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62 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="Parse the input TokenStream of a macro, triggering a compile error if the tokens fail to parse."><meta name="keywords" content="rust, rustlang, rust-lang, parse_macro_input"><title>parse_macro_input in syn - 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="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="../syn/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="../syn/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><div class="sidebar-elems"><h2 class="location"><a href="index.html">In syn</a></h2></div></nav><main><div class="width-limiter"><div class="sub-container"><a class="sub-logo-container" href="../syn/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">Macro <a href="index.html">syn</a>::<wbr><a class="macro" href="#">parse_macro_input</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/syn/parse_macro_input.rs.html#108-128">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><div class="docblock item-decl"><div class="example-wrap"><pre class="rust macro"><code><span class="macro">macro_rules!</span> <span class="ident">parse_macro_input</span> {
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(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">tokenstream</span>:<span class="ident">ident</span> <span class="kw">as</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">ty</span>:<span class="ident">ty</span>) => { ... };
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(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">tokenstream</span>:<span class="ident">ident</span> <span class="ident">with</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">parser</span>:<span class="ident">path</span>) => { ... };
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(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">tokenstream</span>:<span class="ident">ident</span>) => { ... };
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}</code></pre></div>
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</div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Parse the input TokenStream of a macro, triggering a compile error if the
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tokens fail to parse.</p>
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<p>Refer to the <a href="parse/index.html"><code>parse</code> module</a> documentation for more details about parsing
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in Syn.</p>
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<br>
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<h2 id="intended-usage"><a href="#intended-usage">Intended usage</a></h2>
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<p>This macro must be called from a function that returns
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<code>proc_macro::TokenStream</code>. Usually this will be your proc macro entry point,
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the function that has the #[proc_macro] / #[proc_macro_derive] /
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#[proc_macro_attribute] attribute.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><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">syn</span>::{<span class="ident">parse_macro_input</span>, <span class="prelude-ty">Result</span>};
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<span class="kw">use</span> <span class="ident">syn::parse</span>::{<span class="ident">Parse</span>, <span class="ident">ParseStream</span>};
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<span class="kw">struct</span> <span class="ident">MyMacroInput</span> {
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<span class="comment">/* ... */</span>
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}
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<span class="kw">impl</span> <span class="ident">Parse</span> <span class="kw">for</span> <span class="ident">MyMacroInput</span> {
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<span class="kw">fn</span> <span class="ident">parse</span>(<span class="ident">input</span>: <span class="ident">ParseStream</span>) -> <span class="prelude-ty">Result</span><span class="op"><</span><span class="self">Self</span><span class="op">></span> {
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<span class="comment">/* ... */</span>
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}
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}
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<span class="attribute">#[<span class="ident">proc_macro</span>]</span>
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<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">my_macro</span>(<span class="ident">tokens</span>: <span class="ident">TokenStream</span>) -> <span class="ident">TokenStream</span> {
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<span class="kw">let</span> <span class="ident">input</span> <span class="op">=</span> <span class="macro">parse_macro_input!</span>(<span class="ident">tokens</span> <span class="kw">as</span> <span class="ident">MyMacroInput</span>);
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<span class="comment">/* ... */</span>
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}</code></pre></div>
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<br>
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<h2 id="usage-with-parser"><a href="#usage-with-parser">Usage with Parser</a></h2>
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<p>This macro can also be used with the <a href="parse/trait.Parser.html"><code>Parser</code> trait</a> for types that have
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multiple ways that they can be parsed.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">impl</span> <span class="ident">MyMacroInput</span> {
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<span class="kw">fn</span> <span class="ident">parse_alternate</span>(<span class="ident">input</span>: <span class="ident">ParseStream</span>) -> <span class="prelude-ty">Result</span><span class="op"><</span><span class="self">Self</span><span class="op">></span> {
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<span class="comment">/* ... */</span>
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}
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}
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<span class="attribute">#[<span class="ident">proc_macro</span>]</span>
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<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">my_macro</span>(<span class="ident">tokens</span>: <span class="ident">TokenStream</span>) -> <span class="ident">TokenStream</span> {
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<span class="kw">let</span> <span class="ident">input</span> <span class="op">=</span> <span class="macro">parse_macro_input!</span>(<span class="ident">tokens</span> <span class="ident">with</span> <span class="ident">MyMacroInput::parse_alternate</span>);
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<span class="comment">/* ... */</span>
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}</code></pre></div>
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<br>
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<h2 id="expansion"><a href="#expansion">Expansion</a></h2>
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<p><code>parse_macro_input!($variable as $Type)</code> expands to something like:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">match</span> <span class="ident">syn::parse</span>::<span class="op"><</span><span class="macro-nonterminal">$</span><span class="macro-nonterminal">Type</span><span class="op">></span>(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">variable</span>) {
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<span class="prelude-val">Ok</span>(<span class="ident">syntax_tree</span>) => <span class="ident">syntax_tree</span>,
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<span class="prelude-val">Err</span>(<span class="ident">err</span>) => <span class="kw">return</span> <span class="ident">proc_macro::TokenStream::from</span>(<span class="ident">err</span>.<span class="ident">to_compile_error</span>()),
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}</code></pre></div>
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