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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="List of all items in this crate"><title>List of all items in this crate</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="once_cell" 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="../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 sys"><!--[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></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../once_cell/index.html">once_<wbr>cell</a><span class="version">1.20.2</span></h2></div><div class="sidebar-elems"><section id="rustdoc-toc"><h3><a href="#structs">Crate Items</a></h3><ul class="block"><li><a href="#structs" title="Structs">Structs</a></li></ul></section><div id="rustdoc-modnav"></div></div></nav><div class="sidebar-resizer"></div><main><div class="width-limiter"><rustdoc-search></rustdoc-search><section id="main-content" class="content"><h1>List of all items</h1><h3 id="structs">Structs</h3><ul class="all-items"><li><a href="race/struct.OnceBool.html">race::OnceBool</a></li><li><a href="race/struct.OnceBox.html">race::OnceBox</a></li><li><a href="race/struct.OnceNonZeroUsize.html">race::OnceNonZeroUsize</a></li><li><a href="race/struct.OnceRef.html">race::OnceRef</a></li><li><a href="sync/struct.Lazy.html">sync::Lazy</a></li><li><a href="sync/struct.OnceCell.html">sync::OnceCell</a></li><li><a href="unsync/struct.Lazy.html">unsync::Lazy</a></li><li><a href="unsync/struct.OnceCell.html">unsync::OnceCell</a></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="Overview"><title>once_cell - 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="once_cell" 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="../crates.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 crate"><!--[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></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../once_cell/index.html">once_<wbr>cell</a><span class="version">1.20.2</span></h2></div><div class="sidebar-elems"><ul class="block"><li><a id="all-types" href="all.html">All Items</a></li></ul><section id="rustdoc-toc"><h3><a href="#">Sections</a></h3><ul class="block top-toc"><li><a href="#overview" title="Overview">Overview</a></li><li><a href="#recipes" title="Recipes">Recipes</a><ul><li><a href="#safe-initialization-of-global-data" title="Safe Initialization of Global Data">Safe Initialization of Global Data</a></li><li><a href="#lazy-initialized-global-data" title="Lazy Initialized Global Data">Lazy Initialized Global Data</a></li><li><a href="#general-purpose-lazy-evaluation" title="General purpose lazy evaluation">General purpose lazy evaluation</a></li><li><a href="#lazily-compiled-regex" title="Lazily Compiled Regex">Lazily Compiled Regex</a></li><li><a href="#runtime-include_bytes" title="Runtime `include_bytes!`">Runtime <code>include_bytes!</code></a></li><li><a href="#lateinit" title="`lateinit`"><code>lateinit</code></a></li></ul></li><li><a href="#comparison-with-std" title="Comparison with std">Comparison with std</a></li><li><a href="#minimum-supported-rustc-version" title="Minimum Supported `rustc` Version">Minimum Supported <code>rustc</code> Version</a></li><li><a href="#implementation-details" title="Implementation details">Implementation details</a></li><li><a href="#faq" title="F.A.Q.">F.A.Q.</a></li><li><a href="#related-crates" title="Related crates">Related crates</a></li></ul><h3><a href="#modules">Crate Items</a></h3><ul class="block"><li><a href="#modules" title="Modules">Modules</a></li></ul></section><div id="rustdoc-modnav"></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"><h1>Crate <span>once_cell</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/once_cell/lib.rs.html#1-1412">Source</a> </span></div><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><h2 id="overview"><a class="doc-anchor" href="#overview">§</a>Overview</h2>
<p><code>once_cell</code> provides two new cell-like types, <a href="unsync/struct.OnceCell.html"><code>unsync::OnceCell</code></a> and
<a href="sync/struct.OnceCell.html"><code>sync::OnceCell</code></a>. A <code>OnceCell</code> might store arbitrary non-<code>Copy</code> types, can
be assigned to at most once and provides direct access to the stored
contents. The core API looks <em>roughly</em> like this (and there’s much more
inside, read on!):</p>
