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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="github crates-io docs-rs"><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="../light.css" id="themeStyle"><link rel="stylesheet" type="text/css" href="../dark.css" disabled ><link rel="stylesheet" type="text/css" href="../ayu.css" disabled ><script id="default-settings"></script><script src="../storage.js"></script><script src="../crates.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="icon" type="image/svg+xml" href="../favicon.svg">
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<a id="settings-menu" href="../settings.html"><img src="../wheel.svg" width="18" height="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class="fqn"><span class="in-band">Crate <a class="mod" href="">ghost</a><button id="copy-path" onclick="copy_path(this)">⎘</button></span><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#1-331" title="goto source code">[src]</a></span></h1><div class="docblock"><p><a href="https://github.com/dtolnay/ghost"><img src="https://img.shields.io/badge/github-8da0cb?style=for-the-badge&labelColor=555555&logo=github" alt="github" /></a> <a href="https://crates.io/crates/ghost"><img src="https://img.shields.io/badge/crates.io-fc8d62?style=for-the-badge&labelColor=555555&logo=rust" alt="crates-io" /></a> <a href="https://docs.rs/ghost"><img src="https://img.shields.io/badge/docs.rs-66c2a5?style=for-the-badge&labelColor=555555&logoColor=white&logo=data:image/svg+xml;base64,PHN2ZyByb2xlPSJpbWciIHhtbG5zPSJodHRwOi8vd3d3LnczLm9yZy8yMDAwL3N2ZyIgdmlld0JveD0iMCAwIDUxMiA1MTIiPjxwYXRoIGZpbGw9IiNmNWY1ZjUiIGQ9Ik00ODguNiAyNTAuMkwzOTIgMjE0VjEwNS41YzAtMTUtOS4zLTI4LjQtMjMuNC0zMy43bC0xMDAtMzcuNWMtOC4xLTMuMS0xNy4xLTMuMS0yNS4zIDBsLTEwMCAzNy41Yy0xNC4xIDUuMy0yMy40IDE4LjctMjMuNCAzMy43VjIxNGwtOTYuNiAzNi4yQzkuMyAyNTUuNSAwIDI2OC45IDAgMjgzLjlWMzk0YzAgMTMuNiA3LjcgMjYuMSAxOS45IDMyLjJsMTAwIDUwYzEwLjEgNS4xIDIyLjEgNS4xIDMyLjIgMGwxMDMuOS01MiAxMDMuOSA1MmMxMC4xIDUuMSAyMi4xIDUuMSAzMi4yIDBsMTAwLTUwYzEyLjItNi4xIDE5LjktMTguNiAxOS45LTMyLjJWMjgzLjljMC0xNS05LjMtMjguNC0yMy40LTMzLjd6TTM1OCAyMTQuOGwtODUgMzEuOXYtNjguMmw4NS0zN3Y3My4zek0xNTQgMTA0LjFsMTAyLTM4LjIgMTAyIDM4LjJ2LjZsLTEwMiA0MS40LTEwMi00MS40di0uNnptODQgMjkxLjFsLTg1IDQyLjV2LTc5LjFsODUtMzguOHY3NS40em0wLTExMmwtMTAyIDQxLjQtMTAyLTQxLjR2LS42bDEwMi0zOC4yIDEwMiAzOC4ydi42em0yNDAgMTEybC04NSA0Mi41di03OS4xbDg1LTM4Ljh2NzUuNHptMC0xMTJsLTEwMiA0MS40LTEwMi00MS40di0uNmwxMDItMzguMiAxMDIgMzguMnYuNnoiPjwvcGF0aD48L3N2Zz4K" alt="docs-rs" /></a></p>
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<br>
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<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'>ⓘ</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::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="number">0</span>, <span class="ident">std::mem::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::marker::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::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="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::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::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::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::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>
|
||
</td></tr></table></section><section id="search" class="content hidden"></section><div id="rustdoc-vars" data-root-path="../" data-current-crate="ghost" data-search-index-js="../search-index.js" data-search-js="../search.js"></div>
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||
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