<!DOCTYPE html><htmllang="en"><head><metacharset="utf-8"><metaname="viewport"content="width=device-width, initial-scale=1.0"><metaname="generator"content="rustdoc"><metaname="description"content="Support for checking the next token in a stream to decide how to parse."><title>Lookahead1 in syn::parse - Rust</title><linkrel="preload"as="font"type="font/woff2"crossoriginhref="../../static.files/SourceSerif4-Regular-46f98efaafac5295.ttf.woff2"><linkrel="preload"as="font"type="font/woff2"crossoriginhref="../../static.files/FiraSans-Regular-018c141bf0843ffd.woff2"><linkrel="preload"as="font"type="font/woff2"crossoriginhref="../../static.files/FiraSans-Medium-8f9a781e4970d388.woff2"><linkrel="preload"as="font"type="font/woff2"crossoriginhref="../../static.files/SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2"><linkrel="preload"as="font"type="font/woff2"crossoriginhref="../../static.files/SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2"><linkrel="stylesheet"href="../../static.files/normalize-76eba96aa4d2e634.css"><linkrel="stylesheet"href="../../static.files/rustdoc-ac92e1bbe349e143.css"><metaname="rustdoc-vars"data-root-path="../../"data-static-root-path="../../static.files/"data-current-crate="syn"data-themes=""data-resource-suffix=""data-rustdoc-version="1.76.0 (07dca489a 2024-02-04)"data-channel="1.76.0"data-search-js="search-2b6ce74ff89ae146.js"data-settings-js="settings-4313503d2e1961c2.js"><scriptsrc="../../static.files/storage-f2adc0d6ca4d09fb.js"></script><scriptdefersrc="sidebar-items.js"></script><scriptdefersrc="../../static.files/main-305769736d49e732.js"></script><noscript><linkrel="stylesheet"href="../../static.files/noscript-feafe1bb7466e4bd.css"></noscript><linkrel="alternate icon"type="image/png"href="../../static.files/favicon-16x16-8b506e7a72182f1c.png"><linkrel="alternate icon"type="image/png"href="../../static.files/favicon-32x32-422f7d1d52889060.png"><linkrel="icon"type="image/svg+xml"href="../../static.files/favicon-2c020d218678b618.svg"></head><bodyclass="rustdoc struct"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><navclass="mobile-topbar"><buttonclass="sidebar-menu-toggle">☰</button></nav><navclass="sidebar"><divclass="sidebar-crate"><h2><ahref="../../syn/index.html">syn</a><spanclass="version">2.0.51</span></h2></div><h2class="location"><ahref="#">Lookahead1</a></h2><divclass="sidebar-elems"><section><h3><ahref="#implementations">Methods</a></h3><ulclass="block method"><li><ahref="#method.error">error</a></li><li><ahref="#method.peek">peek</a></li></ul><h3><ahref="#synthetic-implementations">Auto Trait Implementations</a></h3><ulclass="block synthetic-implementation"><li><ahref="#impl-RefUnwindSafe-for-Lookahead1%3C'a%3E">!RefUnwindSafe</a></li><li><ahref="#impl-Send-for-Lookahead1%3C'a%3E">!Send</a></li><li><ahref="#impl-Sync-for-Lookahead1%3C'a%3E">!Sync</a></li><li><ahref="#impl-Unpin-for-Lookahead1%3C'a%3E">Unpin</a></li><li><ahref="#impl-UnwindSafe-for-Lookahead1%3C'a%3E">UnwindSafe</a></li></ul><h3><ahref="#blanket-implementations">Blanket Implementations</a></h3><ulclass="block blanket-implementation"><li><ahref="#impl-Any-for-T">Any</a></li><li><ahref="#impl-Borrow%3CT%3E-for-T">Borrow<T></a></li><li><ahref="#impl-BorrowMut%3CT%3E-for-T">BorrowMut<T></a></li><li><ahref="#impl-From%3CT%3E-for-T">From<T></a></li><li><ahref="#impl-Into%3CU%3E-for-T">Into<U></a></li><li><ahref="#impl-TryFrom%3CU%3E-for-T">TryFrom<U></a></li><li><ahref="#impl-TryInto%3CU%3E-for-T">TryInto<U></a></li></ul></section><h2><ahref="index.html">In syn::parse</a></h2></div></nav><divclass="sidebar-resizer"></div>
