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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 `Lookahead1` struct in crate `syn`."><meta name="keywords" content="rust, rustlang, rust-lang, Lookahead1"><title>syn::parse::Lookahead1 - 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></head><body class="rustdoc struct"><!--[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><p class='location'>Struct Lookahead1</p><div class="sidebar-elems"><div class="block items"><a class="sidebar-title" href="#methods">Methods</a><div class="sidebar-links"><a href="#method.peek">peek</a><a href="#method.error">error</a></div><a class="sidebar-title" href="#synthetic-implementations">Auto Trait Implementations</a><div class="sidebar-links"><a href="#impl-Send">!Send</a><a href="#impl-Sync">!Sync</a></div></div><p class='location'><a href='../index.html'>syn</a>::<wbr><a href='index.html'>parse</a></p><script>window.sidebarCurrent = {name: 'Lookahead1', ty: 'struct', 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"><input class="search-input" name="search" autocomplete="off" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><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='in-band'>Struct <a href='../index.html'>syn</a>::<wbr><a href='index.html'>parse</a>::<wbr><a class="struct" href=''>Lookahead1</a></span><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/lookahead.rs.html#65-69' title='goto source code'>[src]</a></span></h1><div class="docblock type-decl"><pre class='rust struct'>pub struct Lookahead1<'a> { /* fields omitted */ }</pre></div><div class='docblock'><p>Support for checking the next token in a stream to decide how to parse.</p>
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<p>An important advantage over <a href="struct.ParseBuffer.html#method.peek"><code>ParseStream::peek</code></a> is that here we
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automatically construct an appropriate error message based on the token
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alternatives that get peeked. If you are producing your own error message,
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go ahead and use <code>ParseStream::peek</code> instead.</p>
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<p>Use <a href="struct.ParseBuffer.html#method.lookahead1"><code>ParseStream::lookahead1</code></a> to construct this object.</p>
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<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
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<pre class="rust rust-example-rendered">
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<span class="attribute">#[<span class="ident">macro_use</span>]</span>
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<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">syn</span>;
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<span class="kw">use</span> <span class="ident">syn</span>::{<span class="ident">ConstParam</span>, <span class="ident">Ident</span>, <span class="ident">Lifetime</span>, <span class="ident">LifetimeDef</span>, <span class="ident">TypeParam</span>};
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<span class="kw">use</span> <span class="ident">syn</span>::<span class="ident">parse</span>::{<span class="ident">Parse</span>, <span class="ident">ParseStream</span>, <span class="prelude-ty">Result</span>};
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<span class="comment">// A generic parameter, a single one of the comma-separated elements inside</span>
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<span class="comment">// angle brackets in:</span>
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<span class="comment">//</span>
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<span class="comment">// fn f<T: Clone, 'a, 'b: 'a, const N: usize>() { ... }</span>
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<span class="comment">//</span>
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<span class="comment">// On invalid input, lookahead gives us a reasonable error message.</span>
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<span class="comment">//</span>
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<span class="comment">// error: expected one of: identifier, lifetime, `const`</span>
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<span class="comment">// |</span>
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<span class="comment">// 5 | fn f<!Sized>() {}</span>
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<span class="comment">// | ^</span>
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<span class="kw">enum</span> <span class="ident">GenericParam</span> {
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<span class="ident">Type</span>(<span class="ident">TypeParam</span>),
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<span class="ident">Lifetime</span>(<span class="ident">LifetimeDef</span>),
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<span class="ident">Const</span>(<span class="ident">ConstParam</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">GenericParam</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="op">-></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">Ident</span>) {
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<span class="ident">input</span>.<span class="ident">parse</span>().<span class="ident">map</span>(<span class="ident">GenericParam</span>::<span class="ident">Type</span>)
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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">Lifetime</span>) {
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<span class="ident">input</span>.<span class="ident">parse</span>().<span class="ident">map</span>(<span class="ident">GenericParam</span>::<span class="ident">Lifetime</span>)
