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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 `dbg` macro in crate `std`."><meta name="keywords" content="rust, rustlang, rust-lang, dbg"><title>std::dbg - Rust</title><link rel="stylesheet" type="text/css" href="../normalize1.35.0.css"><link rel="stylesheet" type="text/css" href="../rustdoc1.35.0.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../dark1.35.0.css"><link rel="stylesheet" type="text/css" href="../light1.35.0.css" id="themeStyle"><script src="../storage1.35.0.js"></script><noscript><link rel="stylesheet" href="../noscript1.35.0.css"></noscript><link rel="shortcut icon" href="../favicon1.35.0.ico"><style type="text/css">#crate-search{background-image:url("../down-arrow1.35.0.svg");}</style></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">&#9776;</div><a href='../std/index.html'><img src='../rust-logo1.35.0.png' alt='logo' width='100'></a><div class="sidebar-elems"><p class='location'><a href='index.html'>std</a></p><script>window.sidebarCurrent = {name: 'dbg', 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="../brush1.35.0.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../theme1.35.0.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" 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="../wheel1.35.0.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 1.32.0'>1.32.0</span><span id='render-detail'><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class='inner'>&#x2212;</span>]</a></span><a class='srclink' href='../src/std/macros.rs.html#322-337' title='goto source code'>[src]</a></span><span class='in-band'>Macro <a href='index.html'>std</a>::<wbr><a class="macro" href=''>dbg</a></span></h1><div class="docblock type-decl hidden-by-usual-hider"><div class="example-wrap"><pre class="rust macro">
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">dbg</span> {
    () <span class="op">=&gt;</span> { ... };
    (<span class="macro-nonterminal">$</span><span class="macro-nonterminal">val</span>:<span class="ident">expr</span>) <span class="op">=&gt;</span> { ... };
}</pre></div>
</div><div class='docblock'><p>Prints and returns the value of a given expression for quick and dirty
debugging.</p>
<p>An example:</p>

<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">let</span> <span class="ident">a</span> <span class="op">=</span> <span class="number">2</span>;
<span class="kw">let</span> <span class="ident">b</span> <span class="op">=</span> <span class="macro">dbg</span><span class="macro">!</span>(<span class="ident">a</span> <span class="op">*</span> <span class="number">2</span>) <span class="op">+</span> <span class="number">1</span>;
<span class="comment">//      ^-- prints: [src/main.rs:2] a * 2 = 4</span>
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">b</span>, <span class="number">5</span>);<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Alet%20a%20%3D%202%3B%0Alet%20b%20%3D%20dbg!(a%20*%202)%20%2B%201%3B%0A%2F%2F%20%20%20%20%20%20%5E--%20prints%3A%20%5Bsrc%2Fmain.rs%3A2%5D%20a%20*%202%20%3D%204%0Aassert_eq!(b%2C%205)%3B%0A%7D">Run</a></pre></div>
<p>The macro works by using the <code>Debug</code> implementation of the type of
the given expression to print the value to <a href="https://en.wikipedia.org/wiki/Standard_streams#Standard_error_(stderr)">stderr</a> along with the
source location of the macro invocation as well as the source code
of the expression.</p>
<p>Invoking the macro on an expression moves and takes ownership of it
before returning the evaluated expression unchanged. If the type
of the expression does not implement <code>Copy</code> and you don't want
to give up ownership, you can instead borrow with <code>dbg!(&amp;expr)</code>
for some expression <code>expr</code>.</p>
<p>The <code>dbg!</code> macro works exactly the same in release builds.
This is useful when debugging issues that only occur in release
builds or when debugging in release mode is significantly faster.</p>
<p>Note that the macro is intended as a debugging tool and therefore you
should avoid having uses of it in version control for longer periods.
Use cases involving debug output that should be added to version control
are better served by macros such as <a href="https://docs.rs/log/*/log/macro.debug.html"><code>debug!</code></a> from the <a href="https://crates.io/crates/log"><code>log</code></a> crate.</p>
<h1 id="stability" class="section-header"><a href="#stability">Stability</a></h1>
<p>The exact output printed by this macro should not be relied upon
and is subject to future changes.</p>
<h1 id="panics" class="section-header"><a href="#panics">Panics</a></h1>
<p>Panics if writing to <code>io::stderr</code> fails.</p>
<h1 id="further-examples" class="section-header"><a href="#further-examples">Further examples</a></h1>
<p>With a method call:</p>

<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">fn</span> <span class="ident">foo</span>(<span class="ident">n</span>: <span class="ident">usize</span>) {
    <span class="kw">if</span> <span class="kw">let</span> <span class="prelude-val">Some</span>(<span class="kw">_</span>) <span class="op">=</span> <span class="macro">dbg</span><span class="macro">!</span>(<span class="ident">n</span>.<span class="ident">checked_sub</span>(<span class="number">4</span>)) {
        <span class="comment">// ...</span>
    }
}