<main><divclass="width-limiter"><navclass="sub"><formclass="search-form"><span></span><divid="sidebar-button"tabindex="-1"><ahref="../../syn/all.html"title="show sidebar"></a></div><inputclass="search-input"name="search"aria-label="Run search in the documentation"autocomplete="off"spellcheck="false"placeholder="Click or press ‘S’ to search, ‘?’ for more options…"type="search"><divid="help-button"tabindex="-1"><ahref="../../help.html"title="help">?</a></div><divid="settings-menu"tabindex="-1"><ahref="../../settings.html"title="settings"><imgwidth="22"height="22"alt="Change settings"src="../../static.files/wheel-7b819b6101059cd0.svg"></a></div></form></nav><sectionid="main-content"class="content"><divclass="main-heading"><h1>Struct <ahref="../index.html">syn</a>::<wbr><ahref="index.html">parse</a>::<wbr><aclass="struct"href="#">Lookahead1</a><buttonid="copy-path"title="Copy item path to clipboard"><imgsrc="../../static.files/clipboard-7571035ce49a181d.svg"width="19"height="18"alt="Copy item path"></button></h1><spanclass="out-of-band"><aclass="src"href="../../src/syn/lookahead.rs.html#62-66">source</a> · <buttonid="toggle-all-docs"title="collapse all docs">[<span>−</span>]</button></span></div><preclass="rust item-decl"><code>pub struct Lookahead1<'a> { <spanclass="comment">/* private fields */</span> }</code></pre><detailsclass="toggle top-doc"open><summaryclass="hideme"><span>Expand description</span></summary><divclass="docblock"><p>Support for checking the next token in a stream to decide how to parse.</p>
<p>An important advantage over <ahref="struct.ParseBuffer.html#method.peek"title="method syn::parse::ParseBuffer::peek"><code>ParseStream::peek</code></a> is that here we
automatically construct an appropriate error message based on the token
alternatives that get peeked. If you are producing your own error message,
go ahead and use <code>ParseStream::peek</code> instead.</p>
<p>Use <ahref="struct.ParseBuffer.html#method.lookahead1"title="method syn::parse::ParseBuffer::lookahead1"><code>ParseStream::lookahead1</code></a> to construct this object.</p>
<p>Consuming tokens from the source stream after constructing a lookahead
object does not also advance the lookahead object.</p>
</div></details><h2id="implementations"class="section-header">Implementations<ahref="#implementations"class="anchor">§</a></h2><divid="implementations-list"><detailsclass="toggle implementors-toggle"open><summary><sectionid="impl-Lookahead1%3C'a%3E"class="impl"><aclass="src rightside"href="../../src/syn/lookahead.rs.html#88-137">source</a><ahref="#impl-Lookahead1%3C'a%3E"class="anchor">§</a><h3class="code-header">impl<'a><aclass="struct"href="struct.Lookahead1.html"title="struct syn::parse::Lookahead1">Lookahead1</a><'a></h3></section></summary><divclass="impl-items"><detailsclass="toggle method-toggle"open><summary><sectionid="method.peek"class="method"><aclass="src rightside"href="../../src/syn/lookahead.rs.html#103-106">source</a><h4class="code-header">pub fn <ahref="#method.peek"class="fn">peek</a><T: <aclass="trait"href="trait.Peek.html"title="trait syn::parse::Peek">Peek</a>>(&self, token: T) -><aclass="primitive"href="https://doc.rust-lang.org/1.76.0/std/primitive.bool.html">bool</a></h4></section></summary><divclass="docblock"><p>Looks at the next token in the parse stream to determine whether it
