278 lines
16 KiB
HTML
278 lines
16 KiB
HTML
<!DOCTYPE HTML>
|
|
<html lang="en" class="light sidebar-visible" dir="ltr">
|
|
<head>
|
|
<!-- Book generated using mdBook -->
|
|
<meta charset="UTF-8">
|
|
<title>Canonicalization - 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 -->
|
|
|
|
|
|
<!-- Provide site root and default themes to javascript -->
|
|
<script>
|
|
const path_to_root = "../";
|
|
const default_light_theme = "light";
|
|
const default_dark_theme = "navy";
|
|
</script>
|
|
<!-- Start loading toc.js asap -->
|
|
<script src="../toc.js"></script>
|
|
</head>
|
|
<body>
|
|
<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 = sidebar === 'visible';
|
|
html.classList.remove('sidebar-visible');
|
|
html.classList.add("sidebar-" + sidebar);
|
|
</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. (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">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/master/src/solve/canonicalization.md" 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>
|
|
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="canonicalization"><a class="header" href="#canonicalization">Canonicalization</a></h1>
|
|
<p>Canonicalization is the process of <em>isolating</em> a value from its context and is necessary
|
|
for global caching of goals which include inference variables.</p>
|
|
<p>The idea is that given the goals <code>u32: Trait<?x></code> and <code>u32: Trait<?y></code>, where <code>?x</code> and <code>?y</code>
|
|
are two different currently unconstrained inference variables, we should get the same result
|
|
for both goals. We can therefore prove <em>the canonical query</em> <code>exists<T> u32: Trait<T></code> once
|
|
and reuse the result.</p>
|
|
<p>Let's first go over the way canonical queries work and then dive into the specifics of
|
|
how canonicalization works.</p>
|
|
<h2 id="a-walkthrough-of-canonical-queries"><a class="header" href="#a-walkthrough-of-canonical-queries">A walkthrough of canonical queries</a></h2>
|
|
<p>To make this a bit easier, let's use the trait goal <code>u32: Trait<?x></code> as an example with the
|
|
assumption that the only relevant impl is <code>impl<T> Trait<Vec<T>> for u32</code>.</p>
|
|
<h3 id="canonicalizing-the-input"><a class="header" href="#canonicalizing-the-input">Canonicalizing the input</a></h3>
|
|
<p>We start by <em>canonicalizing</em> the goal, replacing inference variables with existential and
|
|
placeholders with universal bound variables. This would result in the <em>canonical goal</em>
|
|
<code>exists<T> u32: Trait<T></code>.</p>
|
|
<p>We remember the original values of all bound variables in the original context. Here this would
|
|
map <code>T</code> back to <code>?x</code>. These original values are used later on when dealing with the query
|
|
response.</p>
|
|
<p>We now call the canonical query with the canonical goal.</p>
|
|
<h3 id="instantiating-the-canonical-goal-inside-of-the-query"><a class="header" href="#instantiating-the-canonical-goal-inside-of-the-query">Instantiating the canonical goal inside of the query</a></h3>
|
|
<p>To actually try to prove the canonical goal we start by instantiating the bound variables with
|
|
inference variables and placeholders again.</p>
|
|
<p>This happens inside of the query in a completely separate <code>InferCtxt</code>. Inside of the query we
|
|
now have a goal <code>u32: Trait<?0></code>. We also remember which value we've used to instantiate the bound
|
|
variables in the canonical goal, which maps <code>T</code> to <code>?0</code>.</p>
|
|
<p>We now compute the goal <code>u32: Trait<?0></code> and figure out that this holds, but we've constrained
|
|
<code>?0</code> to <code>Vec<?1></code>. We finally convert this result to something useful to the caller.</p>
|
|
<h3 id="canonicalizing-the-query-response"><a class="header" href="#canonicalizing-the-query-response">Canonicalizing the query response</a></h3>
|
|
<p>We have to return to the caller both whether the goal holds, and the inference constraints
|
|
from inside of the query.</p>
|
|
<p>To return the inference results to the caller we canonicalize the mapping from bound variables
|
|
to the instantiated values in the query. This means that the query response is <code>Certainty::Yes</code>
|
|
and a mapping from <code>T</code> to <code>exists<U> Vec<U></code>.</p>
|
|
<h3 id="instantiating-the-query-response"><a class="header" href="#instantiating-the-query-response">Instantiating the query response</a></h3>
|
|
<p>The caller now has to apply the constraints returned by the query. For this they first
|
|
instantiate the bound variables of the canonical response with inference variables and
|
|
placeholders again, so the mapping in the response is now from <code>T</code> to <code>Vec<?z></code>.</p>
|
|
<p>It now equates the original value of <code>T</code> (<code>?x</code>) with the value for <code>T</code> in the
|
|
response (<code>Vec<?z></code>), which correctly constrains <code>?x</code> to <code>Vec<?z></code>.</p>
|
|
<h2 id="externalconstraints"><a class="header" href="#externalconstraints"><code>ExternalConstraints</code></a></h2>
|
|
<p>Computing a trait goal may not only constrain inference variables, it can also add region
|
|
obligations, e.g. given a goal <code>(): AOutlivesB<'a, 'b></code> we would like to return the fact that
|
|
<code>'a: 'b</code> has to hold.</p>
|
|
<p>This is done by not only returning the mapping from bound variables to the instantiated values
|
|
from the query but also extracting additional <code>ExternalConstraints</code> from the <code>InferCtxt</code> context
|
|
while building the response.</p>
|
|
<h2 id="how-exactly-does-canonicalization-work"><a class="header" href="#how-exactly-does-canonicalization-work">How exactly does canonicalization work</a></h2>
|
|
<p>TODO: link to code once the PR lands and elaborate</p>
|
|
<ul>
|
|
<li>types and consts: infer to existentially bound var, placeholder to universally bound var,
|
|
considering universes</li>
|
|
<li>generic parameters in the input get treated as placeholders in the root universe</li>
|
|
<li>all regions in the input get all mapped to existentially bound vars and we "uniquify" them.
|
|
<code>&'a (): Trait<'a></code> gets canonicalized to <code>exists<'0, '1> &'0 (): Trait<'1></code>. We do not care
|
|
about their universes and simply put all regions into the highest universe of the input.</li>
|
|
<li>in the output everything in a universe of the caller gets put into the root universe and only
|
|
gets its correct universe when we unify the var values with the orig values of the caller</li>
|
|
<li>we do not uniquify regions in the response and don't canonicalize <code>'static</code></li>
|
|
</ul>
|
|
|
|
</main>
|
|
|
|
<nav class="nav-wrapper" aria-label="Page navigation">
|
|
<!-- Mobile navigation buttons -->
|
|
<a rel="prev" href="../solve/the-solver.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/coinduction.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/the-solver.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/coinduction.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>
|
|
|
|
|
|
</div>
|
|
</body>
|
|
</html>
|