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106 lines
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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="Facility to emit dummy implementations (or whatever) in case an error happen."><meta name="keywords" content="rust, rustlang, rust-lang, dummy"><title>proc_macro_error::dummy - 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 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 mod"><!--[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="../../proc_macro_error/index.html"><div class="logo-container"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></div></a><h2 class="location"></h2></nav><nav class="sidebar"><a class="sidebar-logo" href="../../proc_macro_error/index.html"><div class="logo-container"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></div></a><h2 class="location"><a href="#">Module dummy</a></h2><div class="sidebar-elems"><section><div class="block"><ul><li><a href="#functions">Functions</a></li></ul></div></section></div></nav><main><div class="width-limiter"><div class="sub-container"><a class="sub-logo-container" href="../../proc_macro_error/index.html"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></a><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"><button type="button">?</button></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></div><section id="main-content" class="content"><div class="main-heading"><h1 class="fqn"><span class="in-band">Module <a href="../index.html">proc_macro_error</a>::<wbr><a class="mod" href="#">dummy</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></h1><span class="out-of-band"><a class="srclink" href="../../src/proc_macro_error/dummy.rs.html#1-150">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Facility to emit dummy implementations (or whatever) in case
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an error happen.</p>
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<p><code>compile_error!</code> does not abort a compilation right away. This means
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<code>rustc</code> doesn’t just show you the error and abort, it carries on the
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compilation process looking for other errors to report.</p>
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<p>Let’s consider an example:</p>
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<div class='information'><div class='tooltip ignore'>ⓘ</div></div><div class="example-wrap"><pre class="rust rust-example-rendered ignore"><code><span class="kw">use</span> <span class="ident">proc_macro::TokenStream</span>;
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<span class="kw">use</span> <span class="ident">proc_macro_error</span>::<span class="kw-2">*</span>;
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<span class="kw">trait</span> <span class="ident">MyTrait</span> {
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<span class="kw">fn</span> <span class="ident">do_thing</span>();
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}
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<span class="comment">// this proc macro is supposed to generate MyTrait impl</span>
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<span class="attribute">#[<span class="ident">proc_macro_derive</span>(<span class="ident">MyTrait</span>)]</span>
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<span class="attribute">#[<span class="ident">proc_macro_error</span>]</span>
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<span class="kw">fn</span> <span class="ident">example</span>(<span class="ident">input</span>: <span class="ident">TokenStream</span>) -> <span class="ident">TokenStream</span> {
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<span class="comment">// somewhere deep inside</span>
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<span class="macro">abort!</span>(<span class="ident">span</span>, <span class="string">"something's wrong"</span>);
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<span class="comment">// this implementation will be generated if no error happened</span>
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<span class="macro">quote!</span> {
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<span class="kw">impl</span> <span class="ident">MyTrait</span> <span class="kw">for</span> #<span class="ident">name</span> {
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<span class="kw">fn</span> <span class="ident">do_thing</span>() {<span class="comment">/* whatever */</span>}
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}
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}
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}
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<span class="comment">// ================</span>
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<span class="comment">// in main.rs</span>
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<span class="comment">// this derive triggers an error</span>
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<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">MyTrait</span>)]</span> <span class="comment">// first BOOM!</span>
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<span class="kw">struct</span> <span class="ident">Foo</span>;
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<span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="ident">Foo::do_thing</span>(); <span class="comment">// second BOOM!</span>
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}</code></pre></div>
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<p>The problem is: the generated token stream contains only <code>compile_error!</code>
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invocation, the impl was not generated. That means user will see two compilation
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errors:</p>
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<div class="example-wrap"><pre class="language-text"><code>error: something's wrong
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--> $DIR/probe.rs:9:10
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9 |#[proc_macro_derive(MyTrait)]
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| ^^^^^^^
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error[E0599]: no function or associated item named `do_thing` found for type `Foo` in the current scope
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--> src\main.rs:3:10
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1 | struct Foo;
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| ----------- function or associated item `do_thing` not found for this
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2 | fn main() {
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3 | Foo::do_thing(); // second BOOM!