matches the requested type of token.</p>
<h5id="syntax"><ahref="#syntax">Syntax</a></h5>
<p>Note that this method does not use turbofish syntax. Pass the peek type
inside of parentheses.</p>
<ul>
<li><code>input.peek(Token![struct])</code></li>
<li><code>input.peek(Token![==])</code></li>
<li><code>input.peek(Ident)</code><em>(does not accept keywords)</em></li>
<li><code>input.peek(Ident::peek_any)</code></li>
<li><code>input.peek(Lifetime)</code></li>
<li><code>input.peek(token::Brace)</code></li>
</ul>
</div></details><detailsclass="toggle method-toggle"open><summary><sectionid="method.error"class="method"><aclass="src rightside"href="../../src/syn/lookahead.rs.html#112-136">source</a><h4class="code-header">pub fn <ahref="#method.error"class="fn">error</a>(self) -><aclass="struct"href="struct.Error.html"title="struct syn::parse::Error">Error</a></h4></section></summary><divclass="docblock"><p>Triggers an error at the current position of the parse stream.</p>
<p>The error message will identify all of the expected token types that
have been peeked against this lookahead instance.</p>
</div></details></div></details></div><h2id="synthetic-implementations"class="section-header">Auto Trait Implementations<ahref="#synthetic-implementations"class="anchor">§</a></h2><divid="synthetic-implementations-list"><sectionid="impl-RefUnwindSafe-for-Lookahead1%3C'a%3E"class="impl"><ahref="#impl-RefUnwindSafe-for-Lookahead1%3C'a%3E"class="anchor">§</a><h3class="code-header">impl<'a> !<aclass="trait"href="https://doc.rust-lang.org/1.76.0/core/panic/unwind_safe/trait.RefUnwindSafe.html"title="trait core::panic::unwind_safe::RefUnwindSafe">RefUnwindSafe</a> for <aclass="struct"href="struct.Lookahead1.html"title="struct syn::parse::Lookahead1">Lookahead1</a><'a></h3></section><sectionid="impl-Send-for-Lookahead1%3C'a%3E"class="impl"><ahref="#impl-Send-for-Lookahead1%3C'a%3E"class="anchor">§</a><h3class="code-header">impl<'a> !<aclass="trait"href="https://doc.rust-lang.org/1.76.0/core/marker/trait.Send.html"title="trait core::marker::Send">Send</a> for <aclass="struct"href="struct.Lookahead1.html"title="struct syn::parse::Lookahead1">Lookahead1</a><'a></h3></section><sectionid="impl-Sync-for-Lookahead1%3C'a%3E"class="impl"><ahref="#impl-Sync-for-Lookahead1%3C'a%3E"class="anchor">§</a><h3class="code-header">impl<'a> !<aclass="trait"href="https://doc.rust-lang.org/1.76.0/core/marker/trait.Sync.html"title="trait core::marker::Sync">Sync</a> for <aclass="struct"href="struct.Lookahead1.html"title="struct syn::parse::Lookahead1">Lookahead1</a><'a></h3></section><sectionid="impl-Unpin-for-Lookahead1%3C'a%3E"class="impl"><ahref="#impl-Unpin-for-Lookahead1%3C'a%3E"class="anchor">§</a><h3class="code-header">impl<'a><aclass="trait"href="https://doc.rust-lang.org/1.76.0/core/marker/trait.Unpin.html"title="trait core::marker::Unpin">Unpin</a> for <aclass="struct"href="struct.Lookahead1.html"title="struct syn::parse::Lookahead1">Lookahead1</a><'a></h3></section><sectionid="impl-UnwindSafe-for-Lookahead1%3C'a%3E"class="impl"><ahref="#impl-UnwindSafe-for-Lookahead1%3C'a%3E"class="anchor">§</a><h3class="code-header">impl<'a><aclass="trait"href="https://doc.rust-lang.org/1.76.0/core/panic/unwind_safe/trait.UnwindSafe.html"title="trait core::panic::unwind_safe::UnwindSafe">UnwindSafe</a> for <aclass="struct"href="struct.Lookahead1.html"title="struct syn::parse::Lookahead1">Lookahead1</a><'a></h3></section></div><h2id="blanket-implementations"class="section-header">Blanket