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<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="API documentation for the Rust `log` crate."><meta name="keywords" content="rust, rustlang, rust-lang, log"><title>log - Rust</title><link rel="stylesheet" type="text/css" href="../normalize.css"><link rel="stylesheet" type="text/css" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../dark.css"><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><script src="../storage.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="shortcut icon" href="https://www.rust-lang.org/favicon.ico"><style type="text/css">#crate-search{background-image:url("../down-arrow.svg");}</style></head><body class="rustdoc mod"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../log/index.html'><div class='logo-container'><img src='https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png' alt='logo'></div></a><p class='location'>Crate log</p><div class="sidebar-elems"><a id='all-types' href='all.html'><p>See all log's items</p></a><div class="block items"><ul><li><a href="#macros">Macros</a></li><li><a href="#structs">Structs</a></li><li><a href="#enums">Enums</a></li><li><a href="#traits">Traits</a></li><li><a href="#functions">Functions</a></li></ul></div><p class='location'></p><script>window.sidebarCurrent = {name: 'log', ty: 'mod', relpath: '../'};</script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../theme.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><a id="settings-menu" href="../settings.html"><img src="../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class='fqn'><span class='out-of-band'><span id='render-detail'><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class='inner'>−</span>]</a></span><a class='srclink' href='../src/log/lib.rs.html#11-1091' title='goto source code'>[src]</a></span><span class='in-band'>Crate <a class="mod" href=''>log</a></span></h1><div class='docblock'><p>A lightweight logging facade.</p>
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<p>A logging facade provides a single logging API that abstracts over the
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actual logging implementation. Libraries can use the logging API provided
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by this crate, and the consumer of those libraries can choose the logging
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framework that is most suitable for its use case.</p>
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<p>If no logging implementation is selected, the facade falls back to a "noop"
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implementation that ignores all log messages. The overhead in this case
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is very small - just an integer load, comparison and jump.</p>
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<p>A log request consists of a target, a level, and a body. A target is a
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string which defaults to the module path of the location of the log
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request, though that default may be overridden. Logger implementations
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typically use the target to filter requests based on some user
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configuration.</p>
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<h1 id="use" class="section-header"><a href="#use">Use</a></h1><h2 id="in-libraries" class="section-header"><a href="#in-libraries">In libraries</a></h2>
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<p>Libraries should link only to the <code>log</code> crate, and use the provided
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macros to log whatever information will be useful to downstream consumers.</p>
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<h3 id="examples" class="section-header"><a href="#examples">Examples</a></h3>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="attribute">#[<span class="ident">macro_use</span>]</span>
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<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">log</span>;
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<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">shave_the_yak</span>(<span class="ident">yak</span>: <span class="kw-2">&</span><span class="ident">Yak</span>) {
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<span class="macro">info</span><span class="macro">!</span>(<span class="ident">target</span>: <span class="string">"yak_events"</span>, <span class="string">"Commencing yak shaving for {:?}"</span>, <span class="ident">yak</span>);
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<span class="kw">loop</span> {
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<span class="kw">match</span> <span class="ident">find_a_razor</span>() {
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<span class="prelude-val">Ok</span>(<span class="ident">razor</span>) <span class="op">=</span><span class="op">></span> {
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<span class="macro">info</span><span class="macro">!</span>(<span class="string">"Razor located: {}"</span>, <span class="ident">razor</span>);
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<span class="ident">yak</span>.<span class="ident">shave</span>(<span class="ident">razor</span>);
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<span class="kw">break</span>;
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}
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<span class="prelude-val">Err</span>(<span class="ident">err</span>) <span class="op">=</span><span class="op">></span> {
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<span class="macro">warn</span><span class="macro">!</span>(<span class="string">"Unable to locate a razor: {}, retrying"</span>, <span class="ident">err</span>);
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}
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}
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}
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}</pre></div>
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<h2 id="in-executables" class="section-header"><a href="#in-executables">In executables</a></h2>
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<p>Executables should choose a logging framework and initialize it early in the
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runtime of the program. Logging frameworks will typically include a
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function to do this. Any log messages generated before the framework is
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initialized will be ignored.</p>
