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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="Abstractions for creating `io::Write` instances."><title>tracing_subscriber::fmt::writer - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-6b053e98.ttf.woff2,FiraSans-Regular-0fe48ade.woff2,FiraSans-Medium-e1aa3f0a.woff2,SourceCodePro-Regular-8badfe75.ttf.woff2,SourceCodePro-Semibold-aa29a496.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-9960930a.css"><link rel="stylesheet" href="../../../static.files/rustdoc-42caa33d.css"><meta name="rustdoc-vars" data-root-path="../../../" data-static-root-path="../../../static.files/" data-current-crate="tracing_subscriber" data-themes="" data-resource-suffix="" data-rustdoc-version="1.84.1 (e71f9a9a9 2025-01-27)" data-channel="1.84.1" data-search-js="search-92e6798f.js" data-settings-js="settings-0f613d39.js" ><script src="../../../static.files/storage-59e33391.js"></script><script defer src="../sidebar-items.js"></script><script defer src="../../../static.files/main-5f194d8c.js"></script><noscript><link rel="stylesheet" href="../../../static.files/noscript-893ab5e7.css"></noscript><link rel="alternate icon" type="image/png" href="../../../static.files/favicon-32x32-6580c154.png"><link rel="icon" type="image/svg+xml" href="../../../static.files/favicon-044be391.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" title="show sidebar"></button><a class="logo-container" href="../../../tracing_subscriber/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_subscriber/index.html"><img src="https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png" alt="logo"></a><h2><a href="../../../tracing_subscriber/index.html">tracing_<wbr>subscriber</a><span class="version">0.3.19</span></h2></div><div class="sidebar-elems"><section id="rustdoc-toc"><h2 class="location"><a href="#">Module writer</a></h2><h3><a href="#structs">Module Items</a></h3><ul class="block"><li><a href="#structs" title="Structs">Structs</a></li><li><a href="#enums" title="Enums">Enums</a></li><li><a href="#traits" title="Traits">Traits</a></li><li><a href="#types" title="Type Aliases">Type Aliases</a></li></ul></section><div id="rustdoc-modnav"><h2><a href="../index.html">In tracing_<wbr>subscriber::<wbr>fmt</a></h2></div></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"><span class="rustdoc-breadcrumbs"><a href="../../index.html">tracing_subscriber</a>::<wbr><a href="../index.html">fmt</a></span><h1>Module <span>writer</span><button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><rustdoc-toolbar></rustdoc-toolbar><span class="sub-heading"><a class="src" href="../../../src/tracing_subscriber/fmt/writer.rs.html#1-1467">Source</a> </span></div><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Abstractions for creating <a href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write"><code>io::Write</code></a> instances.</p>
</div></details><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.BoxMakeWriter.html" title="struct tracing_subscriber::fmt::writer::BoxMakeWriter">BoxMake<wbr>Writer</a></div><div class="desc docblock-short">A writer that erases the specific <a href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write"><code>io::Write</code></a> and <a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a> types being used.</div></li><li><div class="item-name"><a class="struct" href="struct.MutexGuardWriter.html" title="struct tracing_subscriber::fmt::writer::MutexGuardWriter">Mutex<wbr>Guard<wbr>Writer</a></div><div class="desc docblock-short">A type implementing <a href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write"><code>io::Write</code></a> for a <a href="https://doc.rust-lang.org/1.84.1/std/sync/mutex/struct.MutexGuard.html" title="struct std::sync::mutex::MutexGuard"><code>MutexGuard</code></a> where the type
inside the <a href="https://doc.rust-lang.org/1.84.1/std/sync/mutex/struct.Mutex.html" title="struct std::sync::mutex::Mutex"><code>Mutex</code></a> implements <a href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write"><code>io::Write</code></a>.</div></li><li><div class="item-name"><a class="struct" href="struct.OrElse.html" title="struct tracing_subscriber::fmt::writer::OrElse">OrElse</a></div><div class="desc docblock-short">Combines a <a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a> that returns an <a href="type.OptionalWriter.html" title="type tracing_subscriber::fmt::writer::OptionalWriter"><code>OptionalWriter</code></a> with another
<a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a>, so that the second <a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a> is used when the first
<a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a> returns <a href="enum.EitherWriter.html#method.none" title="associated function tracing_subscriber::fmt::writer::EitherWriter::none"><code>OptionalWriter::none</code></a>.</div></li><li><div class="item-name"><a class="struct" href="struct.Tee.html" title="struct tracing_subscriber::fmt::writer::Tee">Tee</a></div><div class="desc docblock-short">Combines two types implementing <a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a> (or <a href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write"><code>std::io::Write</code></a>) to
produce a writer that writes to both <a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a>’s returned writers.</div></li><li><div class="item-name"><a class="struct" href="struct.TestWriter.html" title="struct tracing_subscriber::fmt::writer::TestWriter">Test<wbr>Writer</a></div><div class="desc docblock-short">A writer intended to support <a href="https://doc.rust-lang.org/book/ch11-02-running-tests.html#showing-function-output"><code>libtest</code>’s output capturing</a> for use in unit tests.</div></li><li><div class="item-name"><a class="struct" href="struct.WithFilter.html" title="struct tracing_subscriber::fmt::writer::WithFilter">With<wbr>Filter</a></div><div class="desc docblock-short">A <a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a> combinator that wraps a <a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a> with a predicate for
span and event <a href="../../../tracing_core/metadata/struct.Metadata.html" title="struct tracing_core::metadata::Metadata"><code>Metadata</code></a>, so that the <a href="../trait.MakeWriter.html#method.make_writer_for" title="method tracing_subscriber::fmt::MakeWriter::make_writer_for"><code>MakeWriter::make_writer_for</code></a>
method returns <a href="enum.EitherWriter.html#method.some" title="associated function tracing_subscriber::fmt::writer::EitherWriter::some"><code>OptionalWriter::some</code></a> when the predicate returns <code>true</code>,
and <a href="enum.EitherWriter.html#method.none" title="associated function tracing_subscriber::fmt::writer::EitherWriter::none"><code>OptionalWriter::none</code></a> when the predicate returns <code>false</code>.</div></li><li><div class="item-name"><a class="struct" href="struct.WithMaxLevel.html" title="struct tracing_subscriber::fmt::writer::WithMaxLevel">With<wbr>MaxLevel</a></div><div class="desc docblock-short">A <a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a> combinator that only returns an enabled <a href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write">writer</a> for spans
and events with metadata at or below a specified verbosity <a href="../../../tracing_core/metadata/struct.Level.html" title="struct tracing_core::metadata::Level"><code>Level</code></a>.</div></li><li><div class="item-name"><a class="struct" href="struct.WithMinLevel.html" title="struct tracing_subscriber::fmt::writer::WithMinLevel">With<wbr>MinLevel</a></div><div class="desc docblock-short">A <a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a> combinator that only returns an enabled <a href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write">writer</a> for spans
and events with metadata at or above a specified verbosity <a href="../../../tracing_core/metadata/struct.Level.html" title="struct tracing_core::metadata::Level"><code>Level</code></a>.</div></li></ul><h2 id="enums" class="section-header">Enums<a href="#enums" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="enum" href="enum.EitherWriter.html" title="enum tracing_subscriber::fmt::writer::EitherWriter">Either<wbr>Writer</a></div><div class="desc docblock-short">A <a href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write">writer</a> that is one of two types implementing <a href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write"><code>io::Write</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.MakeWriter.html" title="trait tracing_subscriber::fmt::writer::MakeWriter">Make<wbr>Writer</a></div><div class="desc docblock-short">A type that can create <a href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write"><code>io::Write</code></a> instances.</div></li><li><div class="item-name"><a class="trait" href="trait.MakeWriterExt.html" title="trait tracing_subscriber::fmt::writer::MakeWriterExt">Make<wbr>Writer<wbr>Ext</a></div><div class="desc docblock-short">Extension trait adding combinators for working with types implementing