Implementations<ahref="#blanket-implementations"class="anchor">§</a></h2><divid="blanket-implementations-list"><detailsclass="toggle implementors-toggle"><summary><sectionid="impl-Any-for-T"class="impl"><aclass="src rightside"href="https://doc.rust-lang.org/1.76.0/src/core/any.rs.html#140">source</a><ahref="#impl-Any-for-T"class="anchor">§</a><h3class="code-header">impl<T><aclass="trait"href="https://doc.rust-lang.org/1.76.0/core/any/trait.Any.html"title="trait core::any::Any">Any</a> for T<divclass="where">where
T: 'static + ?<aclass="trait"href="https://doc.rust-lang.org/1.76.0/core/marker/trait.Sized.html"title="trait core::marker::Sized">Sized</a>,</div></h3></section></summary><divclass="impl-items"><detailsclass="toggle method-toggle"open><summary><sectionid="method.type_id"class="method trait-impl"><aclass="src rightside"href="https://doc.rust-lang.org/1.76.0/src/core/any.rs.html#141">source</a><ahref="#method.type_id"class="anchor">§</a><h4class="code-header">fn <ahref="https://doc.rust-lang.org/1.76.0/core/any/trait.Any.html#tymethod.type_id"class="fn">type_id</a>(&self) -><aclass="struct"href="https://doc.rust-lang.org/1.76.0/core/any/struct.TypeId.html"title="struct core::any::TypeId">TypeId</a></h4></section></summary><divclass='docblock'>Gets the <code>TypeId</code> of <code>self</code>. <ahref="https://doc.rust-lang.org/1.76.0/core/any/trait.Any.html#tymethod.type_id">Read more</a></div></details></div></details><detailsclass="toggle implementors-toggle"><summary><sectionid="impl-Borrow%3CT%3E-for-T"class="impl"><aclass="src rightside"href="https://doc.rust-lang.org/1.76.0/src/core/borrow.rs.html#208">source</a><ahref="#impl-Borrow%3CT%3E-for-T"class="anchor">§</a><h3class="code-header">impl<T><aclass="trait"href="https://doc.rust-lang.org/1.76.0/core/borrow/trait.Borrow.html"title="trait core::borrow::Borrow">Borrow</a><T> for T<divclass="where">where
T: ?<aclass="trait"href="https://doc.rust-lang.org/1.76.0/core/marker/trait.Sized.html"title="trait core::marker::Sized">Sized</a>,</div></h3></section></summary><divclass="impl-items"><detailsclass="toggle method-toggle"open><summary><sectionid="method.borrow"class="method trait-impl"><aclass="src rightside"href="https://doc.rust-lang.org/1.76.0/src/core/borrow.rs.html#210">source</a><ahref="#method.borrow"class="anchor">§</a><h4class="code-header">fn <ahref="https://doc.rust-lang.org/1.76.0/core/borrow/trait.Borrow.html#tymethod.borrow"class="fn">borrow</a>(&self) -><aclass="primitive"href="https://doc.rust-lang.org/1.76.0/std/primitive.reference.html">&T</a></h4></section></summary><divclass='docblock'>Immutably borrows from an owned value. <ahref="https://doc.rust-lang.org/1.76.0/core/borrow/trait.Borrow.html#tymethod.borrow">Read more</a></div></details></div></details><detailsclass="toggle implementors-toggle"><summary><sectionid="impl-BorrowMut%3CT%3E-for-T"class="impl"><aclass="src rightside"href="https://doc.rust-lang.org/1.76.0/src/core/borrow.rs.html#216">source</a><ahref="#impl-BorrowMut%3CT%3E-for-T"class="anchor">§</a><h3class="code-header">impl<T><aclass="trait"href="https://doc.rust-lang.org/1.76.0/core/borrow/trait.BorrowMut.html"title="trait core::borrow::BorrowMut">BorrowMut</a><T> for T<divclass="where">where