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<p>The executable itself may use the <code>log</code> crate to log as well.</p>
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<h3 id="warning" class="section-header"><a href="#warning">Warning</a></h3>
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<p>The logging system may only be initialized once.</p>
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<h3 id="examples-1" class="section-header"><a href="#examples-1">Examples</a></h3>
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<div class='information'><div class='tooltip ignore'>ⓘ<span class='tooltiptext'>This example is not tested</span></div></div><div class="example-wrap"><pre class="rust rust-example-rendered ignore">
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<span class="attribute">#[<span class="ident">macro_use</span>]</span>
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<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">log</span>;
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<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">my_logger</span>;
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<span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="ident">my_logger</span>::<span class="ident">init</span>();
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<span class="macro">info</span><span class="macro">!</span>(<span class="string">"starting up"</span>);
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<span class="comment">// ...</span>
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}</pre></div>
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<h1 id="logger-implementations" class="section-header"><a href="#logger-implementations">Logger implementations</a></h1>
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<p>Loggers implement the <code>Log</code> trait. Here's a very basic example that simply
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logs all messages at the <code>Error</code>, <code>Warn</code> or <code>Info</code> levels to stdout:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">log</span>;
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<span class="kw">use</span> <span class="ident">log</span>::{<span class="ident">LogRecord</span>, <span class="ident">LogLevel</span>, <span class="ident">LogMetadata</span>};
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<span class="kw">struct</span> <span class="ident">SimpleLogger</span>;
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<span class="kw">impl</span> <span class="ident">log</span>::<span class="ident">Log</span> <span class="kw">for</span> <span class="ident">SimpleLogger</span> {
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<span class="kw">fn</span> <span class="ident">enabled</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">metadata</span>: <span class="kw-2">&</span><span class="ident">LogMetadata</span>) <span class="op">-</span><span class="op">></span> <span class="ident">bool</span> {
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<span class="ident">metadata</span>.<span class="ident">level</span>() <span class="op"><</span><span class="op">=</span> <span class="ident">LogLevel</span>::<span class="ident">Info</span>
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}
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<span class="kw">fn</span> <span class="ident">log</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">record</span>: <span class="kw-2">&</span><span class="ident">LogRecord</span>) {
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<span class="kw">if</span> <span class="self">self</span>.<span class="ident">enabled</span>(<span class="ident">record</span>.<span class="ident">metadata</span>()) {
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<span class="macro">println</span><span class="macro">!</span>(<span class="string">"{} - {}"</span>, <span class="ident">record</span>.<span class="ident">level</span>(), <span class="ident">record</span>.<span class="ident">args</span>());
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}
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}
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}
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</pre></div>
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<p>Loggers are installed by calling the <code>set_logger</code> function. It takes a
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closure which is provided a <code>MaxLogLevel</code> token and returns a <code>Log</code> trait
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object. The <code>MaxLogLevel</code> token controls the global maximum log level. The
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logging facade uses this as an optimization to improve performance of log
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messages at levels that are disabled. In the case of our example logger,
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we'll want to set the maximum log level to <code>Info</code>, since we ignore any
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<code>Debug</code> or <code>Trace</code> level log messages. A logging framework should provide a
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function that wraps a call to <code>set_logger</code>, handling initialization of the
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logger:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">init</span>() <span class="op">-</span><span class="op">></span> <span class="prelude-ty">Result</span><span class="op"><</span>(), <span class="ident">SetLoggerError</span><span class="op">></span> {
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<span class="ident">log</span>::<span class="ident">set_logger</span>(<span class="op">|</span><span class="ident">max_log_level</span><span class="op">|</span> {
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<span class="ident">max_log_level</span>.<span class="ident">set</span>(<span class="ident">LogLevelFilter</span>::<span class="ident">Info</span>);
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<span class="ident">Box</span>::<span class="ident">new</span>(<span class="ident">SimpleLogger</span>)
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})
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}</pre></div>
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<h1 id="use-with-no_std" class="section-header"><a href="#use-with-no_std">Use with <code>no_std</code></a></h1>
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<p>To use the <code>log</code> crate without depending on <code>libstd</code>, you need to specify
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<code>default-features = false</code> when specifying the dependency in <code>Cargo.toml</code>.
