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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="github crates-io docs-rs"><meta name="keywords" content="rust, rustlang, rust-lang, async_trait"><title>async_trait - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Semibold.ttf.woff2"><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="../ayu.css" disabled><link rel="stylesheet" type="text/css" href="../dark.css" disabled><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../storage.js"></script><script src="../crates.js"></script><script defer src="../main.js"></script>
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</a><h2 class="location">Crate async_trait</h2><div class="block version"><div class="narrow-helper"></div><p>Version 0.1.52</p></div><div class="sidebar-elems"><a id="all-types" href="all.html"><p>See all async_trait's items</p></a><div id="sidebar-vars" data-name="async_trait" data-ty="mod" data-relpath=""></div><script defer src="sidebar-items.js"></script></div></nav><main><div class="width-limiter"><div class="sub-container"><a class="sub-logo-container" href="../async_trait/index.html"><img class="rust-logo" src="../rust-logo.png" alt="logo"></a><nav class="sub"><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu" title="themes"><img width="18" height="18" alt="Pick another theme!" src="../brush.svg"></button><div id="theme-choices" role="menu"></div></div><form class="search-form"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><button type="button" id="help-button" title="help">?</button><a id="settings-menu" href="../settings.html" title="settings"><img width="18" height="18" alt="Change settings" src="../wheel.svg"></a></div></form></nav></div><section id="main-content" class="content"><h1 class="fqn"><span class="in-band">Crate <a class="mod" href="#">async_trait</a><button id="copy-path" onclick="copy_path(this)" title="Copy item path to clipboard"><img src="../clipboard.svg" width="19" height="18" alt="Copy item path"></button></span><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/async_trait/lib.rs.html#1-338" title="goto source code">[src]</a></span></h1><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p><a href="https://github.com/dtolnay/async-trait"><img src="https://img.shields.io/badge/github-8da0cb?style=for-the-badge&labelColor=555555&logo=github" alt="github" /></a> <a href="https://crates.io/crates/async-trait"><img src="https://img.shields.io/badge/crates.io-fc8d62?style=for-the-badge&labelColor=555555&logo=rust" alt="crates-io" /></a> <a href="https://docs.rs/async-trait"><img src="https://img.shields.io/badge/docs.rs-66c2a5?style=for-the-badge&labelColor=555555&logoColor=white&logo=data:image/svg+xml;base64,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" alt="docs-rs" /></a></p>
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<br>
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<h5>Type erasure for async trait methods</h5>
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<p>The initial round of stabilizations for the async/await language feature in
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Rust 1.39 did not include support for async fn in traits. Trying to include
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an async fn in a trait produces the following error:</p>
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<div class='information'><div class='tooltip compile_fail'>ⓘ</div></div><div class="example-wrap"><pre class="rust rust-example-rendered compile_fail"><code><span class="kw">trait</span> <span class="ident">MyTrait</span> {
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<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">f</span>() {}
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}</code></pre></div>
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<div class="example-wrap"><pre class="language-text"><code>error[E0706]: trait fns cannot be declared `async`
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--> src/main.rs:4:5
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|
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4 | async fn f() {}
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| ^^^^^^^^^^^^^^^</code></pre></div>
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<p>This crate provides an attribute macro to make async fn in traits work.</p>
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<p>Please refer to <a href="https://smallcultfollowing.com/babysteps/blog/2019/10/26/async-fn-in-traits-are-hard/"><em>why async fn in traits are hard</em></a> for a deeper
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analysis of how this implementation differs from what the compiler and
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language hope to deliver in the future.</p>
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<br>
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<h2 id="example" class="section-header"><a href="#example">Example</a></h2>
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<p>This example implements the core of a highly effective advertising platform
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using async fn in a trait.</p>
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<p>The only thing to notice here is that we write an <code>#[async_trait]</code> macro on
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top of traits and trait impls that contain async fn, and then they work.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use</span> <span class="ident">async_trait::async_trait</span>;
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<span class="attribute">#[<span class="ident">async_trait</span>]</span>
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<span class="kw">trait</span> <span class="ident">Advertisement</span> {
