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<p class='location'>Trait Sized</p><div class="block items"><ul><li><a href="#implementors">Implementors</a></li></ul></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>
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<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'>
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<pre class='rust trait'><div class="docblock attributes">#[lang = "sized"]
</div>pub trait Sized { }</pre><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(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, which does not
get an implicit <code>Sized</code> bound. This is because a <code>Sized</code> bound prevents
the trait from being used to form a <a href="../../book/trait-objects.html">trait object</a>:</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_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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