<a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a>.</div></li></ul><h2 id="types" class="section-header">Type Aliases<a href="#types" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="type" href="type.OptionalWriter.html" title="type tracing_subscriber::fmt::writer::OptionalWriter">Optional<wbr>Writer</a></div><div class="desc docblock-short">A <a href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write">writer</a> which may or may not be enabled.</div></li></ul></section></div></main></body></html>

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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="A type that can create `io::Write` instances."><title>MakeWriter in tracing_subscriber::fmt::writer - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-6b053e98.ttf.woff2,FiraSans-Regular-0fe48ade.woff2,FiraSans-Medium-e1aa3f0a.woff2,SourceCodePro-Regular-8badfe75.ttf.woff2,SourceCodePro-Semibold-aa29a496.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-9960930a.css"><link rel="stylesheet" href="../../../static.files/rustdoc-42caa33d.css"><meta name="rustdoc-vars" data-root-path="../../../" data-static-root-path="../../../static.files/" data-current-crate="tracing_subscriber" data-themes="" data-resource-suffix="" data-rustdoc-version="1.84.1 (e71f9a9a9 2025-01-27)" data-channel="1.84.1" data-search-js="search-92e6798f.js" data-settings-js="settings-0f613d39.js" ><script src="../../../static.files/storage-59e33391.js"></script><script defer src="sidebar-items.js"></script><script defer src="../../../static.files/main-5f194d8c.js"></script><noscript><link rel="stylesheet" href="../../../static.files/noscript-893ab5e7.css"></noscript><link rel="alternate icon" type="image/png" href="../../../static.files/favicon-32x32-6580c154.png"><link rel="icon" type="image/svg+xml" href="../../../static.files/favicon-044be391.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_subscriber/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_subscriber/index.html"><img src="https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png" alt="logo"></a><h2><a href="../../../tracing_subscriber/index.html">tracing_<wbr>subscriber</a><span class="version">0.3.19</span></h2></div><div class="sidebar-elems"><section id="rustdoc-toc"><h2 class="location"><a href="#">Make<wbr>Writer</a></h2><h3><a href="#">Sections</a></h3><ul class="block top-toc"><li><a href="#examples" title="Examples">Examples</a></li></ul><h3><a href="#required-associated-types">Required Associated Types</a></h3><ul class="block"><li><a href="#associatedtype.Writer" title="Writer">Writer</a></li></ul><h3><a href="#required-methods">Required Methods</a></h3><ul class="block"><li><a href="#tymethod.make_writer" title="make_writer">make_writer</a></li></ul><h3><a href="#provided-methods">Provided Methods</a></h3><ul class="block"><li><a href="#method.make_writer_for" title="make_writer_for">make_writer_for</a></li></ul><h3><a href="#foreign-impls">Implementations on Foreign Types</a></h3><ul class="block"><li><a href="#impl-MakeWriter%3C'a%3E-for-Arc%3CW%3E" title="Arc&#60;W&#62;">Arc&#60;W&#62;</a></li><li><a href="#impl-MakeWriter%3C'a%3E-for-File" title="File">File</a></li><li><a href="#impl-MakeWriter%3C'a%3E-for-Mutex%3CW%3E" title="Mutex&#60;W&#62;">Mutex&#60;W&#62;</a></li></ul><h3><a href="#implementors">Implementors</a></h3></section><div id="rustdoc-modnav"><h2><a href="index.html">In tracing_<wbr>subscriber::<wbr>fmt::<wbr>writer</a></h2></div></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"><span class="rustdoc-breadcrumbs"><a href="../../index.html">tracing_subscriber</a>::<wbr><a href="../index.html">fmt</a>::<wbr><a href="index.html">writer</a></span><h1>Trait <span class="trait">MakeWriter</span><button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><rustdoc-toolbar></rustdoc-toolbar><span class="sub-heading"><a class="src" href="../../../src/tracing_subscriber/fmt/writer.rs.html#95-209">Source</a> </span></div><pre class="rust item-decl"><code>pub trait MakeWriter&lt;'a&gt; {
type <a href="#associatedtype.Writer" class="associatedtype">Writer</a>: <a class="trait" href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write">Write</a>;
// Required method
fn <a href="#tymethod.make_writer" class="fn">make_writer</a>(&amp;'a self) -&gt; Self::<a class="associatedtype" href="../trait.MakeWriter.html#associatedtype.Writer" title="type tracing_subscriber::fmt::MakeWriter::Writer">Writer</a>;
// Provided method
fn <a href="#method.make_writer_for" class="fn">make_writer_for</a>(&amp;'a self, meta: &amp;<a class="struct" href="../../../tracing_core/metadata/struct.Metadata.html" title="struct tracing_core::metadata::Metadata">Metadata</a>&lt;'_&gt;) -&gt; Self::<a class="associatedtype" href="../trait.MakeWriter.html#associatedtype.Writer" title="type tracing_subscriber::fmt::MakeWriter::Writer">Writer</a> { ... }
}</code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>A type that can create <a href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write"><code>io::Write</code></a> instances.</p>
<p><code>MakeWriter</code> is used by <a href="../struct.Layer.html" title="struct tracing_subscriber::fmt::Layer"><code>fmt::Layer</code></a> or <a href="../struct.Subscriber.html" title="struct tracing_subscriber::fmt::Subscriber"><code>fmt::Subscriber</code></a> to print
formatted text representations of <a href="../../../tracing_core/event/struct.Event.html" title="struct tracing_core::event::Event"><code>Event</code></a>s.</p>
<p>This trait is already implemented for function pointers and
immutably-borrowing closures that return an instance of <a href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write"><code>io::Write</code></a>, such
as <a href="https://doc.rust-lang.org/1.84.1/std/io/stdio/fn.stdout.html" title="fn std::io::stdio::stdout"><code>io::stdout</code></a> and <a href="https://doc.rust-lang.org/1.84.1/std/io/stdio/fn.stderr.html" title="fn std::io::stdio::stderr"><code>io::stderr</code></a>. Additionally, it is implemented for
<a href="https://doc.rust-lang.org/1.84.1/std/sync/mutex/struct.Mutex.html" title="struct std::sync::mutex::Mutex"><code>std::sync::Mutex</code></a> when the type inside the mutex implements
<a href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write"><code>io::Write</code></a>.</p>
<h2 id="examples"><a class="doc-anchor" href="#examples">§</a>Examples</h2>
<p>The simplest usage is to pass in a named function that returns a writer. For
example, to log all events to stderr, we could write:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">let </span>subscriber = tracing_subscriber::fmt()
.with_writer(std::io::stderr)
.finish();</code></pre></div>
<p>Any function that returns a writer can be used:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">fn </span>make_my_great_writer() -&gt; <span class="kw">impl </span>std::io::Write {
<span class="comment">// ...
</span>}
<span class="kw">let </span>subscriber = tracing_subscriber::fmt()
.with_writer(make_my_great_writer)
.finish();</code></pre></div>
<p>A closure can be used to introduce arbitrary logic into how the writer is
created. Consider the (admittedly rather silly) example of sending every 5th
event to stderr, and all other events to stdout:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::io;
<span class="kw">use </span>std::sync::atomic::{AtomicUsize, Ordering::Relaxed};
<span class="kw">let </span>n = AtomicUsize::new(<span class="number">0</span>);
<span class="kw">let </span>subscriber = tracing_subscriber::fmt()
.with_writer(<span class="kw">move </span>|| -&gt; Box&lt;<span class="kw">dyn </span>io::Write&gt; {
<span class="kw">if </span>n.fetch_add(<span class="number">1</span>, Relaxed) % <span class="number">5 </span>== <span class="number">0 </span>{
Box::new(io::stderr())
} <span class="kw">else </span>{
Box::new(io::stdout())
}
})
.finish();</code></pre></div>
<p>A single instance of a type implementing <a href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write"><code>io::Write</code></a> may be used as a
<code>MakeWriter</code> by wrapping it in a <a href="https://doc.rust-lang.org/1.84.1/std/sync/mutex/struct.Mutex.html" title="struct std::sync::mutex::Mutex"><code>Mutex</code></a>. For example, we could
write to a file like so:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::{fs::File, sync::Mutex};
<span class="kw">let </span>log_file = File::create(<span class="string">"my_cool_trace.log"</span>)<span class="question-mark">?</span>;
<span class="kw">let </span>subscriber = tracing_subscriber::fmt()
.with_writer(Mutex::new(log_file))
.finish();</code></pre></div>
</div></details><h2 id="required-associated-types" class="section-header">Required Associated Types<a href="#required-associated-types" class="anchor">§</a></h2><div class="methods"><details class="toggle" open><summary><section id="associatedtype.Writer" class="method"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#100">Source</a><h4 class="code-header">type <a href="#associatedtype.Writer" class="associatedtype">Writer</a>: <a class="trait" href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write">Write</a></h4></section></summary><div class="docblock"><p>The concrete <a href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write"><code>io::Write</code></a> implementation returned by <a href="../trait.MakeWriter.html#tymethod.make_writer" title="method tracing_subscriber::fmt::MakeWriter::make_writer"><code>make_writer</code></a>.</p>
