<!DOCTYPE html><htmllang="en"><head><metacharset="utf-8"><metaname="viewport"content="width=device-width, initial-scale=1.0"><metaname="generator"content="rustdoc"><metaname="description"content="Extensions to the `ParseStream` API to support speculative parsing."><title>Speculative in syn::parse::discouraged - Rust</title><linkrel="preload"as="font"type="font/woff2"crossoriginhref="../../../static.files/SourceSerif4-Regular-46f98efaafac5295.ttf.woff2"><linkrel="preload"as="font"type="font/woff2"crossoriginhref="../../../static.files/FiraSans-Regular-018c141bf0843ffd.woff2"><linkrel="preload"as="font"type="font/woff2"crossoriginhref="../../../static.files/FiraSans-Medium-8f9a781e4970d388.woff2"><linkrel="preload"as="font"type="font/woff2"crossoriginhref="../../../static.files/SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2"><linkrel="preload"as="font"type="font/woff2"crossoriginhref="../../../static.files/SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2"><linkrel="stylesheet"href="../../../static.files/normalize-76eba96aa4d2e634.css"><linkrel="stylesheet"href="../../../static.files/rustdoc-5bc39a1768837dd0.css"><metaname="rustdoc-vars"data-root-path="../../../"data-static-root-path="../../../static.files/"data-current-crate="syn"data-themes=""data-resource-suffix=""data-rustdoc-version="1.77.2 (25ef9e3d8 2024-04-09)"data-channel="1.77.2"data-search-js="search-dd67cee4cfa65049.js"data-settings-js="settings-4313503d2e1961c2.js"><scriptsrc="../../../static.files/storage-4c98445ec4002617.js"></script><scriptdefersrc="sidebar-items.js"></script><scriptdefersrc="../../../static.files/main-48f368f3872407c8.js"></script><noscript><linkrel="stylesheet"href="../../../static.files/noscript-04d5337699b92874.css"></noscript><linkrel="alternate icon"type="image/png"href="../../../static.files/favicon-16x16-8b506e7a72182f1c.png"><linkrel="alternate icon"type="image/png"href="../../../static.files/favicon-32x32-422f7d1d52889060.png"><linkrel="icon"type="image/svg+xml"href="../../../static.files/favicon-2c020d218678b618.svg"></head><bodyclass="rustdoc trait"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><navclass="mobile-topbar"><buttonclass="sidebar-menu-toggle"title="show sidebar"></button></nav><navclass="sidebar"><divclass="sidebar-crate"><h2><ahref="../../../syn/index.html">syn</a><spanclass="version">2.0.58</span></h2></div><h2class="location"><ahref="#">Speculative</a></h2><divclass="sidebar-elems"><section><h3><ahref="#required-methods">Required Methods</a></h3><ulclass="block"><li><ahref="#tymethod.advance_to">advance_to</a></li></ul><h3><ahref="#object-safety">Object Safety</a></h3><h3><ahref="#implementors">Implementors</a></h3></section><h2><ahref="index.html">In syn::parse::discouraged</a></h2></div></nav><divclass="sidebar-resizer"></div>
<main><divclass="width-limiter"><navclass="sub"><formclass="search-form"><span></span><divid="sidebar-button"tabindex="-1"><ahref="../../../syn/all.html"title="show sidebar"></a></div><inputclass="search-input"name="search"aria-label="Run search in the documentation"autocomplete="off"spellcheck="false"placeholder="Click or press ‘S’ to search, ‘?’ for more options…"type="search"><divid="help-button"tabindex="-1"><ahref="../../../help.html"title="help">?</a></div><divid="settings-menu"tabindex="-1"><ahref="../../../settings.html"title="settings"><imgwidth="22"height="22"alt="Change settings"src="../../../static.files/wheel-7b819b6101059cd0.svg"></a></div></form></nav><sectionid="main-content"class="content"><divclass="main-heading"><h1>Trait <ahref="../../index.html">syn</a>::<wbr><ahref="../index.html">parse</a>::<wbr><ahref="index.html">discouraged</a>::<wbr><aclass="trait"href="#">Speculative</a><buttonid="copy-path"title="Copy item path to clipboard"><imgsrc="../../../static.files/clipboard-7571035ce49a181d.svg"width="19"height="18"alt="Copy item path"></button></h1><spanclass="out-of-band"><aclass="src"href="../../../src/syn/discouraged.rs.html#13-165">source</a> · <buttonid="toggle-all-docs"title="collapse all docs">[<span>−</span>]</button></span></div><preclass="rust item-decl"><code>pub trait Speculative {
}</code></pre><detailsclass="toggle top-doc"open><summaryclass="hideme"><span>Expand description</span></summary><divclass="docblock"><p>Extensions to the <code>ParseStream</code> API to support speculative parsing.</p>
</div></details><h2id="required-methods"class="section-header">Required Methods<ahref="#required-methods"class="anchor">§</a></h2><divclass="methods"><detailsclass="toggle method-toggle"open><summary><sectionid="tymethod.advance_to"class="method"><aclass="src rightside"href="../../../src/syn/discouraged.rs.html#164">source</a><h4class="code-header">fn <ahref="#tymethod.advance_to"class="fn">advance_to</a>(&self, fork: <aclass="primitive"href="https://doc.rust-lang.org/1.77.2/std/primitive.reference.html">&Self</a>)</h4></section></summary><divclass="docblock"><p>Advance this parse stream to the position of a forked parse stream.</p>
<p>This is the opposite operation to <ahref="../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>
<p>If you can avoid doing this, you should, as it limits the ability to
generate useful errors. That said, it is often the only way to parse
syntax of the form <code>A* B*</code> for arbitrary syntax <code>A</code> and <code>B</code>. The problem
is that when the fork fails to parse an <code>A</code>, it’s impossible to tell
whether that was because of a syntax error and the user meant to provide
an <code>A</code>, or that the <code>A</code>s are finished and it’s time to start parsing
<code>B</code>s. Use with care.</p>
<p>Also note that if <code>A</code> is a subset of <code>B</code>, <code>A* B*</code> can be parsed by
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
<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 <ahref="https://github.com/rust-lang/rfcs/pull/2544">RFC
2544</a>, <ahref="../../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 <ahref="../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>
