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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="Instruments a function to create and enter a `tracing` span every time the function is called."><title>instrument in tracing - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-46f98efaafac5295.ttf.woff2,FiraSans-Regular-018c141bf0843ffd.woff2,FiraSans-Medium-8f9a781e4970d388.woff2,SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2,SourceCodePro-Semibold-d899c5a5c4aeb14a.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-76eba96aa4d2e634.css"><link rel="stylesheet" href="../static.files/rustdoc-dd39b87e5fcfba68.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="tracing" data-themes="" data-resource-suffix="" data-rustdoc-version="1.80.0 (051478957 2024-07-21)" data-channel="1.80.0" data-search-js="search-d52510db62a78183.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../static.files/storage-118b08c4c78b968e.js"></script><script defer src="sidebar-items.js"></script><script defer src="../static.files/main-20a3ad099b048cf2.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-df360f571f6edeae.css"></noscript><link rel="alternate icon" type="image/png" href="../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../static.files/favicon-2c020d218678b618.svg"></head><body class="rustdoc attr"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle" title="show sidebar"></button><a class="logo-container" href="../tracing/index.html"><img src="https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png" alt=""></a></nav><nav class="sidebar"><div class="sidebar-crate"><a class="logo-container" href="../tracing/index.html"><img src="https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png" alt="logo"></a><h2><a href="../tracing/index.html">tracing</a><span class="version">0.1.40</span></h2></div><div class="sidebar-elems"></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"><h1>Attribute Macro <a href="index.html">tracing</a>::<wbr><a class="attr" href="#">instrument</a><button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><span class="out-of-band"><a class="src" href="../src/tracing_attributes/lib.rs.html#563-566">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</span>]</button></span></div><pre class="rust item-decl"><code>#[instrument]</code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Instruments a function to create and enter a <code>tracing</code> <a href="https://docs.rs/tracing/latest/tracing/span/index.html">span</a> every time
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the function is called.</p>
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<p>Unless overridden, a span with the <a href="https://docs.rs/tracing/latest/tracing/struct.Level.html#associatedconstant.INFO"><code>INFO</code></a> <a href="https://docs.rs/tracing/latest/tracing/struct.Level.html">level</a> will be generated.
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The generated span’s name will be the name of the function.
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By default, all arguments to the function are included as fields on the
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span. Arguments that are <code>tracing</code> <a href="https://docs.rs/tracing/latest/tracing/field/trait.Value.html#foreign-impls">primitive types</a> implementing the
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<a href="https://docs.rs/tracing/latest/tracing/field/trait.Value.html."><code>Value</code> trait</a> will be recorded as fields of that type. Types which do
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not implement <code>Value</code> will be recorded using [<code>std::fmt::Debug</code>].</p>
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<h2 id="overriding-span-attributes"><a class="doc-anchor" href="#overriding-span-attributes">§</a>Overriding Span Attributes</h2>
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<p>To change the <a href="https://docs.rs/tracing/latest/tracing/struct.Metadata.html#method.name">name</a> of the generated span, add a <code>name</code> argument to the
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<code>#[instrument]</code> macro, followed by an equals sign and a string literal. For
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example:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code>
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<span class="comment">// The generated span's name will be "my_span" rather than "my_function".
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</span><span class="attr">#[instrument(name = <span class="string">"my_span"</span>)]
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</span><span class="kw">pub fn </span>my_function() {
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<span class="comment">// ... do something incredibly interesting and important ...
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</span>}</code></pre></div>
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<p>To override the <a href="https://docs.rs/tracing/latest/tracing/struct.Metadata.html#method.target">target</a> of the generated span, add a <code>target</code> argument to
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the <code>#[instrument]</code> macro, followed by an equals sign and a string literal
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for the new target. The <a href="https://docs.rs/tracing/latest/tracing/struct.Metadata.html#method.module_path">module path</a> is still recorded separately. For
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example:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">pub mod </span>my_module {
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<span class="comment">// The generated span's target will be "my_crate::some_special_target",
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// rather than "my_crate::my_module".
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</span><span class="attr">#[instrument(target = <span class="string">"my_crate::some_special_target"</span>)]
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</span><span class="kw">pub fn </span>my_function() {
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<span class="comment">// ... all kinds of neat code in here ...
