| <!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="`Span` and `Event` key-value data."><meta name="keywords" content="rust, rustlang, rust-lang, field"><title>tracing::field - 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" href="../../normalize.css"><link rel="stylesheet" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../ayu.css" disabled><link rel="stylesheet" href="../../dark.css" disabled><link rel="stylesheet" href="../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../storage.js"></script><script defer src="../../main.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../../favicon.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="sidebar-logo" href="../../tracing/index.html"><div class="logo-container"><img src="https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png" alt="logo"></div></a><h2></h2></nav><nav class="sidebar"><a class="sidebar-logo" href="../../tracing/index.html"><div class="logo-container"> |
| <img src="https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png" alt="logo"></div></a><h2 class="location"><a href="#">Module field</a></h2><div class="sidebar-elems"><section><ul class="block"><li><a href="#structs">Structs</a></li><li><a href="#traits">Traits</a></li><li><a href="#functions">Functions</a></li></ul></section></div></nav><main><div class="width-limiter"><nav class="sub"><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1 class="fqn">Module <a href="../index.html">tracing</a>::<wbr><a class="mod" href="#">field</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></h1><span class="out-of-band"><a class="srclink" href="../../src/tracing/field.rs.html#1-170">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p><code>Span</code> and <code>Event</code> key-value data.</p> |
| <p>Spans and events may be annotated with key-value data, referred to as known |
| as <em>fields</em>. These fields consist of a mapping from a key (corresponding to |
| a <code>&str</code> but represented internally as an array index) to a <a href="../trait.Value.html" title="Value"><code>Value</code></a>.</p> |
| <h2 id="values-and-subscribers"><a href="#values-and-subscribers"><code>Value</code>s and <code>Subscriber</code>s</a></h2> |
| <p><code>Subscriber</code>s consume <code>Value</code>s as fields attached to <a href="../span/index.html">span</a>s or <a href="../event/struct.Event.html"><code>Event</code></a>s. |
| The set of field keys on a given span or is defined on its <a href="../struct.Metadata.html"><code>Metadata</code></a>. |
| When a span is created, it provides <a href="../span/struct.Attributes.html"><code>Attributes</code></a> to the <code>Subscriber</code>’s |
| <a href="../trait.Subscriber.html#tymethod.new_span"><code>new_span</code></a> method, containing any fields whose values were provided when |
| the span was created; and may call the <code>Subscriber</code>’s <a href="../span/struct.Record.html"><code>record</code></a> method |
| with additional <a href="../span/struct.Record.html"><code>Record</code></a>s if values are added for more of its fields. |
| Similarly, the <a href="../event/struct.Event.html"><code>Event</code></a> type passed to the subscriber’s <a href="../event/struct.Event.html"><code>event</code></a> method |
| will contain any fields attached to each event.</p> |
| <p><code>tracing</code> represents values as either one of a set of Rust primitives |
| (<code>i64</code>, <code>u64</code>, <code>f64</code>, <code>bool</code>, and <code>&str</code>) or using a <code>fmt::Display</code> or |
| <code>fmt::Debug</code> implementation. <code>Subscriber</code>s are provided these primitive |
| value types as <code>dyn Value</code> trait objects.</p> |
| <p>These trait objects can be formatted using <code>fmt::Debug</code>, but may also be |
| recorded as typed data by calling the <a href="../trait.Value.html#tymethod.record" title="Value::record"><code>Value::record</code></a> method on these |
| trait objects with a <em>visitor</em> implementing the <a href="trait.Visit.html" title="Visit"><code>Visit</code></a> trait. This trait |
| represents the behavior used to record values of various types. For example, |
| an implementation of <code>Visit</code> might record integers by incrementing counters |
| for their field names rather than printing them.</p> |
| <h2 id="using-valuable"><a href="#using-valuable">Using <code>valuable</code></a></h2> |
| <p><code>tracing</code>’s <a href="../trait.Value.html" title="Value"><code>Value</code></a> trait is intentionally minimalist: it supports only a small |
| number of Rust primitives as typed values, and only permits recording |
| user-defined types with their <a href="std::fmt::Debug"><code>fmt::Debug</code></a> or <a href="std::fmt::Debug"><code>fmt::Display</code></a> |
| implementations. However, there are some cases where it may be useful to record |
| nested values (such as arrays, <code>Vec</code>s, or <code>HashMap</code>s containing values), or |
| user-defined <code>struct</code> and <code>enum</code> types without having to format them as |
| unstructured text.</p> |
| <p>To address <code>Value</code>’s limitations, <code>tracing</code> offers experimental support for |
| the <a href="https://crates.io/crates/valuable"><code>valuable</code></a> crate, which provides object-safe inspection of structured |
| values. User-defined types can implement the <a href="https://docs.rs/valuable/latest/valuable/trait.Valuable.html"><code>valuable::Valuable</code></a> trait, |
| and be recorded as a <code>tracing</code> field by calling their <a href="https://docs.rs/valuable/latest/valuable/trait.Valuable.html#tymethod.as_value"><code>as_value</code></a> method. |
| If the <a href="../trait.Subscriber.html"><code>Subscriber</code></a> also supports the <code>valuable</code> crate, it can |