<div class="example-wrap ignore"><a href="#" class="tooltip" title="This example is not tested">ⓘ</a><pre class="rust rust-example-rendered"><code><span class="kw">impl</span>&lt;T&gt; OnceCell&lt;T&gt; {
<span class="kw">const fn </span>new() -&gt; OnceCell&lt;T&gt; { ... }
<span class="kw">fn </span>set(<span class="kw-2">&amp;</span><span class="self">self</span>, value: T) -&gt; <span class="prelude-ty">Result</span>&lt;(), T&gt; { ... }
<span class="kw">fn </span>get(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="prelude-ty">Option</span>&lt;<span class="kw-2">&amp;</span>T&gt; { ... }
}</code></pre></div>
<p>Note that, like with <a href="https://doc.rust-lang.org/std/cell/struct.RefCell.html"><code>RefCell</code></a> and <a href="https://doc.rust-lang.org/std/sync/struct.Mutex.html"><code>Mutex</code></a>, the <code>set</code> method requires
only a shared reference. Because of the single assignment restriction <code>get</code>
can return a <code>&amp;T</code> instead of <code>Ref&lt;T&gt;</code> or <code>MutexGuard&lt;T&gt;</code>.</p>
<p>The <code>sync</code> flavor is thread-safe (that is, implements the <a href="https://doc.rust-lang.org/std/marker/trait.Sync.html"><code>Sync</code></a> trait),
while the <code>unsync</code> one is not.</p>
<h2 id="recipes"><a class="doc-anchor" href="#recipes">§</a>Recipes</h2>
<p><code>OnceCell</code> might be useful for a variety of patterns.</p>
<h3 id="safe-initialization-of-global-data"><a class="doc-anchor" href="#safe-initialization-of-global-data">§</a>Safe Initialization of Global Data</h3>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::{env, io};
<span class="kw">use </span>once_cell::sync::OnceCell;
<span class="attr">#[derive(Debug)]
</span><span class="kw">pub struct </span>Logger {
<span class="comment">// ...
</span>}
<span class="kw">static </span>INSTANCE: OnceCell&lt;Logger&gt; = OnceCell::new();
<span class="kw">impl </span>Logger {
<span class="kw">pub fn </span>global() -&gt; <span class="kw-2">&amp;</span><span class="lifetime">'static </span>Logger {
INSTANCE.get().expect(<span class="string">"logger is not initialized"</span>)
}
<span class="kw">fn </span>from_cli(args: env::Args) -&gt; <span class="prelude-ty">Result</span>&lt;Logger, std::io::Error&gt; {
<span class="comment">// ...
</span>}
}
<span class="kw">fn </span>main() {
<span class="kw">let </span>logger = Logger::from_cli(env::args()).unwrap();
INSTANCE.set(logger).unwrap();
<span class="comment">// use `Logger::global()` from now on
</span>}</code></pre></div>
<h3 id="lazy-initialized-global-data"><a class="doc-anchor" href="#lazy-initialized-global-data">§</a>Lazy Initialized Global Data</h3>
<p>This is essentially the <code>lazy_static!</code> macro, but without a macro.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::{sync::Mutex, collections::HashMap};
<span class="kw">use </span>once_cell::sync::OnceCell;
<span class="kw">fn </span>global_data() -&gt; <span class="kw-2">&amp;</span><span class="lifetime">'static </span>Mutex&lt;HashMap&lt;i32, String&gt;&gt; {
<span class="kw">static </span>INSTANCE: OnceCell&lt;Mutex&lt;HashMap&lt;i32, String&gt;&gt;&gt; = OnceCell::new();
INSTANCE.get_or_init(|| {
<span class="kw">let </span><span class="kw-2">mut </span>m = HashMap::new();
m.insert(<span class="number">13</span>, <span class="string">"Spica"</span>.to_string());
m.insert(<span class="number">74</span>, <span class="string">"Hoyten"</span>.to_string());
Mutex::new(m)
})
}</code></pre></div>
<p>There are also the <a href="sync/struct.Lazy.html"><code>sync::Lazy</code></a> and <a href="unsync/struct.Lazy.html"><code>unsync::Lazy</code></a> convenience types to
streamline this pattern:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::{sync::Mutex, collections::HashMap};
<span class="kw">use </span>once_cell::sync::Lazy;
<span class="kw">static </span>GLOBAL_DATA: Lazy&lt;Mutex&lt;HashMap&lt;i32, String&gt;&gt;&gt; = Lazy::new(|| {
<span class="kw">let </span><span class="kw-2">mut </span>m = HashMap::new();
m.insert(<span class="number">13</span>, <span class="string">"Spica"</span>.to_string());
m.insert(<span class="number">74</span>, <span class="string">"Hoyten"</span>.to_string());
Mutex::new(m)
});
<span class="kw">fn </span>main() {
<span class="macro">println!</span>(<span class="string">"{:?}"</span>, GLOBAL_DATA.lock().unwrap());
