<!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 `concat_idents` macro in crate `std`."><meta name="keywords" content="rust, rustlang, rust-lang, concat_idents"><title>std::concat_idents - 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='../std/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'>std</a></p><script>window.sidebarCurrent = {name: 'concat_idents', 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'>std</a>::<wbr><a class="macro" href=''>concat_idents</a></span><span class='out-of-band'><span class='since' title='Stable since Rust version '></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/std/macros.rs.html#465-468' 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">concat_idents</span> {
($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">e</span>:<span class="ident">ident</span>),<span class="op">+</span>) <span class="op">=></span> { ... };
($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">e</span>:<span class="ident">ident</span>,)<span class="op">+</span>) <span class="op">=></span> { ... };
}</pre>
</div><div class='stability'><div class='stab unstable'><span class=microscope>🔬</span> This is a nightly-only experimental API. (<code>concat_idents_macro </code><a href="https://github.com/rust-lang/rust/issues/29599">#29599</a>)</div></div><div class='docblock'><p>Concatenate identifiers into one identifier.</p>
<p>This macro takes any number of comma-separated identifiers, and
concatenates them all into one, yielding an expression which is a new
identifier. Note that hygiene makes it such that this macro cannot
capture local variables. Also, as a general rule, macros are only
allowed in item, statement or expression position. That means while
you may use this macro for referring to existing variables, functions or
modules etc, you cannot define a new one with it.</p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<pre class="rust rust-example-rendered">
<span class="attribute">#![<span class="ident">feature</span>(<span class="ident">concat_idents</span>)]</span>
<span class="kw">fn</span> <span class="ident">foobar</span>() <span class="op">-></span> <span class="ident">u32</span> { <span class="number">23</span> }
<span class="kw">let</span> <span class="ident">f</span> <span class="op">=</span> <span class="macro">concat_idents</span><span class="macro">!</span>(<span class="ident">foo</span>, <span class="ident">bar</span>);
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"{}"</span>, <span class="ident">f</span>());
<span class="comment">// fn concat_idents!(new, fun, name) { } // not usable in this way!</span><a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0A%23!%5Bfeature(concat_idents)%5D%0A%0Afn%20main()%20%7B%0Afn%20foobar()%20-%3E%20u32%20%7B%2023%20%7D%0A%0Alet%20f%20%3D%20concat_idents!(foo%2C%20bar)%3B%0Aprintln!(%22%7B%7D%22%2C%20f())%3B%0A%0A%2F%2F%20fn%20concat_idents!(new%2C%20fun%2C%20name)%20%7B%20%7D%20%2F%2F%20not%20usable%20in%20this%20way!%0A%7D&version=nightly">Run</a></pre>
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