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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="../../light.css" id="themeStyle"><link rel="stylesheet" type="text/css" href="../../dark.css" disabled ><link rel="stylesheet" type="text/css" href="../../ayu.css" disabled ><script id="default-settings"></script><script src="../../storage.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="icon" type="image/svg+xml" href="../../favicon.svg">
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<link rel="alternate icon" type="image/png" href="../../favicon-32x32.png"><style type="text/css">#crate-search{background-image:url("../../down-arrow.svg");}</style></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><a href='../../syn/index.html'><div class='logo-container rust-logo'><img src='../../rust-logo.png' alt='logo'></div></a><p class="location">Struct Lookahead1</p><div class="sidebar-elems"><div class="block items"><a class="sidebar-title" href="#implementations">Methods</a><div class="sidebar-links"><a href="#method.error">error</a><a href="#method.peek">peek</a></div><a class="sidebar-title" href="#synthetic-implementations">Auto Trait Implementations</a><div class="sidebar-links"><a href="#impl-RefUnwindSafe">!RefUnwindSafe</a><a href="#impl-Send">!Send</a><a href="#impl-Sync">!Sync</a><a href="#impl-Unpin">Unpin</a><a href="#impl-UnwindSafe">UnwindSafe</a></div><a class="sidebar-title" href="#blanket-implementations">Blanket Implementations</a><div class="sidebar-links"><a href="#impl-Any">Any</a><a href="#impl-Borrow%3CT%3E">Borrow<T></a><a href="#impl-BorrowMut%3CT%3E">BorrowMut<T></a><a href="#impl-From%3CT%3E">From<T></a><a href="#impl-Into%3CU%3E">Into<U></a><a href="#impl-TryFrom%3CU%3E">TryFrom<U></a><a href="#impl-TryInto%3CU%3E">TryInto<U></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!" aria-haspopup="menu"><img src="../../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices" role="menu"></div></div><script src="../../theme.js"></script><nav class="sub"><form class="search-form"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" disabled autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><span class="help-button">?</span>
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<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/lookahead.rs.html#59-63" title="goto source code">[src]</a></span><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></h1><div class="docblock type-decl hidden-by-usual-hider"><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="../../syn/parse/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="../../syn/parse/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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<div class="example-wrap"><pre class="rust rust-example-rendered">
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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="prelude-ty">Result</span>, <span class="ident">Token</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>};
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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="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></div>
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</div><h2 id="implementations" class="small-section-header">Implementations<a href="#implementations" class="anchor"></a></h2><h3 id="impl" class="impl"><code class="in-band">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><a class="srclink" href="../../src/syn/lookahead.rs.html#85-134" title="goto source code">[src]</a></h3><div class="impl-items"><h4 id="method.peek" class="method"><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><a class="srclink" href="../../src/syn/lookahead.rs.html#100-103" title="goto source code">[src]</a></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> <em>(does not accept keywords)</em></li>
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<li><code>input.peek(Ident::peek_any)</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"><code>pub fn <a href="#method.error" class="fnname">error</a>(self) -> <a class="struct" href="../../syn/struct.Error.html" title="struct syn::Error">Error</a></code><a class="srclink" href="../../src/syn/lookahead.rs.html#109-133" title="goto source code">[src]</a></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-RefUnwindSafe" class="impl"><code class="in-band">impl<'a> !<a class="trait" href="https://doc.rust-lang.org/nightly/std/panic/trait.RefUnwindSafe.html" title="trait std::panic::RefUnwindSafe">RefUnwindSafe</a> for <a class="struct" href="../../syn/parse/struct.Lookahead1.html" title="struct syn::parse::Lookahead1">Lookahead1</a><'a></code><a href="#impl-RefUnwindSafe" class="anchor"></a></h3><div class="impl-items"></div><h3 id="impl-Send" class="impl"><code class="in-band">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></h3><div class="impl-items"></div><h3 id="impl-Sync" class="impl"><code class="in-band">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></h3><div class="impl-items"></div><h3 id="impl-Unpin" class="impl"><code class="in-band">impl<'a> <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Unpin.html" title="trait core::marker::Unpin">Unpin</a> for <a class="struct" href="../../syn/parse/struct.Lookahead1.html" title="struct syn::parse::Lookahead1">Lookahead1</a><'a></code><a href="#impl-Unpin" class="anchor"></a></h3><div class="impl-items"></div><h3 id="impl-UnwindSafe" class="impl"><code class="in-band">impl<'a> <a class="trait" href="https://doc.rust-lang.org/nightly/std/panic/trait.UnwindSafe.html" title="trait std::panic::UnwindSafe">UnwindSafe</a> for <a class="struct" href="../../syn/parse/struct.Lookahead1.html" title="struct syn::parse::Lookahead1">Lookahead1</a><'a></code><a href="#impl-UnwindSafe" class="anchor"></a></h3><div class="impl-items"></div></div><h2 id="blanket-implementations" class="small-section-header">Blanket Implementations<a href="#blanket-implementations" class="anchor"></a></h2><div id="blanket-implementations-list"><h3 id="impl-Any" class="impl"><code class="in-band">impl<T> <a class="trait" href="https://doc.rust-lang.org/nightly/core/any/trait.Any.html" title="trait core::any::Any">Any</a> for T <span class="where fmt-newline">where<br> T: 'static + ?<a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>, </span></code><a href="#impl-Any" class="anchor"></a><a class="srclink" href="https://doc.rust-lang.org/nightly/src/core/any.rs.html#108-112" title="goto source code">[src]</a></h3><div class="impl-items"><h4 id="method.type_id" class="method hidden"><code>pub fn <a href="https://doc.rust-lang.org/nightly/core/any/trait.Any.html#tymethod.type_id" class="fnname">type_id</a>(&self) -> <a class="struct" href="https://doc.rust-lang.org/nightly/core/any/struct.TypeId.html" title="struct core::any::TypeId">TypeId</a></code><a class="srclink" href="https://doc.rust-lang.org/nightly/src/core/any.rs.html#109-111" title="goto source code">[src]</a></h4><div class='docblock hidden'><p>Gets the <code>TypeId</code> of <code>self</code>. <a href="https://doc.rust-lang.org/nightly/core/any/trait.Any.html#tymethod.type_id">Read more</a></p>
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</div></div><h3 id="impl-Borrow%3CT%3E" class="impl"><code class="in-band">impl<T> <a class="trait" href="https://doc.rust-lang.org/nightly/core/borrow/trait.Borrow.html" title="trait core::borrow::Borrow">Borrow</a><T> for T <span class="where fmt-newline">where<br> T: ?<a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>, </span></code><a href="#impl-Borrow%3CT%3E" class="anchor"></a><a class="srclink" href="https://doc.rust-lang.org/nightly/src/core/borrow.rs.html#210-214" title="goto source code">[src]</a></h3><div class="impl-items"><h4 id="method.borrow" class="method hidden"><code>pub fn <a href="https://doc.rust-lang.org/nightly/core/borrow/trait.Borrow.html#tymethod.borrow" class="fnname">borrow</a>(&self) -> <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.reference.html">&</a>T</code><a class="srclink" href="https://doc.rust-lang.org/nightly/src/core/borrow.rs.html#211-213" title="goto source code">[src]</a></h4><div class='docblock hidden'><p>Immutably borrows from an owned value. <a href="https://doc.rust-lang.org/nightly/core/borrow/trait.Borrow.html#tymethod.borrow">Read more</a></p>
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</div></div><h3 id="impl-BorrowMut%3CT%3E" class="impl"><code class="in-band">impl<T> <a class="trait" href="https://doc.rust-lang.org/nightly/core/borrow/trait.BorrowMut.html" title="trait core::borrow::BorrowMut">BorrowMut</a><T> for T <span class="where fmt-newline">where<br> T: ?<a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>, </span></code><a href="#impl-BorrowMut%3CT%3E" class="anchor"></a><a class="srclink" href="https://doc.rust-lang.org/nightly/src/core/borrow.rs.html#217-221" title="goto source code">[src]</a></h3><div class="impl-items"><h4 id="method.borrow_mut" class="method hidden"><code>pub fn <a href="https://doc.rust-lang.org/nightly/core/borrow/trait.BorrowMut.html#tymethod.borrow_mut" class="fnname">borrow_mut</a>(&mut self) -> <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.reference.html">&mut </a>T</code><a class="srclink" href="https://doc.rust-lang.org/nightly/src/core/borrow.rs.html#218-220" title="goto source code">[src]</a></h4><div class='docblock hidden'><p>Mutably borrows from an owned value. <a href="https://doc.rust-lang.org/nightly/core/borrow/trait.BorrowMut.html#tymethod.borrow_mut">Read more</a></p>
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</div></div><h3 id="impl-From%3CT%3E" class="impl"><code class="in-band">impl<T> <a class="trait" href="https://doc.rust-lang.org/nightly/core/convert/trait.From.html" title="trait core::convert::From">From</a><T> for T</code><a href="#impl-From%3CT%3E" class="anchor"></a><a class="srclink" href="https://doc.rust-lang.org/nightly/src/core/convert/mod.rs.html#552-556" title="goto source code">[src]</a></h3><div class="impl-items"><h4 id="method.from" class="method hidden"><code>pub fn <a href="https://doc.rust-lang.org/nightly/core/convert/trait.From.html#tymethod.from" class="fnname">from</a>(t: T) -> T</code><a class="srclink" href="https://doc.rust-lang.org/nightly/src/core/convert/mod.rs.html#553-555" title="goto source code">[src]</a></h4><div class='docblock hidden'><p>Performs the conversion.</p>