</div></details></div><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.make_writer" class="method"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#115">Source</a><h4 class="code-header">fn <a href="#tymethod.make_writer" class="fn">make_writer</a>(&amp;'a self) -&gt; Self::<a class="associatedtype" href="../trait.MakeWriter.html#associatedtype.Writer" title="type tracing_subscriber::fmt::MakeWriter::Writer">Writer</a></h4></section></summary><div class="docblock"><p>Returns an instance of <a href="../trait.MakeWriter.html#associatedtype.Writer" title="associated type tracing_subscriber::fmt::MakeWriter::Writer"><code>Writer</code></a>.</p>
<h5 id="implementer-notes"><a class="doc-anchor" href="#implementer-notes">§</a>Implementer notes</h5>
<p><a href="../struct.Layer.html" title="struct tracing_subscriber::fmt::Layer"><code>fmt::Layer</code></a> or <a href="../struct.Subscriber.html" title="struct tracing_subscriber::fmt::Subscriber"><code>fmt::Subscriber</code></a> will call this method each time an event is recorded. Ensure any state
that must be saved across writes is not lost when the <a href="../trait.MakeWriter.html#associatedtype.Writer" title="associated type tracing_subscriber::fmt::MakeWriter::Writer"><code>Writer</code></a> instance is dropped. If
creating a <a href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write"><code>io::Write</code></a> instance is expensive, be sure to cache it when implementing
<a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a> to improve performance.</p>
</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.make_writer_for" class="method"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#205-208">Source</a><h4 class="code-header">fn <a href="#method.make_writer_for" class="fn">make_writer_for</a>(&amp;'a self, meta: &amp;<a class="struct" href="../../../tracing_core/metadata/struct.Metadata.html" title="struct tracing_core::metadata::Metadata">Metadata</a>&lt;'_&gt;) -&gt; Self::<a class="associatedtype" href="../trait.MakeWriter.html#associatedtype.Writer" title="type tracing_subscriber::fmt::MakeWriter::Writer">Writer</a></h4></section></summary><div class="docblock"><p>Returns a <a href="../trait.MakeWriter.html#associatedtype.Writer" title="associated type tracing_subscriber::fmt::MakeWriter::Writer"><code>Writer</code></a> for writing data from the span or event described
by the provided <a href="../../../tracing_core/metadata/struct.Metadata.html" title="struct tracing_core::metadata::Metadata"><code>Metadata</code></a>.</p>
<p>By default, this calls <a href="../trait.MakeWriter.html#tymethod.make_writer" title="method tracing_subscriber::fmt::MakeWriter::make_writer"><code>self.make_writer()</code></a>, ignoring
the provided metadata, but implementations can override this to provide
metadata-specific behaviors.</p>
<p>This method allows <code>MakeWriter</code> implementations to implement different
behaviors based on the span or event being written. The <code>MakeWriter</code>
type might return different writers based on the provided metadata, or
might write some values to the writer before or after providing it to
the caller.</p>
<p>For example, we might want to write data from spans and events at the
<a href="../../../tracing_core/metadata/struct.Level.html#associatedconstant.ERROR" title="associated constant tracing_core::metadata::Level::ERROR"><code>ERROR</code></a> and <a href="../../../tracing_core/metadata/struct.Level.html#associatedconstant.WARN" title="associated constant tracing_core::metadata::Level::WARN"><code>WARN</code></a> levels to <code>stderr</code>, and data from spans or events
at lower levels to stdout:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::io::{<span class="self">self</span>, Stdout, Stderr, StdoutLock, StderrLock};
<span class="kw">use </span>tracing_subscriber::fmt::writer::MakeWriter;
<span class="kw">use </span>tracing_core::{Metadata, Level};
<span class="kw">pub struct </span>MyMakeWriter {
stdout: Stdout,
stderr: Stderr,
}
<span class="doccomment">/// A lock on either stdout or stderr, depending on the verbosity level
/// of the event being written.
</span><span class="kw">pub enum </span>StdioLock&lt;<span class="lifetime">'a</span>&gt; {
Stdout(StdoutLock&lt;<span class="lifetime">'a</span>&gt;),
Stderr(StderrLock&lt;<span class="lifetime">'a</span>&gt;),
}
<span class="kw">impl</span>&lt;<span class="lifetime">'a</span>&gt; io::Write <span class="kw">for </span>StdioLock&lt;<span class="lifetime">'a</span>&gt; {
<span class="kw">fn </span>write(<span class="kw-2">&amp;mut </span><span class="self">self</span>, buf: <span class="kw-2">&amp;</span>[u8]) -&gt; io::Result&lt;usize&gt; {
<span class="kw">match </span><span class="self">self </span>{
StdioLock::Stdout(lock) =&gt; lock.write(buf),
StdioLock::Stderr(lock) =&gt; lock.write(buf),
}
}
<span class="kw">fn </span>write_all(<span class="kw-2">&amp;mut </span><span class="self">self</span>, buf: <span class="kw-2">&amp;</span>[u8]) -&gt; io::Result&lt;()&gt; {
<span class="comment">// ...
</span>}
<span class="kw">fn </span>flush(<span class="kw-2">&amp;mut </span><span class="self">self</span>) -&gt; io::Result&lt;()&gt; {
<span class="comment">// ...
</span>}
}
<span class="kw">impl</span>&lt;<span class="lifetime">'a</span>&gt; MakeWriter&lt;<span class="lifetime">'a</span>&gt; <span class="kw">for </span>MyMakeWriter {
<span class="kw">type </span>Writer = StdioLock&lt;<span class="lifetime">'a</span>&gt;;
<span class="kw">fn </span>make_writer(<span class="kw-2">&amp;</span><span class="lifetime">'a </span><span class="self">self</span>) -&gt; <span class="self">Self</span>::Writer {
<span class="comment">// We must have an implementation of `make_writer` that makes
// a "default" writer without any configuring metadata. Let's
// just return stdout in that case.
</span>StdioLock::Stdout(<span class="self">self</span>.stdout.lock())
}
<span class="kw">fn </span>make_writer_for(<span class="kw-2">&amp;</span><span class="lifetime">'a </span><span class="self">self</span>, meta: <span class="kw-2">&amp;</span>Metadata&lt;<span class="lifetime">'_</span>&gt;) -&gt; <span class="self">Self</span>::Writer {
<span class="comment">// Here's where we can implement our special behavior. We'll
// check if the metadata's verbosity level is WARN or ERROR,
// and return stderr in that case.
</span><span class="kw">if </span>meta.level() &lt;= <span class="kw-2">&amp;</span>Level::WARN {
<span class="kw">return </span>StdioLock::Stderr(<span class="self">self</span>.stderr.lock());
}
<span class="comment">// Otherwise, we'll return stdout.
</span>StdioLock::Stdout(<span class="self">self</span>.stdout.lock())
}
}</code></pre></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-MakeWriter%3C'a%3E-for-File" class="impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#697-702">Source</a><a href="#impl-MakeWriter%3C'a%3E-for-File" class="anchor">§</a><h3 class="code-header">impl&lt;'a&gt; <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt; for <a class="struct" href="https://doc.rust-lang.org/1.84.1/std/fs/struct.File.html" title="struct std::fs::File">File</a></h3></section></summary><div class="impl-items"><section id="associatedtype.Writer-1" class="associatedtype trait-impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#698">Source</a><a href="#associatedtype.Writer-1" class="anchor">§</a><h4 class="code-header">type <a href="#associatedtype.Writer" class="associatedtype">Writer</a> = &amp;'a <a class="struct" href="https://doc.rust-lang.org/1.84.1/std/fs/struct.File.html" title="struct std::fs::File">File</a></h4></section><section id="method.make_writer" class="method trait-impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#699-701">Source</a><a href="#method.make_writer" class="anchor">§</a><h4 class="code-header">fn <a href="#tymethod.make_writer" class="fn">make_writer</a>(&amp;'a self) -&gt; Self::<a class="associatedtype" href="../trait.MakeWriter.html#associatedtype.Writer" title="type tracing_subscriber::fmt::MakeWriter::Writer">Writer</a></h4></section></div></details><details class="toggle implementors-toggle"><summary><section id="impl-MakeWriter%3C'a%3E-for-Arc%3CW%3E" class="impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#687-695">Source</a><a href="#impl-MakeWriter%3C'a%3E-for-Arc%3CW%3E" class="anchor">§</a><h3 class="code-header">impl&lt;'a, W&gt; <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt; for <a class="struct" href="https://doc.rust-lang.org/1.84.1/alloc/sync/struct.Arc.html" title="struct alloc::sync::Arc">Arc</a>&lt;W&gt;<div class="where">where
