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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="Extensions to the parsing API with niche applicability."><meta name="keywords" content="rust, rustlang, rust-lang, discouraged"><title>syn::parse::discouraged - 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><script src="../../../crates.js"></script><noscript><link rel="stylesheet" href="../../../noscript.css"></noscript><link rel="icon" type="image/svg+xml" href="../../../favicon.svg">
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<a id="settings-menu" href="../../../settings.html"><img src="../../../wheel.svg" width="18" height="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class="fqn"><span class="in-band">Module <a href="../../index.html">syn</a>::<wbr><a href="../index.html">parse</a>::<wbr><a class="mod" href="">discouraged</a><button id="copy-path" onclick="copy_path(this)"></button></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">&#x2212;</span>]</a></span><a class="srclink" href="../../../src/syn/discouraged.rs.html#1-194" title="goto source code">[src]</a></span></h1><div class="docblock"><p>Extensions to the parsing API with niche applicability.</p>
</div><h2 id="traits" class="section-header"><a href="#traits">Traits</a></h2>
<table><tr class="module-item"><td><a class="trait" href="trait.Speculative.html" title="syn::parse::discouraged::Speculative trait">Speculative</a></td><td class="docblock-short"><p>Extensions to the <code>ParseStream</code> API to support speculative parsing.</p>
</td></tr></table></section><section id="search" class="content hidden"></section><div id="rustdoc-vars" data-root-path="../../../" data-current-crate="syn" data-search-index-js="../../../search-index.js" data-search-js="../../../search.js"></div>
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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="Extensions to the `ParseStream` API to support manipulating invisible delimiters the same as if they were visible."><title>AnyDelimiter in syn::parse::discouraged - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-6b053e98.ttf.woff2,FiraSans-Regular-0fe48ade.woff2,FiraSans-Medium-e1aa3f0a.woff2,SourceCodePro-Regular-8badfe75.ttf.woff2,SourceCodePro-Semibold-aa29a496.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2" crossorigin href="../../../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../../../static.files/normalize-9960930a.css"><link rel="stylesheet" href="../../../static.files/rustdoc-46132b98.css"><meta name="rustdoc-vars" data-root-path="../../../" data-static-root-path="../../../static.files/" data-current-crate="syn" data-themes="" data-resource-suffix="" data-rustdoc-version="1.85.1 (4eb161250 2025-03-15)" data-channel="1.85.1" data-search-js="search-75f5ac3e.js" data-settings-js="settings-0f613d39.js" ><script src="../../../static.files/storage-59e33391.js"></script><script defer src="sidebar-items.js"></script><script defer src="../../../static.files/main-5f194d8c.js"></script><noscript><link rel="stylesheet" href="../../../static.files/noscript-893ab5e7.css"></noscript><link rel="alternate icon" type="image/png" href="../../../static.files/favicon-32x32-6580c154.png"><link rel="icon" type="image/svg+xml" href="../../../static.files/favicon-044be391.svg"></head><body class="rustdoc trait"><!