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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="API documentation for the Rust `transmute` fn in crate `nom`."><meta name="keywords" content="rust, rustlang, rust-lang, transmute"><title>nom::lib::std::mem::transmute - 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><noscript><link rel="stylesheet" href="../../../../noscript.css"></noscript><link rel="shortcut icon" href="../../../../favicon.ico"><style type="text/css">#crate-search{background-image:url("../../../../down-arrow.svg");}</style></head><body class="rustdoc fn"><!--[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='../../../../nom/index.html'><div class='logo-container'><img src='../../../../rust-logo.png' alt='logo'></div></a><div class="sidebar-elems"><p class='location'><a href='../../../index.html'>nom</a>::<wbr><a href='../../index.html'>lib</a>::<wbr><a href='../index.html'>std</a>::<wbr><a href='index.html'>mem</a></p><script>window.sidebarCurrent = {name: 'transmute', ty: 'fn', 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"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" disabled autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><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='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='https://doc.rust-lang.org/nightly/src/core/intrinsics.rs.html#1267' title='goto source code'>[src]</a></span><span class='in-band'>Function <a href='../../../index.html'>nom</a>::<wbr><a href='../../index.html'>lib</a>::<wbr><a href='../index.html'>std</a>::<wbr><a href='index.html'>mem</a>::<wbr><a class="fn" href=''>transmute</a></span></h1><pre class='rust fn'>pub const unsafe extern "rust-intrinsic" fn transmute<T, U>(e: T) -> U</pre><div class='docblock'><p>Reinterprets the bits of a value of one type as another type.</p>
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<p>Both types must have the same size. Neither the original, nor the result,
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may be an <a href="../../nomicon/what-unsafe-does.html">invalid value</a>.</p>
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<p><code>transmute</code> is semantically equivalent to a bitwise move of one type
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into another. It copies the bits from the source value into the
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destination value, then forgets the original. It's equivalent to C's
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<code>memcpy</code> under the hood, just like <code>transmute_copy</code>.</p>
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<p><code>transmute</code> is <strong>incredibly</strong> unsafe. There are a vast number of ways to
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cause <a href="../../reference/behavior-considered-undefined.html">undefined behavior</a> with this function. <code>transmute</code> should be
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the absolute last resort.</p>
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<p>The <a href="../../nomicon/transmutes.html">nomicon</a> has additional
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documentation.</p>
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<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
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<p>There are a few things that <code>transmute</code> is really useful for.</p>
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<p>Turning a pointer into a function pointer. This is <em>not</em> portable to
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machines where function pointers and data pointers have different sizes.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">fn</span> <span class="ident">foo</span>() <span class="op">-</span><span class="op">></span> <span class="ident">i32</span> {
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<span class="number">0</span>
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}
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<span class="kw">let</span> <span class="ident">pointer</span> <span class="op">=</span> <span class="ident">foo</span> <span class="kw">as</span> <span class="kw-2">*</span><span class="kw">const</span> ();
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<span class="kw">let</span> <span class="ident">function</span> <span class="op">=</span> <span class="kw">unsafe</span> {
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<span class="ident">std</span>::<span class="ident">mem</span>::<span class="ident">transmute</span>::<span class="op"><</span><span class="kw-2">*</span><span class="kw">const</span> (), <span class="kw">fn</span>() <span class="op">-</span><span class="op">></span> <span class="ident">i32</span><span class="op">></span>(<span class="ident">pointer</span>)
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};
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">function</span>(), <span class="number">0</span>);</pre></div>
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<p>Extending a lifetime, or shortening an invariant lifetime. This is
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advanced, very unsafe Rust!</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">struct</span> <span class="ident">R</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">></span>(<span class="kw-2">&</span><span class="lifetime">'a</span> <span class="ident">i32</span>);
