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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="A `Subscriber` for formatting and logging `tracing` data."><title>tracing_subscriber::fmt - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/SourceSerif4-Regular-46f98efaafac5295.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/FiraSans-Regular-018c141bf0843ffd.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/FiraSans-Medium-8f9a781e4970d388.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2"><link rel="stylesheet" href="../../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../../static.files/rustdoc-ac92e1bbe349e143.css"><meta name="rustdoc-vars" data-root-path="../../" data-static-root-path="../../static.files/" data-current-crate="tracing_subscriber" data-themes="" data-resource-suffix="" data-rustdoc-version="1.76.0 (07dca489a 2024-02-04)" data-channel="1.76.0" data-search-js="search-2b6ce74ff89ae146.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../../static.files/storage-f2adc0d6ca4d09fb.js"></script><script defer src="../sidebar-items.js"></script><script defer src="../../static.files/main-305769736d49e732.js"></script><noscript><link rel="stylesheet" href="../../static.files/noscript-feafe1bb7466e4bd.css"></noscript><link rel="alternate icon" type="image/png" href="../../static.files/favicon-16x16-8b506e7a72182f1c.png"><link rel="alternate icon" type="image/png" href="../../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../../static.files/favicon-2c020d218678b618.svg"></head><body class="rustdoc mod"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle">☰</button><a class="logo-container" href="../../tracing_subscriber/index.html"><img src="https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png" alt=""></a></nav><nav class="sidebar"><div class="sidebar-crate"><a class="logo-container" href="../../tracing_subscriber/index.html"><img src="https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png" alt="logo"></a><h2><a href="../../tracing_subscriber/index.html">tracing_subscriber</a><span class="version">0.3.18</span></h2></div><h2 class="location"><a href="#">Module fmt</a></h2><div class="sidebar-elems"><section><ul class="block"><li><a href="#modules">Modules</a></li><li><a href="#structs">Structs</a></li><li><a href="#traits">Traits</a></li><li><a href="#functions">Functions</a></li><li><a href="#types">Type Aliases</a></li></ul></section><h2><a href="../index.html">In crate tracing_subscriber</a></h2></div></nav><div class="sidebar-resizer"></div>
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<main><div class="width-limiter"><nav class="sub"><form class="search-form"><span></span><div id="sidebar-button" tabindex="-1"><a href="../../tracing_subscriber/all.html" title="show sidebar"></a></div><input class="search-input" name="search" aria-label="Run search in the documentation" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" tabindex="-1"><a href="../../help.html" title="help">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../static.files/wheel-7b819b6101059cd0.svg"></a></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1>Module <a href="../index.html">tracing_subscriber</a>::<wbr><a class="mod" href="#">fmt</a><button id="copy-path" title="Copy item path to clipboard"><img src="../../static.files/clipboard-7571035ce49a181d.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="src" href="../../src/tracing_subscriber/fmt/mod.rs.html#1-1370">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</span>]</button></span></div><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>A <code>Subscriber</code> for formatting and logging <code>tracing</code> data.</p>
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<h2 id="overview"><a href="#overview">Overview</a></h2>
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<p><a href="https://crates.io/crates/tracing"><code>tracing</code></a> is a framework for instrumenting Rust programs with context-aware,
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structured, event-based diagnostic information. This crate provides an
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implementation of the <a href="https://docs.rs/tracing/latest/tracing/trait.Subscriber.html"><code>Subscriber</code></a> trait that records <code>tracing</code>’s <code>Event</code>s
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and <code>Span</code>s by formatting them as text and logging them to stdout.</p>
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<h2 id="usage"><a href="#usage">Usage</a></h2>
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<p>First, add this to your <code>Cargo.toml</code> file:</p>
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<div class="example-wrap"><pre class="language-toml"><code>[dependencies]
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tracing-subscriber = "0.3"
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</code></pre></div>
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<p><em>Compiler support: <a href="../index.html#supported-rust-versions" title="mod tracing_subscriber">requires <code>rustc</code> 1.63+</a></em></p>
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<p>Add the following to your executable to initialize the default subscriber:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tracing_subscriber;
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tracing_subscriber::fmt::init();</code></pre></div>
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<h3 id="filtering-events-with-environment-variables"><a href="#filtering-events-with-environment-variables">Filtering Events with Environment Variables</a></h3>
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<p>The default subscriber installed by <code>init</code> enables you to filter events
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at runtime using environment variables (using the <a href="../filter/struct.EnvFilter.html" title="struct tracing_subscriber::filter::EnvFilter"><code>EnvFilter</code></a>).</p>
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<p>The filter syntax is a superset of the <a href="https://docs.rs/env_logger/"><code>env_logger</code></a> syntax.</p>
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<p>For example:</p>
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<ul>
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<li>Setting <code>RUST_LOG=debug</code> enables all <code>Span</code>s and <code>Event</code>s
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set to the log level <code>DEBUG</code> or higher</li>
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<li>Setting <code>RUST_LOG=my_crate=trace</code> enables <code>Span</code>s and <code>Event</code>s