--[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" title="show sidebar"></button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../../../syn/index.html">syn</a><span class="version">2.0.100</span></h2></div><div class="sidebar-elems"><section id="rustdoc-toc"><h2 class="location"><a href="#">AnyDelimiter</a></h2><h3><a href="#required-methods">Required Methods</a></h3><ul class="block"><li><a href="#tymethod.parse_any_delimiter" title="parse_any_delimiter">parse_any_delimiter</a></li></ul><h3><a href="#implementors">Implementors</a></h3></section><div id="rustdoc-modnav"><h2><a href="index.html">In syn::<wbr>parse::<wbr>discouraged</a></h2></div></div></nav><div class="sidebar-resizer"></div><main><div class="width-limiter"><rustdoc-search></rustdoc-search><section id="main-content" class="content"><div class="main-heading"><span class="rustdoc-breadcrumbs"><a href="../../index.html">syn</a>::<wbr><a href="../index.html">parse</a>::<wbr><a href="index.html">discouraged</a></span><h1>Trait <span class="trait">AnyDelimiter</span><button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><rustdoc-toolbar></rustdoc-toolbar><span class="sub-heading"><a class="src" href="../../../src/syn/discouraged.rs.html#205-209">Source</a> </span></div><pre class="rust item-decl"><code>pub trait AnyDelimiter {
// Required method
fn <a href="#tymethod.parse_any_delimiter" class="fn">parse_any_delimiter</a>(
&amp;self,
) -&gt; <a class="type" href="../../type.Result.html" title="type syn::Result">Result</a>&lt;(<a class="enum" href="../../../proc_macro2/enum.Delimiter.html" title="enum proc_macro2::Delimiter">Delimiter</a>, <a class="struct" href="../../../proc_macro2/extra/struct.DelimSpan.html" title="struct proc_macro2::extra::DelimSpan">DelimSpan</a>, <a class="struct" href="../struct.ParseBuffer.html" title="struct syn::parse::ParseBuffer">ParseBuffer</a>&lt;'_&gt;)&gt;;
}</code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Extensions to the <code>ParseStream</code> API to support manipulating invisible
delimiters the same as if they were visible.</p>
</div></details><h2 id="required-methods" class="section-header">Required Methods<a href="#required-methods" class="anchor">§</a></h2><div class="methods"><details class="toggle method-toggle" open><summary><section id="tymethod.parse_any_delimiter" class="method"><a class="src rightside" href="../../../src/syn/discouraged.rs.html#208">Source</a><h4 class="code-header">fn <a href="#tymethod.parse_any_delimiter" class="fn">parse_any_delimiter</a>(&amp;self) -&gt; <a class="type" href="../../type.Result.html" title="type syn::Result">Result</a>&lt;(<a class="enum" href="../../../proc_macro2/enum.Delimiter.html" title="enum proc_macro2::Delimiter">Delimiter</a>, <a class="struct" href="../../../proc_macro2/extra/struct.DelimSpan.html" title="struct proc_macro2::extra::DelimSpan">DelimSpan</a>, <a class="struct" href="../struct.ParseBuffer.html" title="struct syn::parse::ParseBuffer">ParseBuffer</a>&lt;'_&gt;)&gt;</h4></section></summary><div class="docblock"><p>Returns the delimiter, the span of the delimiter token, and the nested
contents for further parsing.</p>
</div></details></div><h2 id="implementors" class="section-header">Implementors<a href="#implementors" class="anchor">§</a></h2><div id="implementors-list"><section id="impl-AnyDelimiter-for-ParseBuffer%3C'a%3E" class="impl"><a class="src rightside" href="../../../src/syn/discouraged.rs.html#211-225">Source</a><a href="#impl-AnyDelimiter-for-ParseBuffer%3C'a%3E" class="anchor">§</a><h3 class="code-header">impl&lt;'a&gt; <a class="trait" href="trait.AnyDelimiter.html" title="trait syn::parse::discouraged::AnyDelimiter">AnyDelimiter</a> for <a class="struct" href="../struct.ParseBuffer.html" title="struct syn::parse::ParseBuffer">ParseBuffer</a>&lt;'a&gt;</h3></section></div><script src="../../../trait.impl/syn/parse/discouraged/trait.AnyDelimiter.js" async></script></section></div></main></body></html>
@@ -1,11 +1,9 @@