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| ^^^^^^^^ function or associated item not found in `Foo`</code></pre></div>
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<p>But the second error is meaningless! We definitely need to fix this.</p>
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<p>Most used approach in cases like this is “dummy implementation” -
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omit <code>impl MyTrait for #name</code> and fill functions bodies with <code>unimplemented!()</code>.</p>
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<p>This is how you do it:</p>
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<div class='information'><div class='tooltip ignore'>ⓘ</div></div><div class="example-wrap"><pre class="rust rust-example-rendered ignore"><code><span class="kw">use</span> <span class="ident">proc_macro::TokenStream</span>;
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<span class="kw">use</span> <span class="ident">proc_macro_error</span>::<span class="kw-2">*</span>;
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<span class="kw">trait</span> <span class="ident">MyTrait</span> {
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<span class="kw">fn</span> <span class="ident">do_thing</span>();
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}
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<span class="comment">// this proc macro is supposed to generate MyTrait impl</span>
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<span class="attribute">#[<span class="ident">proc_macro_derive</span>(<span class="ident">MyTrait</span>)]</span>
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<span class="attribute">#[<span class="ident">proc_macro_error</span>]</span>
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<span class="kw">fn</span> <span class="ident">example</span>(<span class="ident">input</span>: <span class="ident">TokenStream</span>) -> <span class="ident">TokenStream</span> {
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<span class="comment">// first of all - we set a dummy impl which will be appended to</span>
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<span class="comment">// `compile_error!` invocations in case a trigger does happen</span>
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<span class="ident">set_dummy</span>(<span class="macro">quote!</span> {
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<span class="kw">impl</span> <span class="ident">MyTrait</span> <span class="kw">for</span> #<span class="ident">name</span> {
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<span class="kw">fn</span> <span class="ident">do_thing</span>() { <span class="macro">unimplemented!</span>() }
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}
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});
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<span class="comment">// somewhere deep inside</span>
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<span class="macro">abort!</span>(<span class="ident">span</span>, <span class="string">"something's wrong"</span>);
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<span class="comment">// this implementation will be generated if no error happened</span>
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<span class="macro">quote!</span> {
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<span class="kw">impl</span> <span class="ident">MyTrait</span> <span class="kw">for</span> #<span class="ident">name</span> {
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<span class="kw">fn</span> <span class="ident">do_thing</span>() {<span class="comment">/* whatever */</span>}
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}
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}
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}
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<span class="comment">// ================</span>
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<span class="comment">// in main.rs</span>
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<span class="comment">// this derive triggers an error</span>
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<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">MyTrait</span>)]</span> <span class="comment">// first BOOM!</span>
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<span class="kw">struct</span> <span class="ident">Foo</span>;
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<span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="ident">Foo::do_thing</span>(); <span class="comment">// no more errors!</span>
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}</code></pre></div>
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</div></details><h2 id="functions" class="small-section-header"><a href="#functions">Functions</a></h2><div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="fn" href="fn.append_dummy.html" title="proc_macro_error::dummy::append_dummy fn">append_dummy</a></div><div class="item-right docblock-short"><p>Same as <a href="fn.set_dummy.html" title="set_dummy"><code>set_dummy</code></a> but, instead of resetting, appends tokens to the
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existing dummy (if any). Behaves as <code>set_dummy</code> if no dummy is present.</p>
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</div></div><div class="item-row"><div class="item-left module-item"><a class="fn" href="fn.set_dummy.html" title="proc_macro_error::dummy::set_dummy fn">set_dummy</a></div><div class="item-right docblock-short"><p>Sets dummy token stream which will be appended to <code>compile_error!(msg);...</code>
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invocations in case you’ll emit any errors.</p>
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</div></div></div></section></div></main><div id="rustdoc-vars" data-root-path="../../" data-current-crate="proc_macro_error" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.64.0 (a55dd71d5 2022-09-19)" ></div></body></html> |