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in <code>my_crate</code> at all log levels</li>
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</ul>
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<p><strong>Note</strong>: This should <strong>not</strong> be called by libraries. Libraries should use
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<a href="https://crates.io/crates/tracing"><code>tracing</code></a> to publish <code>tracing</code> <code>Event</code>s.</p>
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<h2 id="configuration"><a href="#configuration">Configuration</a></h2>
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<p>You can configure a subscriber instead of using the defaults with
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the following functions:</p>
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<h4 id="subscriber"><a href="#subscriber">Subscriber</a></h4>
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<p>The <a href="struct.Subscriber.html" title="struct tracing_subscriber::fmt::Subscriber"><code>FmtSubscriber</code></a> formats and records <code>tracing</code> events as line-oriented logs.
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You can create one by calling:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">let </span>subscriber = tracing_subscriber::fmt()
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<span class="comment">// ... add configuration
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</span>.finish();</code></pre></div>
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<p>You can find the configuration methods for <a href="struct.Subscriber.html" title="struct tracing_subscriber::fmt::Subscriber"><code>FmtSubscriber</code></a> in
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<a href="struct.SubscriberBuilder.html" title="struct tracing_subscriber::fmt::SubscriberBuilder"><code>SubscriberBuilder</code></a>.</p>
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<h3 id="formatters"><a href="#formatters">Formatters</a></h3>
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<p>The output format used by the layer and subscriber in this module is
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represented by implementing the <a href="trait.FormatEvent.html" title="trait tracing_subscriber::fmt::FormatEvent"><code>FormatEvent</code></a> trait, and can be
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customized. This module provides a number of formatter implementations:</p>
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<ul>
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<li>
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<p><a href="format/struct.Full.html" title="struct tracing_subscriber::fmt::format::Full"><code>format::Full</code></a>: The default formatter. This emits human-readable,
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single-line logs for each event that occurs, with the current span context
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displayed before the formatted representation of the event. See
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<a href="format/struct.Full.html#example-output" title="struct tracing_subscriber::fmt::format::Full">here</a> for sample output.</p>
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</li>
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<li>
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<p><a href="format/struct.Compact.html" title="struct tracing_subscriber::fmt::format::Compact"><code>format::Compact</code></a>: A variant of the default formatter, optimized for
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short line lengths. Fields from the current span context are appended to
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the fields of the formatted event. See
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<a href="format/struct.Compact.html#example-output" title="struct tracing_subscriber::fmt::format::Compact">here</a> for sample output.</p>
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</li>
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<li>
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<p><a href="format/struct.Pretty.html" title="struct tracing_subscriber::fmt::format::Pretty"><code>format::Pretty</code></a>: Emits excessively pretty, multi-line logs, optimized
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for human readability. This is primarily intended to be used in local
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development and debugging, or for command-line applications, where
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automated analysis and compact storage of logs is less of a priority than
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readability and visual appeal. See <a href="format/struct.Pretty.html#example-output" title="struct tracing_subscriber::fmt::format::Pretty">here</a>
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for sample output.</p>
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</li>
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<li>
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<p>[<code>format::Json</code>]: Outputs newline-delimited JSON logs. This is intended
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for production use with systems where structured logs are consumed as JSON
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by analysis and viewing tools. The JSON output is not optimized for human
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readability. See <a href="format::Json#example-output">here</a> for sample output.</p>
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</li>
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</ul>
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<h4 id="customizing-formatters"><a href="#customizing-formatters">Customizing Formatters</a></h4>
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<p>The formatting of log lines for spans and events is controlled by two
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traits, <a href="trait.FormatEvent.html" title="trait tracing_subscriber::fmt::FormatEvent"><code>FormatEvent</code></a> and <a href="trait.FormatFields.html" title="trait tracing_subscriber::fmt::FormatFields"><code>FormatFields</code></a>. The <a href="trait.FormatEvent.html" title="trait tracing_subscriber::fmt::FormatEvent"><code>FormatEvent</code></a> trait
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determines the overall formatting of the log line, such as what information
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from the event’s metadata and span context is included and in what order.