}</code></pre></div>
<p>Note that the variable that holds <code>Lazy</code> is declared as <code>static</code>, <em>not</em>
<code>const</code>. This is important: using <code>const</code> instead compiles, but works wrong.</p>
<h3 id="general-purpose-lazy-evaluation"><a class="doc-anchor" href="#general-purpose-lazy-evaluation">§</a>General purpose lazy evaluation</h3>
<p>Unlike <code>lazy_static!</code>, <code>Lazy</code> works with local variables.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>once_cell::unsync::Lazy;
<span class="kw">fn </span>main() {
<span class="kw">let </span>ctx = <span class="macro">vec!</span>[<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>];
<span class="kw">let </span>thunk = Lazy::new(|| {
ctx.iter().sum::&lt;i32&gt;()
});
<span class="macro">assert_eq!</span>(<span class="kw-2">*</span>thunk, <span class="number">6</span>);
}</code></pre></div>
<p>If you need a lazy field in a struct, you probably should use <code>OnceCell</code>
directly, because that will allow you to access <code>self</code> during
initialization.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::{fs, path::PathBuf};
<span class="kw">use </span>once_cell::unsync::OnceCell;
<span class="kw">struct </span>Ctx {
config_path: PathBuf,
config: OnceCell&lt;String&gt;,
}
<span class="kw">impl </span>Ctx {
<span class="kw">pub fn </span>get_config(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="prelude-ty">Result</span>&lt;<span class="kw-2">&amp;</span>str, std::io::Error&gt; {
<span class="kw">let </span>cfg = <span class="self">self</span>.config.get_or_try_init(|| {
fs::read_to_string(<span class="kw-2">&amp;</span><span class="self">self</span>.config_path)
})<span class="question-mark">?</span>;
<span class="prelude-val">Ok</span>(cfg.as_str())
}
}</code></pre></div>
<h3 id="lazily-compiled-regex"><a class="doc-anchor" href="#lazily-compiled-regex">§</a>Lazily Compiled Regex</h3>
<p>This is a <code>regex!</code> macro which takes a string literal and returns an
<em>expression</em> that evaluates to a <code>&amp;'static Regex</code>:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="macro">macro_rules!</span> regex {
(<span class="macro-nonterminal">$re</span>:literal $(,)<span class="question-mark">?</span>) =&gt; {{
<span class="kw">static </span>RE: once_cell::sync::OnceCell&lt;regex::Regex&gt; = once_cell::sync::OnceCell::new();
RE.get_or_init(|| regex::Regex::new(<span class="macro-nonterminal">$re</span>).unwrap())
}};
}</code></pre></div>
<p>This macro can be useful to avoid the “compile regex on every loop
iteration” problem.</p>
<h3 id="runtime-include_bytes"><a class="doc-anchor" href="#runtime-include_bytes">§</a>Runtime <code>include_bytes!</code></h3>
<p>The <code>include_bytes</code> macro is useful to include test resources, but it slows
down test compilation a lot. An alternative is to load the resources at
runtime:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::path::Path;
<span class="kw">use </span>once_cell::sync::OnceCell;
<span class="kw">pub struct </span>TestResource {
path: <span class="kw-2">&amp;</span><span class="lifetime">'static </span>str,
cell: OnceCell&lt;Vec&lt;u8&gt;&gt;,
}
<span class="kw">impl </span>TestResource {
<span class="kw">pub const fn </span>new(path: <span class="kw-2">&amp;</span><span class="lifetime">'static </span>str) -&gt; TestResource {
TestResource { path, cell: OnceCell::new() }
}
<span class="kw">pub fn </span>bytes(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="kw-2">&amp;</span>[u8] {
<span class="self">self</span>.cell.get_or_init(|| {
<span class="kw">let </span>dir = std::env::var(<span class="string">"CARGO_MANIFEST_DIR"</span>).unwrap();
<span class="kw">let </span>path = Path::new(dir.as_str()).join(<span class="self">self</span>.path);
std::fs::read(<span class="kw-2">&amp;</span>path).unwrap_or_else(|_err| {
<span class="macro">panic!</span>(<span class="string">"failed to load test resource: {}"</span>, path.display())
})
}).as_slice()
}
}
<span class="kw">static </span>TEST_IMAGE: TestResource = TestResource::new(<span class="string">"test_data/lena.png"</span>);
<span class="attr">#[test]
</span><span class="kw">fn </span>test_sobel_filter() {
<span class="kw">let </span>rgb: <span class="kw-2">&amp;</span>[u8] = TEST_IMAGE.bytes();
<span class="comment">// ...
