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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."><title>parse_macro_input in syn - 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-492a78a4a87dcc01.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="syn" data-themes="" data-resource-suffix="" data-rustdoc-version="1.82.0 (f6e511eec 2024-10-15)" data-channel="1.82.0" data-search-js="search-a99f1315e7cc5121.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../static.files/storage-118b08c4c78b968e.js"></script><script defer src="sidebar-items.js"></script><script defer src="../static.files/main-921df33f47b8780c.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-3b12f09e550e0385.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 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" title="show sidebar"></button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../syn/index.html">syn</a><span class="version">2.0.85</span></h2></div><div class="sidebar-elems"></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"><h1>Macro <a href="index.html">syn</a>::<wbr><a class="macro" href="#">parse_macro_input</a><button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><span class="out-of-band"><a class="src" href="../src/syn/parse_macro_input.rs.html#108-128">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</span>]</button></span></div><pre class="rust item-decl"><span class="macro">macro_rules!</span> parse_macro_input {
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(<span class="macro-nonterminal">$tokenstream</span>:ident <span class="kw">as </span><span class="macro-nonterminal">$ty</span>:ty) => { ... };
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(<span class="macro-nonterminal">$tokenstream</span>:ident with <span class="macro-nonterminal">$parser</span>:path) => { ... };
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(<span class="macro-nonterminal">$tokenstream</span>:ident) => { ... };
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}</pre><details class="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" title="mod syn::parse"><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 class="doc-anchor" href="#intended-usage">§</a>Intended usage</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>proc_macro::TokenStream;
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<span class="kw">use </span>syn::{parse_macro_input, <span class="prelude-ty">Result</span>};
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<span class="kw">use </span>syn::parse::{Parse, ParseStream};
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<span class="kw">struct </span>MyMacroInput {
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<span class="comment">/* ... */
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</span>}
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<span class="kw">impl </span>Parse <span class="kw">for </span>MyMacroInput {
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<span class="kw">fn </span>parse(input: ParseStream) -> <span class="prelude-ty">Result</span><<span class="self">Self</span>> {
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<span class="comment">/* ... */
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</span>}
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}
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<span class="attr">#[proc_macro]
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</span><span class="kw">pub fn </span>my_macro(tokens: TokenStream) -> TokenStream {
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<span class="kw">let </span>input = <span class="macro">parse_macro_input!</span>(tokens <span class="kw">as </span>MyMacroInput);
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<span class="comment">/* ... */
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</span>}</code></pre></div>
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<br>
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<h2 id="usage-with-parser"><a class="doc-anchor" href="#usage-with-parser">§</a>Usage with Parser</h2>
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<p>This macro can also be used with the <a href="parse/trait.Parser.html" title="trait syn::parse::Parser"><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>MyMacroInput {
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<span class="kw">fn </span>parse_alternate(input: ParseStream) -> <span class="prelude-ty">Result</span><<span class="self">Self</span>> {
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<span class="comment">/* ... */
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</span>}
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}
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<span class="attr">#[proc_macro]
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</span><span class="kw">pub fn </span>my_macro(tokens: TokenStream) -> TokenStream {
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<span class="kw">let </span>input = <span class="macro">parse_macro_input!</span>(tokens with MyMacroInput::parse_alternate);
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<span class="comment">/* ... */
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</span>}</code></pre></div>
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<br>
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<h2 id="expansion"><a class="doc-anchor" href="#expansion">§</a>Expansion</h2>
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<p><code>parse_macro_input!($variable as $Type)</code> expands to something like:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">match </span>syn::parse::<<span class="macro-nonterminal">$Type</span>>(<span class="macro-nonterminal">$variable</span>) {
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<span class="prelude-val">Ok</span>(syntax_tree) => syntax_tree,
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<span class="prelude-val">Err</span>(err) => <span class="kw">return </span>proc_macro::TokenStream::from(err.to_compile_error()),
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}</code></pre></div>
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</div></details></section></div></main></body></html>
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