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<span class="kw">unsafe</span> <span class="kw">fn</span> <span class="ident">extend_lifetime</span><span class="op"><</span><span class="lifetime">'b</span><span class="op">></span>(<span class="ident">r</span>: <span class="ident">R</span><span class="op"><</span><span class="lifetime">'b</span><span class="op">></span>) <span class="op">-</span><span class="op">></span> <span class="ident">R</span><span class="op"><</span><span class="lifetime">'static</span><span class="op">></span> {
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<span class="ident">std</span>::<span class="ident">mem</span>::<span class="ident">transmute</span>::<span class="op"><</span><span class="ident">R</span><span class="op"><</span><span class="lifetime">'b</span><span class="op">></span>, <span class="ident">R</span><span class="op"><</span><span class="lifetime">'static</span><span class="op">></span><span class="op">></span>(<span class="ident">r</span>)
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}
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<span class="kw">unsafe</span> <span class="kw">fn</span> <span class="ident">shorten_invariant_lifetime</span><span class="op"><</span><span class="lifetime">'b</span>, <span class="lifetime">'c</span><span class="op">></span>(<span class="ident">r</span>: <span class="kw-2">&</span><span class="lifetime">'b</span> <span class="kw-2">mut</span> <span class="ident">R</span><span class="op"><</span><span class="lifetime">'static</span><span class="op">></span>)
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<span class="op">-</span><span class="op">></span> <span class="kw-2">&</span><span class="lifetime">'b</span> <span class="kw-2">mut</span> <span class="ident">R</span><span class="op"><</span><span class="lifetime">'c</span><span class="op">></span> {
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<span class="ident">std</span>::<span class="ident">mem</span>::<span class="ident">transmute</span>::<span class="op"><</span><span class="kw-2">&</span><span class="lifetime">'b</span> <span class="kw-2">mut</span> <span class="ident">R</span><span class="op"><</span><span class="lifetime">'static</span><span class="op">></span>, <span class="kw-2">&</span><span class="lifetime">'b</span> <span class="kw-2">mut</span> <span class="ident">R</span><span class="op"><</span><span class="lifetime">'c</span><span class="op">></span><span class="op">></span>(<span class="ident">r</span>)
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}</pre></div>
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<h1 id="alternatives" class="section-header"><a href="#alternatives">Alternatives</a></h1>
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<p>Don't despair: many uses of <code>transmute</code> can be achieved through other means.
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Below are common applications of <code>transmute</code> which can be replaced with safer
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constructs.</p>
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<p>Turning a pointer into a <code>usize</code>:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">let</span> <span class="ident">ptr</span> <span class="op">=</span> <span class="kw-2">&</span><span class="number">0</span>;
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<span class="kw">let</span> <span class="ident">ptr_num_transmute</span> <span class="op">=</span> <span class="kw">unsafe</span> {
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<span class="ident">std</span>::<span class="ident">mem</span>::<span class="ident">transmute</span>::<span class="op"><</span><span class="kw-2">&</span><span class="ident">i32</span>, <span class="ident">usize</span><span class="op">></span>(<span class="ident">ptr</span>)
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};
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<span class="comment">// Use an `as` cast instead</span>
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<span class="kw">let</span> <span class="ident">ptr_num_cast</span> <span class="op">=</span> <span class="ident">ptr</span> <span class="kw">as</span> <span class="kw-2">*</span><span class="kw">const</span> <span class="ident">i32</span> <span class="kw">as</span> <span class="ident">usize</span>;</pre></div>
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<p>Turning a <code>*mut T</code> into an <code>&mut T</code>:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">let</span> <span class="ident">ptr</span>: <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">i32</span> <span class="op">=</span> <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="number">0</span>;
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<span class="kw">let</span> <span class="ident">ref_transmuted</span> <span class="op">=</span> <span class="kw">unsafe</span> {
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<span class="ident">std</span>::<span class="ident">mem</span>::<span class="ident">transmute</span>::<span class="op"><</span><span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">i32</span>, <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">i32</span><span class="op">></span>(<span class="ident">ptr</span>)
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};
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<span class="comment">// Use a reborrow instead</span>
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<span class="kw">let</span> <span class="ident">ref_casted</span> <span class="op">=</span> <span class="kw">unsafe</span> { <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="kw-2">*</span><span class="ident">ptr</span> };</pre></div>