| then visit those types fields as structured values using <code>valuable</code>.</p> |
| <pre class="ignore" style="white-space:normal;font:inherit;"> |
| <strong>Note</strong>: <code>valuable</code> support is an |
| <a href = "../index.html#unstable-features">unstable feature</a>. See |
| the documentation on unstable features for details on how to enable it. |
| </pre> |
| <p>For example:</p> |
| |
| <div class="example-wrap ignore"><div class='tooltip'>ⓘ</div><pre class="rust rust-example-rendered"><code><span class="comment">// Derive `Valuable` for our types: |
| </span><span class="kw">use </span>valuable::Valuable; |
| |
| <span class="attribute">#[derive(Clone, Debug, Valuable)] |
| </span><span class="kw">struct </span>User { |
| name: String, |
| age: u32, |
| address: Address, |
| } |
| |
| <span class="attribute">#[derive(Clone, Debug, Valuable)] |
| </span><span class="kw">struct </span>Address { |
| country: String, |
| city: String, |
| street: String, |
| } |
| |
| <span class="kw">let </span>user = User { |
| name: <span class="string">"Arwen Undomiel"</span>.to_string(), |
| age: <span class="number">3000</span>, |
| address: Address { |
| country: <span class="string">"Middle Earth"</span>.to_string(), |
| city: <span class="string">"Rivendell"</span>.to_string(), |
| street: <span class="string">"leafy lane"</span>.to_string(), |
| }, |
| }; |
| |
| <span class="comment">// Recording `user` as a `valuable::Value` will allow the `tracing` subscriber |
| // to traverse its fields as a nested, typed structure: |
| </span><span class="macro">tracing::info!</span>(current_user = user.as_value());</code></pre></div> |
| <p>Alternatively, the [<code>valuable()</code>] function may be used to convert a type |
| implementing <a href="https://crates.io/crates/valuable"><code>Valuable</code></a> into a <code>tracing</code> field value.</p> |
| <p>When the <code>valuable</code> feature is enabled, the <a href="trait.Visit.html" title="Visit"><code>Visit</code></a> trait will include an |
| optional <a href="Visit::record_value"><code>record_value</code></a> method. <code>Visit</code> implementations that wish to |
| record <code>valuable</code> values can implement this method with custom behavior. |
| If a visitor does not implement <code>record_value</code>, the <a href="https://docs.rs/valuable/latest/valuable/enum.Value.html"><code>valuable::Value</code></a> will |
| be forwarded to the visitor’s <a href="trait.Visit.html#tymethod.record_debug"><code>record_debug</code></a> method.</p> |
| </div></details><h2 id="structs" class="small-section-header"><a href="#structs">Structs</a></h2><div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.DebugValue.html" title="tracing::field::DebugValue struct">DebugValue</a></div><div class="item-right docblock-short">A <code>Value</code> which serializes as a string using <code>fmt::Debug</code>.</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.DisplayValue.html" title="tracing::field::DisplayValue struct">DisplayValue</a></div><div class="item-right docblock-short">A <code>Value</code> which serializes using <code>fmt::Display</code>.</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.Empty.html" title="tracing::field::Empty struct">Empty</a></div><div class="item-right docblock-short">An empty field.</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.Field.html" title="tracing::field::Field struct">Field</a></div><div class="item-right docblock-short">An opaque key allowing <em>O</em>(1) access to a field in a <code>Span</code>’s key-value |
| data.</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.FieldSet.html" title="tracing::field::FieldSet struct">FieldSet</a></div><div class="item-right docblock-short">Describes the fields present on a span.</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.Iter.html" title="tracing::field::Iter struct">Iter</a></div><div class="item-right docblock-short">An iterator over a set of fields.</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.ValueSet.html" title="tracing::field::ValueSet struct">ValueSet</a></div><div class="item-right docblock-short">A set of fields and values for a span.</div></div></div><h2 id="traits" class="small-section-header"><a href="#traits">Traits</a></h2><div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="trait" href="trait.AsField.html" title="tracing::field::AsField trait">AsField</a></div><div class="item-right docblock-short">Trait implemented to allow a type to be used as a field key.</div></div><div class="item-row"><div class="item-left module-item"><a class="trait" href="trait.Value.html" title="tracing::field::Value trait">Value</a></div><div class="item-right docblock-short">A field value of an erased type.</div></div><div class="item-row"><div class="item-left module-item"><a class="trait" href="trait.Visit.html" title="tracing::field::Visit trait">Visit</a></div><div class="item-right docblock-short">Visits typed values.</div></div></div><h2 id="functions" class="small-section-header"><a href="#functions">Functions</a></h2><div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="fn" href="fn.debug.html" title="tracing::field::debug fn">debug</a></div><div class="item-right docblock-short">Wraps a type implementing <code>fmt::Debug</code> as a <code>Value</code> that can be |
| recorded using its <code>Debug</code> implementation.</div></div><div class="item-row"><div class="item-left module-item"><a class="fn" href="fn.display.html" title="tracing::field::display fn">display</a></div><div class="item-right docblock-short">Wraps a type implementing <code>fmt::Display</code> as a <code>Value</code> that can be |
| recorded using its <code>Display</code> implementation.</div></div></div></section></div></main><div id="rustdoc-vars" data-root-path="../../" data-current-crate="tracing" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
