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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="API documentation for the Rust `Visit` trait in crate `tracing`."><meta name="keywords" content="rust, rustlang, rust-lang, Visit"><title>tracing::field::Visit - Rust</title><link rel="stylesheet" type="text/css" href="../../normalize.css"><link rel="stylesheet" type="text/css" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../../light.css" id="themeStyle"><link rel="stylesheet" type="text/css" href="../../dark.css" disabled ><link rel="stylesheet" type="text/css" href="../../ayu.css" disabled ><script id="default-settings"></script><script src="../../storage.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="icon" type="image/svg+xml" href="../../favicon.svg">
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<link rel="alternate icon" type="image/png" href="../../favicon-32x32.png"><style type="text/css">#crate-search{background-image:url("../../down-arrow.svg");}</style></head><body class="rustdoc trait"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a 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><p class="location">Trait Visit</p><div class="sidebar-elems"><div class="block items"><a class="sidebar-title" href="#required-methods">Required Methods</a><div class="sidebar-links"><a href="#tymethod.record_debug">record_debug</a></div><a class="sidebar-title" href="#provided-methods">Provided Methods</a><div class="sidebar-links"><a href="#method.record_bool">record_bool</a><a href="#method.record_error">record_error</a><a href="#method.record_i64">record_i64</a><a href="#method.record_str">record_str</a><a href="#method.record_u64">record_u64</a></div><a class="sidebar-title" href="#foreign-impls">Implementations on Foreign Types</a><div class="sidebar-links"><a href="#impl-Visit-for-DebugMap%3C%27a%2C%20%27b%3E">DebugMap<'a, 'b></a><a href="#impl-Visit-for-DebugStruct%3C%27a%2C%20%27b%3E">DebugStruct<'a, 'b></a></div><a class="sidebar-title" href="#implementors">Implementors</a></div><p class="location"><a href="../index.html">tracing</a>::<wbr><a href="index.html">field</a></p><script>window.sidebarCurrent = {name: "Visit", ty: "trait", relpath: ""};</script><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu"><img src="../../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices" role="menu"></div></div><script src="../../theme.js"></script><nav class="sub"><form class="search-form"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" disabled autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><button type="button" class="help-button">?</button>
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<a id="settings-menu" href="../../settings.html"><img src="../../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class="fqn"><span class="out-of-band"><span id="render-detail"><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span><a class="srclink" href="../../src/tracing_core/field.rs.html#189-228" title="goto source code">[src]</a></span><span class="in-band">Trait <a href="../index.html">tracing</a>::<wbr><a href="index.html">field</a>::<wbr><a class="trait" href="">Visit</a></span></h1><div class="docblock type-decl hidden-by-usual-hider"><pre class="rust trait">pub trait Visit {
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pub fn <a href="#tymethod.record_debug" class="fnname">record_debug</a>(&mut self, field: &<a class="struct" href="../../tracing/field/struct.Field.html" title="struct tracing::field::Field">Field</a>, value: &dyn <a class="trait" href="https://doc.rust-lang.org/nightly/core/fmt/trait.Debug.html" title="trait core::fmt::Debug">Debug</a>);
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pub fn <a href="#method.record_i64" class="fnname">record_i64</a>(&mut self, field: &<a class="struct" href="../../tracing/field/struct.Field.html" title="struct tracing::field::Field">Field</a>, value: <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.i64.html">i64</a>) { ... }
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<div class="item-spacer"></div> pub fn <a href="#method.record_u64" class="fnname">record_u64</a>(&mut self, field: &<a class="struct" href="../../tracing/field/struct.Field.html" title="struct tracing::field::Field">Field</a>, value: <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.u64.html">u64</a>) { ... }
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<div class="item-spacer"></div> pub fn <a href="#method.record_bool" class="fnname">record_bool</a>(&mut self, field: &<a class="struct" href="../../tracing/field/struct.Field.html" title="struct tracing::field::Field">Field</a>, value: <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.bool.html">bool</a>) { ... }
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<div class="item-spacer"></div> pub fn <a href="#method.record_str" class="fnname">record_str</a>(&mut self, field: &<a class="struct" href="../../tracing/field/struct.Field.html" title="struct tracing::field::Field">Field</a>, value: &<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.str.html">str</a>) { ... }