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</span>}
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}</code></pre></div>
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<p>Finally, to override the <a href="https://docs.rs/tracing/latest/tracing/struct.Level.html">level</a> of the generated span, add a <code>level</code>
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argument, followed by an equals sign and a string literal with the name of
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the desired level. Level names are not case sensitive. For example:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="comment">// The span's level will be TRACE rather than INFO.
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</span><span class="attr">#[instrument(level = <span class="string">"trace"</span>)]
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</span><span class="kw">pub fn </span>my_function() {
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<span class="comment">// ... I have written a truly marvelous implementation of this function,
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// which this example is too narrow to contain ...
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</span>}</code></pre></div>
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<h2 id="skipping-fields"><a class="doc-anchor" href="#skipping-fields">§</a>Skipping Fields</h2>
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<p>To skip recording one or more arguments to a function or method, pass
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the argument’s name inside the <code>skip()</code> argument on the <code>#[instrument]</code>
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macro. This can be used when an argument to an instrumented function does
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not implement <a href="std::fmt::Debug"><code>fmt::Debug</code></a>, or to exclude an argument with a verbose or
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costly <code>Debug</code> implementation. Note that:</p>
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<ul>
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<li>multiple argument names can be passed to <code>skip</code>.</li>
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<li>arguments passed to <code>skip</code> do <em>not</em> need to implement <code>fmt::Debug</code>.</li>
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</ul>
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<p>You can also use <code>skip_all</code> to skip all arguments.</p>
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<h3 id="examples"><a class="doc-anchor" href="#examples">§</a>Examples</h3>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="comment">// This type doesn't implement `fmt::Debug`!
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</span><span class="kw">struct </span>NonDebug;
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<span class="comment">// `arg` will be recorded, while `non_debug` will not.
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</span><span class="attr">#[instrument(skip(non_debug))]
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</span><span class="kw">fn </span>my_function(arg: usize, non_debug: NonDebug) {
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<span class="comment">// ...
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</span>}
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<span class="comment">// These arguments are huge
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</span><span class="attr">#[instrument(skip_all)]
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</span><span class="kw">fn </span>my_big_data_function(large: Vec<u8>, also_large: HashMap<String, String>) {
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<span class="comment">// ...
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</span>}</code></pre></div>
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<p>Skipping the <code>self</code> parameter:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[derive(Debug)]
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</span><span class="kw">struct </span>MyType {
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data: Vec<u8>, <span class="comment">// Suppose this buffer is often quite long...
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</span>}
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<span class="kw">impl </span>MyType {
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<span class="comment">// Suppose we don't want to print an entire kilobyte of `data`
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// every time this is called...
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</span><span class="attr">#[instrument(skip(<span class="self">self</span>))]
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</span><span class="kw">pub fn </span>my_method(<span class="kw-2">&mut </span><span class="self">self</span>, an_interesting_argument: usize) {
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<span class="comment">// ... do something (hopefully, using all that `data`!)
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</span>}
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}</code></pre></div>
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<h2 id="adding-fields"><a class="doc-anchor" href="#adding-fields">§</a>Adding Fields</h2>
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<p>Additional fields (key-value pairs with arbitrary data) can be passed to
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to the generated span through the <code>fields</code> argument on the
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<code>#[instrument]</code> macro. Strings, integers or boolean literals are accepted values
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for each field. The name of the field must be a single valid Rust
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identifier, nested (dotted) field names are not supported. Any
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Rust expression can be used as a field value in this manner. These
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expressions will be evaluated at the beginning of the function’s body, so
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arguments to the function may be used in these expressions. Field names may
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also be specified <em>without</em> values. Doing so will result in an <a href="https://docs.rs/tracing/latest/tracing/field/struct.Empty.html">empty field</a>
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whose value may be recorded later within the function body.</p>
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<p>Note that overlap between the names of fields and (non-skipped) arguments
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will result in a compile error.</p>
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<h3 id="examples-1"><a class="doc-anchor" href="#examples-1">§</a>Examples</h3>
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<p>Adding a new field based on the value of an argument:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code>
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<span class="comment">// This will record a field named "i" with the value of `i` *and* a field
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// named "next" with the value of `i` + 1.