<!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="Extensions to the `ParseStream` API to support speculative parsing."><meta name="keywords" content="rust, rustlang, rust-lang, Speculative"><title>Speculative in syn::parse::discouraged - 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><script src="../../../crates.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 trait"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu" role="button">&#9776;</div><a href='../../../syn/index.html'><div class='logo-container rust-logo'><img src='../../../rust-logo.png' alt='logo'></div></a><p class="location">Trait Speculative</p><div class="sidebar-elems"><div class="block items"><a class="sidebar-title" href="#required-methods">Required Methods</a><div class="sidebar-links"><a href="#tymethod.advance_to">advance_to</a></div><a class="sidebar-title" href="#implementors">Implementors</a></div><p class="location"><a href="../../index.html">syn</a>::<wbr><a href="../index.html">parse</a>::<wbr><a href="index.html">discouraged</a></p><div id="sidebar-vars" data-name="Speculative" data-ty="trait" data-relpath=""></div><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" height="18" alt="Pick another theme!"></button><div id="theme-choices" role="menu"></div></div><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><button type="button" class="help-button">?</button>
<a id="settings-menu" href="../../../settings.html"><img src="../../../wheel.svg" width="18" height="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class="fqn"><span class="in-band">Trait <a href="../../index.html">syn</a>::<wbr><a href="../index.html">parse</a>::<wbr><a href="index.html">discouraged</a>::<wbr><a class="trait" href="">Speculative</a><button id="copy-path" onclick="copy_path(this)"></button></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">&#x2212;</span>]</a></span><a class="srclink" href="../../../src/syn/discouraged.rs.html#6-158" title="goto source code">[src]</a></span></h1><div class="docblock type-decl"><pre class="rust trait">pub trait Speculative {
fn <a href="#tymethod.advance_to" class="fnname">advance_to</a>(&amp;self, fork: <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.reference.html">&amp;</a>Self);
}</pre></div><div class="docblock"><p>Extensions to the <code>ParseStream</code> API to support speculative parsing.</p>
</div><h2 id="required-methods" class="small-section-header">Required methods<a href="#required-methods" class="anchor"></a></h2><div class="methods"><h3 id="tymethod.advance_to" class="method"><code>fn <a href="#tymethod.advance_to" class="fnname">advance_to</a>(&amp;self, fork: <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.reference.html">&amp;</a>Self)</code><a class="srclink" href="../../../src/syn/discouraged.rs.html#157" title="goto source code">[src]</a></h3><div class="docblock"><p>Advance this parse stream to the position of a forked parse stream.</p>
<p>This is the opposite operation to <a href="../struct.ParseBuffer.html#method.fork"><code>ParseStream::fork</code></a>. You can fork a
<!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="Extensions to the `ParseStream` API to support speculative parsing."><title>Speculative in syn::parse::discouraged - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-6b053e98.ttf.woff2,FiraSans-Regular-0fe48ade.woff2,FiraSans-Medium-e1aa3f0a.woff2,SourceCodePro-Regular-8badfe75.ttf.woff2,SourceCodePro-Semibold-aa29a496.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2" crossorigin href="../../../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../../../static.files/normalize-9960930a.css"><link rel="stylesheet" href="../../../static.files/rustdoc-46132b98.css"><meta name="rustdoc-vars" data-root-path="../../../" data-static-root-path="../../../static.files/" data-current-crate="syn" data-themes="" data-resource-suffix="" data-rustdoc-version="1.85.1 (4eb161250 2025-03-15)" data-channel="1.85.1" data-search-js="search-75f5ac3e.js" data-settings-js="settings-0f613d39.js" ><script src="../../../static.files/storage-59e33391.js"></script><script defer src="sidebar-items.js"></script><script defer src="../../../static.files/main-5f194d8c.js"></script><noscript><link rel="stylesheet" href="../../../static.files/noscript-893ab5e7.css"></noscript><link rel="alternate icon" type="image/png" href="../../../static.files/favicon-32x32-6580c154.png"><link rel="icon" type="image/svg+xml" href="../../../static.files/favicon-044be391.svg"></head><body class="rustdoc trait"><!