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<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="API documentation for the Rust `ghost` crate."><meta name="keywords" content="rust, rustlang, rust-lang, ghost"><title>ghost - Rust</title><link rel="stylesheet" type="text/css" href="../normalize.css"><link rel="stylesheet" type="text/css" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../dark.css"><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><script src="../storage.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="shortcut icon" href="../favicon.ico"><style type="text/css">#crate-search{background-image:url("../down-arrow.svg");}</style></head><body class="rustdoc mod"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../ghost/index.html'><div class='logo-container'><img src='../rust-logo.png' alt='logo'></div></a><p class='location'>Crate ghost</p><div class="sidebar-elems"><a id='all-types' href='all.html'><p>See all ghost's items</p></a><p class='location'></p><script>window.sidebarCurrent = {name: 'ghost', ty: 'mod', relpath: '../'};</script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../theme.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><a id="settings-menu" href="../settings.html"><img src="../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class='fqn'><span class='out-of-band'><span id='render-detail'><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class='inner'>−</span>]</a></span><a class='srclink' href='../src/ghost/lib.rs.html#1-323' title='goto source code'>[src]</a></span><span class='in-band'>Crate <a class="mod" href=''>ghost</a></span></h1><div class='docblock'><p><strong>Define your own PhantomData and similarly behaved unit types.</strong></p>
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<h1 id="background" class="section-header"><a href="#background">Background</a></h1>
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<p><a href="https://doc.rust-lang.org/std/marker/struct.PhantomData.html"><code>PhantomData</code></a> as defined by the Rust standard library is magical in that
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the same type is impossible to define in ordinary Rust code. It is defined
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in the standard library like this:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="attribute">#[<span class="ident">lang</span> <span class="op">=</span> <span class="string">"phantom_data"</span>]</span>
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<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">PhantomData</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span>;</pre></div>
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<p>The <code>#[lang = "..."]</code> attribute indicates that this is a <a href="https://manishearth.github.io/blog/2017/01/11/rust-tidbits-what-is-a-lang-item/">lang item</a>, a
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special case known to the compiler. It is the only type permitted to carry
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an unused type parameter.</p>
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<p>If we try to define an equivalent unit struct with type parameter, the
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compiler rejects that.</p>
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<div class='information'><div class='tooltip compile_fail'>ⓘ<span class='tooltiptext'>This example deliberately fails to compile</span></div></div><div class="example-wrap"><pre class="rust rust-example-rendered compile_fail">
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<span class="kw">struct</span> <span class="ident">MyPhantom</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span>;</pre></div>
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<pre><code class="language-text">error[E0392]: parameter `T` is never used
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--> src/main.rs:1:18
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|
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1 | struct MyPhantom<T: ?Sized>;
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| ^ unused type parameter
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|
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= help: consider removing `T` or using a marker such as `std::marker::PhantomData`
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</code></pre>
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<p>This crate provides a <code>#[phantom]</code> attribute that makes it possible to
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define unit structs with generic parameters.</p>
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<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">ghost</span>::<span class="ident">phantom</span>;
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<span class="attribute">#[<span class="ident">phantom</span>]</span>
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<span class="kw">struct</span> <span class="ident">MyPhantom</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span>;
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<span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="comment">// Proof that MyPhantom behaves like PhantomData.</span>
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<span class="kw">let</span> <span class="kw">_</span>: <span class="ident">MyPhantom</span><span class="op"><</span><span class="ident">u8</span><span class="op">></span> <span class="op">=</span> <span class="ident">MyPhantom</span>::<span class="op"><</span><span class="ident">u8</span><span class="op">></span>;
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="number">0</span>, <span class="ident">std</span>::<span class="ident">mem</span>::<span class="ident">size_of</span>::<span class="op"><</span><span class="ident">MyPhantom</span><span class="op"><</span><span class="ident">u8</span><span class="op">></span><span class="op">></span>());
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}
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<span class="comment">// Proof that MyPhantom is not just a re-export of PhantomData.</span>
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<span class="comment">// If it were a re-export, these would be conflicting impls.</span>
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<span class="kw">trait</span> <span class="ident">Trait</span> {}
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<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">Trait</span> <span class="kw">for</span> <span class="ident">std</span>::<span class="ident">marker</span>::<span class="ident">PhantomData</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {}
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<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">Trait</span> <span class="kw">for</span> <span class="ident">MyPhantom</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {}