<a class="primitive" href="https://doc.rust-lang.org/1.84.1/std/primitive.reference.html">&amp;'a W</a>: <a class="trait" href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write">Write</a> + 'a,</div></h3></section></summary><div class="impl-items"><section id="associatedtype.Writer-2" class="associatedtype trait-impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#691">Source</a><a href="#associatedtype.Writer-2" class="anchor">§</a><h4 class="code-header">type <a href="#associatedtype.Writer" class="associatedtype">Writer</a> = <a class="primitive" href="https://doc.rust-lang.org/1.84.1/std/primitive.reference.html">&amp;'a W</a></h4></section><section id="method.make_writer-1" class="method trait-impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#692-694">Source</a><a href="#method.make_writer-1" class="anchor">§</a><h4 class="code-header">fn <a href="#tymethod.make_writer" class="fn">make_writer</a>(&amp;'a self) -&gt; Self::<a class="associatedtype" href="../trait.MakeWriter.html#associatedtype.Writer" title="type tracing_subscriber::fmt::MakeWriter::Writer">Writer</a></h4></section></div></details><details class="toggle implementors-toggle"><summary><section id="impl-MakeWriter%3C'a%3E-for-Mutex%3CW%3E" class="impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#792-801">Source</a><a href="#impl-MakeWriter%3C'a%3E-for-Mutex%3CW%3E" class="anchor">§</a><h3 class="code-header">impl&lt;'a, W&gt; <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt; for <a class="struct" href="https://doc.rust-lang.org/1.84.1/std/sync/mutex/struct.Mutex.html" title="struct std::sync::mutex::Mutex">Mutex</a>&lt;W&gt;<div class="where">where
W: <a class="trait" href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write">Write</a> + 'a,</div></h3></section></summary><div class="impl-items"><section id="associatedtype.Writer-3" class="associatedtype trait-impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#796">Source</a><a href="#associatedtype.Writer-3" class="anchor">§</a><h4 class="code-header">type <a href="#associatedtype.Writer" class="associatedtype">Writer</a> = <a class="struct" href="struct.MutexGuardWriter.html" title="struct tracing_subscriber::fmt::writer::MutexGuardWriter">MutexGuardWriter</a>&lt;'a, W&gt;</h4></section><section id="method.make_writer-2" class="method trait-impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#798-800">Source</a><a href="#method.make_writer-2" class="anchor">§</a><h4 class="code-header">fn <a href="#tymethod.make_writer" class="fn">make_writer</a>(&amp;'a self) -&gt; Self::<a class="associatedtype" href="../trait.MakeWriter.html#associatedtype.Writer" title="type tracing_subscriber::fmt::MakeWriter::Writer">Writer</a></h4></section></div></details><h2 id="implementors" class="section-header">Implementors<a href="#implementors" class="anchor">§</a></h2><div id="implementors-list"><details class="toggle implementors-toggle"><summary><section id="impl-MakeWriter%3C'a%3E-for-TestWriter" class="impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#725-731">Source</a><a href="#impl-MakeWriter%3C'a%3E-for-TestWriter" class="anchor">§</a><h3 class="code-header">impl&lt;'a&gt; <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt; for <a class="struct" href="../struct.TestWriter.html" title="struct tracing_subscriber::fmt::TestWriter">TestWriter</a></h3></section></summary><div class="impl-items"><section id="associatedtype.Writer-4" class="associatedtype trait-impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#726">Source</a><a href="#associatedtype.Writer-4" class="anchor">§</a><h4 class="code-header">type <a href="#associatedtype.Writer" class="associatedtype">Writer</a> = <a class="struct" href="../struct.TestWriter.html" title="struct tracing_subscriber::fmt::TestWriter">TestWriter</a></h4></section></div></details><details class="toggle implementors-toggle"><summary><section id="impl-MakeWriter%3C'a%3E-for-BoxMakeWriter" class="impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#757-769">Source</a><a href="#impl-MakeWriter%3C'a%3E-for-BoxMakeWriter" class="anchor">§</a><h3 class="code-header">impl&lt;'a&gt; <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt; for <a class="struct" href="struct.BoxMakeWriter.html" title="struct tracing_subscriber::fmt::writer::BoxMakeWriter">BoxMakeWriter</a></h3></section></summary><div class="impl-items"><section id="associatedtype.Writer-5" class="associatedtype trait-impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#758">Source</a><a href="#associatedtype.Writer-5" class="anchor">§</a><h4 class="code-header">type <a href="#associatedtype.Writer" class="associatedtype">Writer</a> = <a class="struct" href="https://doc.rust-lang.org/1.84.1/alloc/boxed/struct.Box.html" title="struct alloc::boxed::Box">Box</a>&lt;dyn <a class="trait" href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write">Write</a> + 'a&gt;</h4></section></div></details><details class="toggle implementors-toggle"><summary><section id="impl-MakeWriter%3C'a%3E-for-Tee%3CA,+B%3E" class="impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#1035-1051">Source</a><a href="#impl-MakeWriter%3C'a%3E-for-Tee%3CA,+B%3E" class="anchor">§</a><h3 class="code-header">impl&lt;'a, A, B&gt; <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt; for <a class="struct" href="struct.Tee.html" title="struct tracing_subscriber::fmt::writer::Tee">Tee</a>&lt;A, B&gt;<div class="where">where
A: <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt;,
B: <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt;,</div></h3></section></summary><div class="impl-items"><section id="associatedtype.Writer-6" class="associatedtype trait-impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#1040">Source</a><a href="#associatedtype.Writer-6" class="anchor">§</a><h4 class="code-header">type <a href="#associatedtype.Writer" class="associatedtype">Writer</a> = <a class="struct" href="struct.Tee.html" title="struct tracing_subscriber::fmt::writer::Tee">Tee</a>&lt;&lt;A as <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt;&gt;::<a class="associatedtype" href="../trait.MakeWriter.html#associatedtype.Writer" title="type tracing_subscriber::fmt::MakeWriter::Writer">Writer</a>, &lt;B as <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt;&gt;::<a class="associatedtype" href="../trait.MakeWriter.html#associatedtype.Writer" title="type tracing_subscriber::fmt::MakeWriter::Writer">Writer</a>&gt;</h4></section></div></details><details class="toggle implementors-toggle"><summary><section id="impl-MakeWriter%3C'a%3E-for-OrElse%3CA,+B%3E" class="impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#1113-1136">Source</a><a href="#impl-MakeWriter%3C'a%3E-for-OrElse%3CA,+B%3E" class="anchor">§</a><h3 class="code-header">impl&lt;'a, A, B, W&gt; <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt; for <a class="struct" href="struct.OrElse.html" title="struct tracing_subscriber::fmt::writer::OrElse">OrElse</a>&lt;A, B&gt;<div class="where">where
A: <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a, Writer = <a class="type" href="type.OptionalWriter.html" title="type tracing_subscriber::fmt::writer::OptionalWriter">OptionalWriter</a>&lt;W&gt;&gt;,
B: <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt;,
W: <a class="trait" href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write">Write</a>,</div></h3></section></summary><div class="impl-items"><section id="associatedtype.Writer-7" class="associatedtype trait-impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#1119">Source</a><a href="#associatedtype.Writer-7" class="anchor">§</a><h4 class="code-header">type <a href="#associatedtype.Writer" class="associatedtype">Writer</a> = <a class="enum" href="enum.EitherWriter.html" title="enum tracing_subscriber::fmt::writer::EitherWriter">EitherWriter</a>&lt;W, &lt;B as <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt;&gt;::<a class="associatedtype" href="../trait.MakeWriter.html#associatedtype.Writer" title="type tracing_subscriber::fmt::MakeWriter::Writer">Writer</a>&gt;</h4></section></div></details><details class="toggle implementors-toggle"><summary><section id="impl-MakeWriter%3C'a%3E-for-F" class="impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#675-685">Source</a><a href="#impl-MakeWriter%3C'a%3E-for-F" class="anchor">§</a><h3 class="code-header">impl&lt;'a, F, W&gt; <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt; for F<div class="where">where
F: <a class="trait" href="https://doc.rust-lang.org/1.84.1/core/ops/function/trait.Fn.html" title="trait core::ops::function::Fn">Fn</a>() -&gt; W,
W: <a class="trait" href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write">Write</a>,</div></h3></section></summary><div class="impl-items"><section id="associatedtype.Writer-8" class="associatedtype trait-impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#680">Source</a><a href="#associatedtype.Writer-8" class="anchor">§</a><h4 class="code-header">type <a href="#associatedtype.Writer" class="associatedtype">Writer</a> = W</h4></section></div></details><details class="toggle implementors-toggle"><summary><section id="impl-MakeWriter%3C'a%3E-for-WithFilter%3CM,+F%3E" class="impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#998-1018">Source</a><a href="#impl-MakeWriter%3C'a%3E-for-WithFilter%3CM,+F%3E" class="anchor">§</a><h3 class="code-header">impl&lt;'a, M, F&gt; <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt; for <a class="struct" href="struct.WithFilter.html" title="struct tracing_subscriber::fmt::writer::WithFilter">WithFilter</a>&lt;M, F&gt;<div class="where">where
M: <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt;,
