<!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="Source to the Rust file `libcore/hint.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>hint.rs.html -- source</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 source"><!--[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></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"><pre class="line-numbers"><span id="1"> 1</span>
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</pre><pre class="rust ">
<span class="comment">// Copyright 2018 The Rust Project Developers. See the COPYRIGHT</span>
<span class="comment">// file at the top-level directory of this distribution and at</span>
<span class="comment">// http://rust-lang.org/COPYRIGHT.</span>
<span class="comment">//</span>
<span class="comment">// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or</span>
<span class="comment">// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license</span>
<span class="comment">// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your</span>
<span class="comment">// option. This file may not be copied, modified, or distributed</span>
<span class="comment">// except according to those terms.</span>
<span class="attribute">#![<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"core_hint"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.27.0"</span>)]</span>
<span class="doccomment">//! Hints to compiler that affects how code should be emitted or optimized.</span>
<span class="kw">use</span> <span class="ident">intrinsics</span>;
<span class="doccomment">/// Informs the compiler that this point in the code is not reachable, enabling</span>
<span class="doccomment">/// further optimizations.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Safety</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Reaching this function is completely *undefined behavior* (UB). In</span>
<span class="doccomment">/// particular, the compiler assumes that all UB must never happen, and</span>
<span class="doccomment">/// therefore will eliminate all branches that reach to a call to</span>
<span class="doccomment">/// `unreachable_unchecked()`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Like all instances of UB, if this assumption turns out to be wrong, i.e. the</span>
<span class="doccomment">/// `unreachable_unchecked()` call is actually reachable among all possible</span>
<span class="doccomment">/// control flow, the compiler will apply the wrong optimization strategy, and</span>
<span class="doccomment">/// may sometimes even corrupt seemingly unrelated code, causing</span>
<span class="doccomment">/// difficult-to-debug problems.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Use this function only when you can prove that the code will never call it.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The [`unreachable!()`] macro is the safe counterpart of this function, which</span>
<span class="doccomment">/// will panic instead when executed.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`unreachable!()`]: ../macro.unreachable.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Example</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// fn div_1(a: u32, b: u32) -> u32 {</span>
<span class="doccomment">/// use std::hint::unreachable_unchecked;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // `b.saturating_add(1)` is always positive (not zero),</span>
<span class="doccomment">/// // hence `checked_div` will never return None.</span>
<span class="doccomment">/// // Therefore, the else branch is unreachable.</span>
<span class="doccomment">/// a.checked_div(b.saturating_add(1))</span>
<span class="doccomment">/// .unwrap_or_else(|| unsafe { unreachable_unchecked() })</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(div_1(7, 0), 7);</span>
<span class="doccomment">/// assert_eq!(div_1(9, 1), 4);</span>
<span class="doccomment">/// assert_eq!(div_1(11, std::u32::MAX), 0);</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"unreachable"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.27.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">unsafe</span> <span class="kw">fn</span> <span class="ident">unreachable_unchecked</span>() <span class="op">-></span> <span class="op">!</span> {
<span class="ident">intrinsics</span>::<span class="ident">unreachable</span>()
}
</pre>
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