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<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">run</span>(<span class="kw-2">&</span><span class="self">self</span>);
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}
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<span class="kw">struct</span> <span class="ident">Modal</span>;
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<span class="attribute">#[<span class="ident">async_trait</span>]</span>
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<span class="kw">impl</span> <span class="ident">Advertisement</span> <span class="kw">for</span> <span class="ident">Modal</span> {
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<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">run</span>(<span class="kw-2">&</span><span class="self">self</span>) {
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<span class="self">self</span>.<span class="ident">render_fullscreen</span>().<span class="kw">await</span>;
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<span class="kw">for</span> <span class="kw">_</span> <span class="kw">in</span> <span class="number">0</span>..<span class="number">4u16</span> {
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<span class="ident">remind_user_to_join_mailing_list</span>().<span class="kw">await</span>;
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}
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<span class="self">self</span>.<span class="ident">hide_for_now</span>().<span class="kw">await</span>;
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}
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}
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<span class="kw">struct</span> <span class="ident">AutoplayingVideo</span> {
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<span class="ident">media_url</span>: <span class="ident">String</span>,
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}
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<span class="attribute">#[<span class="ident">async_trait</span>]</span>
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<span class="kw">impl</span> <span class="ident">Advertisement</span> <span class="kw">for</span> <span class="ident">AutoplayingVideo</span> {
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<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">run</span>(<span class="kw-2">&</span><span class="self">self</span>) {
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<span class="kw">let</span> <span class="ident">stream</span> <span class="op">=</span> <span class="ident">connect</span>(<span class="kw-2">&</span><span class="self">self</span>.<span class="ident">media_url</span>).<span class="kw">await</span>;
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<span class="ident">stream</span>.<span class="ident">play</span>().<span class="kw">await</span>;
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<span class="comment">// Video probably persuaded user to join our mailing list!</span>
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<span class="ident">Modal</span>.<span class="ident">run</span>().<span class="kw">await</span>;
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}
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}</code></pre></div>
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<p><br><br></p>
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<h2 id="supported-features" class="section-header"><a href="#supported-features">Supported features</a></h2>
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<p>It is the intention that all features of Rust traits should work nicely with
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#[async_trait], but the edge cases are numerous. Please file an issue if
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you see unexpected borrow checker errors, type errors, or warnings. There is
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no use of <code>unsafe</code> in the expanded code, so rest assured that if your code
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compiles it can’t be that badly broken.</p>
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<blockquote>
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<p>☑ Self by value, by reference, by mut reference, or no self;<br>
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☑ Any number of arguments, any return value;<br>
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☑ Generic type parameters and lifetime parameters;<br>
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☑ Associated types;<br>
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☑ Having async and non-async functions in the same trait;<br>
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☑ Default implementations provided by the trait;<br>
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☑ Elided lifetimes;<br>
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☑ Dyn-capable traits.<br></p>
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</blockquote>
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<br>
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<h2 id="explanation" class="section-header"><a href="#explanation">Explanation</a></h2>
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<p>Async fns get transformed into methods that return <code>Pin<Box<dyn Future + Send + 'async>></code> and delegate to a private async freestanding function.</p>
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<p>For example the <code>impl Advertisement for AutoplayingVideo</code> above would be
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expanded as:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">impl</span> <span class="ident">Advertisement</span> <span class="kw">for</span> <span class="ident">AutoplayingVideo</span> {