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The <a href="trait.FormatFields.html" title="trait tracing_subscriber::fmt::FormatFields"><code>FormatFields</code></a> trait determines how fields — both the event’s
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fields and fields on spans — are formatted.</p>
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<p>The <a href="format/index.html" title="mod tracing_subscriber::fmt::format"><code>fmt::format</code></a> module provides several types which implement these traits,
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many of which expose additional configuration options to customize their
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output. The <a href="format/struct.Format.html" title="struct tracing_subscriber::fmt::format::Format"><code>format::Format</code></a> type implements common configuration used by
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all the formatters provided in this crate, and can be used as a builder to
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set specific formatting settings. For example:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tracing_subscriber::fmt;
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<span class="comment">// Configure a custom event formatter
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</span><span class="kw">let </span>format = fmt::format()
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.with_level(<span class="bool-val">false</span>) <span class="comment">// don't include levels in formatted output
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</span>.with_target(<span class="bool-val">false</span>) <span class="comment">// don't include targets
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</span>.with_thread_ids(<span class="bool-val">true</span>) <span class="comment">// include the thread ID of the current thread
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</span>.with_thread_names(<span class="bool-val">true</span>) <span class="comment">// include the name of the current thread
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</span>.compact(); <span class="comment">// use the `Compact` formatting style.
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// Create a `fmt` subscriber that uses our custom event format, and set it
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// as the default.
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</span>tracing_subscriber::fmt()
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.event_format(format)
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.init();</code></pre></div>
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<p>However, if a specific output format is needed, other crates can
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also implement <a href="trait.FormatEvent.html" title="trait tracing_subscriber::fmt::FormatEvent"><code>FormatEvent</code></a> and <a href="trait.FormatFields.html" title="trait tracing_subscriber::fmt::FormatFields"><code>FormatFields</code></a>. See those traits’
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documentation for details on how to implement them.</p>
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<h3 id="filters"><a href="#filters">Filters</a></h3>
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<p>If you want to filter the <code>tracing</code> <code>Events</code> based on environment
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variables, you can use the <a href="../filter/struct.EnvFilter.html" title="struct tracing_subscriber::filter::EnvFilter"><code>EnvFilter</code></a> as follows:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tracing_subscriber::EnvFilter;
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<span class="kw">let </span>filter = EnvFilter::from_default_env();</code></pre></div>
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<p>As mentioned above, the <a href="../filter/struct.EnvFilter.html" title="struct tracing_subscriber::filter::EnvFilter"><code>EnvFilter</code></a> allows <code>Span</code>s and <code>Event</code>s to
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be filtered at runtime by setting the <code>RUST_LOG</code> environment variable.</p>
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<p>You can find the other available <a href="../filter/index.html" title="mod tracing_subscriber::filter"><code>filter</code></a>s in the documentation.</p>
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<h4 id="using-your-subscriber"><a href="#using-your-subscriber">Using Your Subscriber</a></h4>
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<p>Finally, once you have configured your <code>Subscriber</code>, you need to
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configure your executable to use it.</p>