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This makes no difference to libraries using <code>log</code> since the logging API
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remains the same. However executables will need to use the <code>set_logger_raw</code>
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function to initialize a logger and the <code>shutdown_logger_raw</code> function to
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shut down the global logger before exiting:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">init</span>() <span class="op">-</span><span class="op">></span> <span class="prelude-ty">Result</span><span class="op"><</span>(), <span class="ident">SetLoggerError</span><span class="op">></span> {
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<span class="kw">unsafe</span> {
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<span class="ident">log</span>::<span class="ident">set_logger_raw</span>(<span class="op">|</span><span class="ident">max_log_level</span><span class="op">|</span> {
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<span class="kw">static</span> <span class="ident">LOGGER</span>: <span class="ident">SimpleLogger</span> <span class="op">=</span> <span class="ident">SimpleLogger</span>;
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<span class="ident">max_log_level</span>.<span class="ident">set</span>(<span class="ident">LogLevelFilter</span>::<span class="ident">Info</span>);
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<span class="kw-2">&</span><span class="ident">SimpleLogger</span>
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})
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}
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}
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<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">shutdown</span>() <span class="op">-</span><span class="op">></span> <span class="prelude-ty">Result</span><span class="op"><</span>(), <span class="ident">ShutdownLoggerError</span><span class="op">></span> {
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<span class="ident">log</span>::<span class="ident">shutdown_logger_raw</span>().<span class="ident">map</span>(<span class="op">|</span><span class="ident">logger</span><span class="op">|</span> {
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<span class="kw">let</span> <span class="ident">logger</span> <span class="op">=</span> <span class="kw">unsafe</span> { <span class="kw-2">&</span><span class="kw-2">*</span>(<span class="ident">logger</span> <span class="kw">as</span> <span class="kw-2">*</span><span class="kw">const</span> <span class="ident">SimpleLogger</span>) };
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<span class="ident">logger</span>.<span class="ident">flush</span>();
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})
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}</pre></div>
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</div><h2 id='macros' class='section-header'><a href="#macros">Macros</a></h2>
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<table><tr class='module-item'><td><a class="macro" href="macro.debug.html" title='log::debug macro'>debug</a></td><td class='docblock-short'><p>Logs a message at the debug level.</p>
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</td></tr><tr class='module-item'><td><a class="macro" href="macro.error.html" title='log::error macro'>error</a></td><td class='docblock-short'><p>Logs a message at the error level.</p>
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</td></tr><tr class='module-item'><td><a class="macro" href="macro.info.html" title='log::info macro'>info</a></td><td class='docblock-short'><p>Logs a message at the info level.</p>
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</td></tr><tr class='module-item'><td><a class="macro" href="macro.log.html" title='log::log macro'>log</a></td><td class='docblock-short'><p>The standard logging macro.</p>
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</td></tr><tr class='module-item'><td><a class="macro" href="macro.log_enabled.html" title='log::log_enabled macro'>log_enabled</a></td><td class='docblock-short'><p>Determines if a message logged at the specified level in that module will
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be logged.</p>
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</td></tr><tr class='module-item'><td><a class="macro" href="macro.trace.html" title='log::trace macro'>trace</a></td><td class='docblock-short'><p>Logs a message at the trace level.</p>
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</td></tr><tr class='module-item'><td><a class="macro" href="macro.warn.html" title='log::warn macro'>warn</a></td><td class='docblock-short'><p>Logs a message at the warn level.</p>