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</span><span class="attr">#[instrument(fields(next = i + <span class="number">1</span>))]
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</span><span class="kw">pub fn </span>my_function(i: usize) {
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<span class="comment">// ...
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</span>}</code></pre></div>
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<p>Recording specific properties of a struct as their own fields:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code>
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<span class="comment">// This will record the request's URI and HTTP method as their own separate
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// fields.
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</span><span class="attr">#[instrument(fields(http.uri = req.uri(), http.method = req.method()))]
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</span><span class="kw">pub fn </span>handle_request<B>(req: http::Request<B>) -> http::Response<B> {
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<span class="comment">// ... handle the request ...
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</span>}</code></pre></div>
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<p>This can be used in conjunction with <code>skip</code> or <code>skip_all</code> to record only
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some fields of a struct:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="comment">// Remember the struct with the very large `data` field from the earlier
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// example? Now it also has a `name`, which we might want to include in
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// our span.
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</span><span class="attr">#[derive(Debug)]
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</span><span class="kw">struct </span>MyType {
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name: <span class="kw-2">&</span><span class="lifetime">'static </span>str,
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data: Vec<u8>,
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}
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<span class="kw">impl </span>MyType {
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<span class="comment">// This will skip the `data` field, but will include `self.name`,
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// formatted using `fmt::Display`.
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</span><span class="attr">#[instrument(skip(<span class="self">self</span>), fields(<span class="self">self</span>.name = %<span class="self">self</span>.name))]
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</span><span class="kw">pub fn </span>my_method(<span class="kw-2">&mut </span><span class="self">self</span>, an_interesting_argument: usize) {
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<span class="comment">// ... do something (hopefully, using all that `data`!)
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</span>}
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}</code></pre></div>
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<p>Adding an empty field to be recorded later:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code>
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<span class="comment">// This function does a very interesting and important mathematical calculation.
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// Suppose we want to record both the inputs to the calculation *and* its result...
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</span><span class="attr">#[instrument(fields(result))]
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</span><span class="kw">pub fn </span>do_calculation(input_1: usize, input_2: usize) -> usize {
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<span class="comment">// Rerform the calculation.
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</span><span class="kw">let </span>result = input_1 + input_2;
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<span class="comment">// Record the result as part of the current span.
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</span>tracing::Span::current().record(<span class="string">"result"</span>, <span class="kw-2">&</span>result);
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<span class="comment">// Now, the result will also be included on this event!
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</span><span class="macro">tracing::info!</span>(<span class="string">"calculation complete!"</span>);
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<span class="comment">// ... etc ...
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</span>}</code></pre></div>
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<h2 id="examples-2"><a class="doc-anchor" href="#examples-2">§</a>Examples</h2>
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<p>Instrumenting a function:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[instrument]
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</span><span class="kw">pub fn </span>my_function(my_arg: usize) {
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<span class="comment">// This event will be recorded inside a span named `my_function` with the
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// field `my_arg`.
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</span><span class="macro">tracing::info!</span>(<span class="string">"inside my_function!"</span>);
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<span class="comment">// ...
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</span>}</code></pre></div>
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<p>Setting the level for the generated span:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[instrument(level = Level::DEBUG)]
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</span><span class="kw">pub fn </span>my_function() {
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<span class="comment">// ...
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</span>}</code></pre></div>
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<p>Levels can be specified either with <a href="https://docs.rs/tracing/latest/tracing/struct.Level.html"><code>Level</code></a> constants, literal strings
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(e.g., <code>"debug"</code>, <code>"info"</code>) or numerically (1—5, corresponding to <a href="https://docs.rs/tracing/latest/tracing/struct.Level.html#associatedconstant.TRACE"><code>Level::TRACE</code></a>—<a href="https://docs.rs/tracing/latest/tracing/struct.Level.html#associatedconstant.ERROR"><code>Level::ERROR</code></a>).</p>
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<p>Overriding the generated span’s name:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[instrument(name = <span class="string">"my_name"</span>)]
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</span><span class="kw">pub fn </span>my_function() {
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<span class="comment">// ...