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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="macro">Token</span><span class="macro">!</span>[<span class="kw">const</span>]) {
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<span class="ident">input</span>.<span class="ident">parse</span>().<span class="ident">map</span>(<span class="ident">GenericParam</span>::<span class="ident">Const</span>)
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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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}</pre>
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</div><h2 id='methods' class='small-section-header'>Methods<a href='#methods' class='anchor'></a></h2><h3 id='impl' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl<'a> <a class="struct" href="../../syn/parse/struct.Lookahead1.html" title="struct syn::parse::Lookahead1">Lookahead1</a><'a></code><a href='#impl' class='anchor'></a></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/syn/lookahead.rs.html#91-139' title='goto source code'>[src]</a></span></td></tr></tbody></table></h3><div class='impl-items'><h4 id='method.peek' class="method"><span id='peek.v' class='invisible'><table class='table-display'><tbody><tr><td><code>pub fn <a href='#method.peek' class='fnname'>peek</a><T: <a class="trait" href="../../syn/parse/trait.Peek.html" title="trait syn::parse::Peek">Peek</a>>(&self, token: T) -> <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.bool.html">bool</a></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/syn/lookahead.rs.html#105-108' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Looks at the next token in the parse stream to determine whether it
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matches the requested type of token.</p>
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<h1 id="syntax" class="section-header"><a href="#syntax">Syntax</a></h1>
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<p>Note that this method does not use turbofish syntax. Pass the peek type
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inside of parentheses.</p>
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<ul>
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<li><code>input.peek(Token![struct])</code></li>
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<li><code>input.peek(Token![==])</code></li>
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<li><code>input.peek(Ident)</code></li>
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<li><code>input.peek(Lifetime)</code></li>
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<li><code>input.peek(token::Brace)</code></li>
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</ul>
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</div><h4 id='method.error' class="method"><span id='error.v' class='invisible'><table class='table-display'><tbody><tr><td><code>pub fn <a href='#method.error' class='fnname'>error</a>(self) -> <a class="struct" href="../../syn/parse/struct.Error.html" title="struct syn::parse::Error">Error</a></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/syn/lookahead.rs.html#114-138' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Triggers an error at the current position of the parse stream.</p>
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<p>The error message will identify all of the expected token types that
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have been peeked against this lookahead instance.</p>
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</div></div><h2 id='synthetic-implementations' class='small-section-header'>Auto Trait Implementations<a href='#synthetic-implementations' class='anchor'></a></h2><div id='synthetic-implementations-list'><h3 id='impl-Send' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl<'a> !<a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> for <a class="struct" href="../../syn/parse/struct.Lookahead1.html" title="struct syn::parse::Lookahead1">Lookahead1</a><'a></code><a href='#impl-Send' class='anchor'></a></span></td><td><span class='out-of-band'></span></td></tr></tbody></table></h3><div class='impl-items'></div><h3 id='impl-Sync' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl<'a> !<a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> for <a class="struct" href="../../syn/parse/struct.Lookahead1.html" title="struct syn::parse::Lookahead1">Lookahead1</a><'a></code><a href='#impl-Sync' class='anchor'></a></span></td><td><span class='out-of-band'></span></td></tr></tbody></table></h3><div class='impl-items'></div></div></section><section id="search" class="content hidden"></section><section class="footer"></section><aside id="help" class="hidden"><div><h1 class="hidden">Help</h1><div class="shortcuts"><h2>Keyboard Shortcuts</h2><dl><dt><kbd>?</kbd></dt><dd>Show this help dialog</dd><dt><kbd>S</kbd></dt><dd>Focus the search field</dd><dt><kbd>↑</kbd></dt><dd>Move up in search results</dd><dt><kbd>↓</kbd></dt><dd>Move down in search results</dd><dt><kbd>↹</kbd></dt><dd>Switch tab</dd><dt><kbd>⏎</kbd></dt><dd>Go to active search result</dd><dt><kbd>+</kbd></dt><dd>Expand all sections</dd><dt><kbd>-</kbd></dt><dd>Collapse all sections</dd></dl></div><div class="infos"><h2>Search Tricks</h2><p>Prefix searches with a type followed by a colon (e.g. <code>fn:</code>) to restrict the search to a given type.</p><p>Accepted types are: <code>fn</code>, <code>mod</code>, <code>struct</code>, <code>enum</code>, <code>trait</code>, <code>type</code>, <code>macro</code>, and <code>const</code>.</p><p>Search functions by type signature (e.g. <code>vec -> usize</code> or <code>* -> vec</code>)</p><p>Search multiple things at once by splitting your query with comma (e.g. <code>str,u8</code> or <code>String,struct:Vec,test</code>)</p></div></div></aside><script>window.rootPath = "../../";window.currentCrate = "syn";</script><script src="../../aliases.js"></script><script src="../../main.js"></script><script defer src="../../search-index.js"></script></body></html> |