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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="Core primitives for `tracing`."><title>tracing_core - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-46f98efaafac5295.ttf.woff2,FiraSans-Regular-018c141bf0843ffd.woff2,FiraSans-Medium-8f9a781e4970d388.woff2,SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2,SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../static.files/rustdoc-dd39b87e5fcfba68.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="tracing_core" data-themes="" data-resource-suffix="" data-rustdoc-version="1.80.0 (051478957 2024-07-21)" data-channel="1.80.0" data-search-js="search-d52510db62a78183.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../static.files/storage-118b08c4c78b968e.js"></script><script defer src="../crates.js"></script><script defer src="../static.files/main-20a3ad099b048cf2.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-df360f571f6edeae.css"></noscript><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 crate"><!--[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" title="show sidebar"></button><a class="logo-container" href="../tracing_core/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_core/index.html"><img src="https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png" alt="logo"></a><h2><a href="../tracing_core/index.html">tracing_core</a><span class="version">0.1.32</span></h2></div><div class="sidebar-elems"><ul class="block"><li><a id="all-types" href="all.html">All Items</a></li></ul><section><ul class="block"><li><a href="#reexports">Re-exports</a></li><li><a href="#modules">Modules</a></li><li><a href="#macros">Macros</a></li><li><a href="#structs">Structs</a></li><li><a href="#traits">Traits</a></li></ul></section></div></nav><div class="sidebar-resizer"></div><main><div class="width-limiter"><rustdoc-search></rustdoc-search><section id="main-content" class="content"><div class="main-heading"><h1>Crate <a class="mod" href="#">tracing_core</a><button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><span class="out-of-band"><a class="src" href="../src/tracing_core/lib.rs.html#1-293">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>&#x2212;</span>]</button></span></div><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Core primitives for <code>tracing</code>.</p>
<p><a href="https://crates.io/crates/tracing"><code>tracing</code></a> is a framework for instrumenting Rust programs to collect
structured, event-based diagnostic information. This crate defines the core
primitives of <code>tracing</code>.</p>
<p>This crate provides:</p>
<ul>
<li>
<p><a href="span/struct.Id.html" title="struct tracing_core::span::Id"><code>span::Id</code></a> identifies a span within the execution of a program.</p>
</li>
<li>
<p><a href="struct.Event.html" title="struct tracing_core::Event"><code>Event</code></a> represents a single event within a trace.</p>
</li>
<li>
<p><a href="trait.Subscriber.html" title="trait tracing_core::Subscriber"><code>Subscriber</code></a>, the trait implemented to collect trace data.</p>
</li>
<li>
<p><a href="struct.Metadata.html" title="struct tracing_core::Metadata"><code>Metadata</code></a> and <a href="trait.Callsite.html" title="trait tracing_core::Callsite"><code>Callsite</code></a> provide information describing spans and
<code>Event</code>s.</p>
</li>
<li>
<p><a href="struct.Field.html" title="struct tracing_core::Field"><code>Field</code></a>, <a href="field/struct.FieldSet.html" title="struct tracing_core::field::FieldSet"><code>FieldSet</code></a>, <a href="field/trait.Value.html" title="trait tracing_core::field::Value"><code>Value</code></a>, and <a href="field/struct.ValueSet.html" title="struct tracing_core::field::ValueSet"><code>ValueSet</code></a> represent the
structured data attached to a span.</p>
</li>
<li>
<p><a href="struct.Dispatch.html" title="struct tracing_core::Dispatch"><code>Dispatch</code></a> allows spans and events to be dispatched to <code>Subscriber</code>s.</p>
</li>
</ul>
<p>In addition, it defines the global callsite registry and per-thread current
dispatcher which other components of the tracing system rely on.</p>
<p><em>Compiler support: <a href="#supported-rust-versions">requires <code>rustc</code> 1.56+</a></em></p>
<h3 id="usage"><a class="doc-anchor" href="#usage">§</a>Usage</h3>
<p>Application authors will typically not use this crate directly. Instead,
they will use the <a href="https://crates.io/crates/tracing"><code>tracing</code></a> crate, which provides a much more
fully-featured API. However, this crates API will change very infrequently,
so it may be used when dependencies must be very stable.</p>