</span>}</code></pre></div>
<h3 id="lateinit"><a class="doc-anchor" href="#lateinit">§</a><code>lateinit</code></h3>
<p><code>LateInit</code> type for delayed initialization. It is reminiscent of Kotlin’s
<code>lateinit</code> keyword and allows construction of cyclic data structures:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>once_cell::sync::OnceCell;
<span class="kw">pub struct </span>LateInit&lt;T&gt; { cell: OnceCell&lt;T&gt; }
<span class="kw">impl</span>&lt;T&gt; LateInit&lt;T&gt; {
<span class="kw">pub fn </span>init(<span class="kw-2">&amp;</span><span class="self">self</span>, value: T) {
<span class="macro">assert!</span>(<span class="self">self</span>.cell.set(value).is_ok())
}
}
<span class="kw">impl</span>&lt;T&gt; Default <span class="kw">for </span>LateInit&lt;T&gt; {
<span class="kw">fn </span>default() -&gt; <span class="self">Self </span>{ LateInit { cell: OnceCell::default() } }
}
<span class="kw">impl</span>&lt;T&gt; std::ops::Deref <span class="kw">for </span>LateInit&lt;T&gt; {
<span class="kw">type </span>Target = T;
<span class="kw">fn </span>deref(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="kw-2">&amp;</span>T {
<span class="self">self</span>.cell.get().unwrap()
}
}
<span class="attr">#[derive(Default)]
</span><span class="kw">struct </span>A&lt;<span class="lifetime">'a</span>&gt; {
b: LateInit&lt;<span class="kw-2">&amp;</span><span class="lifetime">'a </span>B&lt;<span class="lifetime">'a</span>&gt;&gt;,
}
<span class="attr">#[derive(Default)]
</span><span class="kw">struct </span>B&lt;<span class="lifetime">'a</span>&gt; {
a: LateInit&lt;<span class="kw-2">&amp;</span><span class="lifetime">'a </span>A&lt;<span class="lifetime">'a</span>&gt;&gt;
}
<span class="kw">fn </span>build_cycle() {
<span class="kw">let </span>a = A::default();
<span class="kw">let </span>b = B::default();
a.b.init(<span class="kw-2">&amp;</span>b);
b.a.init(<span class="kw-2">&amp;</span>a);
<span class="kw">let </span>_a = <span class="kw-2">&amp;</span>a.b.a.b.a;
}</code></pre></div>
<h2 id="comparison-with-std"><a class="doc-anchor" href="#comparison-with-std">§</a>Comparison with std</h2><div><table><thead><tr><th><code>!Sync</code> types</th><th>Access Mode</th><th>Drawbacks</th></tr></thead><tbody>
<tr><td><code>Cell&lt;T&gt;</code></td><td><code>T</code></td><td>requires <code>T: Copy</code> for <code>get</code></td></tr>
<tr><td><code>RefCell&lt;T&gt;</code></td><td><code>RefMut&lt;T&gt;</code> / <code>Ref&lt;T&gt;</code></td><td>may panic at runtime</td></tr>
<tr><td><code>unsync::OnceCell&lt;T&gt;</code></td><td><code>&amp;T</code></td><td>assignable only once</td></tr>
</tbody></table>
</div><div><table><thead><tr><th><code>Sync</code> types</th><th>Access Mode</th><th>Drawbacks</th></tr></thead><tbody>
<tr><td><code>AtomicT</code></td><td><code>T</code></td><td>works only with certain <code>Copy</code> types</td></tr>
<tr><td><code>Mutex&lt;T&gt;</code></td><td><code>MutexGuard&lt;T&gt;</code></td><td>may deadlock at runtime, may block the thread</td></tr>
<tr><td><code>sync::OnceCell&lt;T&gt;</code></td><td><code>&amp;T</code></td><td>assignable only once, may block the thread</td></tr>
