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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."><title>proc_macro_error::dummy - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-46f98efaafac5295.ttf.woff2,FiraSans-Regular-018c141bf0843ffd.woff2,FiraSans-Medium-8f9a781e4970d388.woff2,SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2,SourceCodePro-Semibold-d899c5a5c4aeb14a.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-76eba96aa4d2e634.css"><link rel="stylesheet" href="../../static.files/rustdoc-b0742ba02757f159.css"><meta name="rustdoc-vars" data-root-path="../../" data-static-root-path="../../static.files/" data-current-crate="proc_macro_error" data-themes="" data-resource-suffix="" data-rustdoc-version="1.83.0 (90b35a623 2024-11-26)" data-channel="1.83.0" data-search-js="search-f0d225181b97f9a4.js" data-settings-js="settings-805db61a62df4bd2.js" ><script src="../../static.files/storage-1d39b6787ed640ff.js"></script><script defer src="../sidebar-items.js"></script><script defer src="../../static.files/main-f070b9041d14864c.js"></script><noscript><link rel="stylesheet" href="../../static.files/noscript-0111fcff984fae8f.css"></noscript><link rel="alternate icon" type="image/png" href="../../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../../static.files/favicon-2c020d218678b618.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" title="show sidebar"></button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../../proc_macro_error/index.html">proc_<wbr>macro_<wbr>error</a><span class="version">1.0.4</span></h2></div><div class="sidebar-elems"><section id="rustdoc-toc"><h2 class="location"><a href="#">Module dummy</a></h2><h3><a href="#functions">Module Items</a></h3><ul class="block"><li><a href="#functions" title="Functions">Functions</a></li></ul></section><div id="rustdoc-modnav"><h2 class="in-crate"><a href="../index.html">In crate proc_<wbr>macro_<wbr>error</a></h2></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"><span class="rustdoc-breadcrumbs"><a href="../index.html">proc_macro_error</a></span><h1>Module <span>dummy</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/proc_macro_error/dummy.rs.html#1-150">source</a> </span></div><details class="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="example-wrap ignore"><a href="#" class="tooltip" title="This example is not tested">ⓘ</a><pre class="rust rust-example-rendered"><code><span class="kw">use </span>proc_macro::TokenStream;
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<span class="kw">use </span>proc_macro_error::<span class="kw-2">*</span>;
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<span class="kw">trait </span>MyTrait {
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<span class="kw">fn </span>do_thing();
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}
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<span class="comment">// this proc macro is supposed to generate MyTrait impl
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</span><span class="attr">#[proc_macro_derive(MyTrait)]
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#[proc_macro_error]
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</span><span class="kw">fn </span>example(input: TokenStream) -> TokenStream {
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<span class="comment">// somewhere deep inside
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</span><span class="macro">abort!</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
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</span><span class="macro">quote!</span> {
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<span class="kw">impl </span>MyTrait <span class="kw">for </span>#name {
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<span class="kw">fn </span>do_thing() {<span class="comment">/* whatever */</span>}
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}
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}
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}
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<span class="comment">// ================
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// in main.rs
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// this derive triggers an error
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</span><span class="attr">#[derive(MyTrait)] </span><span class="comment">// first BOOM!
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</span><span class="kw">struct </span>Foo;
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<span class="kw">fn </span>main() {
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Foo::do_thing(); <span class="comment">// second BOOM!
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</span>}</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="example-wrap ignore"><a href="#" class="tooltip" title="This example is not tested">ⓘ</a><pre class="rust rust-example-rendered"><code><span class="kw">use </span>proc_macro::TokenStream;
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<span class="kw">use </span>proc_macro_error::<span class="kw-2">*</span>;
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<span class="kw">trait </span>MyTrait {
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<span class="kw">fn </span>do_thing();
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}
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<span class="comment">// this proc macro is supposed to generate MyTrait impl
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</span><span class="attr">#[proc_macro_derive(MyTrait)]
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#[proc_macro_error]
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</span><span class="kw">fn </span>example(input: TokenStream) -> TokenStream {
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<span class="comment">// first of all - we set a dummy impl which will be appended to
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// `compile_error!` invocations in case a trigger does happen
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</span>set_dummy(<span class="macro">quote!</span> {
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<span class="kw">impl </span>MyTrait <span class="kw">for </span>#name {
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<span class="kw">fn </span>do_thing() { <span class="macro">unimplemented!</span>() }
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}
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});
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<span class="comment">// somewhere deep inside
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</span><span class="macro">abort!</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
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</span><span class="macro">quote!</span> {
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<span class="kw">impl </span>MyTrait <span class="kw">for </span>#name {
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<span class="kw">fn </span>do_thing() {<span class="comment">/* whatever */</span>}
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}
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}
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}
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<span class="comment">// ================
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// in main.rs
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// this derive triggers an error
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</span><span class="attr">#[derive(MyTrait)] </span><span class="comment">// first BOOM!
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</span><span class="kw">struct </span>Foo;
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<span class="kw">fn </span>main() {
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Foo::do_thing(); <span class="comment">// no more errors!
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</span>}</code></pre></div>
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</div></details><h2 id="functions" class="section-header">Functions<a href="#functions" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="fn" href="fn.append_dummy.html" title="fn proc_macro_error::dummy::append_dummy">append_<wbr>dummy</a></div><div class="desc docblock-short">Same as <a href="fn.set_dummy.html" title="fn proc_macro_error::dummy::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.</div></li><li><div class="item-name"><a class="fn" href="fn.set_dummy.html" title="fn proc_macro_error::dummy::set_dummy">set_<wbr>dummy</a></div><div class="desc docblock-short">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.</div></li></ul></section></div></main></body></html>
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