F: <a class="trait" href="https://doc.rust-lang.org/1.84.1/core/ops/function/trait.Fn.html" title="trait core::ops::function::Fn">Fn</a>(&amp;<a class="struct" href="../../../tracing_core/metadata/struct.Metadata.html" title="struct tracing_core::metadata::Metadata">Metadata</a>&lt;'_&gt;) -&gt; <a class="primitive" href="https://doc.rust-lang.org/1.84.1/std/primitive.bool.html">bool</a>,</div></h3></section></summary><div class="impl-items"><section id="associatedtype.Writer-9" class="associatedtype trait-impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#1003">Source</a><a href="#associatedtype.Writer-9" class="anchor">§</a><h4 class="code-header">type <a href="#associatedtype.Writer" class="associatedtype">Writer</a> = <a class="enum" href="enum.EitherWriter.html" title="enum tracing_subscriber::fmt::writer::EitherWriter">EitherWriter</a>&lt;&lt;M as <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt;&gt;::<a class="associatedtype" href="../trait.MakeWriter.html#associatedtype.Writer" title="type tracing_subscriber::fmt::MakeWriter::Writer">Writer</a>, <a class="struct" href="https://doc.rust-lang.org/1.84.1/std/io/util/struct.Sink.html" title="struct std::io::util::Sink">Sink</a>&gt;</h4></section></div></details><details class="toggle implementors-toggle"><summary><section id="impl-MakeWriter%3C'a%3E-for-WithMaxLevel%3CM%3E" class="impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#928-944">Source</a><a href="#impl-MakeWriter%3C'a%3E-for-WithMaxLevel%3CM%3E" class="anchor">§</a><h3 class="code-header">impl&lt;'a, M: <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt;&gt; <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt; for <a class="struct" href="struct.WithMaxLevel.html" title="struct tracing_subscriber::fmt::writer::WithMaxLevel">WithMaxLevel</a>&lt;M&gt;</h3></section></summary><div class="impl-items"><section id="associatedtype.Writer-10" class="associatedtype trait-impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#929">Source</a><a href="#associatedtype.Writer-10" class="anchor">§</a><h4 class="code-header">type <a href="#associatedtype.Writer" class="associatedtype">Writer</a> = <a class="enum" href="enum.EitherWriter.html" title="enum tracing_subscriber::fmt::writer::EitherWriter">EitherWriter</a>&lt;&lt;M as <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt;&gt;::<a class="associatedtype" href="../trait.MakeWriter.html#associatedtype.Writer" title="type tracing_subscriber::fmt::MakeWriter::Writer">Writer</a>, <a class="struct" href="https://doc.rust-lang.org/1.84.1/std/io/util/struct.Sink.html" title="struct std::io::util::Sink">Sink</a>&gt;</h4></section></div></details><details class="toggle implementors-toggle"><summary><section id="impl-MakeWriter%3C'a%3E-for-WithMinLevel%3CM%3E" class="impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#961-977">Source</a><a href="#impl-MakeWriter%3C'a%3E-for-WithMinLevel%3CM%3E" class="anchor">§</a><h3 class="code-header">impl&lt;'a, M: <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt;&gt; <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt; for <a class="struct" href="struct.WithMinLevel.html" title="struct tracing_subscriber::fmt::writer::WithMinLevel">WithMinLevel</a>&lt;M&gt;</h3></section></summary><div class="impl-items"><section id="associatedtype.Writer-11" class="associatedtype trait-impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#962">Source</a><a href="#associatedtype.Writer-11" class="anchor">§</a><h4 class="code-header">type <a href="#associatedtype.Writer" class="associatedtype">Writer</a> = <a class="enum" href="enum.EitherWriter.html" title="enum tracing_subscriber::fmt::writer::EitherWriter">EitherWriter</a>&lt;&lt;M as <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt;&gt;::<a class="associatedtype" href="../trait.MakeWriter.html#associatedtype.Writer" title="type tracing_subscriber::fmt::MakeWriter::Writer">Writer</a>, <a class="struct" href="https://doc.rust-lang.org/1.84.1/std/io/util/struct.Sink.html" title="struct std::io::util::Sink">Sink</a>&gt;</h4></section></div></details></div><script src="../../../trait.impl/tracing_subscriber/fmt/writer/trait.MakeWriter.js" data-ignore-extern-crates="alloc,std" async></script></section></div></main></body></html>

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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="Extension trait adding combinators for working with types implementing `MakeWriter`."><title>MakeWriterExt in tracing_subscriber::fmt::writer - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-6b053e98.ttf.woff2,FiraSans-Regular-0fe48ade.woff2,FiraSans-Medium-e1aa3f0a.woff2,SourceCodePro-Regular-8badfe75.ttf.woff2,SourceCodePro-Semibold-aa29a496.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-9960930a.css"><link rel="stylesheet" href="../../../static.files/rustdoc-42caa33d.css"><meta name="rustdoc-vars" data-root-path="../../../" data-static-root-path="../../../static.files/" data-current-crate="tracing_subscriber" data-themes="" data-resource-suffix="" data-rustdoc-version="1.84.1 (e71f9a9a9 2025-01-27)" data-channel="1.84.1" data-search-js="search-92e6798f.js" data-settings-js="settings-0f613d39.js" ><script src="../../../static.files/storage-59e33391.js"></script><script defer src="sidebar-items.js"></script><script defer src="../../../static.files/main-5f194d8c.js"></script><noscript><link rel="stylesheet" href="../../../static.files/noscript-893ab5e7.css"></noscript><link rel="alternate icon" type="image/png" href="../../../static.files/favicon-32x32-6580c154.png"><link rel="icon" type="image/svg+xml" href="../../../static.files/favicon-044be391.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_subscriber/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_subscriber/index.html"><img src="https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png" alt="logo"></a><h2><a href="../../../tracing_subscriber/index.html">tracing_<wbr>subscriber</a><span class="version">0.3.19</span></h2></div><div class="sidebar-elems"><section id="rustdoc-toc"><h2 class="location"><a href="#">Make<wbr>Writer<wbr>Ext</a></h2><h3><a href="#provided-methods">Provided Methods</a></h3><ul class="block"><li><a href="#method.and" title="and">and</a></li><li><a href="#method.or_else" title="or_else">or_else</a></li><li><a href="#method.with_filter" title="with_filter">with_filter</a></li><li><a href="#method.with_max_level" title="with_max_level">with_max_level</a></li><li><a href="#method.with_min_level" title="with_min_level">with_min_level</a></li></ul><h3><a href="#implementors">Implementors</a></h3></section><div id="rustdoc-modnav"><h2><a href="index.html">In tracing_<wbr>subscriber::<wbr>fmt::<wbr>writer</a></h2></div></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"><span class="rustdoc-breadcrumbs"><a href="../../index.html">tracing_subscriber</a>::<wbr><a href="../index.html">fmt</a>::<wbr><a href="index.html">writer</a></span><h1>Trait <span class="trait">MakeWriterExt</span><button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><rustdoc-toolbar></rustdoc-toolbar><span class="sub-heading"><a class="src" href="../../../src/tracing_subscriber/fmt/writer.rs.html#217-491">Source</a> </span></div><pre class="rust item-decl"><code>pub trait MakeWriterExt&lt;'a&gt;: <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt; {
// Provided methods
fn <a href="#method.with_max_level" class="fn">with_max_level</a>(self, level: <a class="struct" href="../../../tracing_core/metadata/struct.Level.html" title="struct tracing_core::metadata::Level">Level</a>) -&gt; <a class="struct" href="struct.WithMaxLevel.html" title="struct tracing_subscriber::fmt::writer::WithMaxLevel">WithMaxLevel</a>&lt;Self&gt;
<span class="where">where Self: <a class="trait" href="https://doc.rust-lang.org/1.84.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a></span> { ... }
<span class="item-spacer"></span> fn <a href="#method.with_min_level" class="fn">with_min_level</a>(self, level: <a class="struct" href="../../../tracing_core/metadata/struct.Level.html" title="struct tracing_core::metadata::Level">Level</a>) -&gt; <a class="struct" href="struct.WithMinLevel.html" title="struct tracing_subscriber::fmt::writer::WithMinLevel">WithMinLevel</a>&lt;Self&gt;
<span class="where">where Self: <a class="trait" href="https://doc.rust-lang.org/1.84.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a></span> { ... }
<span class="item-spacer"></span> fn <a href="#method.with_filter" class="fn">with_filter</a>&lt;F&gt;(self, filter: F) -&gt; <a class="struct" href="struct.WithFilter.html" title="struct tracing_subscriber::fmt::writer::WithFilter">WithFilter</a>&lt;Self, F&gt;
<span class="where">where Self: <a class="trait" href="https://doc.rust-lang.org/1.84.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>,
F: <a class="trait" href="https://doc.rust-lang.org/1.84.1/core/ops/function/trait.Fn.html" title="trait core::ops::function::Fn">Fn</a>(&amp;<a class="struct" href="../../../tracing_core/metadata/struct.Metadata.html" title="struct tracing_core::metadata::Metadata">Metadata</a>&lt;'_&gt;) -&gt; <a class="primitive" href="https://doc.rust-lang.org/1.84.1/std/primitive.bool.html">bool</a></span> { ... }