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<p>A subscriber can be installed globally using:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tracing;
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<span class="kw">use </span>tracing_subscriber::FmtSubscriber;
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<span class="kw">let </span>subscriber = FmtSubscriber::new();
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tracing::subscriber::set_global_default(subscriber)
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.map_err(|_err| <span class="macro">eprintln!</span>(<span class="string">"Unable to set global default subscriber"</span>));
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<span class="comment">// Note this will only fail if you try to set the global default
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// subscriber multiple times</span></code></pre></div>
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<h4 id="composing-layers"><a href="#composing-layers">Composing Layers</a></h4>
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<p>Composing an <a href="../filter/struct.EnvFilter.html" title="struct tracing_subscriber::filter::EnvFilter"><code>EnvFilter</code></a> <code>Layer</code> and a <a href="struct.Layer.html" title="struct tracing_subscriber::fmt::Layer">format <code>Layer</code></a>:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tracing_subscriber::{fmt, EnvFilter};
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<span class="kw">use </span>tracing_subscriber::prelude::<span class="kw-2">*</span>;
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<span class="kw">let </span>fmt_layer = fmt::layer()
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.with_target(<span class="bool-val">false</span>);
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<span class="kw">let </span>filter_layer = EnvFilter::try_from_default_env()
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.or_else(|<span class="kw">_</span>| EnvFilter::try_new(<span class="string">"info"</span>))
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.unwrap();
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tracing_subscriber::registry()
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.with(filter_layer)
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.with(fmt_layer)
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.init();</code></pre></div>
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</div></details><h2 id="modules" class="section-header"><a href="#modules">Modules</a></h2><ul class="item-table"><li><div class="item-name"><a class="mod" href="format/index.html" title="mod tracing_subscriber::fmt::format">format</a></div><div class="desc docblock-short">Formatters for logging <code>tracing</code> events.</div></li><li><div class="item-name"><a class="mod" href="time/index.html" title="mod tracing_subscriber::fmt::time">time</a></div><div class="desc docblock-short">Formatters for event timestamps.</div></li><li><div class="item-name"><a class="mod" href="writer/index.html" title="mod tracing_subscriber::fmt::writer">writer</a></div><div class="desc docblock-short">Abstractions for creating <a href="https://doc.rust-lang.org/1.76.0/std/io/trait.Write.html" title="trait std::io::Write"><code>io::Write</code></a> instances.</div></li></ul><h2 id="structs" class="section-header"><a href="#structs">Structs</a></h2><ul class="item-table"><li><div class="item-name"><a class="struct" href="struct.FmtContext.html" title="struct tracing_subscriber::fmt::FmtContext">FmtContext</a></div><div class="desc docblock-short">Provides the current span context to a formatter.</div></li><li><div class="item-name"><a class="struct" href="struct.FormattedFields.html" title="struct tracing_subscriber::fmt::FormattedFields">FormattedFields</a></div><div class="desc docblock-short">A formatted representation of a span’s fields stored in its <a href="../registry/struct.Extensions.html" title="struct tracing_subscriber::registry::Extensions">extensions</a>.</div></li><li><div class="item-name"><a class="struct" href="struct.Layer.html" title="struct tracing_subscriber::fmt::Layer">Layer</a></div><div class="desc docblock-short">A <a href="../layer/trait.Layer.html" title="trait tracing_subscriber::layer::Layer"><code>Layer</code></a> that logs formatted representations of <code>tracing</code> events.</div></li><li><div class="item-name"><a class="struct" href="struct.Subscriber.html" title="struct tracing_subscriber::fmt::Subscriber">Subscriber</a></div><div class="desc docblock-short">A <code>Subscriber</code> that logs formatted representations of <code>tracing</code> events.</div></li><li><div class="item-name"><a class="struct" href="struct.SubscriberBuilder.html" title="struct tracing_subscriber::fmt::SubscriberBuilder">SubscriberBuilder</a></div><div class="desc docblock-short">Configures and constructs <code>Subscriber</code>s.