314 lines
16 KiB
HTML
314 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>Method lookup - 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/method-lookup.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="method-lookup"><a class="header" href="#method-lookup">Method lookup</a></h1>
|
|
<p>Method lookup can be rather complex due to the interaction of a number
|
|
of factors, such as self types, autoderef, trait lookup, etc. This
|
|
file provides an overview of the process. More detailed notes are in
|
|
the code itself, naturally.</p>
|
|
<p>One way to think of method lookup is that we convert an expression of
|
|
the form <code>receiver.method(...)</code> into a more explicit <a href="https://doc.rust-lang.org/nightly/book/ch19-03-advanced-traits.html#fully-qualified-syntax-for-disambiguation-calling-methods-with-the-same-name">fully-qualified syntax</a>
|
|
(formerly called <a href="https://github.com/rust-lang/rfcs/blob/master/text/0132-ufcs.md">UFCS</a>):</p>
|
|
<ul>
|
|
<li><code>Trait::method(ADJ(receiver), ...)</code> for a trait call</li>
|
|
<li><code>ReceiverType::method(ADJ(receiver), ...)</code> for an inherent method call</li>
|
|
</ul>
|
|
<p>Here <code>ADJ</code> is some kind of adjustment, which is typically a series of
|
|
autoderefs and then possibly an autoref (e.g., <code>&**receiver</code>). However
|
|
we sometimes do other adjustments and coercions along the way, in
|
|
particular unsizing (e.g., converting from <code>[T; n]</code> to <code>[T]</code>).</p>
|
|
<p>Method lookup is divided into two major phases:</p>
|
|
<ol>
|
|
<li>Probing (<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_typeck/method/probe/"><code>probe.rs</code></a>). The probe phase is when we decide what method
|
|
to call and how to adjust the receiver.</li>
|
|
<li>Confirmation (<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_typeck/method/confirm/"><code>confirm.rs</code></a>). The confirmation phase "applies"
|
|
this selection, updating the side-tables, unifying type variables, and
|
|
otherwise doing side-effectful things.</li>
|
|
</ol>
|
|
<p>One reason for this division is to be more amenable to caching. The
|
|
probe phase produces a "pick" (<code>probe::Pick</code>), which is designed to be
|
|
cacheable across method-call sites. Therefore, it does not include
|
|
inference variables or other information.</p>
|
|
<h2 id="the-probe-phase"><a class="header" href="#the-probe-phase">The Probe phase</a></h2>
|
|
<h3 id="steps"><a class="header" href="#steps">Steps</a></h3>
|
|
<p>The first thing that the probe phase does is to create a series of
|
|
<em>steps</em>. This is done by progressively dereferencing the receiver type
|
|
until it cannot be deref'd anymore, as well as applying an optional
|
|
"unsize" step. So if the receiver has type <code>Rc<Box<[T; 3]>></code>, this
|
|
might yield:</p>
|
|
<ol>
|
|
<li><code>Rc<Box<[T; 3]>></code></li>
|
|
<li><code>Box<[T; 3]></code></li>
|
|
<li><code>[T; 3]</code></li>
|
|
<li><code>[T]</code></li>
|
|
</ol>
|
|
<h3 id="candidate-assembly"><a class="header" href="#candidate-assembly">Candidate assembly</a></h3>
|
|
<p>We then search along those steps to create a list of <em>candidates</em>. A
|
|
<code>Candidate</code> is a method item that might plausibly be the method being
|
|
invoked. For each candidate, we'll derive a "transformed self type"
|
|
that takes into account explicit self.</p>
|
|
<p>Candidates are grouped into two kinds, inherent and extension.</p>
|
|
<p><strong>Inherent candidates</strong> are those that are derived from the
|
|
type of the receiver itself. So, if you have a receiver of some
|
|
nominal type <code>Foo</code> (e.g., a struct), any methods defined within an
|
|
impl like <code>impl Foo</code> are inherent methods. Nothing needs to be
|
|
imported to use an inherent method, they are associated with the type
|
|
itself (note that inherent impls can only be defined in the same
|
|
crate as the type itself).</p>
|
|
<!--
|
|
FIXME: Inherent candidates are not always derived from impls. If you
|
|
have a trait object, such as a value of type `Box<ToString>`, then the
|
|
trait methods (`to_string()`, in this case) are inherently associated
|
|
with it. Another case is type parameters, in which case the methods of
|
|
their bounds are inherent. However, this part of the rules is subject
|
|
to change: when DST's "impl Trait for Trait" is complete, trait object
|
|
dispatch could be subsumed into trait matching, and the type parameter
|
|
behavior should be reconsidered in light of where clauses.
|
|
|
|
Is this still accurate?
|
|
-->
|
|
<p><strong>Extension candidates</strong> are derived from imported traits. If I have
|
|
the trait <code>ToString</code> imported, and I call <code>to_string()</code> as a method,
|
|
then we will list the <code>to_string()</code> definition in each impl of
|
|
<code>ToString</code> as a candidate. These kinds of method calls are called
|
|
"extension methods".</p>
|
|
<p>So, let's continue our example. Imagine that we were calling a method
|
|
<code>foo</code> with the receiver <code>Rc<Box<[T; 3]>></code> and there is a trait <code>Foo</code>
|
|
that defines it with <code>&self</code> for the type <code>Rc<U></code> as well as a method
|
|
on the type <code>Box</code> that defines <code>foo</code> but with <code>&mut self</code>. Then we
|
|
might have two candidates:</p>
|
|
<ul>
|
|
<li><code>&Rc<U></code> as an extension candidate</li>
|
|
<li><code>&mut Box<U></code> as an inherent candidate</li>
|
|
</ul>
|
|
<h3 id="candidate-search"><a class="header" href="#candidate-search">Candidate search</a></h3>
|
|
<p>Finally, to actually pick the method, we will search down the steps,
|
|
trying to match the receiver type against the candidate types. At
|
|
each step, we also consider an auto-ref and auto-mut-ref to see whether
|
|
that makes any of the candidates match. For each resulting receiver
|
|
type, we consider inherent candidates before extension candidates.
|
|
If there are multiple matching candidates in a group, we report an
|
|
error, except that multiple impls of the same trait are treated as a
|
|
single match. Otherwise we pick the first match we find.</p>
|
|
<p>In the case of our example, the first step is <code>Rc<Box<[T; 3]>></code>,
|
|
which does not itself match any candidate. But when we autoref it, we
|
|
get the type <code>&Rc<Box<[T; 3]>></code> which matches <code>&Rc<U></code>. We would then
|
|
recursively consider all where-clauses that appear on the impl: if
|
|
those match (or we cannot rule out that they do), then this is the
|
|
method we would pick. Otherwise, we would continue down the series of
|
|
steps.</p>
|
|
|
|
</main>
|
|
|
|
<nav class="nav-wrapper" aria-label="Page navigation">
|
|
<!-- Mobile navigation buttons -->
|
|
<a rel="prev" href="type-checking.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="variance.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="type-checking.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="variance.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>
|