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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="Define a type that supports parsing and printing a given identifier as if it were a keyword."><meta name="keywords" content="rust, rustlang, rust-lang, custom_keyword"><title>custom_keyword 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="#">custom_keyword</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/custom_keyword.rs.html#90-120">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">custom_keyword</span> {
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(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">ident</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>Define a type that supports parsing and printing a given identifier as if it
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were a keyword.</p>
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<h2 id="usage"><a href="#usage">Usage</a></h2>
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<p>As a convention, it is recommended that this macro be invoked within a
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module called <code>kw</code> or <code>keyword</code> and that the resulting parser be invoked
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with a <code>kw::</code> or <code>keyword::</code> prefix.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">mod</span> <span class="ident">kw</span> {
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<span class="macro">syn::custom_keyword!</span>(<span class="ident">whatever</span>);
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}</code></pre></div>
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<p>The generated syntax tree node supports the following operations just like
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any built-in keyword token.</p>
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<ul>
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<li>
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<p><a href="parse/struct.ParseBuffer.html#method.peek">Peeking</a> — <code>input.peek(kw::whatever)</code></p>
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</li>
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<li>
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<p><a href="parse/struct.ParseBuffer.html#method.parse">Parsing</a> — <code>input.parse::<kw::whatever>()?</code></p>
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</li>
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<li>
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<p><a href="../quote/to_tokens/trait.ToTokens.html">Printing</a> — <code>quote!( ... #whatever_token ... )</code></p>
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</li>
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<li>
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<p>Construction from a <a href="../proc_macro2/struct.Span.html"><code>Span</code></a> — <code>let whatever_token = kw::whatever(sp)</code></p>
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</li>
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<li>
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<p>Field access to its span — <code>let sp = whatever_token.span</code></p>
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</li>
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</ul>
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<h2 id="example"><a href="#example">Example</a></h2>
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<p>This example parses input that looks like <code>bool = true</code> or <code>str = "value"</code>.
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The key must be either the identifier <code>bool</code> or the identifier <code>str</code>. If
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<code>bool</code>, the value may be either <code>true</code> or <code>false</code>. If <code>str</code>, the value may
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be any string literal.</p>
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<p>The symbols <code>bool</code> and <code>str</code> are not reserved keywords in Rust so these are
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not considered keywords in the <code>syn::token</code> module. Like any other
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identifier that is not a keyword, these can be declared as custom keywords
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by crates that need to use them as such.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use</span> <span class="ident">syn</span>::{<span class="ident">LitBool</span>, <span class="ident">LitStr</span>, <span class="prelude-ty">Result</span>, <span class="ident">Token</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">mod</span> <span class="ident">kw</span> {
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<span class="macro">syn::custom_keyword!</span>(<span class="ident">bool</span>);
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<span class="macro">syn::custom_keyword!</span>(<span class="ident">str</span>);
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}
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<span class="kw">enum</span> <span class="ident">Argument</span> {
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<span class="ident">Bool</span> {
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<span class="ident">bool_token</span>: <span class="ident">kw::bool</span>,
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<span class="ident">eq_token</span>: <span class="macro">Token!</span>[<span class="op">=</span>],
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<span class="ident">value</span>: <span class="ident">LitBool</span>,
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},
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<span class="ident">Str</span> {
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<span class="ident">str_token</span>: <span class="ident">kw::str</span>,
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<span class="ident">eq_token</span>: <span class="macro">Token!</span>[<span class="op">=</span>],
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<span class="ident">value</span>: <span class="ident">LitStr</span>,
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},
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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">Argument</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="kw">let</span> <span class="ident">lookahead</span> <span class="op">=</span> <span class="ident">input</span>.<span class="ident">lookahead1</span>();
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<span class="kw">if</span> <span class="ident">lookahead</span>.<span class="ident">peek</span>(<span class="ident">kw::bool</span>) {
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<span class="prelude-val">Ok</span>(<span class="ident">Argument::Bool</span> {
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<span class="ident">bool_token</span>: <span class="ident">input</span>.<span class="ident">parse</span>::<span class="op"><</span><span class="ident">kw::bool</span><span class="op">></span>()<span class="question-mark">?</span>,
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<span class="ident">eq_token</span>: <span class="ident">input</span>.<span class="ident">parse</span>()<span class="question-mark">?</span>,
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<span class="ident">value</span>: <span class="ident">input</span>.<span class="ident">parse</span>()<span class="question-mark">?</span>,
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})
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} <span class="kw">else</span> <span class="kw">if</span> <span class="ident">lookahead</span>.<span class="ident">peek</span>(<span class="ident">kw::str</span>) {
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<span class="prelude-val">Ok</span>(<span class="ident">Argument::Str</span> {
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<span class="ident">str_token</span>: <span class="ident">input</span>.<span class="ident">parse</span>::<span class="op"><</span><span class="ident">kw::str</span><span class="op">></span>()<span class="question-mark">?</span>,
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<span class="ident">eq_token</span>: <span class="ident">input</span>.<span class="ident">parse</span>()<span class="question-mark">?</span>,
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<span class="ident">value</span>: <span class="ident">input</span>.<span class="ident">parse</span>()<span class="question-mark">?</span>,
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})
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} <span class="kw">else</span> {
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<span class="prelude-val">Err</span>(<span class="ident">lookahead</span>.<span class="ident">error</span>())
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}
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}
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}</code></pre></div>
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