<p><code>Subscriber</code> implementations may depend on <code>tracing-core</code> rather than
<code>tracing</code>, as the additional APIs provided by <code>tracing</code> are primarily useful
for instrumenting libraries and applications, and are generally not
necessary for <code>Subscriber</code> implementations.</p>
<p>The <a href="https://github.com/tokio-rs/tracing"><code>tokio-rs/tracing</code></a> repository contains less stable crates designed to
be used with the <code>tracing</code> ecosystem. It includes a collection of
<code>Subscriber</code> implementations, as well as utility and adapter crates.</p>
<h3 id="crate-feature-flags"><a class="doc-anchor" href="#crate-feature-flags">§</a>Crate Feature Flags</h3>
<p>The following crate <a href="https://doc.rust-lang.org/cargo/reference/manifest.html#the-features-section">feature flags</a> are available:</p>
<ul>
<li>
<p><code>std</code>: Depend on the Rust standard library (enabled by default).</p>
<p><code>no_std</code> users may disable this feature with <code>default-features = false</code>:</p>
<div class="example-wrap"><pre class="language-toml"><code>[dependencies]
tracing-core = { version = &quot;0.1.22&quot;, default-features = false }
</code></pre></div>
<p><strong>Note</strong>:<code>tracing-core</code>s <code>no_std</code> support requires <code>liballoc</code>.</p>
</li>
</ul>
<h4 id="unstable-features"><a class="doc-anchor" href="#unstable-features">§</a>Unstable Features</h4>
<p>These feature flags enable <strong>unstable</strong> features. The public API may break in 0.1.x
releases. To enable these features, the <code>--cfg tracing_unstable</code> must be passed to
<code>rustc</code> when compiling.</p>
<p>The following unstable feature flags are currently available:</p>
<ul>
<li><code>valuable</code>: Enables support for recording <a href="field/index.html" title="mod tracing_core::field">field values</a> using the
<a href="https://crates.io/crates/valuable"><code>valuable</code></a> crate.</li>
</ul>
<h5 id="enabling-unstable-features"><a class="doc-anchor" href="#enabling-unstable-features">§</a>Enabling Unstable Features</h5>
<p>The easiest way to set the <code>tracing_unstable</code> cfg is to use the <code>RUSTFLAGS</code>
env variable when running <code>cargo</code> commands:</p>
<div class="example-wrap"><pre class="language-shell"><code>RUSTFLAGS=&quot;--cfg tracing_unstable&quot; cargo build
</code></pre></div>
<p>Alternatively, the following can be added to the <code>.cargo/config</code> file in a
project to automatically enable the cfg flag for that project:</p>
<div class="example-wrap"><pre class="language-toml"><code>[build]
rustflags = [&quot;--cfg&quot;, &quot;tracing_unstable&quot;]
</code></pre></div><h3 id="supported-rust-versions"><a class="doc-anchor" href="#supported-rust-versions">§</a>Supported Rust Versions</h3>
<p>Tracing is built against the latest stable release. The minimum supported
version is 1.56. The current Tracing version is not guaranteed to build on
Rust versions earlier than the minimum supported version.</p>
<p>Tracing follows the same compiler support policies as the rest of the Tokio
project. The current stable Rust compiler and the three most recent minor
versions before it will always be supported. For example, if the current
stable compiler version is 1.69, the minimum supported version will not be
increased past 1.66, three minor versions prior. Increasing the minimum
supported compiler version is not considered a semver breaking change as
long as doing so complies with this policy.</p>
</div></details><h2 id="reexports" class="section-header">Re-exports<a href="#reexports" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name" id="reexport.Kind"><code>pub use self::metadata::<a class="struct" href="metadata/struct.Kind.html" title="struct tracing_core::metadata::Kind">Kind</a>;</code></div></li><li><div class="item-name" id="reexport.Interest"><code>pub use self::subscriber::<a class="struct" href="subscriber/struct.Interest.html" title="struct tracing_core::subscriber::Interest">Interest</a>;</code></div></li></ul><h2 id="modules" class="section-header">Modules<a href="#modules" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="mod" href="callsite/index.html" title="mod tracing_core::callsite">callsite</a></div><div class="desc docblock-short">Callsites represent the source locations from which spans or events
