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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="API documentation for the Rust `custom_keyword` macro in crate `syn`."><meta name="keywords" content="rust, rustlang, rust-lang, custom_keyword"><title>syn::custom_keyword - Rust</title><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="../dark.css"><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><script src="../storage.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="shortcut icon" href="../favicon.ico"><style type="text/css">#crate-search{background-image:url("../down-arrow.svg");}</style></head><body class="rustdoc macro"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../syn/index.html'><div class='logo-container'><img src='../rust-logo.png' alt='logo'></div></a><div class="sidebar-elems"><p class='location'><a href='index.html'>syn</a></p><script>window.sidebarCurrent = {name: 'custom_keyword', ty: 'macro', relpath: ''};</script><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../theme.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><a id="settings-menu" href="../settings.html"><img src="../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class='fqn'><span class='out-of-band'><span id='render-detail'><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class='inner'>−</span>]</a></span><a class='srclink' href='../src/syn/custom_keyword.rs.html#90-120' title='goto source code'>[src]</a></span><span class='in-band'>Macro <a href='index.html'>syn</a>::<wbr><a class="macro" href=''>custom_keyword</a></span></h1><div class="docblock type-decl hidden-by-usual-hider"><div class="example-wrap"><pre class="rust macro">
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<span class="macro">macro_rules</span><span class="macro">!</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>) <span class="op">=</span><span class="op">></span> { ... };
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}</pre></div>
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</div><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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<h1 id="usage" class="section-header"><a href="#usage">Usage</a></h1>
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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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<pre><code class="language-edition2018">mod kw {
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syn::custom_keyword!(whatever);
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}
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</code></pre>
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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="https://docs.rs/quote/0.6/quote/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="https://docs.rs/proc-macro2/0.4/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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<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
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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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<pre><code class="language-edition2018">use syn::{LitBool, LitStr, Result, Token};
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use syn::parse::{Parse, ParseStream};
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mod kw {
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syn::custom_keyword!(bool);
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syn::custom_keyword!(str);
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}
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enum Argument {
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Bool {
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bool_token: kw::bool,
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eq_token: Token![=],
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value: LitBool,
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},
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Str {
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str_token: kw::str,
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eq_token: Token![=],
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value: LitStr,
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},
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}
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impl Parse for Argument {
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fn parse(input: ParseStream) -> Result<Self> {
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let lookahead = input.lookahead1();
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if lookahead.peek(kw::bool) {
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Ok(Argument::Bool {
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bool_token: input.parse::<kw::bool>()?,
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eq_token: input.parse()?,
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value: input.parse()?,
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})
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} else if lookahead.peek(kw::str) {
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Ok(Argument::Str {
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str_token: input.parse::<kw::str>()?,
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eq_token: input.parse()?,
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value: input.parse()?,
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})
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} else {
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Err(lookahead.error())
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}
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}
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}
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</code></pre>
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