--[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" title="show sidebar"></button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../../../syn/index.html">syn</a><span class="version">2.0.100</span></h2></div><div class="sidebar-elems"><section id="rustdoc-toc"><h2 class="location"><a href="#">Speculative</a></h2><h3><a href="#required-methods">Required Methods</a></h3><ul class="block"><li><a href="#tymethod.advance_to" title="advance_to">advance_to</a></li></ul><h3><a href="#dyn-compatibility">Dyn Compatibility</a></h3><h3><a href="#implementors">Implementors</a></h3></section><div id="rustdoc-modnav"><h2><a href="index.html">In syn::<wbr>parse::<wbr>discouraged</a></h2></div></div></nav><div class="sidebar-resizer"></div><main><div class="width-limiter"><rustdoc-search></rustdoc-search><section id="main-content" class="content"><div class="main-heading"><span class="rustdoc-breadcrumbs"><a href="../../index.html">syn</a>::<wbr><a href="../index.html">parse</a>::<wbr><a href="index.html">discouraged</a></span><h1>Trait <span class="trait">Speculative</span><button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><rustdoc-toolbar></rustdoc-toolbar><span class="sub-heading"><a class="src" href="../../../src/syn/discouraged.rs.html#13-165">Source</a> </span></div><pre class="rust item-decl"><code>pub trait Speculative {
// Required method
fn <a href="#tymethod.advance_to" class="fn">advance_to</a>(&amp;self, fork: &amp;Self);
}</code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Extensions to the <code>ParseStream</code> API to support speculative parsing.</p>
</div></details><h2 id="required-methods" class="section-header">Required Methods<a href="#required-methods" class="anchor">§</a></h2><div class="methods"><details class="toggle method-toggle" open><summary><section id="tymethod.advance_to" class="method"><a class="src rightside" href="../../../src/syn/discouraged.rs.html#164">Source</a><h4 class="code-header">fn <a href="#tymethod.advance_to" class="fn">advance_to</a>(&amp;self, fork: &amp;Self)</h4></section></summary><div class="docblock"><p>Advance this parse stream to the position of a forked parse stream.</p>
<p>This is the opposite operation to <a href="../struct.ParseBuffer.html#method.fork" title="method syn::parse::ParseBuffer::fork"><code>ParseStream::fork</code></a>. You can fork a
parse stream, perform some speculative parsing, then join the original
stream to the fork to “commit” the parsing from the fork to the main
stream.</p>
@@ -20,95 +18,90 @@ an <code>A</code>, or that the <code>A</code>s are finished and its time to s
parsing <code>B*</code> and removing the leading members of <code>A</code> from the
repetition, bypassing the need to involve the downsides associated with
speculative parsing.</p>
<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
<h5 id="example"><a class="doc-anchor" href="#example">§</a>Example</h5>
<p>There has been chatter about the possibility of making the colons in the
turbofish syntax like <code>path::to::&lt;T&gt;</code> no longer required by accepting
<code>path::to&lt;T&gt;</code> in expression position. Specifically, according to <a href="https://github.com/rust-lang/rfcs/pull/2544">RFC
2544</a>, <a href="../../struct.PathSegment.html"><code>PathSegment</code></a> parsing should always try to consume a following
2544</a>, <a href="../../struct.PathSegment.html" title="struct syn::PathSegment"><code>PathSegment</code></a> parsing should always try to consume a following
<code>&lt;</code> token as the start of generic arguments, and reset to the <code>&lt;</code> if
that fails (e.g. the token is acting as a less-than operator).</p>
<p>This is the exact kind of parsing behavior which requires the “fork,
try, commit” behavior that <a href="../struct.ParseBuffer.html#method.fork"><code>ParseStream::fork</code></a> discourages. With