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<span class="kw">fn</span> <span class="ident">run</span><span class="op"><</span><span class="lifetime">'async</span><span class="op">></span>(
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<span class="kw-2">&</span><span class="lifetime">'async</span> <span class="self">self</span>,
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) -> <span class="ident">Pin</span><span class="op"><</span><span class="ident">Box</span><span class="op"><</span><span class="kw">dyn</span> <span class="ident">core::future::Future</span><span class="op"><</span><span class="ident">Output</span> <span class="op">=</span> ()<span class="op">></span> <span class="op">+</span> <span class="ident">Send</span> <span class="op">+</span> <span class="lifetime">'async</span><span class="op">></span><span class="op">></span>
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<span class="kw">where</span>
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<span class="self">Self</span>: <span class="ident">Sync</span> <span class="op">+</span> <span class="lifetime">'async</span>,
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{
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<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">run</span>(<span class="ident">_self</span>: <span class="kw-2">&</span><span class="ident">AutoplayingVideo</span>) {
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<span class="comment">/* the original method body */</span>
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}
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<span class="ident">Box::pin</span>(<span class="ident">run</span>(<span class="self">self</span>))
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}
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}</code></pre></div>
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<p><br><br></p>
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<h2 id="non-threadsafe-futures" class="section-header"><a href="#non-threadsafe-futures">Non-threadsafe futures</a></h2>
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<p>Not all async traits need futures that are <code>dyn Future + Send</code>. To avoid
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having Send and Sync bounds placed on the async trait methods, invoke the
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async trait macro as <code>#[async_trait(?Send)]</code> on both the trait and the impl
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blocks.</p>
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<br>
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<h2 id="elided-lifetimes" class="section-header"><a href="#elided-lifetimes">Elided lifetimes</a></h2>
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<p>Be aware that async fn syntax does not allow lifetime elision outside of <code>&</code>
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and <code>&mut</code> references. (This is true even when not using #[async_trait].)
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Lifetimes must be named or marked by the placeholder <code>'_</code>.</p>
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<p>Fortunately the compiler is able to diagnose missing lifetimes with a good
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error message.</p>
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<div class='information'><div class='tooltip compile_fail'>ⓘ</div></div><div class="example-wrap"><pre class="rust rust-example-rendered compile_fail"><code><span class="kw">type</span> <span class="ident">Elided</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">></span> <span class="op">=</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="ident">usize</span>;
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<span class="attribute">#[<span class="ident">async_trait</span>]</span>
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<span class="kw">trait</span> <span class="ident">Test</span> {
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<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">test</span>(<span class="ident">not_okay</span>: <span class="ident">Elided</span>, <span class="ident">okay</span>: <span class="kw-2">&</span><span class="ident">usize</span>) {}
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}</code></pre></div>
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<div class="example-wrap"><pre class="language-text"><code>error[E0726]: implicit elided lifetime not allowed here
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--> src/main.rs:9:29
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|
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9 | async fn test(not_okay: Elided, okay: &usize) {}
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| ^^^^^^- help: indicate the anonymous lifetime: `<'_>`</code></pre></div>
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<p>The fix is to name the lifetime or use <code>'_</code>.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attribute">#[<span class="ident">async_trait</span>]</span>
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<span class="kw">trait</span> <span class="ident">Test</span> {
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<span class="comment">// either</span>
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<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">test</span><span class="op"><</span><span class="lifetime">'e</span><span class="op">></span>(<span class="ident">elided</span>: <span class="ident">Elided</span><span class="op"><</span><span class="lifetime">'e</span><span class="op">></span>) {}
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<span class="comment">// or</span>