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<span class="comment">// Proof that MyPhantom is local to the current crate.</span>
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<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">MyPhantom</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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}</pre></div>
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<p>The implementation accepts where-clauses, lifetimes, multiple generic
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parameters, and derives. Here is a contrived invocation that demonstrates
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everything at once:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">ghost</span>::<span class="ident">phantom</span>;
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<span class="attribute">#[<span class="ident">phantom</span>]</span>
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<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Copy</span>, <span class="ident">Clone</span>, <span class="ident">Default</span>, <span class="ident">Hash</span>, <span class="ident">PartialOrd</span>, <span class="ident">Ord</span>, <span class="ident">PartialEq</span>, <span class="ident">Eq</span>, <span class="ident">Debug</span>)]</span>
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<span class="kw">struct</span> <span class="ident">Crazy</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">V</span>: <span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> <span class="kw">where</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="ident">V</span>: <span class="ident">IntoIterator</span><span class="op"><</span><span class="ident">Item</span> <span class="op">=</span> <span class="ident">T</span><span class="op">></span>;
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<span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="kw">let</span> <span class="kw">_</span> <span class="op">=</span> <span class="ident">Crazy</span>::<span class="op"><</span><span class="lifetime">'static</span>, <span class="ident">Vec</span><span class="op"><</span><span class="ident">String</span><span class="op">></span>, <span class="kw-2">&</span><span class="lifetime">'static</span> <span class="ident">String</span><span class="op">></span>;
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<span class="comment">// Lifetime elision.</span>
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<span class="kw">let</span> <span class="ident">crazy</span> <span class="op">=</span> <span class="ident">Crazy</span>::<span class="op"><</span><span class="ident">Vec</span><span class="op"><</span><span class="ident">String</span><span class="op">></span>, <span class="kw-2">&</span><span class="ident">String</span><span class="op">></span>;
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<span class="macro">println</span><span class="macro">!</span>(<span class="string">"{:?}"</span>, <span class="ident">crazy</span>);
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}</pre></div>
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<h1 id="variance" class="section-header"><a href="#variance">Variance</a></h1>
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<p>The <code>#[phantom]</code> attribute accepts attributes on individual generic
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parameters (both lifetime and type parameters) to make them contravariant or
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invariant. The default is covariance.</p>
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<ul>
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<li><code>#[contra]</code> — contravariant generic parameter</li>
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<li><code>#[invariant]</code> — invariant generic parameter</li>
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</ul>
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<p>The implications of variance are explained in more detail by the <a href="https://doc.rust-lang.org/nomicon/subtyping.html">Subtyping
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chapter</a> of the Rustonomicon.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">ghost</span>::<span class="ident">phantom</span>;
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<span class="attribute">#[<span class="ident">phantom</span>]</span>
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<span class="kw">struct</span> <span class="ident">ContravariantLifetime</span><span class="op"><</span><span class="attribute">#[<span class="ident">contra</span>]</span> <span class="lifetime">'a</span><span class="op">></span>;
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<span class="kw">fn</span> <span class="ident">f</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">></span>(<span class="ident">arg</span>: <span class="ident">ContravariantLifetime</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">></span>) <span class="op">-</span><span class="op">></span> <span class="ident">ContravariantLifetime</span><span class="op"><</span><span class="lifetime">'static</span><span class="op">></span> {
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<span class="comment">// This coercion is only legal because the lifetime parameter is</span>
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<span class="comment">// contravariant. If it were covariant (the default) or invariant,</span>
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<span class="comment">// this would not compile.</span>
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<span class="ident">arg</span>
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}
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<span class="attribute">#[<span class="ident">phantom</span>]</span>
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<span class="kw">struct</span> <span class="ident">Demo</span><span class="op"><</span><span class="ident">A</span>, <span class="attribute">#[<span class="ident">contra</span>]</span> <span class="ident">B</span>, <span class="attribute">#[<span class="ident">invariant</span>]</span> <span class="ident">C</span><span class="op">></span>;</pre></div>
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<h1 id="documentation" class="section-header"><a href="#documentation">Documentation</a></h1>
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<p>There are two alternatives for how to handle Rustdoc documentation on
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publicly exposed phantom types.</p>
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<p>You may provide documentation directly on the phantom struct in the obvious
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way, but Rustdoc will blithely display the somewhat distracting
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implementation details of the mechanism emitted by the <code>#[phantom]</code> macro.
