<!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 `Sized` trait in crate `core`."><meta name="keywords" content="rust, rustlang, rust-lang, Sized"><title>core::marker::Sized - 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><link rel="shortcut icon" href="https://doc.rust-lang.org/favicon.ico"></head><body class="rustdoc trait"><!--[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='../../core/index.html'><img src='https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png' alt='logo' width='100'></a><p class='location'>Trait Sized</p><div class="sidebar-elems"><div class="block items"><a class="sidebar-title" href="#implementors">Implementors</a></div><p class='location'><a href='../index.html'>core</a>::<wbr><a href='index.html'>marker</a></p><script>window.sidebarCurrent = {name: 'Sized', ty: 'trait', relpath: ''};</script><script defer src="sidebar-items.js"></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"><input class="search-input" name="search" autocomplete="off" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><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='in-band'>Trait <a href='../index.html'>core</a>::<wbr><a href='index.html'>marker</a>::<wbr><a class="trait" href=''>Sized</a></span><span class='out-of-band'><span class='since' title='Stable since Rust version 1.0.0'>1.0.0</span><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/core/marker.rs.html#93-95' title='goto source code'>[src]</a></span></h1><div class="docblock type-decl"><pre class='rust trait'><div class="docblock attributes">#[lang = "sized"]
</div>pub trait Sized { }</pre></div><div class='docblock'><p>Types with a constant size known at compile time.</p>
<p>All type parameters have an implicit bound of <code>Sized</code>. The special syntax
<code>?Sized</code> can be used to remove this bound if it's not appropriate.</p>
<pre class="rust rust-example-rendered">
<span class="kw">struct</span> <span class="ident">Foo</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>(<span class="ident">T</span>);
<span class="kw">struct</span> <span class="ident">Bar</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">T</span>);
<span class="comment">// struct FooUse(Foo<[i32]>); // error: Sized is not implemented for [i32]</span>
<span class="kw">struct</span> <span class="ident">BarUse</span>(<span class="ident">Bar</span><span class="op"><</span>[<span class="ident">i32</span>]<span class="op">></span>); <span class="comment">// OK</span><a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0A%23!%5Ballow(dead_code)%5D%0Afn%20main()%20%7B%0Astruct%20Foo%3CT%3E(T)%3B%0Astruct%20Bar%3CT%3A%20%3FSized%3E(T)%3B%0A%0A%2F%2F%20struct%20FooUse(Foo%3C%5Bi32%5D%3E)%3B%20%2F%2F%20error%3A%20Sized%20is%20not%20implemented%20for%20%5Bi32%5D%0Astruct%20BarUse(Bar%3C%5Bi32%5D%3E)%3B%20%2F%2F%20OK%0A%7D">Run</a></pre>
<p>The one exception is the implicit <code>Self</code> type of a trait. A trait does not
have an implicit <code>Sized</code> bound as this is incompatible with <a href="../../book/first-edition/trait-objects.html">trait object</a>s
where, by definition, the trait needs to work with all possible implementors,
and thus could be any size.</p>
<p>Although Rust will let you bind <code>Sized</code> to a trait, you won't
be able to use it to form a trait object later:</p>
<pre class="rust rust-example-rendered">
<span class="kw">trait</span> <span class="ident">Foo</span> { }
<span class="kw">trait</span> <span class="ident">Bar</span>: <span class="ident">Sized</span> { }
<span class="kw">struct</span> <span class="ident">Impl</span>;
<span class="kw">impl</span> <span class="ident">Foo</span> <span class="kw">for</span> <span class="ident">Impl</span> { }
<span class="kw">impl</span> <span class="ident">Bar</span> <span class="kw">for</span> <span class="ident">Impl</span> { }
<span class="kw">let</span> <span class="ident">x</span>: <span class="kw-2">&</span><span class="ident">Foo</span> <span class="op">=</span> <span class="kw-2">&</span><span class="ident">Impl</span>; <span class="comment">// OK</span>
<span class="comment">// let y: &Bar = &Impl; // error: the trait `Bar` cannot</span>
<span class="comment">// be made into an object</span><a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0A%23!%5Ballow(unused_variables)%5D%0Afn%20main()%20%7B%0Atrait%20Foo%20%7B%20%7D%0Atrait%20Bar%3A%20Sized%20%7B%20%7D%0A%0Astruct%20Impl%3B%0Aimpl%20Foo%20for%20Impl%20%7B%20%7D%0Aimpl%20Bar%20for%20Impl%20%7B%20%7D%0A%0Alet%20x%3A%20%26Foo%20%3D%20%26Impl%3B%20%20%20%20%2F%2F%20OK%0A%2F%2F%20let%20y%3A%20%26Bar%20%3D%20%26Impl%3B%20%2F%2F%20error%3A%20the%20trait%20%60Bar%60%20cannot%0A%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%2F%2F%20be%20made%20into%20an%20object%0A%7D">Run</a></pre>
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