| <!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="Asserts that the traits support dynamic dispatch (object-safety)."><meta name="keywords" content="rust, rustlang, rust-lang, assert_obj_safe"><title>assert_obj_safe in static_assertions - 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" href="../normalize.css"><link rel="stylesheet" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../ayu.css" disabled><link rel="stylesheet" href="../dark.css" disabled><link rel="stylesheet" href="../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../storage.js"></script><script defer src="sidebar-items.js"></script><script defer src="../main.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../favicon.svg"></head><body class="rustdoc macro"><!--[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">☰</button><a class="sidebar-logo" href="../static_assertions/index.html"><div class="logo-container"><img src="https://raw.githubusercontent.com/nvzqz/static-assertions-rs/assets/Icon.png" alt="logo"></div></a><h2></h2></nav><nav class="sidebar"><a class="sidebar-logo" href="../static_assertions/index.html"><div class="logo-container"> |
| <img src="https://raw.githubusercontent.com/nvzqz/static-assertions-rs/assets/Icon.png" alt="logo"></div></a><div class="sidebar-elems"><h2><a href="index.html">In static_assertions</a></h2></div></nav><main><div class="width-limiter"><nav class="sub"><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1 class="fqn">Macro <a href="index.html">static_assertions</a>::<wbr><a class="macro" href="#">assert_obj_safe</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></h1><span class="out-of-band"><a class="srclink" href="../src/static_assertions/assert_obj_safe.rs.html#72-76">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><div class="item-decl"><div class="example-wrap"><pre class="rust macro"><code><span class="macro">macro_rules! </span>assert_obj_safe { |
| ($(<span class="macro-nonterminal">$xs</span>:path),+ $(,)<span class="question-mark">?</span>) => { ... }; |
| }</code></pre></div> |
| </div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Asserts that the traits support dynamic dispatch |
| (<a href="https://doc.rust-lang.org/book/ch17-02-trait-objects.html#object-safety-is-required-for-trait-objects">object-safety</a>).</p> |
| <p>This is useful for when changes are made to a trait that accidentally |
| prevent it from being used as an <a href="https://doc.rust-lang.org/book/ch17-02-trait-objects.html#object-safety-is-required-for-trait-objects">object</a>. Such a case would be adding a |
| generic method and forgetting to add <code>where Self: Sized</code> after it. If left |
| unnoticed, that mistake will affect crate users and break both forward and |
| backward compatibility.</p> |
| <h2 id="examples"><a href="#examples">Examples</a></h2> |
| <p>When exposing a public API, it’s important that traits that could previously |
| use dynamic dispatch can still do so in future compatible crate versions.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">trait </span>MySafeTrait { |
| <span class="kw">fn </span>foo(<span class="kw-2">&</span><span class="self">self</span>) -> u32; |
| } |
| |
| <span class="macro">assert_obj_safe!</span>(std::fmt::Write, MySafeTrait);</code></pre></div> |
| <p>Works with traits that are not in the calling module:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">mod </span>inner { |
| <span class="kw">pub trait </span>BasicTrait { |
| <span class="kw">fn </span>bar(<span class="kw-2">&</span><span class="self">self</span>); |
| } |
| <span class="macro">assert_obj_safe!</span>(BasicTrait); |
| } |
| |
| <span class="macro">assert_obj_safe!</span>(inner::BasicTrait);</code></pre></div> |
| <p>The following example fails to compile because raw pointers cannot be sent |
| between threads safely:</p> |
| |
| <div class="example-wrap compile_fail"><div class='tooltip'>ⓘ</div><pre class="rust rust-example-rendered"><code><span class="macro">assert_impl!</span>(<span class="kw-2">*const </span>u8, Send);</code></pre></div> |
| <p>The following example fails to compile because generics without |
| <code>where Self: Sized</code> are not allowed in <a href="https://doc.rust-lang.org/book/ch17-02-trait-objects.html#object-safety-is-required-for-trait-objects">object-safe</a> trait methods:</p> |
| |
| <div class="example-wrap compile_fail"><div class='tooltip'>ⓘ</div><pre class="rust rust-example-rendered"><code><span class="kw">trait </span>MyUnsafeTrait { |
| <span class="kw">fn </span>baz<T>(<span class="kw-2">&</span><span class="self">self</span>) -> T; |
| } |
| |
| <span class="macro">assert_obj_safe!</span>(MyUnsafeTrait);</code></pre></div> |
| <p>When we fix that, the previous code will compile:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">trait </span>MyUnsafeTrait { |
| <span class="kw">fn </span>baz<T>(<span class="kw-2">&</span><span class="self">self</span>) -> T <span class="kw">where </span><span class="self">Self</span>: Sized; |
| } |
| |
| <span class="macro">assert_obj_safe!</span>(MyUnsafeTrait);</code></pre></div> |
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