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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="Trait implemented to allow a type to be used as a field key."><title>AsField in tracing::field - Rust</title><script> if (window.location.protocol !== "file:") document.write(`<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">`)</script><link rel="stylesheet" href="../../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../../static.files/rustdoc-e935ef01ae1c1829.css"><meta name="rustdoc-vars" data-root-path="../../" data-static-root-path="../../static.files/" data-current-crate="tracing" data-themes="" data-resource-suffix="" data-rustdoc-version="1.78.0 (9b00956e5 2024-04-29)" data-channel="1.78.0" data-search-js="search-42d8da7a6b9792c2.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../../static.files/storage-4c98445ec4002617.js"></script><script defer src="sidebar-items.js"></script><script defer src="../../static.files/main-12cf3b4f4f9dc36d.js"></script><noscript><link rel="stylesheet" href="../../static.files/noscript-04d5337699b92874.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 trait"><!--[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/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/index.html"><img src="https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png" alt="logo"></a><h2><a href="../../tracing/index.html">tracing</a><span class="version">0.1.40</span></h2></div><h2 class="location"><a href="#">AsField</a></h2><div class="sidebar-elems"><section><h3><a href="#required-methods">Required Methods</a></h3><ul class="block"><li><a href="#tymethod.as_field">as_field</a></li></ul><h3><a href="#foreign-impls">Implementations on Foreign Types</a></h3><ul class="block"><li><a href="#impl-AsField-for-str">str</a></li></ul><h3><a href="#implementors">Implementors</a></h3></section><h2><a href="index.html">In tracing::field</a></h2></div></nav><div class="sidebar-resizer"></div>
<main><div class="width-limiter"><nav class="sub"><form class="search-form"><span></span><div id="sidebar-button" tabindex="-1"><a href="../../tracing/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>Trait <a href="../index.html">tracing</a>::<wbr><a href="index.html">field</a>::<wbr><a class="trait" href="#">AsField</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/field.rs.html#129-135">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>&#x2212;</span>]</button></span></div><pre class="rust item-decl"><code>pub trait AsField: Sealed {
// Required method
fn <a href="#tymethod.as_field" class="fn">as_field</a>(&amp;self, metadata: &amp;<a class="struct" href="../struct.Metadata.html" title="struct tracing::Metadata">Metadata</a>&lt;'_&gt;) -&gt; <a class="enum" href="https://doc.rust-lang.org/1.78.0/core/option/enum.Option.html" title="enum core::option::Option">Option</a>&lt;<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>&gt;;
}</code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Trait implemented to allow a type to be used as a field key.</p>
<pre class="ignore" style="white-space:normal;font:inherit;">
<strong>Note</strong>: Although this is implemented for both the
<a href="./struct.Field.html"><code>Field</code></a> type <em>and</em> any
type that can be borrowed as an <code>&str</code>, only <code>Field</code>
allows <em>O</em>(1) access.
Indexing a field with a string results in an iterative search that performs
string comparisons. Thus, if possible, once the key for a field is known, it
should be used whenever possible.
</pre></div></details><h2 id="required-methods" class="section-header">Required Methods<a href="#required-methods" class="anchor">§</a></h2><div class="methods"><details class="toggle method-toggle" open><summary><section id="tymethod.as_field" class="method"><a class="src rightside" href="../../src/tracing/field.rs.html#134">source</a><h4 class="code-header">fn <a href="#tymethod.as_field" class="fn">as_field</a>(&amp;self, metadata: &amp;<a class="struct" href="../struct.Metadata.html" title="struct tracing::Metadata">Metadata</a>&lt;'_&gt;) -&gt; <a class="enum" href="https://doc.rust-lang.org/1.78.0/core/option/enum.Option.html" title="enum core::option::Option">Option</a>&lt;<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>&gt;</h4></section></summary><div class="docblock"><p>Attempts to convert <code>&amp;self</code> into a <code>Field</code> with the specified <code>metadata</code>.</p>
<p>If <code>metadata</code> defines this field, then the field is returned. Otherwise,
this returns <code>None</code>.</p>
</div></details></div><h2 id="foreign-impls" class="section-header">Implementations on Foreign Types<a href="#foreign-impls" class="anchor">§</a></h2><details class="toggle implementors-toggle"><summary><section id="impl-AsField-for-str" class="impl"><a class="src rightside" href="../../src/tracing/field.rs.html#161-166">source</a><a href="#impl-AsField-for-str" class="anchor">§</a><h3 class="code-header">impl <a class="trait" href="trait.AsField.html" title="trait tracing::field::AsField">AsField</a> for <a class="primitive" href="https://doc.rust-lang.org/1.78.0/std/primitive.str.html">str</a></h3></section></summary><div class="impl-items"><section id="method.as_field" class="method trait-impl"><a class="src rightside" href="../../src/tracing/field.rs.html#163-165">source</a><a href="#method.as_field" class="anchor">§</a><h4 class="code-header">fn <a href="#tymethod.as_field" class="fn">as_field</a>(&amp;self, metadata: &amp;<a class="struct" href="../struct.Metadata.html" title="struct tracing::Metadata">Metadata</a>&lt;'_&gt;) -&gt; <a class="enum" href="https://doc.rust-lang.org/1.78.0/core/option/enum.Option.html" title="enum core::option::Option">Option</a>&lt;<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>&gt;</h4></section></div></details><h2 id="implementors" class="section-header">Implementors<a href="#implementors" class="anchor">§</a></h2><div id="implementors-list"><section id="impl-AsField-for-Field" class="impl"><a class="src rightside" href="../../src/tracing/field.rs.html#139-148">source</a><a href="#impl-AsField-for-Field" class="anchor">§</a><h3 class="code-header">impl <a class="trait" href="trait.AsField.html" title="trait tracing::field::AsField">AsField</a> for <a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a></h3></section><section id="impl-AsField-for-%26Field" class="impl"><a class="src rightside" href="../../src/tracing/field.rs.html#150-159">source</a><a href="#impl-AsField-for-%26Field" class="anchor">§</a><h3 class="code-header">impl&lt;'a&gt; <a class="trait" href="trait.AsField.html" title="trait tracing::field::AsField">AsField</a> for &amp;'a <a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a></h3></section></div><script src="../../trait.impl/tracing/field/trait.AsField.js" data-ignore-extern-crates="tracing_core,std" async></script></section></div></main></body></html>