<!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 `env` macro in crate `std`."><meta name="keywords" content="rust, rustlang, rust-lang, env"><title>std::env - 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: 'env', 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=''>env</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/std/macros.rs.html#408-411' 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">env</span> {
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">name</span>:<span class="ident">expr</span>) <span class="op">=></span> { ... };
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">name</span>:<span class="ident">expr</span>,) <span class="op">=></span> { ... };
}</pre>
</div><div class='docblock'><p>Inspect an environment variable at compile time.</p>
<p>This macro will expand to the value of the named environment variable at
compile time, yielding an expression of type <code>&'static str</code>.</p>
<p>If the environment variable is not defined, then a compilation error
will be emitted. To not emit a compile error, use the <a href="../std/macro.option_env.html"><code>option_env!</code></a>
macro instead.</p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<pre class="rust rust-example-rendered">
<span class="kw">let</span> <span class="ident">path</span>: <span class="kw-2">&</span><span class="lifetime">'static</span> <span class="ident">str</span> <span class="op">=</span> <span class="macro">env</span><span class="macro">!</span>(<span class="string">"PATH"</span>);
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"the $PATH variable at the time of compiling was: {}"</span>, <span class="ident">path</span>);<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Alet%20path%3A%20%26'static%20str%20%3D%20env!(%22PATH%22)%3B%0Aprintln!(%22the%20%24PATH%20variable%20at%20the%20time%20of%20compiling%20was%3A%20%7B%7D%22%2C%20path)%3B%0A%7D">Run</a></pre>
<p>You can customize the error message by passing a string as the second
parameter:</p>
<div class='information'><div class='tooltip compile_fail'>ⓘ<span class='tooltiptext'>This example deliberately fails to compile</span></div></div><pre class="rust rust-example-rendered compile_fail">
<span class="kw">let</span> <span class="ident">doc</span>: <span class="kw-2">&</span><span class="lifetime">'static</span> <span class="ident">str</span> <span class="op">=</span> <span class="macro">env</span><span class="macro">!</span>(<span class="string">"documentation"</span>, <span class="string">"what's that?!"</span>);<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Alet%20doc%3A%20%26'static%20str%20%3D%20env!(%22documentation%22%2C%20%22what's%20that%3F!%22)%3B%0A%7D">Run</a></pre>
<p>If the <code>documentation</code> environment variable is not defined, you'll get
the following error:</p>
<pre><code class="language-text">error: what's that?!
</code></pre>
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