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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">☰</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>
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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">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">−</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>(&self, fork: <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.reference.html">&</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>(&self, fork: <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.reference.html">&</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
|
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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 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 {
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// Required method
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||||
fn <a href="#tymethod.advance_to" class="fn">advance_to</a>(&self, fork: &Self);
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}</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>(&self, fork: &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>
|
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@@ -20,95 +18,90 @@ an <code>A</code>, or that the <code>A</code>s are finished and it’s time to s
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parsing <code>B*</code> and removing the leading members of <code>A</code> from the
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repetition, bypassing the need to involve the downsides associated with
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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::<T></code> no longer required by accepting
|
||||
<code>path::to<T></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
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||||
2544</a>, <a href="../../struct.PathSegment.html" title="struct syn::PathSegment"><code>PathSegment</code></a> parsing should always try to consume a following
|
||||
<code><</code> token as the start of generic arguments, and reset to the <code><</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
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||||
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
|
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content a second time.</p>
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||||
<p>This change in behavior can be implemented in syn by replacing just the
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||||
<code>Parse</code> implementation for <code>PathSegment</code>:</p>
|
||||
|
||||
<div class="example-wrap"><pre class="rust rust-example-rendered">
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||||
<span class="kw">use</span> <span class="ident">syn::parse::discouraged::Speculative</span>;
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>syn::parse::discouraged::Speculative;
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<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">PathSegment</span> {
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<span class="kw">pub</span> <span class="ident">ident</span>: <span class="ident">Ident</span>,
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<span class="kw">pub</span> <span class="ident">arguments</span>: <span class="ident">PathArguments</span>,
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<span class="kw">pub struct </span>PathSegment {
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<span class="kw">pub </span>ident: Ident,
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<span class="kw">pub </span>arguments: PathArguments,
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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">PathSegment</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">if</span> <span class="ident">input</span>.<span class="ident">peek</span>(<span class="macro">Token!</span>[<span class="kw">super</span>])
|
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<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>])
|
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<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>])
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<span class="kw">impl </span>Parse <span class="kw">for </span>PathSegment {
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||||
<span class="kw">fn </span>parse(input: ParseStream) -> <span class="prelude-ty">Result</span><<span class="self">Self</span>> {
|
||||
<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>])
|
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{
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||||
<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">&&</span> <span class="ident">input</span>.<span class="ident">peek3</span>(<span class="macro">Token!</span>[<span class="op"><</span>]) {
|
||||
<span class="kw">return</span> <span class="prelude-val">Ok</span>(<span class="ident">PathSegment</span> {
|
||||
<span class="ident">ident</span>,
|
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<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>),
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<span class="kw">let </span>ident = input.parse()<span class="question-mark">?</span>;
|
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<span class="kw">if </span>input.peek(<span class="macro">Token!</span>[::]) && input.peek3(<span class="macro">Token!</span>[<]) {
|
||||
<span class="kw">return </span><span class="prelude-val">Ok</span>(PathSegment {
|
||||
ident,
|
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arguments: PathArguments::AngleBracketed(input.parse()<span class="question-mark">?</span>),
|
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});
|
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}
|
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<span class="kw">if</span> <span class="ident">input</span>.<span class="ident">peek</span>(<span class="macro">Token!</span>[<span class="op"><</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="op"><</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">&</span><span class="ident">fork</span>);
|
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<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>[<]) && !input.peek(<span class="macro">Token!</span>[<=]) {
|
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<span class="kw">let </span>fork = input.fork();
|
||||
<span class="kw">if let </span><span class="prelude-val">Ok</span>(arguments) = fork.parse() {
|
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input.advance_to(<span class="kw-2">&</span>fork);
|
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<span class="kw">return </span><span class="prelude-val">Ok</span>(PathSegment {
|
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ident,
|
||||
arguments: PathArguments::AngleBracketed(arguments),
|
||||
});
|
||||
}
|
||||
}
|
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<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))
|
||||
}
|
||||
}
|
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</pre></div>
|
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<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<&'a fn(), for<'a> serde::>();
|
||||
</code></pre>
|
||||
<div class="example-wrap"><pre class="language-text"><code>let _ = f<&'a fn(), for<'a> serde::>();</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
|
||||
--> src.rs:L:C
|
||||
|
|
||||
L | let _ = f<&'a fn(), for<'a> serde::>();
|
||||
| ^
|
||||
</code></pre>
|
||||
L | let _ = f<&'a fn(), for<'a> serde::>();
|
||||
| ^</code></pre></div>
|
||||
<p>but if parsed using the above speculative parsing, it falls back to
|
||||
assuming that the <code><</code> is a less-than when it fails to parse the generic
|
||||
arguments, and tries to interpret the <code>&'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 `:`
|
||||
--> src.rs:L:C
|
||||
|
|
||||
L | let _ = f<&'a fn(), for<'a> serde::>();
|
||||
| ^^
|
||||
</code></pre>
|
||||
L | let _ = f<&'a fn(), for<'a> serde::>();
|
||||
| ^^</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<'a> <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><'a></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>(&self, fork: <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.reference.html">&</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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<script src="../../../main.js"></script></body></html>
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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<'a> <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><'a></h3></section></div><script src="../../../trait.impl/syn/parse/discouraged/trait.Speculative.js" async></script></section></div></main></body></html>
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