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<p>Turning an <code>&mut T</code> into an <code>&mut U</code>:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">let</span> <span class="ident">ptr</span> <span class="op">=</span> <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="number">0</span>;
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<span class="kw">let</span> <span class="ident">val_transmuted</span> <span class="op">=</span> <span class="kw">unsafe</span> {
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<span class="ident">std</span>::<span class="ident">mem</span>::<span class="ident">transmute</span>::<span class="op"><</span><span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">i32</span>, <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">u32</span><span class="op">></span>(<span class="ident">ptr</span>)
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};
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<span class="comment">// Now, put together `as` and reborrowing - note the chaining of `as`</span>
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<span class="comment">// `as` is not transitive</span>
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<span class="kw">let</span> <span class="ident">val_casts</span> <span class="op">=</span> <span class="kw">unsafe</span> { <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="kw-2">*</span>(<span class="ident">ptr</span> <span class="kw">as</span> <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">i32</span> <span class="kw">as</span> <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">u32</span>) };</pre></div>
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<p>Turning an <code>&str</code> into an <code>&[u8]</code>:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="comment">// this is not a good way to do this.</span>
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<span class="kw">let</span> <span class="ident">slice</span> <span class="op">=</span> <span class="kw">unsafe</span> { <span class="ident">std</span>::<span class="ident">mem</span>::<span class="ident">transmute</span>::<span class="op"><</span><span class="kw-2">&</span><span class="ident">str</span>, <span class="kw-2">&</span>[<span class="ident">u8</span>]<span class="op">></span>(<span class="string">"Rust"</span>) };
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">slice</span>, <span class="kw-2">&</span>[<span class="number">82</span>, <span class="number">117</span>, <span class="number">115</span>, <span class="number">116</span>]);
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<span class="comment">// You could use `str::as_bytes`</span>
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<span class="kw">let</span> <span class="ident">slice</span> <span class="op">=</span> <span class="string">"Rust"</span>.<span class="ident">as_bytes</span>();
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">slice</span>, <span class="kw-2">&</span>[<span class="number">82</span>, <span class="number">117</span>, <span class="number">115</span>, <span class="number">116</span>]);
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<span class="comment">// Or, just use a byte string, if you have control over the string</span>
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<span class="comment">// literal</span>
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="string">b"Rust"</span>, <span class="kw-2">&</span>[<span class="number">82</span>, <span class="number">117</span>, <span class="number">115</span>, <span class="number">116</span>]);</pre></div>
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<p>Turning a <code>Vec<&T></code> into a <code>Vec<Option<&T>></code>:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">let</span> <span class="ident">store</span> <span class="op">=</span> [<span class="number">0</span>, <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>];
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<span class="kw">let</span> <span class="ident">v_orig</span> <span class="op">=</span> <span class="ident">store</span>.<span class="ident">iter</span>().<span class="ident">collect</span>::<span class="op"><</span><span class="ident">Vec</span><span class="op"><</span><span class="kw-2">&</span><span class="ident">i32</span><span class="op">></span><span class="op">></span>();
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<span class="comment">// clone the vector as we will reuse them later</span>
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<span class="kw">let</span> <span class="ident">v_clone</span> <span class="op">=</span> <span class="ident">v_orig</span>.<span class="ident">clone</span>();
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<span class="comment">// Using transmute: this relies on the unspecified data layout of `Vec`, which is a</span>
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<span class="comment">// bad idea and could cause Undefined Behavior.</span>
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<span class="comment">// However, it is no-copy.</span>
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<span class="kw">let</span> <span class="ident">v_transmuted</span> <span class="op">=</span> <span class="kw">unsafe</span> {