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</span>}</code></pre></div>
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<p>Overriding the generated span’s target:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[instrument(target = <span class="string">"my_target"</span>)]
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</span><span class="kw">pub fn </span>my_function() {
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<span class="comment">// ...
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</span>}</code></pre></div>
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<p>Overriding the generated span’s parent:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[instrument(parent = <span class="prelude-val">None</span>)]
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</span><span class="kw">pub fn </span>my_function() {
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<span class="comment">// ...
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</span>}</code></pre></div>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="comment">// A struct which owns a span handle.
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</span><span class="kw">struct </span>MyStruct
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{
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span: tracing::Span
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}
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<span class="kw">impl </span>MyStruct
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{
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<span class="comment">// Use the struct's `span` field as the parent span
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</span><span class="attr">#[instrument(parent = <span class="kw-2">&</span><span class="self">self</span>.span, skip(<span class="self">self</span>))]
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</span><span class="kw">fn </span>my_method(<span class="kw-2">&</span><span class="self">self</span>) {}
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}</code></pre></div>
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<p>Specifying <a href="https://docs.rs/tracing/latest/tracing/struct.Span.html#method.follows_from"><code>follows_from</code></a> relationships:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[instrument(follows_from = causes)]
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</span><span class="kw">pub fn </span>my_function(causes: <span class="kw-2">&</span>[tracing::Id]) {
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<span class="comment">// ...
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</span>}</code></pre></div>
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<p>Any expression of type <code>impl IntoIterator<Item = impl Into<Option<Id>>></code>
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may be provided to <code>follows_from</code>; e.g.:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[instrument(follows_from = [cause]</span>)]
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<span class="kw">pub fn </span>my_function(cause: <span class="kw-2">&</span>tracing::span::EnteredSpan) {
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<span class="comment">// ...
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</span>}</code></pre></div>
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<p>To skip recording an argument, pass the argument’s name to the <code>skip</code>:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">struct </span>NonDebug;
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<span class="attr">#[instrument(skip(non_debug))]
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</span><span class="kw">fn </span>my_function(arg: usize, non_debug: NonDebug) {
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<span class="comment">// ...
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</span>}</code></pre></div>
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<p>To add additional context to the span, pass key-value pairs to <code>fields</code>:</p>
|
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[instrument(fields(foo=<span class="string">"bar"</span>, id=<span class="number">1</span>, show=<span class="bool-val">true</span>))]
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</span><span class="kw">fn </span>my_function(arg: usize) {
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<span class="comment">// ...
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</span>}</code></pre></div>
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<p>Adding the <code>ret</code> argument to <code>#[instrument]</code> will emit an event with the function’s
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return value when the function returns:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[instrument(ret)]
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</span><span class="kw">fn </span>my_function() -> i32 {
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<span class="number">42
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</span>}</code></pre></div>
|
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<p>The return value event will have the same level as the span generated by <code>#[instrument]</code>.
|
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By default, this will be <a href="https://docs.rs/tracing/latest/tracing/struct.Level.html#associatedconstant.INFO"><code>INFO</code></a>, but if the level is overridden, the event will be at the same
|
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level.</p>
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<p>It’s also possible to override the level for the <code>ret</code> event independently:</p>
|
||
|
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[instrument(ret(level = Level::WARN))]
|
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</span><span class="kw">fn </span>my_function() -> i32 {
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<span class="number">42
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||
</span>}</code></pre></div>
|
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<p><strong>Note</strong>: if the function returns a <code>Result<T, E></code>, <code>ret</code> will record returned values if and
|
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only if the function returns [<code>Result::Ok</code>].</p>
|
||
<p>By default, returned values will be recorded using their [<code>std::fmt::Debug</code>] implementations.