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This way should be preferred if you need to document any public methods, as
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methods will not be visible in the other alternative.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">ghost</span>::<span class="ident">phantom</span>;
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<span class="doccomment">/// Documentation.</span>
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<span class="attribute">#[<span class="ident">phantom</span>]</span>
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<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">MyPhantom</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span>;
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<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">MyPhantom</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="doccomment">/// Documentation on methods.</span>
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<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">foo</span>() {}
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}</pre></div>
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<p>If you aren't adding methods or don't need methods to be rendered in the
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documentation, the recommended idiom is as follows. Rustdoc will show a much
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less distracting type signature and all of your trait impls, but will not
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show inherent methods.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">mod</span> <span class="ident">private</span> {
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<span class="kw">use</span> <span class="ident">ghost</span>::<span class="ident">phantom</span>;
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<span class="attribute">#[<span class="ident">phantom</span>]</span>
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<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">MyPhantom</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span>;
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}
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<span class="doccomment">/// Documentation goes here.</span>
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<span class="attribute">#[<span class="ident">allow</span>(<span class="ident">type_alias_bounds</span>)]</span>
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<span class="kw">pub</span> <span class="kw">type</span> <span class="ident">MyPhantom</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="op">=</span> <span class="ident">private</span>::<span class="ident">MyPhantom</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>;
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<span class="attribute">#[<span class="ident">doc</span>(<span class="ident">hidden</span>)]</span>
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<span class="kw">pub</span> <span class="kw">use</span> <span class="self">self</span>::<span class="ident">private</span>::<span class="kw-2">*</span>;</pre></div>
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<h1 id="use-cases" class="section-header"><a href="#use-cases">Use cases</a></h1>
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<p>Entirely up to your imagination. Just to name one, how about a typed
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registry library that admits the following syntax for iterating over values
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registered of a particular type:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">for</span> <span class="ident">flag</span> <span class="kw">in</span> <span class="ident">Registry</span>::<span class="op"><</span><span class="ident">Flag</span><span class="op">></span> {
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<span class="comment">/* ... */</span>
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}</pre></div>
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</div><h2 id='attributes' class='section-header'><a href="#attributes">Attribute Macros</a></h2>
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<table><tr class='module-item'><td><a class="attr" href="attr.phantom.html" title='ghost::phantom attr'>phantom</a></td><td class='docblock-short'><p>Define your own PhantomData and similarly behaved unit types.</p>
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</td></tr></table></section><section id="search" class="content hidden"></section><section class="footer"></section><aside id="help" class="hidden"><div><h1 class="hidden">Help</h1><div class="shortcuts"><h2>Keyboard Shortcuts</h2><dl><dt><kbd>?</kbd></dt><dd>Show this help dialog</dd><dt><kbd>S</kbd></dt><dd>Focus the search field</dd><dt><kbd>↑</kbd></dt><dd>Move up in search results</dd><dt><kbd>↓</kbd></dt><dd>Move down in search results</dd><dt><kbd>↹</kbd></dt><dd>Switch tab</dd><dt><kbd>⏎</kbd></dt><dd>Go to active search result</dd><dt><kbd>+</kbd></dt><dd>Expand all sections</dd><dt><kbd>-</kbd></dt><dd>Collapse all sections</dd></dl></div><div class="infos"><h2>Search Tricks</h2><p>Prefix searches with a type followed by a colon (e.g., <code>fn:</code>) to restrict the search to a given type.</p><p>Accepted types are: <code>fn</code>, <code>mod</code>, <code>struct</code>, <code>enum</code>, <code>trait</code>, <code>type</code>, <code>macro</code>, and <code>const</code>.</p><p>Search functions by type signature (e.g., <code>vec -> usize</code> or <code>* -> vec</code>)</p><p>Search multiple things at once by splitting your query with comma (e.g., <code>str,u8</code> or <code>String,struct:Vec,test</code>)</p></div></div></aside><script>window.rootPath = "../";window.currentCrate = "ghost";</script><script src="../aliases.js"></script><script src="../main.js"></script><script defer src="../search-index.js"></script></body></html> |