originate.</div></li><li><div class="item-name"><a class="mod" href="dispatcher/index.html" title="mod tracing_core::dispatcher">dispatcher</a></div><div class="desc docblock-short">Dispatches trace events to <a href="trait.Subscriber.html" title="trait tracing_core::Subscriber"><code>Subscriber</code></a>s.</div></li><li><div class="item-name"><a class="mod" href="event/index.html" title="mod tracing_core::event">event</a></div><div class="desc docblock-short">Events represent single points in time during the execution of a program.</div></li><li><div class="item-name"><a class="mod" href="field/index.html" title="mod tracing_core::field">field</a></div><div class="desc docblock-short"><code>Span</code> and <code>Event</code> key-value data.</div></li><li><div class="item-name"><a class="mod" href="metadata/index.html" title="mod tracing_core::metadata">metadata</a></div><div class="desc docblock-short">Metadata describing trace data.</div></li><li><div class="item-name"><a class="mod" href="span/index.html" title="mod tracing_core::span">span</a></div><div class="desc docblock-short">Spans represent periods of time in the execution of a program.</div></li><li><div class="item-name"><a class="mod" href="subscriber/index.html" title="mod tracing_core::subscriber">subscriber</a></div><div class="desc docblock-short">Collectors collect and record trace data.</div></li></ul><h2 id="macros" class="section-header">Macros<a href="#macros" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="macro" href="macro.identify_callsite.html" title="macro tracing_core::identify_callsite">identify_callsite</a></div><div class="desc docblock-short">Statically constructs an <a href="callsite/struct.Identifier.html" title="struct tracing_core::callsite::Identifier"><code>Identifier</code></a> for the provided <a href="trait.Callsite.html" title="trait tracing_core::Callsite"><code>Callsite</code></a>.</div></li><li><div class="item-name"><a class="macro" href="macro.metadata.html" title="macro tracing_core::metadata">metadata</a></div><div class="desc docblock-short">Statically constructs new span <a href="struct.Metadata.html" title="struct tracing_core::Metadata">metadata</a>.</div></li></ul><h2 id="structs" class="section-header">Structs<a href="#structs" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="struct" href="struct.Dispatch.html" title="struct tracing_core::Dispatch">Dispatch</a></div><div class="desc docblock-short"><code>Dispatch</code> trace data to a <a href="trait.Subscriber.html" title="trait tracing_core::Subscriber"><code>Subscriber</code></a>.</div></li><li><div class="item-name"><a class="struct" href="struct.Event.html" title="struct tracing_core::Event">Event</a></div><div class="desc docblock-short"><code>Event</code>s represent single points in time where something occurred during the
execution of a program.</div></li><li><div class="item-name"><a class="struct" href="struct.Field.html" title="struct tracing_core::Field">Field</a></div><div class="desc docblock-short">An opaque key allowing <em>O</em>(1) access to a field in a <code>Span</code>s key-value
data.</div></li><li><div class="item-name"><a class="struct" href="struct.Level.html" title="struct tracing_core::Level">Level</a></div><div class="desc docblock-short">Describes the level of verbosity of a span or event.</div></li><li><div class="item-name"><a class="struct" href="struct.LevelFilter.html" title="struct tracing_core::LevelFilter">LevelFilter</a></div><div class="desc docblock-short">A filter comparable to a verbosity <a href="struct.Level.html" title="struct tracing_core::Level"><code>Level</code></a>.</div></li><li><div class="item-name"><a class="struct" href="struct.Metadata.html" title="struct tracing_core::Metadata">Metadata</a></div><div class="desc docblock-short">Metadata describing a <a href="span/index.html" title="mod tracing_core::span">span</a> or <a href="event/index.html" title="mod tracing_core::event">event</a>.</div></li><li><div class="item-name"><a class="struct" href="struct.Once.html" title="struct tracing_core::Once">Once</a></div><div class="desc docblock-short">A synchronization primitive which can be used to run a one-time global
initialization. Useful for one-time initialization for FFI or related
functionality. This type can only be constructed with <a href="struct.Once.html#method.new" title="associated function tracing_core::Once::new"><code>Once::new()</code></a>.</div></li></ul><h2 id="traits" class="section-header">Traits<a href="#traits" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="trait" href="trait.Callsite.html" title="trait tracing_core::Callsite">Callsite</a></div><div class="desc docblock-short">Trait implemented by callsites.</div></li><li><div class="item-name"><a class="trait" href="trait.Subscriber.html" title="trait tracing_core::Subscriber">Subscriber</a></div><div class="desc docblock-short">Trait representing the functions required to collect trace data.</div></li></ul></section></div></main></body></html>