</div></li><li><div class="item-name"><a class="struct" href="struct.TestWriter.html" title="struct tracing_subscriber::fmt::TestWriter">TestWriter</a></div><div class="desc docblock-short">A writer intended to support <a href="https://doc.rust-lang.org/book/ch11-02-running-tests.html#showing-function-output"><code>libtest</code>’s output capturing</a> for use in unit tests.</div></li></ul><h2 id="traits" class="section-header"><a href="#traits">Traits</a></h2><ul class="item-table"><li><div class="item-name"><a class="trait" href="trait.FormatEvent.html" title="trait tracing_subscriber::fmt::FormatEvent">FormatEvent</a></div><div class="desc docblock-short">A type that can format a tracing <a href="../../tracing_core/event/struct.Event.html" title="struct tracing_core::event::Event"><code>Event</code></a> to a <a href="format/struct.Writer.html" title="struct tracing_subscriber::fmt::format::Writer"><code>Writer</code></a>.</div></li><li><div class="item-name"><a class="trait" href="trait.FormatFields.html" title="trait tracing_subscriber::fmt::FormatFields">FormatFields</a></div><div class="desc docblock-short">A type that can format a <a href="../field/trait.RecordFields.html" title="trait tracing_subscriber::field::RecordFields">set of fields</a> to a <a href="format/struct.Writer.html" title="struct tracing_subscriber::fmt::format::Writer"><code>Writer</code></a>.</div></li><li><div class="item-name"><a class="trait" href="trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a></div><div class="desc docblock-short">A type that can create <a href="https://doc.rust-lang.org/1.76.0/std/io/trait.Write.html" title="trait std::io::Write"><code>io::Write</code></a> instances.</div></li></ul><h2 id="functions" class="section-header"><a href="#functions">Functions</a></h2><ul class="item-table"><li><div class="item-name"><a class="fn" href="fn.fmt.html" title="fn tracing_subscriber::fmt::fmt">fmt</a></div><div class="desc docblock-short">Returns a new <a href="struct.SubscriberBuilder.html" title="struct tracing_subscriber::fmt::SubscriberBuilder"><code>SubscriberBuilder</code></a> for configuring a <a href="struct.Subscriber.html" title="struct tracing_subscriber::fmt::Subscriber">formatting subscriber</a>.</div></li><li><div class="item-name"><a class="fn" href="fn.format.html" title="fn tracing_subscriber::fmt::format">format</a></div><div class="desc docblock-short">Returns the default configuration for an [event formatter].</div></li><li><div class="item-name"><a class="fn" href="fn.init.html" title="fn tracing_subscriber::fmt::init">init</a></div><div class="desc docblock-short">Install a global tracing subscriber that listens for events and
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filters based on the value of the <a href="../filter/struct.EnvFilter.html#associatedconstant.DEFAULT_ENV" title="associated constant tracing_subscriber::filter::EnvFilter::DEFAULT_ENV"><code>RUST_LOG</code> environment variable</a>.</div></li><li><div class="item-name"><a class="fn" href="fn.layer.html" title="fn tracing_subscriber::fmt::layer">layer</a></div><div class="desc docblock-short">Returns a new <a href="struct.Layer.html" title="struct tracing_subscriber::fmt::Layer">formatting layer</a> that can be <a href="../layer/index.html" title="mod tracing_subscriber::layer">composed</a> with other layers to
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construct a <a href="struct.Subscriber.html" title="struct tracing_subscriber::fmt::Subscriber"><code>Subscriber</code></a>.</div></li><li><div class="item-name"><a class="fn" href="fn.time.html" title="fn tracing_subscriber::fmt::time">time</a></div><div class="desc docblock-short">Returns a new <code>SystemTime</code> timestamp provider.</div></li><li><div class="item-name"><a class="fn" href="fn.try_init.html" title="fn tracing_subscriber::fmt::try_init">try_init</a></div><div class="desc docblock-short">Install a global tracing subscriber that listens for events and
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filters based on the value of the <a href="../filter/struct.EnvFilter.html#associatedconstant.DEFAULT_ENV" title="associated constant tracing_subscriber::filter::EnvFilter::DEFAULT_ENV"><code>RUST_LOG</code> environment variable</a>,
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if one is not already set.</div></li></ul><h2 id="types" class="section-header"><a href="#types">Type Aliases</a></h2><ul class="item-table"><li><div class="item-name"><a class="type" href="type.Formatter.html" title="type tracing_subscriber::fmt::Formatter">Formatter</a></div><div class="desc docblock-short">A <code>Subscriber</code> that logs formatted representations of <code>tracing</code> events.
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This type only logs formatted events; it does not perform any filtering.</div></li></ul></section></div></main></body></html> |