<!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 `unreachable` macro in crate `core`."><meta name="keywords" content="rust, rustlang, rust-lang, unreachable"><title>core::unreachable - 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 macro"><!--[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><div class="sidebar-elems"><p class='location'><a href='index.html'>core</a></p><script>window.sidebarCurrent = {name: 'unreachable', ty: 'macro', 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'>Macro <a href='index.html'>core</a>::<wbr><a class="macro" href=''>unreachable</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/macros.rs.html#467-480' title='goto source code'>[src]</a></span></h1><div class="docblock type-decl"><pre class="rust macro">
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">unreachable</span> {
() <span class="op">=></span> { ... };
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">msg</span>:<span class="ident">expr</span>) <span class="op">=></span> { ... };
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">msg</span>:<span class="ident">expr</span>,) <span class="op">=></span> { ... };
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">fmt</span>:<span class="ident">expr</span>, $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">arg</span>:<span class="ident">tt</span>)<span class="kw-2">*</span>) <span class="op">=></span> { ... };
}</pre>
</div><div class='docblock'><p>A utility macro for indicating unreachable code.</p>
<p>This is useful any time that the compiler can't determine that some code is unreachable. For
example:</p>
<ul>
<li>Match arms with guard conditions.</li>
<li>Loops that dynamically terminate.</li>
<li>Iterators that dynamically terminate.</li>
</ul>
<p>If the determination that the code is unreachable proves incorrect, the
program immediately terminates with a <a href="../std/macro.panic.html"><code>panic!</code></a>. The function <a href="../std/hint/fn.unreachable_unchecked.html"><code>unreachable_unchecked</code></a>,
which belongs to the <a href="../std/hint/index.html"><code>std::hint</code></a> module, informs the compilier to
optimize the code out of the release version entirely.</p>
<h1 id="panics" class="section-header"><a href="#panics">Panics</a></h1>
<p>This will always <a href="../std/macro.panic.html"><code>panic!</code></a></p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<p>Match arms:</p>
<pre class="rust rust-example-rendered">
<span class="kw">fn</span> <span class="ident">foo</span>(<span class="ident">x</span>: <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">i32</span><span class="op">></span>) {
<span class="kw">match</span> <span class="ident">x</span> {
<span class="prelude-val">Some</span>(<span class="ident">n</span>) <span class="kw">if</span> <span class="ident">n</span> <span class="op">>=</span> <span class="number">0</span> <span class="op">=></span> <span class="macro">println</span><span class="macro">!</span>(<span class="string">"Some(Non-negative)"</span>),
<span class="prelude-val">Some</span>(<span class="ident">n</span>) <span class="kw">if</span> <span class="ident">n</span> <span class="op"><</span> <span class="number">0</span> <span class="op">=></span> <span class="macro">println</span><span class="macro">!</span>(<span class="string">"Some(Negative)"</span>),
<span class="prelude-val">Some</span>(<span class="kw">_</span>) <span class="op">=></span> <span class="macro">unreachable</span><span class="macro">!</span>(), <span class="comment">// compile error if commented out</span>
<span class="prelude-val">None</span> <span class="op">=></span> <span class="macro">println</span><span class="macro">!</span>(<span class="string">"None"</span>)
}
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0A%23%5Ballow(dead_code)%5D%0Afn%20foo(x%3A%20Option%3Ci32%3E)%20%7B%0A%20%20%20%20match%20x%20%7B%0A%20%20%20%20%20%20%20%20Some(n)%20if%20n%20%3E%3D%200%20%3D%3E%20println!(%22Some(Non-negative)%22)%2C%0A%20%20%20%20%20%20%20%20Some(n)%20if%20n%20%3C%20%200%20%3D%3E%20println!(%22Some(Negative)%22)%2C%0A%20%20%20%20%20%20%20%20Some(_)%20%20%20%20%20%20%20%20%20%20%20%3D%3E%20unreachable!()%2C%20%2F%2F%20compile%20error%20if%20commented%20out%0A%20%20%20%20%20%20%20%20None%20%20%20%20%20%20%20%20%20%20%20%20%20%20%3D%3E%20println!(%22None%22)%0A%20%20%20%20%7D%0A%7D%0A%7D">Run</a></pre>
<p>Iterators:</p>
<pre class="rust rust-example-rendered">
<span class="kw">fn</span> <span class="ident">divide_by_three</span>(<span class="ident">x</span>: <span class="ident">u32</span>) <span class="op">-></span> <span class="ident">u32</span> { <span class="comment">// one of the poorest implementations of x/3</span>
<span class="kw">for</span> <span class="ident">i</span> <span class="kw">in</span> <span class="number">0</span>.. {
<span class="kw">if</span> <span class="number">3</span><span class="kw-2">*</span><span class="ident">i</span> <span class="op"><</span> <span class="ident">i</span> { <span class="macro">panic</span><span class="macro">!</span>(<span class="string">"u32 overflow"</span>); }
<span class="kw">if</span> <span class="ident">x</span> <span class="op"><</span> <span class="number">3</span><span class="kw-2">*</span><span class="ident">i</span> { <span class="kw">return</span> <span class="ident">i</span><span class="op">-</span><span class="number">1</span>; }
}
<span class="macro">unreachable</span><span class="macro">!</span>();
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0A%23%5Ballow(dead_code)%5D%0Afn%20divide_by_three(x%3A%20u32)%20-%3E%20u32%20%7B%20%2F%2F%20one%20of%20the%20poorest%20implementations%20of%20x%2F3%0A%20%20%20%20for%20i%20in%200..%20%7B%0A%20%20%20%20%20%20%20%20if%203*i%20%3C%20i%20%7B%20panic!(%22u32%20overflow%22)%3B%20%7D%0A%20%20%20%20%20%20%20%20if%20x%20%3C%203*i%20%7B%20return%20i-1%3B%20%7D%0A%20%20%20%20%7D%0A%20%20%20%20unreachable!()%3B%0A%7D%0A%7D">Run</a></pre>
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