<span class="item-spacer"></span> fn <a href="#method.and" class="fn">and</a>&lt;B&gt;(self, other: B) -&gt; <a class="struct" href="struct.Tee.html" title="struct tracing_subscriber::fmt::writer::Tee">Tee</a>&lt;Self, B&gt; <a href="#" class="tooltip" data-notable-ty="Tee&lt;Self, B&gt;">ⓘ</a>
<span class="where">where Self: <a class="trait" href="https://doc.rust-lang.org/1.84.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>,
B: <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt; + <a class="trait" href="https://doc.rust-lang.org/1.84.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a></span> { ... }
<span class="item-spacer"></span> fn <a href="#method.or_else" class="fn">or_else</a>&lt;W, B&gt;(self, other: B) -&gt; <a class="struct" href="struct.OrElse.html" title="struct tracing_subscriber::fmt::writer::OrElse">OrElse</a>&lt;Self, B&gt;
<span class="where">where Self: <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a, Writer = <a class="type" href="type.OptionalWriter.html" title="type tracing_subscriber::fmt::writer::OptionalWriter">OptionalWriter</a>&lt;W&gt;&gt; + <a class="trait" href="https://doc.rust-lang.org/1.84.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>,
B: <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt; + <a class="trait" href="https://doc.rust-lang.org/1.84.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>,
W: <a class="trait" href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write">Write</a></span> { ... }
}</code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Extension trait adding combinators for working with types implementing
<a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a>.</p>
<p>This is not intended to be implemented directly for user-defined
<a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a>s; instead, it should be imported when the desired methods are
used.</p>
</div></details><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.with_max_level" class="method"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#276-281">Source</a><h4 class="code-header">fn <a href="#method.with_max_level" class="fn">with_max_level</a>(self, level: <a class="struct" href="../../../tracing_core/metadata/struct.Level.html" title="struct tracing_core::metadata::Level">Level</a>) -&gt; <a class="struct" href="struct.WithMaxLevel.html" title="struct tracing_subscriber::fmt::writer::WithMaxLevel">WithMaxLevel</a>&lt;Self&gt;<div class="where">where
Self: <a class="trait" href="https://doc.rust-lang.org/1.84.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>,</div></h4></section></summary><div class="docblock"><p>Wraps <code>self</code> and returns a <a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a> that will only write output
for events at or below the provided verbosity <a href="../../../tracing_core/metadata/struct.Level.html" title="struct tracing_core::metadata::Level"><code>Level</code></a>. For instance,
<code>Level::TRACE</code> is considered to be _more verbose<code>than</code>Level::INFO`.</p>
<p>Events whose level is more verbose than <code>level</code> will be ignored, and no
output will be written.</p>
<h5 id="examples"><a class="doc-anchor" href="#examples">§</a>Examples</h5>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tracing::Level;
<span class="kw">use </span>tracing_subscriber::fmt::writer::MakeWriterExt;
<span class="comment">// Construct a writer that outputs events to `stderr` only if the span or
// event's level is &gt;= WARN (WARN and ERROR).
</span><span class="kw">let </span>mk_writer = std::io::stderr.with_max_level(Level::WARN);
tracing_subscriber::fmt().with_writer(mk_writer).init();</code></pre></div>
<p>Writing the <code>ERROR</code> and <code>WARN</code> levels to <code>stderr</code>, and everything else
to <code>stdout</code>:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code>
<span class="kw">let </span>mk_writer = std::io::stderr
.with_max_level(Level::WARN)
.or_else(std::io::stdout);
tracing_subscriber::fmt().with_writer(mk_writer).init();</code></pre></div>
<p>Writing the <code>ERROR</code> level to <code>stderr</code>, the <code>INFO</code> and <code>WARN</code> levels to
<code>stdout</code>, and the <code>INFO</code> and DEBUG` levels to a file:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::{sync::Arc, fs::File};
<span class="kw">let </span>debug_log = Arc::new(File::create(<span class="string">"debug.log"</span>)<span class="question-mark">?</span>);
<span class="kw">let </span>mk_writer = std::io::stderr
.with_max_level(Level::ERROR)
.or_else(std::io::stdout
.with_max_level(Level::INFO)
.and(debug_log.with_max_level(Level::DEBUG))
);
tracing_subscriber::fmt().with_writer(mk_writer).init();</code></pre></div>
</div></details><details class="toggle method-toggle" open><summary><section id="method.with_min_level" class="method"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#318-323">Source</a><h4 class="code-header">fn <a href="#method.with_min_level" class="fn">with_min_level</a>(self, level: <a class="struct" href="../../../tracing_core/metadata/struct.Level.html" title="struct tracing_core::metadata::Level">Level</a>) -&gt; <a class="struct" href="struct.WithMinLevel.html" title="struct tracing_subscriber::fmt::writer::WithMinLevel">WithMinLevel</a>&lt;Self&gt;<div class="where">where
Self: <a class="trait" href="https://doc.rust-lang.org/1.84.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>,</div></h4></section></summary><div class="docblock"><p>Wraps <code>self</code> and returns a <a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a> that will only write output
for events at or above the provided verbosity <a href="../../../tracing_core/metadata/struct.Level.html" title="struct tracing_core::metadata::Level"><code>Level</code></a>.</p>
<p>Events whose level is less verbose than <code>level</code> will be ignored, and no
output will be written.</p>
<h5 id="examples-1"><a class="doc-anchor" href="#examples-1">§</a>Examples</h5>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tracing::Level;
<span class="kw">use </span>tracing_subscriber::fmt::writer::MakeWriterExt;
<span class="comment">// Construct a writer that outputs events to `stdout` only if the span or
// event's level is &lt;= DEBUG (DEBUG and TRACE).
</span><span class="kw">let </span>mk_writer = std::io::stdout.with_min_level(Level::DEBUG);
tracing_subscriber::fmt().with_writer(mk_writer).init();</code></pre></div>
<p>This can be combined with <a href="trait.MakeWriterExt.html#method.with_max_level" title="method tracing_subscriber::fmt::writer::MakeWriterExt::with_max_level"><code>MakeWriterExt::with_max_level</code></a> to write
only within a range of levels:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="comment">// Only write the `DEBUG` and `INFO` levels to stdout.
</span><span class="kw">let </span>mk_writer = std::io::stdout
.with_max_level(Level::DEBUG)
.with_min_level(Level::INFO)
<span class="comment">// Write the `WARN` and `ERROR` levels to stderr.
</span>.and(std::io::stderr.with_min_level(Level::WARN));
tracing_subscriber::fmt().with_writer(mk_writer).init();</code></pre></div>
</div></details><details class="toggle method-toggle" open><summary><section id="method.with_filter" class="method"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#397-403">Source</a><h4 class="code-header">fn <a href="#method.with_filter" class="fn">with_filter</a>&lt;F&gt;(self, filter: F) -&gt; <a class="struct" href="struct.WithFilter.html" title="struct tracing_subscriber::fmt::writer::WithFilter">WithFilter</a>&lt;Self, F&gt;<div class="where">where
Self: <a class="trait" href="https://doc.rust-lang.org/1.84.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>,
F: <a class="trait" href="https://doc.rust-lang.org/1.84.1/core/ops/function/trait.Fn.html" title="trait core::ops::function::Fn">Fn</a>(&amp;<a class="struct" href="../../../tracing_core/metadata/struct.Metadata.html" title="struct tracing_core::metadata::Metadata">Metadata</a>&lt;'_&gt;) -&gt; <a class="primitive" href="https://doc.rust-lang.org/1.84.1/std/primitive.bool.html">bool</a>,</div></h4></section></summary><div class="docblock"><p>Wraps <code>self</code> with a predicate that takes a span or event’s <a href="../../../tracing_core/metadata/struct.Metadata.html" title="struct tracing_core::metadata::Metadata"><code>Metadata</code></a>
and returns a <code>bool</code>. The returned <a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a>’s
<a href="../trait.MakeWriter.html#method.make_writer_for" title="method tracing_subscriber::fmt::MakeWriter::make_writer_for"><code>MakeWriter::make_writer_for</code></a> method will check the predicate to
determine if a writer should be produced for a given span or event.</p>
<p>If the predicate returns <code>false</code>, the wrapped <a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a>’s
<a href="../trait.MakeWriter.html#method.make_writer_for" title="method tracing_subscriber::fmt::MakeWriter::make_writer_for"><code>make_writer_for</code></a> will return <a href="enum.EitherWriter.html#method.none" title="associated function tracing_subscriber::fmt::writer::EitherWriter::none"><code>OptionalWriter::none</code></a>.