try, commit” behavior that <a href="../struct.ParseBuffer.html#method.fork" title="method syn::parse::ParseBuffer::fork"><code>ParseStream::fork</code></a> discourages. With
<code>advance_to</code>, we can avoid having to parse the speculatively parsed
content a second time.</p>
<p>This change in behavior can be implemented in syn by replacing just the
<code>Parse</code> implementation for <code>PathSegment</code>:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">syn::parse::discouraged::Speculative</span>;
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>syn::parse::discouraged::Speculative;
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">PathSegment</span> {
<span class="kw">pub</span> <span class="ident">ident</span>: <span class="ident">Ident</span>,
<span class="kw">pub</span> <span class="ident">arguments</span>: <span class="ident">PathArguments</span>,
<span class="kw">pub struct </span>PathSegment {
<span class="kw">pub </span>ident: Ident,
<span class="kw">pub </span>arguments: PathArguments,
}
<span class="kw">impl</span> <span class="ident">Parse</span> <span class="kw">for</span> <span class="ident">PathSegment</span> {
<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">&gt;</span> <span class="prelude-ty">Result</span><span class="op">&lt;</span><span class="self">Self</span><span class="op">&gt;</span> {
<span class="kw">if</span> <span class="ident">input</span>.<span class="ident">peek</span>(<span class="macro">Token!</span>[<span class="kw">super</span>])
<span class="op">|</span><span class="op">|</span> <span class="ident">input</span>.<span class="ident">peek</span>(<span class="macro">Token!</span>[<span class="self">self</span>])
<span class="op">|</span><span class="op">|</span> <span class="ident">input</span>.<span class="ident">peek</span>(<span class="macro">Token!</span>[<span class="self">Self</span>])
<span class="op">|</span><span class="op">|</span> <span class="ident">input</span>.<span class="ident">peek</span>(<span class="macro">Token!</span>[<span class="kw">crate</span>])
<span class="kw">impl </span>Parse <span class="kw">for </span>PathSegment {
<span class="kw">fn </span>parse(input: ParseStream) -&gt; <span class="prelude-ty">Result</span>&lt;<span class="self">Self</span>&gt; {
<span class="kw">if </span>input.peek(<span class="macro">Token!</span>[<span class="kw">super</span>])
|| input.peek(<span class="macro">Token!</span>[<span class="self">self</span>])
|| input.peek(<span class="macro">Token!</span>[<span class="self">Self</span>])
|| input.peek(<span class="macro">Token!</span>[<span class="kw">crate</span>])
{
<span class="kw">let</span> <span class="ident">ident</span> <span class="op">=</span> <span class="ident">input</span>.<span class="ident">call</span>(<span class="ident">Ident::parse_any</span>)<span class="question-mark">?</span>;
<span class="kw">return</span> <span class="prelude-val">Ok</span>(<span class="ident">PathSegment::from</span>(<span class="ident">ident</span>));
<span class="kw">let </span>ident = input.call(Ident::parse_any)<span class="question-mark">?</span>;
<span class="kw">return </span><span class="prelude-val">Ok</span>(PathSegment::from(ident));
}
<span class="kw">let</span> <span class="ident">ident</span> <span class="op">=</span> <span class="ident">input</span>.<span class="ident">parse</span>()<span class="question-mark">?</span>;
<span class="kw">if</span> <span class="ident">input</span>.<span class="ident">peek</span>(<span class="macro">Token!</span>[::]) <span class="op">&amp;&amp;</span> <span class="ident">input</span>.<span class="ident">peek3</span>(<span class="macro">Token!</span>[<span class="op">&lt;</span>]) {
<span class="kw">return</span> <span class="prelude-val">Ok</span>(<span class="ident">PathSegment</span> {
<span class="ident">ident</span>,
<span class="ident">arguments</span>: <span class="ident">PathArguments::AngleBracketed</span>(<span class="ident">input</span>.<span class="ident">parse</span>()<span class="question-mark">?</span>),