T: ?<aclass="trait"href="https://doc.rust-lang.org/1.76.0/core/marker/trait.Sized.html"title="trait core::marker::Sized">Sized</a>,</div></h3></section></summary><divclass="impl-items"><detailsclass="toggle method-toggle"open><summary><sectionid="method.borrow_mut"class="method trait-impl"><aclass="src rightside"href="https://doc.rust-lang.org/1.76.0/src/core/borrow.rs.html#217">source</a><ahref="#method.borrow_mut"class="anchor">§</a><h4class="code-header">fn <ahref="https://doc.rust-lang.org/1.76.0/core/borrow/trait.BorrowMut.html#tymethod.borrow_mut"class="fn">borrow_mut</a>(&mut self) -><aclass="primitive"href="https://doc.rust-lang.org/1.76.0/std/primitive.reference.html">&mut T</a></h4></section></summary><divclass='docblock'>Mutably borrows from an owned value. <ahref="https://doc.rust-lang.org/1.76.0/core/borrow/trait.BorrowMut.html#tymethod.borrow_mut">Read more</a></div></details></div></details><detailsclass="toggle implementors-toggle"><summary><sectionid="impl-From%3CT%3E-for-T"class="impl"><aclass="src rightside"href="https://doc.rust-lang.org/1.76.0/src/core/convert/mod.rs.html#763">source</a><ahref="#impl-From%3CT%3E-for-T"class="anchor">§</a><h3class="code-header">impl<T><aclass="trait"href="https://doc.rust-lang.org/1.76.0/core/convert/trait.From.html"title="trait core::convert::From">From</a><T> for T</h3></section></summary><divclass="impl-items"><detailsclass="toggle method-toggle"open><summary><sectionid="method.from"class="method trait-impl"><aclass="src rightside"href="https://doc.rust-lang.org/1.76.0/src/core/convert/mod.rs.html#766">source</a><ahref="#method.from"class="anchor">§</a><h4class="code-header">fn <ahref="https://doc.rust-lang.org/1.76.0/core/convert/trait.From.html#tymethod.from"class="fn">from</a>(t: T) -> T</h4></section></summary><divclass="docblock"><p>Returns the argument unchanged.</p>
</div></details></div></details><detailsclass="toggle implementors-toggle"><summary><sectionid="impl-Into%3CU%3E-for-T"class="impl"><aclass="src rightside"href="https://doc.rust-lang.org/1.76.0/src/core/convert/mod.rs.html#747-749">source</a><ahref="#impl-Into%3CU%3E-for-T"class="anchor">§</a><h3class="code-header">impl<T, U><aclass="trait"href="https://doc.rust-lang.org/1.76.0/core/convert/trait.Into.html"title="trait core::convert::Into">Into</a><U> for T<divclass="where">where
<p>That is, this conversion is whatever the implementation of
<code><ahref="https://doc.rust-lang.org/1.76.0/core/convert/trait.From.html"title="trait core::convert::From">From</a><T> for U</code> chooses to do.</p>
</div></details></div></details><detailsclass="toggle implementors-toggle"><summary><sectionid="impl-TryFrom%3CU%3E-for-T"class="impl"><aclass="src rightside"href="https://doc.rust-lang.org/1.76.0/src/core/convert/mod.rs.html#803-805">source</a><ahref="#impl-TryFrom%3CU%3E-for-T"class="anchor">§</a><h3class="code-header">impl<T, U><aclass="trait"href="https://doc.rust-lang.org/1.76.0/core/convert/trait.TryFrom.html"title="trait core::convert::TryFrom">TryFrom</a><U> for T<divclass="where">where
U: <aclass="trait"href="https://doc.rust-lang.org/1.76.0/core/convert/trait.Into.html"title="trait core::convert::Into">Into</a><T>,</div></h3></section></summary><divclass="impl-items"><detailsclass="toggle"open><summary><sectionid="associatedtype.Error"class="associatedtype trait-impl"><ahref="#associatedtype.Error"class="anchor">§</a><h4class="code-header">type <ahref="https://doc.rust-lang.org/1.76.0/core/convert/trait.TryFrom.html#associatedtype.Error"class="associatedtype">Error</a> = <aclass="enum"href="https://doc.rust-lang.org/1.76.0/core/convert/enum.Infallible.html"title="enum core::convert::Infallible">Infallible</a></h4></section></summary><divclass='docblock'>The type returned in the event of a conversion error.