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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="Visits typed values."><title>Visit in tracing::field - 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" 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 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">☰</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="#">Visit</a></h2><div class="sidebar-elems"><section><h3><a href="#required-methods">Required Methods</a></h3><ul class="block"><li><a href="#tymethod.record_debug">record_debug</a></li></ul><h3><a href="#provided-methods">Provided Methods</a></h3><ul class="block"><li><a href="#method.record_bool">record_bool</a></li><li><a href="#method.record_error">record_error</a></li><li><a href="#method.record_f64">record_f64</a></li><li><a href="#method.record_i128">record_i128</a></li><li><a href="#method.record_i64">record_i64</a></li><li><a href="#method.record_str">record_str</a></li><li><a href="#method.record_u128">record_u128</a></li><li><a href="#method.record_u64">record_u64</a></li></ul><h3><a href="#foreign-impls">Implementations on Foreign Types</a></h3><ul class="block"><li><a href="#impl-Visit-for-DebugMap%3C'a,+'b%3E">DebugMap<'a, 'b></a></li><li><a href="#impl-Visit-for-DebugStruct%3C'a,+'b%3E">DebugStruct<'a, 'b></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>
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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/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="#">Visit</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_core/field.rs.html#266">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</span>]</button></span></div><pre class="rust item-decl"><code>pub trait Visit {
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// Required method
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fn <a href="#tymethod.record_debug" class="fn">record_debug</a>(&mut self, field: &<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>, value: &dyn <a class="trait" href="https://doc.rust-lang.org/1.76.0/core/fmt/trait.Debug.html" title="trait core::fmt::Debug">Debug</a>);
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// Provided methods
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fn <a href="#method.record_f64" class="fn">record_f64</a>(&mut self, field: &<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>, value: <a class="primitive" href="https://doc.rust-lang.org/1.76.0/std/primitive.f64.html">f64</a>) { ... }
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<span class="item-spacer"></span> fn <a href="#method.record_i64" class="fn">record_i64</a>(&mut self, field: &<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>, value: <a class="primitive" href="https://doc.rust-lang.org/1.76.0/std/primitive.i64.html">i64</a>) { ... }
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<span class="item-spacer"></span> fn <a href="#method.record_u64" class="fn">record_u64</a>(&mut self, field: &<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>, value: <a class="primitive" href="https://doc.rust-lang.org/1.76.0/std/primitive.u64.html">u64</a>) { ... }
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<span class="item-spacer"></span> fn <a href="#method.record_i128" class="fn">record_i128</a>(&mut self, field: &<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>, value: <a class="primitive" href="https://doc.rust-lang.org/1.76.0/std/primitive.i128.html">i128</a>) { ... }
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<span class="item-spacer"></span> fn <a href="#method.record_u128" class="fn">record_u128</a>(&mut self, field: &<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>, value: <a class="primitive" href="https://doc.rust-lang.org/1.76.0/std/primitive.u128.html">u128</a>) { ... }
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<span class="item-spacer"></span> fn <a href="#method.record_bool" class="fn">record_bool</a>(&mut self, field: &<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>, value: <a class="primitive" href="https://doc.rust-lang.org/1.76.0/std/primitive.bool.html">bool</a>) { ... }
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<span class="item-spacer"></span> fn <a href="#method.record_str" class="fn">record_str</a>(&mut self, field: &<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>, value: &<a class="primitive" href="https://doc.rust-lang.org/1.76.0/std/primitive.str.html">str</a>) { ... }
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<span class="item-spacer"></span> fn <a href="#method.record_error" class="fn">record_error</a>(&mut self, field: &<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>, value: &(dyn <a class="trait" href="https://doc.rust-lang.org/1.76.0/core/error/trait.Error.html" title="trait core::error::Error">Error</a> + 'static)) { ... }
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}</code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Visits typed values.</p>
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<p>An instance of <code>Visit</code> (“a visitor”) represents the logic necessary to
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record field values of various types. When an implementor of <a href="../trait.Value.html" title="trait tracing::Value"><code>Value</code></a> is
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<a href="../trait.Value.html#tymethod.record" title="method tracing::Value::record">recorded</a>, it calls the appropriate method on the provided visitor to
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indicate the type that value should be recorded as.</p>
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<p>When a <a href="../trait.Subscriber.html" title="trait tracing::Subscriber"><code>Subscriber</code></a> implementation <a href="../trait.Subscriber.html#tymethod.event" title="method tracing::Subscriber::event">records an <code>Event</code></a> or a
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<a href="../trait.Subscriber.html#tymethod.record" title="method tracing::Subscriber::record">set of <code>Value</code>s added to a <code>Span</code></a>, it can pass an <code>&mut Visit</code> to the
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<code>record</code> method on the provided <a href="struct.ValueSet.html" title="struct tracing::field::ValueSet"><code>ValueSet</code></a> or <a href="../event/struct.Event.html" title="struct tracing::event::Event"><code>Event</code></a>. This visitor