Otherwise, it calls the wrapped <a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a>’s
<a href="../trait.MakeWriter.html#method.make_writer_for" title="method tracing_subscriber::fmt::MakeWriter::make_writer_for"><code>make_writer_for</code></a> method, and returns the produced writer.</p>
<p>This can be used to filter an output based on arbitrary <a href="../../../tracing_core/metadata/struct.Metadata.html" title="struct tracing_core::metadata::Metadata"><code>Metadata</code></a>
parameters.</p>
<h5 id="examples-2"><a class="doc-anchor" href="#examples-2">§</a>Examples</h5>
<p>Writing events with a specific target to an HTTP access log, and other
events to stdout:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tracing_subscriber::fmt::writer::MakeWriterExt;
<span class="kw">use </span>std::{sync::Arc, fs::File};
<span class="kw">let </span>access_log = Arc::new(File::create(<span class="string">"access.log"</span>)<span class="question-mark">?</span>);
<span class="kw">let </span>mk_writer = access_log
<span class="comment">// Only write events with the target "http::access_log" to the
// access log file.
</span>.with_filter(|meta| meta.target() == <span class="string">"http::access_log"</span>)
<span class="comment">// Write events with all other targets to stdout.
</span>.or_else(std::io::stdout);
tracing_subscriber::fmt().with_writer(mk_writer).init();</code></pre></div>
<p>Conditionally enabling or disabling a log file:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tracing_subscriber::fmt::writer::MakeWriterExt;
<span class="kw">use </span>std::{
sync::{Arc, atomic::{AtomicBool, Ordering}},
fs::File,
};
<span class="kw">static </span>DEBUG_LOG_ENABLED: AtomicBool = AtomicBool::new(<span class="bool-val">false</span>);
<span class="comment">// Create the debug log file
</span><span class="kw">let </span>debug_file = Arc::new(File::create(<span class="string">"debug.log"</span>)<span class="question-mark">?</span>)
<span class="comment">// Enable the debug log only if the flag is enabled.
</span>.with_filter(|<span class="kw">_</span>| DEBUG_LOG_ENABLED.load(Ordering::Acquire));
<span class="comment">// Always write to stdout
</span><span class="kw">let </span>mk_writer = std::io::stdout
<span class="comment">// Write to the debug file if it's enabled
</span>.and(debug_file);
tracing_subscriber::fmt().with_writer(mk_writer).init();
<span class="comment">// ...
// Later, we can toggle on or off the debug log file.
</span>DEBUG_LOG_ENABLED.store(<span class="bool-val">true</span>, Ordering::Release);</code></pre></div>
</div></details><details class="toggle method-toggle" open><summary><section id="method.and" class="method"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#452-458">Source</a><h4 class="code-header">fn <a href="#method.and" class="fn">and</a>&lt;B&gt;(self, other: B) -&gt; <a class="struct" href="struct.Tee.html" title="struct tracing_subscriber::fmt::writer::Tee">Tee</a>&lt;Self, B&gt; <a href="#" class="tooltip" data-notable-ty="Tee&lt;Self, B&gt;">ⓘ</a><div class="where">where
Self: <a class="trait" href="https://doc.rust-lang.org/1.84.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>,
B: <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt; + <a class="trait" href="https://doc.rust-lang.org/1.84.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>,</div></h4></section></summary><div class="docblock"><p>Combines <code>self</code> with another type implementing <a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a>, returning
a new <a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a> that produces <a href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write">writers</a> that write to <em>both</em>
outputs.</p>
<p>If writing to either writer returns an error, the returned writer will
return that error. However, both writers will still be written to before
the error is returned, so it is possible for one writer to fail while
the other is written to successfully.</p>
<h5 id="examples-3"><a class="doc-anchor" href="#examples-3">§</a>Examples</h5>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tracing_subscriber::fmt::writer::MakeWriterExt;
<span class="comment">// Construct a writer that outputs events to `stdout` *and* `stderr`.
</span><span class="kw">let </span>mk_writer = std::io::stdout.and(std::io::stderr);
tracing_subscriber::fmt().with_writer(mk_writer).init();</code></pre></div>
<p><code>and</code> can be used in conjunction with filtering combinators. For
example, if we want to write to a number of outputs depending on the
level of an event, we could write:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tracing::Level;
<span class="kw">use </span>std::{sync::Arc, fs::File};
<span class="kw">let </span>debug_log = Arc::new(File::create(<span class="string">"debug.log"</span>)<span class="question-mark">?</span>);
<span class="comment">// Write everything to the debug log.
</span><span class="kw">let </span>mk_writer = debug_log
<span class="comment">// Write the `ERROR` and `WARN` levels to stderr.
</span>.and(std::io::stderr.with_max_level(Level::WARN))
<span class="comment">// Write `INFO` to `stdout`.
</span>.and(std::io::stdout
.with_max_level(Level::INFO)
.with_min_level(Level::INFO)
);
tracing_subscriber::fmt().with_writer(mk_writer).init();</code></pre></div>
</div></details><details class="toggle method-toggle" open><summary><section id="method.or_else" class="method"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#483-490">Source</a><h4 class="code-header">fn <a href="#method.or_else" class="fn">or_else</a>&lt;W, B&gt;(self, other: B) -&gt; <a class="struct" href="struct.OrElse.html" title="struct tracing_subscriber::fmt::writer::OrElse">OrElse</a>&lt;Self, B&gt;<div class="where">where
Self: <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a, Writer = <a class="type" href="type.OptionalWriter.html" title="type tracing_subscriber::fmt::writer::OptionalWriter">OptionalWriter</a>&lt;W&gt;&gt; + <a class="trait" href="https://doc.rust-lang.org/1.84.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>,
B: <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt; + <a class="trait" href="https://doc.rust-lang.org/1.84.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>,
W: <a class="trait" href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write">Write</a>,</div></h4></section></summary><div class="docblock"><p>Combines <code>self</code> with another type implementing <a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a>, returning
a new <a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a> that calls <code>other</code>’s <a href="../trait.MakeWriter.html#tymethod.make_writer" title="method tracing_subscriber::fmt::MakeWriter::make_writer"><code>make_writer</code></a> if <code>self</code>’s
<code>make_writer</code> returns <a href="enum.EitherWriter.html#method.none" title="associated function tracing_subscriber::fmt::writer::EitherWriter::none"><code>OptionalWriter::none</code></a>.</p>
<h5 id="examples-4"><a class="doc-anchor" href="#examples-4">§</a>Examples</h5>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tracing::Level;
<span class="kw">use </span>tracing_subscriber::fmt::writer::MakeWriterExt;
<span class="comment">// Produces a writer that writes to `stderr` if the level is &gt;= WARN,
// or returns `OptionalWriter::none()` otherwise.