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</td></tr></table><h2 id='structs' class='section-header'><a href="#structs">Structs</a></h2>
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<table><tr class='module-item'><td><a class="struct" href="struct.LogLocation.html" title='log::LogLocation struct'>LogLocation</a></td><td class='docblock-short'><p>The location of a log message.</p>
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</td></tr><tr class='module-item'><td><a class="struct" href="struct.LogMetadata.html" title='log::LogMetadata struct'>LogMetadata</a></td><td class='docblock-short'><p>Metadata about a log message.</p>
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</td></tr><tr class='module-item'><td><a class="struct" href="struct.LogRecord.html" title='log::LogRecord struct'>LogRecord</a></td><td class='docblock-short'><p>The "payload" of a log message.</p>
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</td></tr><tr class='module-item'><td><a class="struct" href="struct.MaxLogLevelFilter.html" title='log::MaxLogLevelFilter struct'>MaxLogLevelFilter</a></td><td class='docblock-short'><p>A token providing read and write access to the global maximum log level
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filter.</p>
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</td></tr><tr class='module-item'><td><a class="struct" href="struct.SetLoggerError.html" title='log::SetLoggerError struct'>SetLoggerError</a></td><td class='docblock-short'><p>The type returned by <code>set_logger</code> if <code>set_logger</code> has already been called.</p>
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</td></tr><tr class='module-item'><td><a class="struct" href="struct.ShutdownLoggerError.html" title='log::ShutdownLoggerError struct'>ShutdownLoggerError</a></td><td class='docblock-short'><p>The type returned by <code>shutdown_logger_raw</code> if <code>shutdown_logger_raw</code> has
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already been called or if <code>set_logger_raw</code> has not been called yet.</p>
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</td></tr></table><h2 id='enums' class='section-header'><a href="#enums">Enums</a></h2>
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<table><tr class='module-item'><td><a class="enum" href="enum.LogLevel.html" title='log::LogLevel enum'>LogLevel</a></td><td class='docblock-short'><p>An enum representing the available verbosity levels of the logging framework</p>
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</td></tr><tr class='module-item'><td><a class="enum" href="enum.LogLevelFilter.html" title='log::LogLevelFilter enum'>LogLevelFilter</a></td><td class='docblock-short'><p>An enum representing the available verbosity level filters of the logging
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framework.</p>
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</td></tr></table><h2 id='traits' class='section-header'><a href="#traits">Traits</a></h2>
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<table><tr class='module-item'><td><a class="trait" href="trait.Log.html" title='log::Log trait'>Log</a></td><td class='docblock-short'><p>A trait encapsulating the operations required of a logger</p>
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</td></tr></table><h2 id='functions' class='section-header'><a href="#functions">Functions</a></h2>
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<table><tr class='module-item'><td><a class="fn" href="fn.max_log_level.html" title='log::max_log_level fn'>max_log_level</a></td><td class='docblock-short'><p>Returns the current maximum log level.</p>
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</td></tr><tr class='module-item'><td><a class="fn" href="fn.set_logger.html" title='log::set_logger fn'>set_logger</a></td><td class='docblock-short'><p>Sets the global logger.</p>
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</td></tr><tr class='module-item'><td><a class="fn" href="fn.set_logger_raw.html" title='log::set_logger_raw fn'>set_logger_raw</a><a title='unsafe function' href='#'><sup>⚠</sup></a></td><td class='docblock-short'><p>Sets the global logger from a raw pointer.</p>
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</td></tr><tr class='module-item'><td><a class="fn" href="fn.shutdown_logger.html" title='log::shutdown_logger fn'>shutdown_logger</a></td><td class='docblock-short'><p>Shuts down the global logger.</p>
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</td></tr><tr class='module-item'><td><a class="fn" href="fn.shutdown_logger_raw.html" title='log::shutdown_logger_raw fn'>shutdown_logger_raw</a></td><td class='docblock-short'><p>Shuts down the global logger.</p>
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