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<span class="ident">std</span>::<span class="ident">mem</span>::<span class="ident">transmute</span>::<span class="op"><</span><span class="ident">Vec</span><span class="op"><</span><span class="kw-2">&</span><span class="ident">i32</span><span class="op">></span>, <span class="ident">Vec</span><span class="op"><</span><span class="prelude-ty">Option</span><span class="op"><</span><span class="kw-2">&</span><span class="ident">i32</span><span class="op">></span><span class="op">></span><span class="op">></span>(<span class="ident">v_clone</span>)
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};
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<span class="kw">let</span> <span class="ident">v_clone</span> <span class="op">=</span> <span class="ident">v_orig</span>.<span class="ident">clone</span>();
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<span class="comment">// This is the suggested, safe way.</span>
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<span class="comment">// It does copy the entire vector, though, into a new array.</span>
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<span class="kw">let</span> <span class="ident">v_collected</span> <span class="op">=</span> <span class="ident">v_clone</span>.<span class="ident">into_iter</span>()
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.<span class="ident">map</span>(<span class="prelude-val">Some</span>)
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.<span class="ident">collect</span>::<span class="op"><</span><span class="ident">Vec</span><span class="op"><</span><span class="prelude-ty">Option</span><span class="op"><</span><span class="kw-2">&</span><span class="ident">i32</span><span class="op">></span><span class="op">></span><span class="op">></span>();
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<span class="kw">let</span> <span class="ident">v_clone</span> <span class="op">=</span> <span class="ident">v_orig</span>.<span class="ident">clone</span>();
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<span class="comment">// The no-copy, unsafe way, still using transmute, but not relying on the data layout.</span>
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<span class="comment">// Like the first approach, this reuses the `Vec` internals.</span>
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<span class="comment">// Therefore, the new inner type must have the</span>
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<span class="comment">// exact same size, *and the same alignment*, as the old type.</span>
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<span class="comment">// The same caveats exist for this method as transmute, for</span>
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<span class="comment">// the original inner type (`&i32`) to the converted inner type</span>
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<span class="comment">// (`Option<&i32>`), so read the nomicon pages linked above and also</span>
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<span class="comment">// consult the [`from_raw_parts`] documentation.</span>
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<span class="kw">let</span> <span class="ident">v_from_raw</span> <span class="op">=</span> <span class="kw">unsafe</span> {
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<span class="comment">// Ensure the original vector is not dropped.</span>
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">v_clone</span> <span class="op">=</span> <span class="ident">std</span>::<span class="ident">mem</span>::<span class="ident">ManuallyDrop</span>::<span class="ident">new</span>(<span class="ident">v_clone</span>);
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<span class="ident">Vec</span>::<span class="ident">from_raw_parts</span>(<span class="ident">v_clone</span>.<span class="ident">as_mut_ptr</span>() <span class="kw">as</span> <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="kw-2">&</span><span class="ident">i32</span><span class="op">></span>,
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<span class="ident">v_clone</span>.<span class="ident">len</span>(),
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<span class="ident">v_clone</span>.<span class="ident">capacity</span>())
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};</pre></div>
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<p>Implementing <code>split_at_mut</code>:</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">std</span>::{<span class="ident">slice</span>, <span class="ident">mem</span>};
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<span class="comment">// There are multiple ways to do this, and there are multiple problems</span>
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<span class="comment">// with the following (transmute) way.</span>
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<span class="kw">fn</span> <span class="ident">split_at_mut_transmute</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>(<span class="ident">slice</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> [<span class="ident">T</span>], <span class="ident">mid</span>: <span class="ident">usize</span>)
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<span class="op">-</span><span class="op">></span> (<span class="kw-2">&</span><span class="kw-2">mut</span> [<span class="ident">T</span>], <span class="kw-2">&</span><span class="kw-2">mut</span> [<span class="ident">T</span>]) {
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<span class="kw">let</span> <span class="ident">len</span> <span class="op">=</span> <span class="ident">slice</span>.<span class="ident">len</span>();
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<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">mid</span> <span class="op"><</span><span class="op">=</span> <span class="ident">len</span>);