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will then be used to record all the field-value pairs present on that
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<code>Event</code> or <code>ValueSet</code>.</p>
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<h2 id="examples"><a href="#examples">Examples</a></h2>
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<p>A simple visitor that writes to a string might be implemented like so:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::fmt::{<span class="self">self</span>, Write};
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<span class="kw">use </span>tracing::field::{Value, Visit, Field};
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<span class="kw">pub struct </span>StringVisitor<<span class="lifetime">'a</span>> {
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string: <span class="kw-2">&</span><span class="lifetime">'a </span><span class="kw-2">mut </span>String,
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}
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<span class="kw">impl</span><<span class="lifetime">'a</span>> Visit <span class="kw">for </span>StringVisitor<<span class="lifetime">'a</span>> {
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<span class="kw">fn </span>record_debug(<span class="kw-2">&mut </span><span class="self">self</span>, field: <span class="kw-2">&</span>Field, value: <span class="kw-2">&</span><span class="kw">dyn </span>fmt::Debug) {
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<span class="macro">write!</span>(<span class="self">self</span>.string, <span class="string">"{} = {:?}; "</span>, field.name(), value).unwrap();
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}
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}</code></pre></div>
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<p>This visitor will format each recorded value using <code>fmt::Debug</code>, and
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append the field name and formatted value to the provided string,
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regardless of the type of the recorded value. When all the values have
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been recorded, the <code>StringVisitor</code> may be dropped, allowing the string
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to be printed or stored in some other data structure.</p>
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<p>The <code>Visit</code> trait provides default implementations for <code>record_i64</code>,
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<code>record_u64</code>, <code>record_bool</code>, <code>record_str</code>, and <code>record_error</code>, which simply
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forward the recorded value to <code>record_debug</code>. Thus, <code>record_debug</code> is the
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only method which a <code>Visit</code> implementation <em>must</em> implement. However,
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visitors may override the default implementations of these functions in
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order to implement type-specific behavior.</p>
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<p>Additionally, when a visitor receives a value of a type it does not care
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about, it is free to ignore those values completely. For example, a
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visitor which only records numeric data might look like this:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">pub struct </span>SumVisitor {
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sum: i64,
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}
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<span class="kw">impl </span>Visit <span class="kw">for </span>SumVisitor {
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<span class="kw">fn </span>record_i64(<span class="kw-2">&mut </span><span class="self">self</span>, _field: <span class="kw-2">&</span>Field, value: i64) {
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<span class="self">self</span>.sum += value;
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}
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<span class="kw">fn </span>record_u64(<span class="kw-2">&mut </span><span class="self">self</span>, _field: <span class="kw-2">&</span>Field, value: u64) {
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<span class="self">self</span>.sum += value <span class="kw">as </span>i64;
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}
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<span class="kw">fn </span>record_debug(<span class="kw-2">&mut </span><span class="self">self</span>, _field: <span class="kw-2">&</span>Field, _value: <span class="kw-2">&</span>fmt::Debug) {
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<span class="comment">// Do nothing
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</span>}
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}</code></pre></div>
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<p>This visitor (which is probably not particularly useful) keeps a running
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sum of all the numeric values it records, and ignores all other values. A
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more practical example of recording typed values is presented in
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<code>examples/counters.rs</code>, which demonstrates a very simple metrics system
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implemented using <code>tracing</code>.</p>
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<div class="example-wrap" style="display:inline-block">
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<pre class="ignore" style="white-space:normal;font:inherit;">
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<strong>Note</strong>: The <code>record_error</code> trait method is only
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available when the Rust standard library is present, as it requires the
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<code>std::error::Error</code> trait.