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<div class="item-spacer"></div> pub fn <a href="#method.record_error" class="fnname">record_error</a>(&mut self, field: &<a class="struct" href="../../tracing/field/struct.Field.html" title="struct tracing::field::Field">Field</a>, value: &(dyn <a class="trait" href="https://doc.rust-lang.org/nightly/std/error/trait.Error.html" title="trait std::error::Error">Error</a> + 'static)) { ... }
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}</pre></div><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"><code>Value</code></a> is
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<a href="trait.Value.html#method.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="../subscriber/trait.Subscriber.html"><code>Subscriber</code></a> implementation <a href="../subscriber/trait.Subscriber.html#method.event">records an <code>Event</code></a> or a
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<a href="../subscriber/trait.Subscriber.html#method.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"><code>ValueSet</code></a> or <a href="../event/struct.Event.html"><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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<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
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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">
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<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">fmt</span>::{<span class="self">self</span>, <span class="ident">Write</span>};
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<span class="kw">use</span> <span class="ident">tracing</span>::<span class="ident">field</span>::{<span class="ident">Value</span>, <span class="ident">Visit</span>, <span class="ident">Field</span>};
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<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">StringVisitor</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">></span> {
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<span class="ident">string</span>: <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="kw-2">mut</span> <span class="ident">String</span>,
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}
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<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">></span> <span class="ident">Visit</span> <span class="kw">for</span> <span class="ident">StringVisitor</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">></span> {
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<span class="kw">fn</span> <span class="ident">record_debug</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>, <span class="ident">field</span>: <span class="kw-2">&</span><span class="ident">Field</span>, <span class="ident">value</span>: <span class="kw-2">&</span><span class="ident">fmt</span>::<span class="ident">Debug</span>) {
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<span class="macro">write</span><span class="macro">!</span>(<span class="self">self</span>.<span class="ident">string</span>, <span class="string">"{} = {:?}; "</span>, <span class="ident">field</span>.<span class="ident">name</span>(), <span class="ident">value</span>).<span class="ident">unwrap</span>();
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}
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}</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">
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<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">SumVisitor</span> {
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<span class="ident">sum</span>: <span class="ident">i64</span>,
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}
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<span class="kw">impl</span> <span class="ident">Visit</span> <span class="kw">for</span> <span class="ident">SumVisitor</span> {
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<span class="kw">fn</span> <span class="ident">record_i64</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>, <span class="ident">_field</span>: <span class="kw-2">&</span><span class="ident">Field</span>, <span class="ident">value</span>: <span class="ident">i64</span>) {
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<span class="self">self</span>.<span class="ident">sum</span> <span class="op">+</span><span class="op">=</span> <span class="ident">value</span>;
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}
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<span class="kw">fn</span> <span class="ident">record_u64</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>, <span class="ident">_field</span>: <span class="kw-2">&</span><span class="ident">Field</span>, <span class="ident">value</span>: <span class="ident">u64</span>) {
