<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/1.78.0/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>