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</div></div><h3 id="impl-Into%3CU%3E" class="impl"><code class="in-band">impl<T, U> <a class="trait" href="https://doc.rust-lang.org/nightly/core/convert/trait.Into.html" title="trait core::convert::Into">Into</a><U> for T <span class="where fmt-newline">where<br> U: <a class="trait" href="https://doc.rust-lang.org/nightly/core/convert/trait.From.html" title="trait core::convert::From">From</a><T>, </span></code><a href="#impl-Into%3CU%3E" class="anchor"></a><a class="srclink" href="https://doc.rust-lang.org/nightly/src/core/convert/mod.rs.html#541-548" title="goto source code">[src]</a></h3><div class="impl-items"><h4 id="method.into" class="method hidden"><code>pub fn <a href="https://doc.rust-lang.org/nightly/core/convert/trait.Into.html#tymethod.into" class="fnname">into</a>(self) -> U</code><a class="srclink" href="https://doc.rust-lang.org/nightly/src/core/convert/mod.rs.html#545-547" title="goto source code">[src]</a></h4><div class='docblock hidden'><p>Performs the conversion.</p>
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</div></div><h3 id="impl-TryFrom%3CU%3E" class="impl"><code class="in-band">impl<T, U> <a class="trait" href="https://doc.rust-lang.org/nightly/core/convert/trait.TryFrom.html" title="trait core::convert::TryFrom">TryFrom</a><U> for T <span class="where fmt-newline">where<br> U: <a class="trait" href="https://doc.rust-lang.org/nightly/core/convert/trait.Into.html" title="trait core::convert::Into">Into</a><T>, </span></code><a href="#impl-TryFrom%3CU%3E" class="anchor"></a><a class="srclink" href="https://doc.rust-lang.org/nightly/src/core/convert/mod.rs.html#589-598" title="goto source code">[src]</a></h3><div class="impl-items"><h4 id="associatedtype.Error" class="type"><code>type <a href="https://doc.rust-lang.org/nightly/core/convert/trait.TryFrom.html#associatedtype.Error" class="type">Error</a> = <a class="enum" href="https://doc.rust-lang.org/nightly/core/convert/enum.Infallible.html" title="enum core::convert::Infallible">Infallible</a></code></h4><div class='docblock'><p>The type returned in the event of a conversion error.</p>
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</div><h4 id="method.try_from" class="method hidden"><code>pub fn <a href="https://doc.rust-lang.org/nightly/core/convert/trait.TryFrom.html#tymethod.try_from" class="fnname">try_from</a>(value: U) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/result/enum.Result.html" title="enum core::result::Result">Result</a><T, <T as <a class="trait" href="https://doc.rust-lang.org/nightly/core/convert/trait.TryFrom.html" title="trait core::convert::TryFrom">TryFrom</a><U>>::<a class="type" href="https://doc.rust-lang.org/nightly/core/convert/trait.TryFrom.html#associatedtype.Error" title="type core::convert::TryFrom::Error">Error</a>></code><a class="srclink" href="https://doc.rust-lang.org/nightly/src/core/convert/mod.rs.html#595-597" title="goto source code">[src]</a></h4><div class='docblock hidden'><p>Performs the conversion.</p>
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</div></div><h3 id="impl-TryInto%3CU%3E" class="impl"><code class="in-band">impl<T, U> <a class="trait" href="https://doc.rust-lang.org/nightly/core/convert/trait.TryInto.html" title="trait core::convert::TryInto">TryInto</a><U> for T <span class="where fmt-newline">where<br> U: <a class="trait" href="https://doc.rust-lang.org/nightly/core/convert/trait.TryFrom.html" title="trait core::convert::TryFrom">TryFrom</a><T>, </span></code><a href="#impl-TryInto%3CU%3E" class="anchor"></a><a class="srclink" href="https://doc.rust-lang.org/nightly/src/core/convert/mod.rs.html#575-584" title="goto source code">[src]</a></h3><div class="impl-items"><h4 id="associatedtype.Error-1" class="type"><code>type <a href="https://doc.rust-lang.org/nightly/core/convert/trait.TryInto.html#associatedtype.Error" class="type">Error</a> = <U as <a class="trait" href="https://doc.rust-lang.org/nightly/core/convert/trait.TryFrom.html" title="trait core::convert::TryFrom">TryFrom</a><T>>::<a class="type" href="https://doc.rust-lang.org/nightly/core/convert/trait.TryFrom.html#associatedtype.Error" title="type core::convert::TryFrom::Error">Error</a></code></h4><div class='docblock'><p>The type returned in the event of a conversion error.</p>
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</div><h4 id="method.try_into" class="method hidden"><code>pub fn <a href="https://doc.rust-lang.org/nightly/core/convert/trait.TryInto.html#tymethod.try_into" class="fnname">try_into</a>(self) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/result/enum.Result.html" title="enum core::result::Result">Result</a><U, <U as <a class="trait" href="https://doc.rust-lang.org/nightly/core/convert/trait.TryFrom.html" title="trait core::convert::TryFrom">TryFrom</a><T>>::<a class="type" href="https://doc.rust-lang.org/nightly/core/convert/trait.TryFrom.html#associatedtype.Error" title="type core::convert::TryFrom::Error">Error</a>></code><a class="srclink" href="https://doc.rust-lang.org/nightly/src/core/convert/mod.rs.html#581-583" title="goto source code">[src]</a></h4><div class='docblock hidden'><p>Performs the conversion.</p>
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