| <!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="Make a parser that is usable with `parse_macro_input!` in a `#[proc_macro_attribute]` macro."><meta name="keywords" content="rust, rustlang, rust-lang, parser"><title>parser in syn::meta - 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 fn"><!--[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></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><a href="index.html">In syn::meta</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">Function <a href="../index.html">syn</a>::<wbr><a href="index.html">meta</a>::<wbr><a class="fn" href="#">parser</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/syn/meta.rs.html#131-139">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"><pre class="rust fn"><code>pub fn parser(<br> logic: impl <a class="trait" href="https://doc.rust-lang.org/nightly/core/ops/function/trait.FnMut.html" title="trait core::ops::function::FnMut">FnMut</a>(<a class="struct" href="struct.ParseNestedMeta.html" title="struct syn::meta::ParseNestedMeta">ParseNestedMeta</a><'_>) -> <a class="type" href="../parse/type.Result.html" title="type syn::parse::Result">Result</a><<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.unit.html">()</a>><br>) -> impl <a class="trait" href="../parse/trait.Parser.html" title="trait syn::parse::Parser">Parser</a><Output = <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.unit.html">()</a>></code></pre></div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Make a parser that is usable with <code>parse_macro_input!</code> in a |
| <code>#[proc_macro_attribute]</code> macro.</p> |
| <p><em>Warning:</em> When parsing attribute args <strong>other than</strong> the |
| <code>proc_macro::TokenStream</code> input of a <code>proc_macro_attribute</code>, you do <strong>not</strong> |
| need this function. In several cases your callers will get worse error |
| messages if you use this function, because the surrounding delimiter’s span |
| is concealed from attribute macros by rustc. Use |
| <a href="../struct.Attribute.html#method.parse_nested_meta"><code>Attribute::parse_nested_meta</code></a> instead.</p> |
| <h2 id="example"><a href="#example">Example</a></h2> |
| <p>This example implements an attribute macro whose invocations look like this:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attribute">#[tea(kind = <span class="string">"EarlGrey"</span>, hot)] |
| </span><span class="kw">struct </span>Picard {...}</code></pre></div> |
| <p>The “parameters” supported by the attribute are:</p> |
| <ul> |
| <li><code>kind = "..."</code></li> |
| <li><code>hot</code></li> |
| <li><code>with(sugar, milk, ...)</code>, a comma-separated list of ingredients</li> |
| </ul> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>proc_macro::TokenStream; |
| <span class="kw">use </span>syn::{parse_macro_input, LitStr, Path}; |
| |
| <span class="attribute">#[proc_macro_attribute] |
| </span><span class="kw">pub fn </span>tea(args: TokenStream, input: TokenStream) -> TokenStream { |
| <span class="kw">let </span><span class="kw-2">mut </span>kind: <span class="prelude-ty">Option</span><LitStr> = <span class="prelude-val">None</span>; |
| <span class="kw">let </span><span class="kw-2">mut </span>hot: bool = <span class="bool-val">false</span>; |
| <span class="kw">let </span><span class="kw-2">mut </span>with: Vec<Path> = Vec::new(); |
| <span class="kw">let </span>tea_parser = syn::meta::parser(|meta| { |
| <span class="kw">if </span>meta.path.is_ident(<span class="string">"kind"</span>) { |
| kind = <span class="prelude-val">Some</span>(meta.value()<span class="question-mark">?</span>.parse()<span class="question-mark">?</span>); |
| <span class="prelude-val">Ok</span>(()) |
| } <span class="kw">else if </span>meta.path.is_ident(<span class="string">"hot"</span>) { |
| hot = <span class="bool-val">true</span>; |
| <span class="prelude-val">Ok</span>(()) |
| } <span class="kw">else if </span>meta.path.is_ident(<span class="string">"with"</span>) { |
| meta.parse_nested_meta(|meta| { |
| with.push(meta.path); |
| <span class="prelude-val">Ok</span>(()) |
| }) |
| } <span class="kw">else </span>{ |
| <span class="prelude-val">Err</span>(meta.error(<span class="string">"unsupported tea property"</span>)) |
| } |
| }); |
| |
| <span class="macro">parse_macro_input!</span>(args with tea_parser); |
| <span class="macro">eprintln!</span>(<span class="string">"kind={kind:?} hot={hot} with={with:?}"</span>); |
| |
| <span class="comment">/* ... */ |
| </span>}</code></pre></div> |
| <p>The <code>syn::meta</code> library will take care of dealing with the commas including |
| trailing commas, and producing sensible error messages on unexpected input.</p> |
| <div class="example-wrap"><pre class="language-console"><code>error: expected `,` |
| --> src/main.rs:3:37 |
| | |
| 3 | #[tea(kind = "EarlGrey", with(sugar = "lol", milk))] |
| | ^</code></pre></div><h2 id="example-1"><a href="#example-1">Example</a></h2> |
| <p>Same as above but we factor out most of the logic into a separate function.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>proc_macro::TokenStream; |
| <span class="kw">use </span>syn::meta::ParseNestedMeta; |
| <span class="kw">use </span>syn::parse::{Parser, <span class="prelude-ty">Result</span>}; |
| <span class="kw">use </span>syn::{parse_macro_input, LitStr, Path}; |
| |
| <span class="attribute">#[proc_macro_attribute] |
| </span><span class="kw">pub fn </span>tea(args: TokenStream, input: TokenStream) -> TokenStream { |
| <span class="kw">let </span><span class="kw-2">mut </span>attrs = TeaAttributes::default(); |
| <span class="kw">let </span>tea_parser = syn::meta::parser(|meta| attrs.parse(meta)); |
| <span class="macro">parse_macro_input!</span>(args with tea_parser); |
| |
| <span class="comment">/* ... */ |
| </span>} |
| |
| <span class="attribute">#[derive(Default)] |
| </span><span class="kw">struct </span>TeaAttributes { |
| kind: <span class="prelude-ty">Option</span><LitStr>, |
| hot: bool, |
| with: Vec<Path>, |
| } |
| |
| <span class="kw">impl </span>TeaAttributes { |
| <span class="kw">fn </span>parse(<span class="kw-2">&mut </span><span class="self">self</span>, meta: ParseNestedMeta) -> <span class="prelude-ty">Result</span><()> { |
| <span class="kw">if </span>meta.path.is_ident(<span class="string">"kind"</span>) { |
| <span class="self">self</span>.kind = <span class="prelude-val">Some</span>(meta.value()<span class="question-mark">?</span>.parse()<span class="question-mark">?</span>); |
| <span class="prelude-val">Ok</span>(()) |
| } <span class="kw">else </span><span class="comment">/* just like in last example */ |
| |
| </span>} |
| }</code></pre></div> |
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