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</pre></div>
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</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.record_debug" class="method"><a class="src rightside" href="../../src/tracing_core/field.rs.html#326">source</a><h4 class="code-header">fn <a href="#tymethod.record_debug" class="fn">record_debug</a>(&mut self, field: &<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>, value: &dyn <a class="trait" href="https://doc.rust-lang.org/1.76.0/core/fmt/trait.Debug.html" title="trait core::fmt::Debug">Debug</a>)</h4></section></summary><div class="docblock"><p>Visit a value implementing <code>fmt::Debug</code>.</p>
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</div></details></div><h2 id="provided-methods" class="section-header">Provided Methods<a href="#provided-methods" class="anchor">§</a></h2><div class="methods"><details class="toggle method-toggle" open><summary><section id="method.record_f64" class="method"><a class="src rightside" href="../../src/tracing_core/field.rs.html#277">source</a><h4 class="code-header">fn <a href="#method.record_f64" class="fn">record_f64</a>(&mut self, field: &<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>, value: <a class="primitive" href="https://doc.rust-lang.org/1.76.0/std/primitive.f64.html">f64</a>)</h4></section></summary><div class="docblock"><p>Visit a double-precision floating point value.</p>
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</div></details><details class="toggle method-toggle" open><summary><section id="method.record_i64" class="method"><a class="src rightside" href="../../src/tracing_core/field.rs.html#282">source</a><h4 class="code-header">fn <a href="#method.record_i64" class="fn">record_i64</a>(&mut self, field: &<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>, value: <a class="primitive" href="https://doc.rust-lang.org/1.76.0/std/primitive.i64.html">i64</a>)</h4></section></summary><div class="docblock"><p>Visit a signed 64-bit integer value.</p>
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</div></details><details class="toggle method-toggle" open><summary><section id="method.record_u64" class="method"><a class="src rightside" href="../../src/tracing_core/field.rs.html#287">source</a><h4 class="code-header">fn <a href="#method.record_u64" class="fn">record_u64</a>(&mut self, field: &<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>, value: <a class="primitive" href="https://doc.rust-lang.org/1.76.0/std/primitive.u64.html">u64</a>)</h4></section></summary><div class="docblock"><p>Visit an unsigned 64-bit integer value.</p>
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</div></details><details class="toggle method-toggle" open><summary><section id="method.record_i128" class="method"><a class="src rightside" href="../../src/tracing_core/field.rs.html#292">source</a><h4 class="code-header">fn <a href="#method.record_i128" class="fn">record_i128</a>(&mut self, field: &<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>, value: <a class="primitive" href="https://doc.rust-lang.org/1.76.0/std/primitive.i128.html">i128</a>)</h4></section></summary><div class="docblock"><p>Visit a signed 128-bit integer value.</p>
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</div></details><details class="toggle method-toggle" open><summary><section id="method.record_u128" class="method"><a class="src rightside" href="../../src/tracing_core/field.rs.html#297">source</a><h4 class="code-header">fn <a href="#method.record_u128" class="fn">record_u128</a>(&mut self, field: &<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>, value: <a class="primitive" href="https://doc.rust-lang.org/1.76.0/std/primitive.u128.html">u128</a>)</h4></section></summary><div class="docblock"><p>Visit an unsigned 128-bit integer value.</p>
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</div></details><details class="toggle method-toggle" open><summary><section id="method.record_bool" class="method"><a class="src rightside" href="../../src/tracing_core/field.rs.html#302">source</a><h4 class="code-header">fn <a href="#method.record_bool" class="fn">record_bool</a>(&mut self, field: &<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>, value: <a class="primitive" href="https://doc.rust-lang.org/1.76.0/std/primitive.bool.html">bool</a>)</h4></section></summary><div class="docblock"><p>Visit a boolean value.</p>
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</div></details><details class="toggle method-toggle" open><summary><section id="method.record_str" class="method"><a class="src rightside" href="../../src/tracing_core/field.rs.html#307">source</a><h4 class="code-header">fn <a href="#method.record_str" class="fn">record_str</a>(&mut self, field: &<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>, value: &<a class="primitive" href="https://doc.rust-lang.org/1.76.0/std/primitive.str.html">str</a>)</h4></section></summary><div class="docblock"><p>Visit a string value.</p>
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</div></details><details class="toggle method-toggle" open><summary><section id="method.record_error" class="method"><a class="src rightside" href="../../src/tracing_core/field.rs.html#321">source</a><h4 class="code-header">fn <a href="#method.record_error" class="fn">record_error</a>(&mut self, field: &<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>, value: &(dyn <a class="trait" href="https://doc.rust-lang.org/1.76.0/core/error/trait.Error.html" title="trait core::error::Error">Error</a> + 'static))</h4></section></summary><div class="docblock"><p>Records a type implementing <code>Error</code>.</p>
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<div class="example-wrap" style="display:inline-block">
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<pre class="ignore" style="white-space:normal;font:inherit;">
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<strong>Note</strong>: This is only enabled when the Rust standard library is
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present.