<span class="kw">let </span>ident = input.parse()<span class="question-mark">?</span>;
<span class="kw">if </span>input.peek(<span class="macro">Token!</span>[::]) &amp;&amp; input.peek3(<span class="macro">Token!</span>[&lt;]) {
<span class="kw">return </span><span class="prelude-val">Ok</span>(PathSegment {
ident,
arguments: PathArguments::AngleBracketed(input.parse()<span class="question-mark">?</span>),
});
}
<span class="kw">if</span> <span class="ident">input</span>.<span class="ident">peek</span>(<span class="macro">Token!</span>[<span class="op">&lt;</span>]) <span class="op">&amp;&amp;</span> <span class="op">!</span><span class="ident">input</span>.<span class="ident">peek</span>(<span class="macro">Token!</span>[<span class="op">&lt;</span><span class="op">=</span>]) {
<span class="kw">let</span> <span class="ident">fork</span> <span class="op">=</span> <span class="ident">input</span>.<span class="ident">fork</span>();
<span class="kw">if</span> <span class="kw">let</span> <span class="prelude-val">Ok</span>(<span class="ident">arguments</span>) <span class="op">=</span> <span class="ident">fork</span>.<span class="ident">parse</span>() {
<span class="ident">input</span>.<span class="ident">advance_to</span>(<span class="kw-2">&amp;</span><span class="ident">fork</span>);
<span class="kw">return</span> <span class="prelude-val">Ok</span>(<span class="ident">PathSegment</span> {
<span class="ident">ident</span>,
<span class="ident">arguments</span>: <span class="ident">PathArguments::AngleBracketed</span>(<span class="ident">arguments</span>),
<span class="kw">if </span>input.peek(<span class="macro">Token!</span>[&lt;]) &amp;&amp; !input.peek(<span class="macro">Token!</span>[&lt;=]) {
<span class="kw">let </span>fork = input.fork();
<span class="kw">if let </span><span class="prelude-val">Ok</span>(arguments) = fork.parse() {
input.advance_to(<span class="kw-2">&amp;</span>fork);
<span class="kw">return </span><span class="prelude-val">Ok</span>(PathSegment {
ident,
arguments: PathArguments::AngleBracketed(arguments),
});
}
}
<span class="prelude-val">Ok</span>(<span class="ident">PathSegment::from</span>(<span class="ident">ident</span>))
<span class="prelude-val">Ok</span>(PathSegment::from(ident))
}
}
</pre></div>
<h1 id="drawbacks" class="section-header"><a href="#drawbacks">Drawbacks</a></h1>
</code></pre></div>
<h5 id="drawbacks"><a class="doc-anchor" href="#drawbacks">§</a>Drawbacks</h5>
<p>The main drawback of this style of speculative parsing is in error
presentation. Even if the lookahead is the “correct” parse, the error
that is shown is that of the “fallback” parse. To use the same example
as the turbofish above, take the following unfinished “turbofish”:</p>
<pre><code class="language-text">let _ = f&lt;&amp;'a fn(), for&lt;'a&gt; serde::&gt;();
</code></pre>
<div class="example-wrap"><pre class="language-text"><code>let _ = f&lt;&amp;&#39;a fn(), for&lt;&#39;a&gt; serde::&gt;();</code></pre></div>
<p>If this is parsed as generic arguments, we can provide the error message</p>
<pre><code class="language-text">error: expected identifier
<div class="example-wrap"><pre class="language-text"><code>error: expected identifier
--&gt; src.rs:L:C
|
L | let _ = f&lt;&amp;'a fn(), for&lt;'a&gt; serde::&gt;();
| ^
</code></pre>
L | let _ = f&lt;&amp;&#39;a fn(), for&lt;&#39;a&gt; serde::&gt;();
| ^</code></pre></div>
<p>but if parsed using the above speculative parsing, it falls back to
assuming that the <code>&lt;</code> is a less-than when it fails to parse the generic
arguments, and tries to interpret the <code>&amp;'a</code> as the start of a labelled
loop, resulting in the much less helpful error</p>
<pre><code class="language-text">error: expected `:`
<div class="example-wrap"><pre class="language-text"><code>error: expected `:`
--&gt; src.rs:L:C
|
L | let _ = f&lt;&amp;'a fn(), for&lt;'a&gt; serde::&gt;();