</div></details><detailsclass="toggle method-toggle"open><summary><sectionid="method.try_from"class="method trait-impl"><aclass="src rightside"href="https://doc.rust-lang.org/1.76.0/src/core/convert/mod.rs.html#810">source</a><ahref="#method.try_from"class="anchor">§</a><h4class="code-header">fn <ahref="https://doc.rust-lang.org/1.76.0/core/convert/trait.TryFrom.html#tymethod.try_from"class="fn">try_from</a>(value: U) -><aclass="enum"href="https://doc.rust-lang.org/1.76.0/core/result/enum.Result.html"title="enum core::result::Result">Result</a><T, <T as <aclass="trait"href="https://doc.rust-lang.org/1.76.0/core/convert/trait.TryFrom.html"title="trait core::convert::TryFrom">TryFrom</a><U>>::<aclass="associatedtype"href="https://doc.rust-lang.org/1.76.0/core/convert/trait.TryFrom.html#associatedtype.Error"title="type core::convert::TryFrom::Error">Error</a>></h4></section></summary><divclass='docblock'>Performs the conversion.</div></details></div></details><detailsclass="toggle implementors-toggle"><summary><sectionid="impl-TryInto%3CU%3E-for-T"class="impl"><aclass="src rightside"href="https://doc.rust-lang.org/1.76.0/src/core/convert/mod.rs.html#788-790">source</a><ahref="#impl-TryInto%3CU%3E-for-T"class="anchor">§</a><h3class="code-header">impl<T, U><aclass="trait"href="https://doc.rust-lang.org/1.76.0/core/convert/trait.TryInto.html"title="trait core::convert::TryInto">TryInto</a><U> for T<divclass="where">where
U: <aclass="trait"href="https://doc.rust-lang.org/1.76.0/core/convert/trait.TryFrom.html"title="trait core::convert::TryFrom">TryFrom</a><T>,</div></h3></section></summary><divclass="impl-items"><detailsclass="toggle"open><summary><sectionid="associatedtype.Error-1"class="associatedtype trait-impl"><ahref="#associatedtype.Error-1"class="anchor">§</a><h4class="code-header">type <ahref="https://doc.rust-lang.org/1.76.0/core/convert/trait.TryInto.html#associatedtype.Error"class="associatedtype">Error</a> = <U as <aclass="trait"href="https://doc.rust-lang.org/1.76.0/core/convert/trait.TryFrom.html"title="trait core::convert::TryFrom">TryFrom</a><T>>::<aclass="associatedtype"href="https://doc.rust-lang.org/1.76.0/core/convert/trait.TryFrom.html#associatedtype.Error"title="type core::convert::TryFrom::Error">Error</a></h4></section></summary><divclass='docblock'>The type returned in the event of a conversion error.</div></details><detailsclass="toggle method-toggle"open><summary><sectionid="method.try_into"class="method trait-impl"><aclass="src rightside"href="https://doc.rust-lang.org/1.76.0/src/core/convert/mod.rs.html#795">source</a><ahref="#method.try_into"class="anchor">§</a><h4class="code-header">fn <ahref="https://doc.rust-lang.org/1.76.0/core/convert/trait.TryInto.html#tymethod.try_into"class="fn">try_into</a>(self) -><aclass="enum"href="https://doc.rust-lang.org/1.76.0/core/result/enum.Result.html"title="enum core::result::Result">Result</a><U, <U as <aclass="trait"href="https://doc.rust-lang.org/1.76.0/core/convert/trait.TryFrom.html"title="trait core::convert::TryFrom">TryFrom</a><T>>::<aclass="associatedtype"href="https://doc.rust-lang.org/1.76.0/core/convert/trait.TryFrom.html#associatedtype.Error"title="type core::convert::TryFrom::Error">Error</a>></h4></section></summary><divclass='docblock'>Performs the conversion.</div></details></div></details></div></section></div></main></body></html>