|
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If a returned value implements [<code>std::fmt::Display</code>], it can be recorded using its <code>Display</code>
|
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implementation instead, by writing <code>ret(Display)</code>:</p>
|
||
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[instrument(ret(Display))]
|
||
</span><span class="kw">fn </span>my_function() -> i32 {
|
||
<span class="number">42
|
||
</span>}</code></pre></div>
|
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<p>If the function returns a <code>Result<T, E></code> and <code>E</code> implements <code>std::fmt::Display</code>, adding
|
||
<code>err</code> or <code>err(Display)</code> will emit error events when the function returns <code>Err</code>:</p>
|
||
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[instrument(err)]
|
||
</span><span class="kw">fn </span>my_function(arg: usize) -> <span class="prelude-ty">Result</span><(), std::io::Error> {
|
||
<span class="prelude-val">Ok</span>(())
|
||
}</code></pre></div>
|
||
<p>The level of the error value event defaults to <code>ERROR</code>.</p>
|
||
<p>Similarly, overriding the level of the <code>err</code> event :</p>
|
||
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[instrument(err(level = Level::INFO))]
|
||
</span><span class="kw">fn </span>my_function(arg: usize) -> <span class="prelude-ty">Result</span><(), std::io::Error> {
|
||
<span class="prelude-val">Ok</span>(())
|
||
}</code></pre></div>
|
||
<p>By default, error values will be recorded using their <code>std::fmt::Display</code> implementations.
|
||
If an error implements <code>std::fmt::Debug</code>, it can be recorded using its <code>Debug</code> implementation
|
||
instead by writing <code>err(Debug)</code>:</p>
|
||
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[instrument(err(Debug))]
|
||
</span><span class="kw">fn </span>my_function(arg: usize) -> <span class="prelude-ty">Result</span><(), std::io::Error> {
|
||
<span class="prelude-val">Ok</span>(())
|
||
}</code></pre></div>
|
||
<p>If a <code>target</code> is specified, both the <code>ret</code> and <code>err</code> arguments will emit outputs to
|
||
the declared target (or the default channel if <code>target</code> is not specified).</p>
|
||
<p>The <code>ret</code> and <code>err</code> arguments can be combined in order to record an event if a
|
||
function returns [<code>Result::Ok</code>] or [<code>Result::Err</code>]:</p>
|
||
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[instrument(err, ret)]
|
||
</span><span class="kw">fn </span>my_function(arg: usize) -> <span class="prelude-ty">Result</span><(), std::io::Error> {
|
||
<span class="prelude-val">Ok</span>(())
|
||
}</code></pre></div>
|
||
<p><code>async fn</code>s may also be instrumented:</p>
|
||
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[instrument]
|
||
</span><span class="kw">pub async fn </span>my_function() -> <span class="prelude-ty">Result</span><(), ()> {
|
||
<span class="comment">// ...
|
||
</span>}</code></pre></div>
|
||
<p>It also works with <a href="https://crates.io/crates/async-trait">async-trait</a>
|
||
(a crate that allows defining async functions in traits,
|
||
something not currently possible in Rust),
|
||
and hopefully most libraries that exhibit similar behaviors:</p>
|
||
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>async_trait::async_trait;
|
||
|
||
<span class="attr">#[async_trait]
|
||
</span><span class="kw">pub trait </span>Foo {
|
||
<span class="kw">async fn </span>foo(<span class="kw-2">&</span><span class="self">self</span>, arg: usize);
|
||
}
|
||
|
||
<span class="attr">#[derive(Debug)]
|
||
</span><span class="kw">struct </span>FooImpl(usize);
|
||
|
||
<span class="attr">#[async_trait]
|
||
</span><span class="kw">impl </span>Foo <span class="kw">for </span>FooImpl {
|
||
<span class="attr">#[instrument(fields(value = <span class="self">self</span>.<span class="number">0</span>, tmp = std::any::type_name::<<span class="self">Self</span>>()))]
|
||
</span><span class="kw">async fn </span>foo(<span class="kw-2">&</span><span class="self">self</span>, arg: usize) {}
|
||
}</code></pre></div>
|
||
<p><code>const fn</code> cannot be instrumented, and will result in a compilation failure:</p>
|
||
|
||
<div class="example-wrap compile_fail"><a href="#" class="tooltip" title="This example deliberately fails to compile">ⓘ</a><pre class="rust rust-example-rendered"><code><span class="attr">#[instrument]
|
||
</span><span class="kw">const fn </span>my_const_function() {}</code></pre></div>
|
||
</div></details></section></div></main></body></html> |