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<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">test</span>(<span class="ident">elided</span>: <span class="ident">Elided</span><span class="op"><</span><span class="lifetime">'_</span><span class="op">></span>) {}
|
||
}</code></pre></div>
|
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<p><br><br></p>
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<h2 id="dyn-traits" class="section-header"><a href="#dyn-traits">Dyn traits</a></h2>
|
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<p>Traits with async methods can be used as trait objects as long as they meet
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the usual requirements for dyn – no methods with type parameters, no self
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by value, no associated types, etc.</p>
|
||
|
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attribute">#[<span class="ident">async_trait</span>]</span>
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<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">ObjectSafe</span> {
|
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<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">f</span>(<span class="kw-2">&</span><span class="self">self</span>);
|
||
<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">g</span>(<span class="kw-2">&mut</span> <span class="self">self</span>);
|
||
}
|
||
|
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<span class="kw">impl</span> <span class="ident">ObjectSafe</span> <span class="kw">for</span> <span class="ident">MyType</span> {...}
|
||
|
||
<span class="kw">let</span> <span class="ident">value</span>: <span class="ident">MyType</span> <span class="op">=</span> ...;
|
||
<span class="kw">let</span> <span class="ident">object</span> <span class="op">=</span> <span class="kw-2">&</span><span class="ident">value</span> <span class="kw">as</span> <span class="kw-2">&</span><span class="kw">dyn</span> <span class="ident">ObjectSafe</span>; <span class="comment">// make trait object</span></code></pre></div>
|
||
<p>The one wrinkle is in traits that provide default implementations of async
|
||
methods. In order for the default implementation to produce a future that is
|
||
Send, the async_trait macro must emit a bound of <code>Self: Sync</code> on trait
|
||
methods that take <code>&self</code> and a bound <code>Self: Send</code> on trait methods that
|
||
take <code>&mut self</code>. An example of the former is visible in the expanded code
|
||
in the explanation section above.</p>
|
||
<p>If you make a trait with async methods that have default implementations,
|
||
everything will work except that the trait cannot be used as a trait object.
|
||
Creating a value of type <code>&dyn Trait</code> will produce an error that looks like
|
||
this:</p>
|
||
<div class="example-wrap"><pre class="language-text"><code>error: the trait `Test` cannot be made into an object
|
||
--> src/main.rs:8:5
|
||
|
|
||
8 | async fn cannot_dyn(&self) {}
|
||
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^</code></pre></div>
|
||
<p>For traits that need to be object safe and need to have default
|
||
implementations for some async methods, there are two resolutions. Either
|
||
you can add Send and/or Sync as supertraits (Send if there are <code>&mut self</code>
|
||
methods with default implementations, Sync if there are <code>&self</code> methods with
|
||
default implementations) to constrain all implementors of the trait such that
|
||
the default implementations are applicable to them:</p>
|
||
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attribute">#[<span class="ident">async_trait</span>]</span>
|
||
<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">ObjectSafe</span>: <span class="ident">Sync</span> { <span class="comment">// added supertrait</span>
|
||
<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">can_dyn</span>(<span class="kw-2">&</span><span class="self">self</span>) {}
|
||
}
|
||
|
||
<span class="kw">let</span> <span class="ident">object</span> <span class="op">=</span> <span class="kw-2">&</span><span class="ident">value</span> <span class="kw">as</span> <span class="kw-2">&</span><span class="kw">dyn</span> <span class="ident">ObjectSafe</span>;</code></pre></div>
|
||
<p>or you can strike the problematic methods from your trait object by
|
||
bounding them with <code>Self: Sized</code>:</p>
|
||
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attribute">#[<span class="ident">async_trait</span>]</span>
|
||
<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">ObjectSafe</span> {
|
||
<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">cannot_dyn</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="kw">where</span> <span class="self">Self</span>: <span class="ident">Sized</span> {}
|
||
|
||
<span class="comment">// presumably other methods</span>
|
||
}
|
||
|
||
<span class="kw">let</span> <span class="ident">object</span> <span class="op">=</span> <span class="kw-2">&</span><span class="ident">value</span> <span class="kw">as</span> <span class="kw-2">&</span><span class="kw">dyn</span> <span class="ident">ObjectSafe</span>;</code></pre></div>
|
||
</div></details><h2 id="attributes" class="small-section-header"><a href="#attributes">Attribute Macros</a></h2>
|
||
<div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="attr" href="attr.async_trait.html" title="async_trait::async_trait attr">async_trait</a></div><div class="item-right docblock-short"></div></div></div></section><section id="search" class="content hidden"></section></div></main><div id="rustdoc-vars" data-root-path="../" data-current-crate="async_trait" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.59.0 (9d1b2106e 2022-02-23)" ></div>
|
||
</body></html> |