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<span class="self">self</span>.<span class="ident">sum</span> <span class="op">+</span><span class="op">=</span> <span class="ident">value</span> <span class="kw">as</span> <span class="ident">i64</span>;
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}
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<span class="kw">fn</span> <span class="ident">record_debug</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>, <span class="ident">_field</span>: <span class="kw-2">&</span><span class="ident">Field</span>, <span class="ident">_value</span>: <span class="kw-2">&</span><span class="ident">fmt</span>::<span class="ident">Debug</span>) {
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<span class="comment">// Do nothing</span>
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}
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}</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="information">
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<div class="tooltip ignore" style="">ⓘ<span class="tooltiptext">Note</span></div>
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</div>
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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><h2 id="required-methods" class="small-section-header">Required methods<a href="#required-methods" class="anchor"></a></h2><div class="methods"><h3 id="tymethod.record_debug" class="method"><code>pub fn <a href="#tymethod.record_debug" class="fnname">record_debug</a>(&mut self, field: &<a class="struct" href="../../tracing/field/struct.Field.html" title="struct tracing::field::Field">Field</a>, value: &dyn <a class="trait" href="https://doc.rust-lang.org/nightly/core/fmt/trait.Debug.html" title="trait core::fmt::Debug">Debug</a>)</code><a class="srclink" href="../../src/tracing_core/field.rs.html#227" title="goto source code">[src]</a></h3><div class="docblock"><p>Visit a value implementing <code>fmt::Debug</code>.</p>
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</div></div><span class="loading-content">Loading content...</span><h2 id="provided-methods" class="small-section-header">Provided methods<a href="#provided-methods" class="anchor"></a></h2><div class="methods"><h3 id="method.record_i64" class="method"><code>pub fn <a href="#method.record_i64" class="fnname">record_i64</a>(&mut self, field: &<a class="struct" href="../../tracing/field/struct.Field.html" title="struct tracing::field::Field">Field</a>, value: <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.i64.html">i64</a>)</code><a class="srclink" href="../../src/tracing_core/field.rs.html#191-193" title="goto source code">[src]</a></h3><div class="docblock"><p>Visit a signed 64-bit integer value.</p>
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</div><h3 id="method.record_u64" class="method"><code>pub fn <a href="#method.record_u64" class="fnname">record_u64</a>(&mut self, field: &<a class="struct" href="../../tracing/field/struct.Field.html" title="struct tracing::field::Field">Field</a>, value: <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.u64.html">u64</a>)</code><a class="srclink" href="../../src/tracing_core/field.rs.html#196-198" title="goto source code">[src]</a></h3><div class="docblock"><p>Visit an unsigned 64-bit integer value.</p>
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</div><h3 id="method.record_bool" class="method"><code>pub fn <a href="#method.record_bool" class="fnname">record_bool</a>(&mut self, field: &<a class="struct" href="../../tracing/field/struct.Field.html" title="struct tracing::field::Field">Field</a>, value: <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.bool.html">bool</a>)</code><a class="srclink" href="../../src/tracing_core/field.rs.html#201-203" title="goto source code">[src]</a></h3><div class="docblock"><p>Visit a boolean value.</p>
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</div><h3 id="method.record_str" class="method"><code>pub fn <a href="#method.record_str" class="fnname">record_str</a>(&mut self, field: &<a class="struct" href="../../tracing/field/struct.Field.html" title="struct tracing::field::Field">Field</a>, value: &<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.str.html">str</a>)</code><a class="srclink" href="../../src/tracing_core/field.rs.html#206-208" title="goto source code">[src]</a></h3><div class="docblock"><p>Visit a string value.</p>
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</div><h3 id="method.record_error" class="method"><code>pub fn <a href="#method.record_error" class="fnname">record_error</a>(&mut self, field: &<a class="struct" href="../../tracing/field/struct.Field.html" title="struct tracing::field::Field">Field</a>, value: &(dyn <a class="trait" href="https://doc.rust-lang.org/nightly/std/error/trait.Error.html" title="trait std::error::Error">Error</a> + 'static))</code><a class="srclink" href="../../src/tracing_core/field.rs.html#222-224" title="goto source code">[src]</a></h3><div class="docblock"><p>Records a type implementing <code>Error</code>.</p>