</tbody></table>
</div>
<p>Technically, calling <code>get_or_init</code> will also cause a panic or a deadlock if
it recursively calls itself. However, because the assignment can happen only
once, such cases should be more rare than equivalents with <code>RefCell</code> and
<code>Mutex</code>.</p>
<h2 id="minimum-supported-rustc-version"><a class="doc-anchor" href="#minimum-supported-rustc-version">§</a>Minimum Supported <code>rustc</code> Version</h2>
<p>If only the <code>std</code>, <code>alloc</code>, or <code>race</code> features are enabled, MSRV will be
updated conservatively, supporting at least latest 8 versions of the compiler.
When using other features, like <code>parking_lot</code>, MSRV might be updated more
frequently, up to the latest stable. In both cases, increasing MSRV is <em>not</em>
considered a semver-breaking change and requires only a minor version bump.</p>
<h2 id="implementation-details"><a class="doc-anchor" href="#implementation-details">§</a>Implementation details</h2>
<p>The implementation is based on the
<a href="https://github.com/rust-lang-nursery/lazy-static.rs/"><code>lazy_static</code></a> and
<a href="https://github.com/indiv0/lazycell/"><code>lazy_cell</code></a> crates and
<a href="https://doc.rust-lang.org/std/sync/struct.Once.html"><code>std::sync::Once</code></a>. In some sense, <code>once_cell</code> just streamlines and unifies
those APIs.</p>
<p>To implement a sync flavor of <code>OnceCell</code>, this crates uses either a custom
re-implementation of <code>std::sync::Once</code> or <code>parking_lot::Mutex</code>. This is
controlled by the <code>parking_lot</code> feature (disabled by default). Performance
is the same for both cases, but the <code>parking_lot</code> based <code>OnceCell&lt;T&gt;</code> is
smaller by up to 16 bytes.</p>
<p>This crate uses <code>unsafe</code>.</p>
<h2 id="faq"><a class="doc-anchor" href="#faq">§</a>F.A.Q.</h2>
<p><strong>Should I use the sync or unsync flavor?</strong></p>
<p>Because Rust compiler checks thread safety for you, it’s impossible to
accidentally use <code>unsync</code> where <code>sync</code> is required. So, use <code>unsync</code> in
single-threaded code and <code>sync</code> in multi-threaded. It’s easy to switch
between the two if code becomes multi-threaded later.</p>
<p>At the moment, <code>unsync</code> has an additional benefit that reentrant
initialization causes a panic, which might be easier to debug than a
deadlock.</p>
<p><strong>Does this crate support async?</strong></p>
<p>No, but you can use
<a href="https://crates.io/crates/async_once_cell"><code>async_once_cell</code></a> instead.</p>
<p><strong>Does this crate support <code>no_std</code>?</strong></p>
<p>Yes, but with caveats. <code>OnceCell</code> is a synchronization primitive which
<em>semantically</em> relies on blocking. <code>OnceCell</code> guarantees that at most one
<code>f</code> will be called to compute the value. If two threads of execution call
<code>get_or_init</code> concurrently, one of them has to wait.</p>
<p>Waiting fundamentally requires OS support. Execution environment needs to
understand who waits on whom to prevent deadlocks due to priority inversion.
