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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="Fern supports logging most things by default, except for one kind of struct: structs which make log calls to the global logger from within their `Display` or `Debug` implementations."><meta name="keywords" content="rust, rustlang, rust-lang, meta"><title>fern::meta - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Semibold.ttf.woff2"><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="../../ayu.css" disabled><link rel="stylesheet" type="text/css" href="../../dark.css" disabled><link rel="stylesheet" type="text/css" href="../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../storage.js"></script><script src="../../crates.js"></script><script defer src="../../main.js"></script>
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</a><h2 class="location">Module meta</h2><div class="sidebar-elems"><div id="sidebar-vars" data-name="meta" data-ty="mod" data-relpath="./"></div><script defer src="./sidebar-items.js"></script></div></nav><main><div class="width-limiter"><div class="sub-container"><a class="sub-logo-container" href="../../fern/index.html"><img class="rust-logo" src="../../rust-logo.png" alt="logo"></a><nav class="sub"><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu" title="themes"><img width="18" height="18" alt="Pick another theme!" src="../../brush.svg"></button><div id="theme-choices" role="menu"></div></div><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" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><button type="button" id="help-button" title="help">?</button><a id="settings-menu" href="../../settings.html" title="settings"><img width="18" height="18" alt="Change settings" src="../../wheel.svg"></a></div></form></nav></div><section id="main-content" class="content"><h1 class="fqn"><span class="in-band">Module <a href="../index.html">fern</a>::<wbr><a class="mod" href="#">meta</a><button id="copy-path" onclick="copy_path(this)" title="Copy item path to clipboard"><img src="../../clipboard.svg" width="19" height="18" alt="Copy item path"></button></span><span class="out-of-band"><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/fern/meta.rs.html#1-79" title="goto source code">[src]</a></span></h1><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Fern supports logging most things by default, except for one kind of struct: structs which make log
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calls to the global logger from within their <code>Display</code> or <code>Debug</code> implementations.</p>
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<p>Here’s an example of such a structure:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">struct</span> <span class="ident">Thing</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">str</span>);
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<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">></span> <span class="ident">fmt::Display</span> <span class="kw">for</span> <span class="ident">Thing</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">></span> {
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<span class="kw">fn</span> <span class="ident">fmt</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">f</span>: <span class="kw-2">&mut</span> <span class="ident">fmt::Formatter</span>) -> <span class="ident">fmt::Result</span> {
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<span class="macro">debug!</span>(<span class="string">"just displayed a Thing wrapping {}"</span>, <span class="self">self</span>.<span class="number">0</span>);
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<span class="ident">f</span>.<span class="ident">write_str</span>(<span class="self">self</span>.<span class="number">0</span>)
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}
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}
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</code></pre></div>
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<p>This structure, and this structure alone, will cause some problems when logging in fern. There are
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mitigations, but since it’s a fairly niche use case, they are disabled by default.</p>
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<p>The problems are, depending on which backend you use:</p>
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<ul>
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<li>stdout/stderr: logging will ‘stutter’, with the logs output inside the <code>Display</code> implementation
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cutting other log lines down the center</li>
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<li>file: thread will deadlock, and all future logs will also deadlock</li>
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</ul>
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<p>There are two mitigations you can make, both completely fix this error.</p>
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<p>The simplest mitigation to this is to enable the <code>meta-logging-in-format</code> feature of <code>fern</code>. The
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disadvantage is that this means fern makes an additional allocation per log call per affected
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backend. Not a huge cost, but enough to mean it’s disabled by default. To enable this, use the
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following in your <code>Cargo.toml</code>:</p>
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<div class="example-wrap"><pre class="language-toml"><code>[dependencies]
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fern = { version = "0.5", features = ["meta-logging-in-format"] }</code></pre></div>
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<p>The second mitigation is one you can make inside a formatting closure. This means extra code
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complexity, but it also means you can enable it per-logger: the fix isn’t global. This fix is also
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redundant if you’ve already enable the above feature. To add the second mitigation, replacec
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<code>format_args!()</code> with <code>format!()</code> as displayed below:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="ident">fern::Dispatch::new</span>()
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...
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<span class="comment">// instead of doing this:</span>
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.<span class="ident">format</span>(<span class="kw">move</span> <span class="op">|</span><span class="ident">out</span>, <span class="ident">message</span>, <span class="ident">record</span><span class="op">|</span> {
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<span class="ident">out</span>.<span class="ident">finish</span>(<span class="macro">format_args!</span>(<span class="string">"[{}] {}"</span>, <span class="ident">record</span>.<span class="ident">level</span>(), <span class="ident">message</span>))
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})
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<span class="comment">// do this:</span>
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.<span class="ident">format</span>(<span class="kw">move</span> <span class="op">|</span><span class="ident">out</span>, <span class="ident">message</span>, <span class="ident">record</span><span class="op">|</span> {
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<span class="kw">let</span> <span class="ident">formatted</span> <span class="op">=</span> <span class="macro">format!</span>(<span class="string">"[{}] {}"</span>, <span class="ident">record</span>.<span class="ident">level</span>(), <span class="ident">message</span>);
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<span class="ident">out</span>.<span class="ident">finish</span>(<span class="macro">format_args!</span>(<span class="string">"{}"</span>, <span class="ident">formatted</span>))
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})</code></pre></div>
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<p>This second mitigation works by forcing the <code>Display</code> implementation to run before any text has
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started to log to the backend. There’s an additional allocation per log, but it no longer deadlocks!</p>
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<p>This mitigation also has the advantage of ensuring there’s only one call to <code>Display::fmt</code>. If youc
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use <code>meta-logging-in-format</code> and have multiple backends, <code>Display::fmt</code> will still be called once
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per backend. With this, it will only be called once.</p>
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<hr />
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<p>If you’ve never experienced this problem, there’s no need to fix it - <code>Display::fmt</code> and
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<code>Debug::fmt</code> are normally implemented as “pure” functions with no side effects.</p>
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