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<span class="kw">unsafe</span> {
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<span class="kw">let</span> <span class="ident">slice2</span> <span class="op">=</span> <span class="ident">mem</span>::<span class="ident">transmute</span>::<span class="op"><</span><span class="kw-2">&</span><span class="kw-2">mut</span> [<span class="ident">T</span>], <span class="kw-2">&</span><span class="kw-2">mut</span> [<span class="ident">T</span>]<span class="op">></span>(<span class="ident">slice</span>);
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<span class="comment">// first: transmute is not typesafe; all it checks is that T and</span>
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<span class="comment">// U are of the same size. Second, right here, you have two</span>
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<span class="comment">// mutable references pointing to the same memory.</span>
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(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">slice</span>[<span class="number">0</span>..<span class="ident">mid</span>], <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">slice2</span>[<span class="ident">mid</span>..<span class="ident">len</span>])
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}
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}
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<span class="comment">// This gets rid of the typesafety problems; `&mut *` will *only* give</span>
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<span class="comment">// you an `&mut T` from an `&mut T` or `*mut T`.</span>
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<span class="kw">fn</span> <span class="ident">split_at_mut_casts</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>(<span class="ident">slice</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> [<span class="ident">T</span>], <span class="ident">mid</span>: <span class="ident">usize</span>)
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<span class="op">-</span><span class="op">></span> (<span class="kw-2">&</span><span class="kw-2">mut</span> [<span class="ident">T</span>], <span class="kw-2">&</span><span class="kw-2">mut</span> [<span class="ident">T</span>]) {
|
||
<span class="kw">let</span> <span class="ident">len</span> <span class="op">=</span> <span class="ident">slice</span>.<span class="ident">len</span>();
|
||
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">mid</span> <span class="op"><</span><span class="op">=</span> <span class="ident">len</span>);
|
||
<span class="kw">unsafe</span> {
|
||
<span class="kw">let</span> <span class="ident">slice2</span> <span class="op">=</span> <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="kw-2">*</span>(<span class="ident">slice</span> <span class="kw">as</span> <span class="kw-2">*</span><span class="kw-2">mut</span> [<span class="ident">T</span>]);
|
||
<span class="comment">// however, you still have two mutable references pointing to</span>
|
||
<span class="comment">// the same memory.</span>
|
||
(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">slice</span>[<span class="number">0</span>..<span class="ident">mid</span>], <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">slice2</span>[<span class="ident">mid</span>..<span class="ident">len</span>])
|
||
}
|
||
}
|
||
|
||
<span class="comment">// This is how the standard library does it. This is the best method, if</span>
|
||
<span class="comment">// you need to do something like this</span>
|
||
<span class="kw">fn</span> <span class="ident">split_at_stdlib</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>(<span class="ident">slice</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> [<span class="ident">T</span>], <span class="ident">mid</span>: <span class="ident">usize</span>)
|
||
<span class="op">-</span><span class="op">></span> (<span class="kw-2">&</span><span class="kw-2">mut</span> [<span class="ident">T</span>], <span class="kw-2">&</span><span class="kw-2">mut</span> [<span class="ident">T</span>]) {
|
||
<span class="kw">let</span> <span class="ident">len</span> <span class="op">=</span> <span class="ident">slice</span>.<span class="ident">len</span>();
|
||
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">mid</span> <span class="op"><</span><span class="op">=</span> <span class="ident">len</span>);
|
||
<span class="kw">unsafe</span> {
|
||
<span class="kw">let</span> <span class="ident">ptr</span> <span class="op">=</span> <span class="ident">slice</span>.<span class="ident">as_mut_ptr</span>();
|
||
<span class="comment">// This now has three mutable references pointing at the same</span>
|
||
<span class="comment">// memory. `slice`, the rvalue ret.0, and the rvalue ret.1.</span>
|
||
<span class="comment">// `slice` is never used after `let ptr = ...`, and so one can</span>
|
||
<span class="comment">// treat it as "dead", and therefore, you only have two real</span>
|
||
<span class="comment">// mutable slices.</span>
|
||
(<span class="ident">slice</span>::<span class="ident">from_raw_parts_mut</span>(<span class="ident">ptr</span>, <span class="ident">mid</span>),
|
||
<span class="ident">slice</span>::<span class="ident">from_raw_parts_mut</span>(<span class="ident">ptr</span>.<span class="ident">add</span>(<span class="ident">mid</span>), <span class="ident">len</span> <span class="op">-</span> <span class="ident">mid</span>))
|
||
}
|
||
}</pre></div>
|
||
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