| ^^
</code></pre>
L | let _ = f&lt;&amp;&#39;a fn(), for&lt;&#39;a&gt; serde::&gt;();
| ^^</code></pre></div>
<p>This can be mitigated with various heuristics (two examples: show both
forks parse errors, or show the one that consumed more tokens), but
when you can control the grammar, sticking to something that can be
parsed LL(3) and without the LL(*) speculative parsing this makes
possible, displaying reasonable errors becomes much more simple.</p>
<h1 id="performance" class="section-header"><a href="#performance">Performance</a></h1>
<h5 id="performance"><a class="doc-anchor" href="#performance">§</a>Performance</h5>
<p>This method performs a cheap fixed amount of work that does not depend
on how far apart the two streams are positioned.</p>
<h1 id="panics" class="section-header"><a href="#panics">Panics</a></h1>
<h5 id="panics"><a class="doc-anchor" href="#panics">§</a>Panics</h5>
<p>The forked stream in the argument of <code>advance_to</code> must have been
obtained by forking <code>self</code>. Attempting to advance to any other stream
will cause a panic.</p>
</div></div><span class="loading-content">Loading content...</span><h2 id="implementors" class="small-section-header">Implementors<a href="#implementors" class="anchor"></a></h2><div class="item-list" id="implementors-list"><details class="rustdoc-toggle implementors-toggle" open><summary><h3 id="impl-Speculative" class="impl"><code class="in-band">impl&lt;'a&gt; <a class="trait" href="trait.Speculative.html" title="trait syn::parse::discouraged::Speculative">Speculative</a> for <a class="struct" href="../struct.ParseBuffer.html" title="struct syn::parse::ParseBuffer">ParseBuffer</a>&lt;'a&gt;</code><a href="#impl-Speculative" class="anchor"></a><a class="srclink" href="../../../src/syn/discouraged.rs.html#160-194" title="goto source code">[src]</a></h3></summary><div class="impl-items"><h4 id="method.advance_to" class="method hidden trait-impl"><code>fn <a href="#tymethod.advance_to" class="fnname">advance_to</a>(&amp;self, fork: <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.reference.html">&amp;</a>Self)</code><a href="#method.advance_to" class="anchor"></a><a class="srclink" href="../../../src/syn/discouraged.rs.html#161-193" title="goto source code">[src]</a></h4></div></details></div><span class="loading-content">Loading content...</span><script type="text/javascript" src="../../../implementors/syn/parse/discouraged/trait.Speculative.js" async></script></section><section id="search" class="content hidden"></section><div id="rustdoc-vars" data-root-path="../../../" data-current-crate="syn" data-search-index-js="../../../search-index.js" data-search-js="../../../search.js"></div>
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</div></details></div><h2 id="dyn-compatibility" class="section-header">Dyn Compatibility<a href="#dyn-compatibility" class="anchor">§</a></h2><div class="dyn-compatibility-info"><p>This trait is <b>not</b> <a href="https://doc.rust-lang.org/1.85.1/reference/items/traits.html#object-safety">dyn compatible</a>.</p><p><i>In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.</i></p></div><h2 id="implementors" class="section-header">Implementors<a href="#implementors" class="anchor">§</a></h2><div id="implementors-list"><section id="impl-Speculative-for-ParseBuffer%3C'a%3E" class="impl"><a class="src rightside" href="../../../src/syn/discouraged.rs.html#167-201">Source</a><a href="#impl-Speculative-for-ParseBuffer%3C'a%3E" class="anchor">§</a><h3 class="code-header">impl&lt;'a&gt; <a class="trait" href="trait.Speculative.html" title="trait syn::parse::discouraged::Speculative">Speculative</a> for <a class="struct" href="../struct.ParseBuffer.html" title="struct syn::parse::ParseBuffer">ParseBuffer</a>&lt;'a&gt;</h3></section></div><script src="../../../trait.impl/syn/parse/discouraged/trait.Speculative.js" async></script></section></div></main></body></html>