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<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>
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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 `Speculative` trait in crate `syn`."><meta name="keywords" content="rust, rustlang, rust-lang, Speculative"><title>syn::parse::discouraged::Speculative - 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 trait"><!--[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">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" 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><button type="button" class="help-button">?</button>
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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="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></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 hidden-by-usual-hider"><pre class="rust trait">pub trait Speculative {
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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 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">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><script>window.sidebarCurrent = {name: "Speculative", ty: "trait", 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><button type="button" class="help-button">?</button>
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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/discouraged.rs.html#6-158" title="goto source code">[src]</a></span><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></span></h1><div class="docblock type-decl hidden-by-usual-hider"><pre class="rust trait">pub trait Speculative {
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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);
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}</pre></div><div class="docblock"><p>Extensions to the <code>ParseStream</code> API to support speculative parsing.</p>
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</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>
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<p>This is the opposite operation to <a href="../../../syn/parse/struct.ParseBuffer.html#method.fork"><code>ParseStream::fork</code></a>. You can fork a
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parse stream, perform some speculative parsing, then join the original
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stream to the fork to “commit” the parsing from the fork to the main
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stream to the fork to "commit" the parsing from the fork to the main
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stream.</p>
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<p>If you can avoid doing this, you should, as it limits the ability to
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generate useful errors. That said, it is often the only way to parse
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syntax of the form <code>A* B*</code> for arbitrary syntax <code>A</code> and <code>B</code>. The problem
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is that when the fork fails to parse an <code>A</code>, it’s impossible to tell
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is that when the fork fails to parse an <code>A</code>, it's impossible to tell
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whether that was because of a syntax error and the user meant to provide
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an <code>A</code>, or that the <code>A</code>s are finished and it’s time to start parsing
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an <code>A</code>, or that the <code>A</code>s are finished and it's time to start parsing
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<code>B</code>s. Use with care.</p>
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<p>Also note that if <code>A</code> is a subset of <code>B</code>, <code>A* B*</code> can be parsed by
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parsing <code>B*</code> and removing the leading members of <code>A</code> from the
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@@ -27,8 +27,8 @@ turbofish syntax like <code>path::to::<T></code> no longer required by acc
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2544</a>, <a href="../../../syn/struct.PathSegment.html"><code>PathSegment</code></a> parsing should always try to consume a following
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<code><</code> token as the start of generic arguments, and reset to the <code><</code> if
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that fails (e.g. the token is acting as a less-than operator).</p>
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<p>This is the exact kind of parsing behavior which requires the “fork,
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try, commit” behavior that <a href="../../../syn/parse/struct.ParseBuffer.html#method.fork"><code>ParseStream::fork</code></a> discourages. With
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<p>This is the exact kind of parsing behavior which requires the "fork,
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try, commit" behavior that <a href="../../../syn/parse/struct.ParseBuffer.html#method.fork"><code>ParseStream::fork</code></a> discourages. With
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<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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@@ -76,9 +76,9 @@ content a second time.</p>
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</pre></div>
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<h1 id="drawbacks" class="section-header"><a href="#drawbacks">Drawbacks</a></h1>
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<p>The main drawback of this style of speculative parsing is in error
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presentation. Even if the lookahead is the “correct” parse, the error
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that is shown is that of the “fallback” parse. To use the same example
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as the turbofish above, take the following unfinished “turbofish”:</p>
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presentation. Even if the lookahead is the "correct" parse, the error
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that is shown is that of the "fallback" parse. To use the same example
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as the turbofish above, take the following unfinished "turbofish":</p>
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<pre><code class="language-text">let _ = f<&'a fn(), for<'a> serde::>();
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</code></pre>
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<p>If this is parsed as generic arguments, we can provide the error message</p>
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@@ -99,7 +99,7 @@ L | let _ = f<&'a fn(), for<'a> serde::>();
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| ^^
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</code></pre>
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<p>This can be mitigated with various heuristics (two examples: show both
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forks’ parse errors, or show the one that consumed more tokens), but
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forks' parse errors, or show the one that consumed more tokens), but
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when you can control the grammar, sticking to something that can be
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parsed LL(3) and without the LL(*) speculative parsing this makes
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possible, displaying reasonable errors becomes much more simple.</p>
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@@ -110,5 +110,4 @@ on how far apart the two streams are positioned.</p>
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<p>The forked stream in the argument of <code>advance_to</code> must have been
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obtained by forking <code>self</code>. Attempting to advance to any other stream
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will cause a panic.</p>
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</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"><h3 id="impl-Speculative" class="impl"><code class="in-band">impl<'a> <a class="trait" href="../../../syn/parse/discouraged/trait.Speculative.html" title="trait syn::parse::discouraged::Speculative">Speculative</a> for <a class="struct" href="../../../syn/parse/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><div class="impl-items"><h4 id="method.advance_to" class="method hidden"><code>fn <a href="#method.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#161-193" title="goto source code">[src]</a></h4></div></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><section class="footer"></section><div id="rustdoc-vars" data-root-path="../../../" data-current-crate="syn"></div>
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<script src="../../../main.js"></script><script defer src="../../../search-index.js"></script></body></html>
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</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"><h3 id="impl-Speculative" class="impl"><code class="in-band">impl<'a> Speculative for <a class="struct" href="../../../syn/parse/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><div class="impl-items"><h4 id="method.advance_to" class="method hidden"><code>fn <a href="#method.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#161-193" title="goto source code">[src]</a></h4></div></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><section class="footer"></section><script>window.rootPath = "../../../";window.currentCrate = "syn";</script><script src="../../../main.js"></script><script defer src="../../../search-index.js"></script></body></html>
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