<span class="ident">foo</span>(<span class="number">3</span>)<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Afn%20foo(n%3A%20usize)%20%7B%0A%20%20%20%20if%20let%20Some(_)%20%3D%20dbg!(n.checked_sub(4))%20%7B%0A%20%20%20%20%20%20%20%20%2F%2F%20...%0A%20%20%20%20%7D%0A%7D%0A%0Afoo(3)%0A%7D">Run</a></pre></div>
<p>This prints to <a href="https://en.wikipedia.org/wiki/Standard_streams#Standard_error_(stderr)">stderr</a>:</p>
<pre><code class="language-text,ignore">[src/main.rs:4] n.checked_sub(4) = None
</code></pre>
<p>Naive factorial implementation:</p>

<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">fn</span> <span class="ident">factorial</span>(<span class="ident">n</span>: <span class="ident">u32</span>) <span class="op">-&gt;</span> <span class="ident">u32</span> {
    <span class="kw">if</span> <span class="macro">dbg</span><span class="macro">!</span>(<span class="ident">n</span> <span class="op">&lt;=</span> <span class="number">1</span>) {
        <span class="macro">dbg</span><span class="macro">!</span>(<span class="number">1</span>)
    } <span class="kw">else</span> {
        <span class="macro">dbg</span><span class="macro">!</span>(<span class="ident">n</span> <span class="op">*</span> <span class="ident">factorial</span>(<span class="ident">n</span> <span class="op">-</span> <span class="number">1</span>))
    }
}

<span class="macro">dbg</span><span class="macro">!</span>(<span class="ident">factorial</span>(<span class="number">4</span>));<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Afn%20factorial(n%3A%20u32)%20-%3E%20u32%20%7B%0A%20%20%20%20if%20dbg!(n%20%3C%3D%201)%20%7B%0A%20%20%20%20%20%20%20%20dbg!(1)%0A%20%20%20%20%7D%20else%20%7B%0A%20%20%20%20%20%20%20%20dbg!(n%20*%20factorial(n%20-%201))%0A%20%20%20%20%7D%0A%7D%0A%0Adbg!(factorial(4))%3B%0A%7D">Run</a></pre></div>
<p>This prints to <a href="https://en.wikipedia.org/wiki/Standard_streams#Standard_error_(stderr)">stderr</a>:</p>
<pre><code class="language-text,ignore">[src/main.rs:3] n &lt;= 1 = false
[src/main.rs:3] n &lt;= 1 = false
[src/main.rs:3] n &lt;= 1 = false
[src/main.rs:3] n &lt;= 1 = true
[src/main.rs:4] 1 = 1
[src/main.rs:5] n * factorial(n - 1) = 2
[src/main.rs:5] n * factorial(n - 1) = 6
[src/main.rs:5] n * factorial(n - 1) = 24
[src/main.rs:11] factorial(4) = 24
</code></pre>
<p>The <code>dbg!(..)</code> macro moves the input:</p>

<div class='information'><div class='tooltip compile_fail'>ⓘ<span class='tooltiptext'>This example deliberately fails to compile</span></div></div><div class="example-wrap"><pre class="rust rust-example-rendered compile_fail">
<span class="doccomment">/// A wrapper around `usize` which importantly is not Copyable.</span>
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Debug</span>)]</span>
<span class="kw">struct</span> <span class="ident">NoCopy</span>(<span class="ident">usize</span>);

<span class="kw">let</span> <span class="ident">a</span> <span class="op">=</span> <span class="ident">NoCopy</span>(<span class="number">42</span>);
<span class="kw">let</span> <span class="kw">_</span> <span class="op">=</span> <span class="macro">dbg</span><span class="macro">!</span>(<span class="ident">a</span>); <span class="comment">// &lt;-- `a` is moved here.</span>
<span class="kw">let</span> <span class="kw">_</span> <span class="op">=</span> <span class="macro">dbg</span><span class="macro">!</span>(<span class="ident">a</span>); <span class="comment">// &lt;-- `a` is moved again; error!</span><a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0A%2F%2F%2F%20A%20wrapper%20around%20%60usize%60%20which%20importantly%20is%20not%20Copyable.%0A%23%5Bderive(Debug)%5D%0Astruct%20NoCopy(usize)%3B%0A%0Alet%20a%20%3D%20NoCopy(42)%3B%0Alet%20_%20%3D%20dbg!(a)%3B%20%2F%2F%20%3C--%20%60a%60%20is%20moved%20here.%0Alet%20_%20%3D%20dbg!(a)%3B%20%2F%2F%20%3C--%20%60a%60%20is%20moved%20again%3B%20error!%0A%7D">Run</a></pre></div>
<p>You can also use <code>dbg!()</code> without a value to just print the
file and line whenever it's reached.</p>
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