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</pre>
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</div></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-Visit-for-DebugMap%3C'a,+'b%3E" class="impl"><a class="src rightside" href="../../src/tracing_core/field.rs.html#391">source</a><a href="#impl-Visit-for-DebugMap%3C'a,+'b%3E" class="anchor">§</a><h3 class="code-header">impl<'a, 'b> <a class="trait" href="trait.Visit.html" title="trait tracing::field::Visit">Visit</a> for <a class="struct" href="https://doc.rust-lang.org/1.76.0/core/fmt/builders/struct.DebugMap.html" title="struct core::fmt::builders::DebugMap">DebugMap</a><'a, 'b></h3></section></summary><div class="impl-items"><section id="method.record_debug" class="method trait-impl"><a class="src rightside" href="../../src/tracing_core/field.rs.html#392">source</a><a href="#method.record_debug" class="anchor">§</a><h4 class="code-header">fn <a href="#tymethod.record_debug" class="fn">record_debug</a>(&mut self, field: &<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>, value: &dyn <a class="trait" href="https://doc.rust-lang.org/1.76.0/core/fmt/trait.Debug.html" title="trait core::fmt::Debug">Debug</a>)</h4></section></div></details><details class="toggle implementors-toggle"><summary><section id="impl-Visit-for-DebugStruct%3C'a,+'b%3E" class="impl"><a class="src rightside" href="../../src/tracing_core/field.rs.html#385">source</a><a href="#impl-Visit-for-DebugStruct%3C'a,+'b%3E" class="anchor">§</a><h3 class="code-header">impl<'a, 'b> <a class="trait" href="trait.Visit.html" title="trait tracing::field::Visit">Visit</a> for <a class="struct" href="https://doc.rust-lang.org/1.76.0/core/fmt/builders/struct.DebugStruct.html" title="struct core::fmt::builders::DebugStruct">DebugStruct</a><'a, 'b></h3></section></summary><div class="impl-items"><section id="method.record_debug-1" class="method trait-impl"><a class="src rightside" href="../../src/tracing_core/field.rs.html#386">source</a><a href="#method.record_debug-1" class="anchor">§</a><h4 class="code-header">fn <a href="#tymethod.record_debug" class="fn">record_debug</a>(&mut self, field: &<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>, value: &dyn <a class="trait" href="https://doc.rust-lang.org/1.76.0/core/fmt/trait.Debug.html" title="trait core::fmt::Debug">Debug</a>)</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-Visit-for-F" class="impl"><a class="src rightside" href="../../src/tracing_core/field.rs.html#397-399">source</a><a href="#impl-Visit-for-F" class="anchor">§</a><h3 class="code-header">impl<F> <a class="trait" href="trait.Visit.html" title="trait tracing::field::Visit">Visit</a> for F<div class="where">where
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F: <a class="trait" href="https://doc.rust-lang.org/1.76.0/core/ops/function/trait.FnMut.html" title="trait core::ops::function::FnMut">FnMut</a>(&<a class="struct" href="struct.Field.html" title="struct tracing::field::Field">Field</a>, &dyn <a class="trait" href="https://doc.rust-lang.org/1.76.0/core/fmt/trait.Debug.html" title="trait core::fmt::Debug">Debug</a>),</div></h3></section></div><script src="../../trait.impl/tracing_core/field/trait.Visit.js" data-ignore-extern-crates="core" async></script></section></div></main></body></html> |