</span><span class="kw">let </span>stderr = std::io::stderr.with_max_level(Level::WARN);
<span class="comment">// If the `stderr` `MakeWriter` is disabled by the max level filter,
// write to stdout instead:
</span><span class="kw">let </span>mk_writer = stderr.or_else(std::io::stdout);
tracing_subscriber::fmt().with_writer(mk_writer).init();</code></pre></div>
</div></details></div><h2 id="implementors" class="section-header">Implementors<a href="#implementors" class="anchor">§</a></h2><div id="implementors-list"><section id="impl-MakeWriterExt%3C'a%3E-for-M" class="impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#1205">Source</a><a href="#impl-MakeWriterExt%3C'a%3E-for-M" class="anchor">§</a><h3 class="code-header">impl&lt;'a, M&gt; <a class="trait" href="trait.MakeWriterExt.html" title="trait tracing_subscriber::fmt::writer::MakeWriterExt">MakeWriterExt</a>&lt;'a&gt; for M<div class="where">where
M: <a class="trait" href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter">MakeWriter</a>&lt;'a&gt;,</div></h3></section></div><script src="../../../trait.impl/tracing_subscriber/fmt/writer/trait.MakeWriterExt.js" async></script><script type="text/json" id="notable-traits-data">{"Tee<Self, B>":"<h3>Notable traits for <code><a class=\"struct\" href=\"struct.Tee.html\" title=\"struct tracing_subscriber::fmt::writer::Tee\">Tee</a>&lt;A, B&gt;</code></h3><pre><code><div class=\"where\">impl&lt;A, B&gt; <a class=\"trait\" href=\"https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html\" title=\"trait std::io::Write\">Write</a> for <a class=\"struct\" href=\"struct.Tee.html\" title=\"struct tracing_subscriber::fmt::writer::Tee\">Tee</a>&lt;A, B&gt;<div class=\"where\">where\n A: <a class=\"trait\" href=\"https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html\" title=\"trait std::io::Write\">Write</a>,\n B: <a class=\"trait\" href=\"https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html\" title=\"trait std::io::Write\">Write</a>,</div></div>"}</script></section></div></main></body></html>

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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="A writer which may or may not be enabled."><title>OptionalWriter in tracing_subscriber::fmt::writer - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-6b053e98.ttf.woff2,FiraSans-Regular-0fe48ade.woff2,FiraSans-Medium-e1aa3f0a.woff2,SourceCodePro-Regular-8badfe75.ttf.woff2,SourceCodePro-Semibold-aa29a496.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-9960930a.css"><link rel="stylesheet" href="../../../static.files/rustdoc-42caa33d.css"><meta name="rustdoc-vars" data-root-path="../../../" data-static-root-path="../../../static.files/" data-current-crate="tracing_subscriber" data-themes="" data-resource-suffix="" data-rustdoc-version="1.84.1 (e71f9a9a9 2025-01-27)" data-channel="1.84.1" data-search-js="search-92e6798f.js" data-settings-js="settings-0f613d39.js" ><script src="../../../static.files/storage-59e33391.js"></script><script defer src="sidebar-items.js"></script><script defer src="../../../static.files/main-5f194d8c.js"></script><noscript><link rel="stylesheet" href="../../../static.files/noscript-893ab5e7.css"></noscript><link rel="alternate icon" type="image/png" href="../../../static.files/favicon-32x32-6580c154.png"><link rel="icon" type="image/svg+xml" href="../../../static.files/favicon-044be391.svg"></head><body class="rustdoc type"><!--[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_subscriber/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_subscriber/index.html"><img src="https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png" alt="logo"></a><h2><a href="../../../tracing_subscriber/index.html">tracing_<wbr>subscriber</a><span class="version">0.3.19</span></h2></div><div class="sidebar-elems"><section id="rustdoc-toc"><h2 class="location"><a href="#">Optional<wbr>Writer</a></h2><h3><a href="#aliased-type">Aliased type</a></h3><h3><a href="#variants">Variants</a></h3><ul class="block variant"><li><a href="#variant.A" title="A">A</a></li><li><a href="#variant.B" title="B">B</a></li></ul><h3><a href="#implementations">Methods</a></h3><ul class="block method"><li><a href="#method.none" title="none">none</a></li><li><a href="#method.some" title="some">some</a></li></ul><h3><a href="#trait-implementations">Trait Implementations</a></h3><ul class="block trait-implementation"><li><a href="#impl-From%3COption%3CT%3E%3E-for-EitherWriter%3CT,+Sink%3E" title="From&#60;Option&#60;T&#62;&#62;">From&#60;Option&#60;T&#62;&#62;</a></li></ul></section><div id="rustdoc-modnav"><h2><a href="index.html">In tracing_<wbr>subscriber::<wbr>fmt::<wbr>writer</a></h2></div></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"><span class="rustdoc-breadcrumbs"><a href="../../index.html">tracing_subscriber</a>::<wbr><a href="../index.html">fmt</a>::<wbr><a href="index.html">writer</a></span><h1>Type Alias <span class="type">OptionalWriter</span><button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><rustdoc-toolbar></rustdoc-toolbar><span class="sub-heading"><a class="src" href="../../../src/tracing_subscriber/fmt/writer.rs.html#567">Source</a> </span></div><pre class="rust item-decl"><code>pub type OptionalWriter&lt;T&gt; = <a class="enum" href="enum.EitherWriter.html" title="enum tracing_subscriber::fmt::writer::EitherWriter">EitherWriter</a>&lt;T, <a class="struct" href="https://doc.rust-lang.org/1.84.1/std/io/util/struct.Sink.html" title="struct std::io::util::Sink">Sink</a>&gt;;</code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>A <a href="https://doc.rust-lang.org/1.84.1/std/io/trait.Write.html" title="trait std::io::Write">writer</a> which may or may not be enabled.</p>
<p>This may be used by <a href="../trait.MakeWriter.html" title="trait tracing_subscriber::fmt::MakeWriter"><code>MakeWriter</code></a> implementations that wish to
conditionally enable or disable the returned writer based on a span or
event’s <a href="../../../tracing_core/metadata/struct.Metadata.html" title="struct tracing_core::metadata::Metadata"><code>Metadata</code></a>.</p>
</div></details><h2 id="aliased-type" class="section-header">Aliased Type<a href="#aliased-type" class="anchor">§</a></h2><pre class="rust item-decl"><code>enum OptionalWriter&lt;T&gt; {
A(T),
B(<a class="struct" href="https://doc.rust-lang.org/1.84.1/std/io/util/struct.Sink.html" title="struct std::io::util::Sink">Sink</a>),
}</code></pre><h2 id="variants" class="variants section-header">Variants<a href="#variants" class="anchor">§</a></h2><div class="variants"><section id="variant.A" class="variant"><a href="#variant.A" class="anchor">§</a><h3 class="code-header">A(T)</h3></section><div class="docblock"><p>A writer of type <code>A</code>.</p>
</div><section id="variant.B" class="variant"><a href="#variant.B" class="anchor">§</a><h3 class="code-header">B(<a class="struct" href="https://doc.rust-lang.org/1.84.1/std/io/util/struct.Sink.html" title="struct std::io::util::Sink">Sink</a>)</h3></section><div class="docblock"><p>A writer of type <code>B</code>.</p>
</div></div><h2 id="implementations" class="section-header">Implementations<a href="#implementations" class="anchor">§</a></h2><div id="implementations-list"><details class="toggle implementors-toggle" open><summary><section id="impl-EitherWriter%3CT,+Sink%3E" class="impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#881-901">Source</a><a href="#impl-EitherWriter%3CT,+Sink%3E" class="anchor">§</a><h3 class="code-header">impl&lt;T&gt; <a class="type" href="type.OptionalWriter.html" title="type tracing_subscriber::fmt::writer::OptionalWriter">OptionalWriter</a>&lt;T&gt;</h3></section></summary><div class="impl-items"><details class="toggle method-toggle" open><summary><section id="method.none" class="method"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#890-892">Source</a><h4 class="code-header">pub fn <a href="#method.none" class="fn">none</a>() -&gt; Self</h4></section></summary><div class="docblock"><p>Returns a <a href="https://doc.rust-lang.org/1.84.1/std/io/util/fn.sink.html" title="fn std::io::util::sink">disabled writer</a>.</p>
<p>Any bytes written to the returned writer are discarded.</p>
<p>This is equivalent to returning <a href="https://doc.rust-lang.org/1.84.1/core/option/enum.Option.html#variant.None" title="variant core::option::Option::None"><code>Option::None</code></a>.</p>
</div></details><details class="toggle method-toggle" open><summary><section id="method.some" class="method"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#898-900">Source</a><h4 class="code-header">pub fn <a href="#method.some" class="fn">some</a>(t: T) -&gt; Self</h4></section></summary><div class="docblock"><p>Returns an enabled writer of type <code>T</code>.</p>
<p>This is equivalent to returning <a href="https://doc.rust-lang.org/1.84.1/core/option/enum.Option.html#variant.Some" title="variant core::option::Option::Some"><code>Option::Some</code></a>.</p>
</div></details></div></details></div><h2 id="trait-implementations" class="section-header">Trait Implementations<a href="#trait-implementations" class="anchor">§</a></h2><div id="trait-implementations-list"><details class="toggle implementors-toggle" open><summary><section id="impl-From%3COption%3CT%3E%3E-for-EitherWriter%3CT,+Sink%3E" class="impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#903-911">Source</a><a href="#impl-From%3COption%3CT%3E%3E-for-EitherWriter%3CT,+Sink%3E" class="anchor">§</a><h3 class="code-header">impl&lt;T&gt; <a class="trait" href="https://doc.rust-lang.org/1.84.1/core/convert/trait.From.html" title="trait core::convert::From">From</a>&lt;<a class="enum" href="https://doc.rust-lang.org/1.84.1/core/option/enum.Option.html" title="enum core::option::Option">Option</a>&lt;T&gt;&gt; for <a class="type" href="type.OptionalWriter.html" title="type tracing_subscriber::fmt::writer::OptionalWriter">OptionalWriter</a>&lt;T&gt;</h3></section></summary><div class="impl-items"><details class="toggle method-toggle" open><summary><section id="method.from" class="method trait-impl"><a class="src rightside" href="../../../src/tracing_subscriber/fmt/writer.rs.html#905-910">Source</a><a href="#method.from" class="anchor">§</a><h4 class="code-header">fn <a href="https://doc.rust-lang.org/1.84.1/core/convert/trait.From.html#tymethod.from" class="fn">from</a>(opt: <a class="enum" href="https://doc.rust-lang.org/1.84.1/core/option/enum.Option.html" title="enum core::option::Option">Option</a>&lt;T&gt;) -&gt; Self</h4></section></summary><div class='docblock'>Converts to this type from the input type.</div></details></div></details></div><script src="../../../type.impl/tracing_subscriber/fmt/writer/enum.EitherWriter.js" data-self-path="tracing_subscriber::fmt::writer::OptionalWriter" async></script></section></div></main></body></html>