then prove all nested |
| goals together in either <code>EvalCtxt::try_evaluate_added_goals</code> or |
| <code>EvalCtxt::evaluate_added_goals_and_make_canonical_response</code>. This allows us to handle |
| inference constraints from later goals.</p> |
| <p>E.g. if we have both <code>?x: Debug</code> and <code>(): ConstrainToU8<?x></code> as nested goals, |
| then proving <code>?x: Debug</code> is initially ambiguous, but after proving <code>(): ConstrainToU8<?x></code> |
| we constrain <code>?x</code> to <code>u8</code> and proving <code>u8: Debug</code> succeeds.</p> |
| <h3 id="matching-on-tykind"><a class="header" href="#matching-on-tykind">Matching on <code>TyKind</code></a></h3> |
| <p>We lazily normalize types in the solver, so we always have to assume that any types |
| and constants are potentially unnormalized. This means that matching on <code>TyKind</code> can easily |
| be incorrect.</p> |
| <p>We handle normalization in two different ways. When proving <code>Trait</code> goals when normalizing |
| associated types, we separately assemble candidates depending on whether they structurally |
| match the self type. Candidates which match on the self type are handled in |
| <code>EvalCtxt::assemble_candidates_via_self_ty</code> which recurses via |
| <code>EvalCtxt::assemble_candidates_after_normalizing_self_ty</code>, which normalizes the self type |
| by one level. In all other cases we have to match on a <code>TyKind</code> we first use |
| <code>EvalCtxt::try_normalize_ty</code> to normalize the type as much as possible.</p> |
| <h3 id="higher-ranked-goals"><a class="header" href="#higher-ranked-goals">Higher ranked goals</a></h3> |
| <p>In case the goal is higher-ranked, e.g. <code>for<'a> F: FnOnce(&'a ())</code>, <code>EvalCtxt::compute_goal</code> |
| eagerly instantiates <code>'a</code> with a placeholder and then recursively proves |
| <code>F: FnOnce(&'!a ())</code> as a nested goal.</p> |
| <h3 id="dealing-with-choice"><a class="header" href="#dealing-with-choice">Dealing with choice</a></h3> |
| <p>Some goals can be proven in multiple ways. In these cases we try each option in |
| a separate "probe" and then attempt to merge the resulting responses by using |
| <code>EvalCtxt::try_merge_responses</code>. If merging the responses fails, we use |
| <code>EvalCtxt::flounder</code> instead, returning ambiguity. For some goals, we try to |
| incompletely prefer some choices over others in case <code>EvalCtxt::try_merge_responses</code> |
| fails.</p> |
| <h2 id="learning-more"><a class="header" href="#learning-more">Learning more</a></h2> |
| <p>The solver should be fairly self-contained. I hope that the above information provides a |
| good foundation when looking at the code itself. Please reach out on Zulip if you get stuck |
| while doing so or there are some quirks and design decisions which were unclear and deserve |
| better comments or should be mentioned here.</p> |
| |
| </main> |
| |
| <nav class="nav-wrapper" aria-label="Page navigation"> |
| <!-- Mobile navigation buttons --> |
| <a rel="prev" href="../solve/invariants.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 prefetch" href="../solve/candidate-preference.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="../solve/invariants.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 prefetch" href="../solve/candidate-preference.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> |
| window.playground_copyable = true; |
| </script> |
| |
| |
| <script src="../elasticlunr.min.js"></script> |
| <script src="../mark.min.js"></script> |
| <script src="../searcher.js"></script> |
| |
| <script src="../clipboard.min.js"></script> |
| <script src="../highlight.js"></script> |
| <script src="../book.js"></script> |
| |
| <!-- Custom JS scripts --> |
| <script src="../mermaid.min.js"></script> |
| <script src="../mermaid-init.js"></script> |
| <script src="../pagetoc.js"></script> |
| |
| |
| |
| </div> |
| </body> |
| </html> |