You <em>could</em> make code to compile by blindly using pure spinlocks, but the
runtime behavior would be subtly wrong.</p>
<p>Given these constraints, <code>once_cell</code> provides the following options:</p>
<ul>
<li>The <code>race</code> module provides similar, but distinct synchronization primitive
which is compatible with <code>no_std</code>. With <code>race</code>, the <code>f</code> function can be
called multiple times by different threads, but only one thread will win
to install the value.</li>
<li><code>critical-section</code> feature (with a <code>-</code>, not <code>_</code>) uses <code>critical_section</code>
to implement blocking.</li>
</ul>
<p><strong>Can I bring my own mutex?</strong></p>
<p>There is <a href="https://crates.io/crates/generic_once_cell">generic_once_cell</a> to
allow just that.</p>
<p><strong>Should I use <code>std::cell::OnceCell</code>, <code>once_cell</code>, or <code>lazy_static</code>?</strong></p>
<p>If you can use <code>std</code> version (your MSRV is at least 1.70, and you don’t need
extra features <code>once_cell</code> provides), use <code>std</code>. Otherwise, use <code>once_cell</code>.
Don’t use <code>lazy_static</code>.</p>
<h2 id="related-crates"><a class="doc-anchor" href="#related-crates">§</a>Related crates</h2>
<ul>
<li>Most of this crate’s functionality is available in <code>std</code> starting with
Rust 1.70. See <code>std::cell::OnceCell</code> and <code>std::sync::OnceLock</code>.</li>
<li><a href="https://github.com/niklasf/double-checked-cell">double-checked-cell</a></li>
<li><a href="https://crates.io/crates/lazy-init">lazy-init</a></li>
<li><a href="https://crates.io/crates/lazycell">lazycell</a></li>
<li><a href="https://crates.io/crates/mitochondria">mitochondria</a></li>
<li><a href="https://crates.io/crates/lazy_static">lazy_static</a></li>
<li><a href="https://crates.io/crates/async_once_cell">async_once_cell</a></li>
<li><a href="https://crates.io/crates/generic_once_cell">generic_once_cell</a> (bring
your own mutex)</li>
</ul>
</div></details><h2 id="modules" class="section-header">Modules<a href="#modules" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="mod" href="race/index.html" title="mod once_cell::race">race</a></div><div class="desc docblock-short">Thread-safe, non-blocking, “first one wins” flavor of <code>OnceCell</code>.</div></li><li><div class="item-name"><a class="mod" href="sync/index.html" title="mod once_cell::sync">sync</a></div><div class="desc docblock-short">Thread-safe, blocking version of <code>OnceCell</code>.</div></li><li><div class="item-name"><a class="mod" href="unsync/index.html" title="mod once_cell::unsync">unsync</a></div><div class="desc docblock-short">Single-threaded version of <code>OnceCell</code>.</div></li></ul></section></div></main></body></html>

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<p>If two threads race to initialize a type from the <code>race</code> module, they
don’t block, execute initialization function together, but only one of
them stores the result.</p>
<p>This module does not require <code>std</code> feature.</p>
<h2 id="atomic-orderings"><a class="doc-anchor" href="#atomic-orderings">§</a>Atomic orderings</h2>
<p>All types in this module use <code>Acquire</code> and <code>Release</code>
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getter thread’s perspective. This is an acceptable tradeoff to make since
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architectures versus <code>Relaxed</code>.</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.OnceBool.html" title="struct once_cell::race::OnceBool">Once<wbr>Bool</a></div><div class="desc docblock-short">A thread-safe cell which can be written to only once.</div></li><li><div class="item-name"><a class="struct" href="struct.OnceBox.html" title="struct once_cell::race::OnceBox">OnceBox</a></div><div class="desc docblock-short">A thread-safe cell which can be written to only once.</div></li><li><div class="item-name"><a class="struct" href="struct.OnceNonZeroUsize.html" title="struct once_cell::race::OnceNonZeroUsize">Once<wbr>NonZero<wbr>Usize</a></div><div class="desc docblock-short">A thread-safe cell which can be written to only once.</div></li><li><div class="item-name"><a class="struct" href="struct.OnceRef.html" title="struct once_cell::race::OnceRef">OnceRef</a></div><div class="desc docblock-short">A thread-safe cell which can be written to only once.</div></li></ul></section></div></main></body></html>

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