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<div class="information">
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<div class="tooltip ignore" style="">ⓘ<span class="tooltiptext">Note</span></div>
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</div>
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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></div></div><span class="loading-content">Loading content...</span><h2 id="foreign-impls" class="small-section-header">Implementations on Foreign Types<a href="#foreign-impls" class="anchor"></a></h2><h3 id="impl-Visit-for-DebugStruct%3C%27a%2C%20%27b%3E" class="impl"><code class="in-band">impl<'a, 'b> <a class="trait" href="../../tracing/field/trait.Visit.html" title="trait tracing::field::Visit">Visit</a> for <a class="struct" href="https://doc.rust-lang.org/nightly/core/fmt/builders/struct.DebugStruct.html" title="struct core::fmt::builders::DebugStruct">DebugStruct</a><'a, 'b></code><a href="#impl-Visit-for-DebugStruct%3C%27a%2C%20%27b%3E" class="anchor"></a><a class="srclink" href="../../src/tracing_core/field.rs.html#273-277" title="goto source code">[src]</a></h3><div class="impl-items"><h4 id="method.record_debug" class="method hidden"><code>pub fn <a href="#method.record_debug" class="fnname">record_debug</a>(&mut self, field: &<a class="struct" href="../../tracing/field/struct.Field.html" title="struct tracing::field::Field">Field</a>, value: &dyn <a class="trait" href="https://doc.rust-lang.org/nightly/core/fmt/trait.Debug.html" title="trait core::fmt::Debug">Debug</a>)</code><a class="srclink" href="../../src/tracing_core/field.rs.html#274-276" title="goto source code">[src]</a></h4></div><h3 id="impl-Visit-for-DebugMap%3C%27a%2C%20%27b%3E" class="impl"><code class="in-band">impl<'a, 'b> <a class="trait" href="../../tracing/field/trait.Visit.html" title="trait tracing::field::Visit">Visit</a> for <a class="struct" href="https://doc.rust-lang.org/nightly/core/fmt/builders/struct.DebugMap.html" title="struct core::fmt::builders::DebugMap">DebugMap</a><'a, 'b></code><a href="#impl-Visit-for-DebugMap%3C%27a%2C%20%27b%3E" class="anchor"></a><a class="srclink" href="../../src/tracing_core/field.rs.html#279-283" title="goto source code">[src]</a></h3><div class="impl-items"><h4 id="method.record_debug-1" class="method hidden"><code>pub fn <a href="#method.record_debug" class="fnname">record_debug</a>(&mut self, field: &<a class="struct" href="../../tracing/field/struct.Field.html" title="struct tracing::field::Field">Field</a>, value: &dyn <a class="trait" href="https://doc.rust-lang.org/nightly/core/fmt/trait.Debug.html" title="trait core::fmt::Debug">Debug</a>)</code><a class="srclink" href="../../src/tracing_core/field.rs.html#280-282" title="goto source code">[src]</a></h4></div><span class="loading-content">Loading content...</span><h2 id="implementors" class="small-section-header">Implementors<a href="#implementors" class="anchor"></a></h2><div class="item-list" id="implementors-list"><h3 id="impl-Visit" class="impl"><code class="in-band">impl<F> Visit for F <span class="where fmt-newline">where<br> F: <a class="trait" href="https://doc.rust-lang.org/nightly/core/ops/function/trait.FnMut.html" title="trait core::ops::function::FnMut">FnMut</a>(&<a class="struct" href="../../tracing/field/struct.Field.html" title="struct tracing::field::Field">Field</a>, &dyn <a class="trait" href="https://doc.rust-lang.org/nightly/core/fmt/trait.Debug.html" title="trait core::fmt::Debug">Debug</a>), </span></code><a href="#impl-Visit" class="anchor"></a><a class="srclink" href="../../src/tracing_core/field.rs.html#285-292" title="goto source code">[src]</a></h3><div class="impl-items"><h4 id="method.record_debug-2" class="method hidden"><code>pub fn <a href="#method.record_debug-2" class="fnname">record_debug</a>(&mut self, field: &<a class="struct" href="../../tracing/field/struct.Field.html" title="struct tracing::field::Field">Field</a>, value: &dyn <a class="trait" href="https://doc.rust-lang.org/nightly/core/fmt/trait.Debug.html" title="trait core::fmt::Debug">Debug</a>)</code><a class="srclink" href="../../src/tracing_core/field.rs.html#289-291" title="goto source code">[src]</a></h4></div></div><span class="loading-content">Loading content...</span><script type="text/javascript" src="../../implementors/tracing_core/field/trait.Visit.js" async></script></section><section id="search" class="content hidden"></section><section class="footer"></section><script>window.rootPath = "../../";window.currentCrate = "tracing";</script><script src="../../main.js"></script><script defer src="../../search-index.js"></script></body></html> |