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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-46f98efaafac5295.ttf.woff2,FiraSans-Regular-018c141bf0843ffd.woff2,FiraSans-Medium-8f9a781e4970d388.woff2,SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2,SourceCodePro-Semibold-d899c5a5c4aeb14a.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-76eba96aa4d2e634.css"><link rel="stylesheet" href="../../../static.files/rustdoc-492a78a4a87dcc01.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="1.82.0 (f6e511eec 2024-10-15)" data-channel="1.82.0" data-search-js="search-a99f1315e7cc5121.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../../../static.files/storage-118b08c4c78b968e.js"></script><script defer src="sidebar-items.js"></script><script defer src="../../../static.files/main-921df33f47b8780c.js"></script><noscript><link rel="stylesheet" href="../../../static.files/noscript-3b12f09e550e0385.css"></noscript><link rel="alternate icon" type="image/png" href="../../../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../../../static.files/favicon-2c020d218678b618.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">2.0.85</span></h2></div><h2 class="location"><a href="#">Speculative</a></h2><div class="sidebar-elems"><section><h3><a href="#required-methods">Required Methods</a></h3><ul class="block"><li><a href="#tymethod.advance_to">advance_to</a></li></ul><h3><a href="#object-safety">Object Safety</a></h3><h3><a href="#implementors">Implementors</a></h3></section><h2><a href="index.html">In syn::<wbr>parse::<wbr>discouraged</a></h2></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"><h1>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" title="Copy item path to clipboard">Copy item path</button></h1><span class="out-of-band"><a class="src" href="../../../src/syn/discouraged.rs.html#13-165">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</span>]</button></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>
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</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>
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<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
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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.</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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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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<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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repetition, bypassing the need to involve the downsides associated with
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speculative parsing.</p>
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<h5 id="example"><a class="doc-anchor" href="#example">§</a>Example</h5>
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<p>There has been chatter about the possibility of making the colons in the
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turbofish syntax like <code>path::to::<T></code> no longer required by accepting
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<code>path::to<T></code> in expression position. Specifically, according to <a href="https://github.com/rust-lang/rfcs/pull/2544">RFC
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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
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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="../struct.ParseBuffer.html#method.fork" title="method syn::parse::ParseBuffer::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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<code>Parse</code> implementation for <code>PathSegment</code>:</p>
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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 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>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>> {
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<span class="kw">if </span>input.peek(<span class="macro">Token!</span>[<span class="kw">super</span>])
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|| input.peek(<span class="macro">Token!</span>[<span class="self">self</span>])
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|| input.peek(<span class="macro">Token!</span>[<span class="self">Self</span>])
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|| 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>ident = input.call(Ident::parse_any)<span class="question-mark">?</span>;
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<span class="kw">return </span><span class="prelude-val">Ok</span>(PathSegment::from(ident));
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}
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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>[<]) {
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<span class="kw">return </span><span class="prelude-val">Ok</span>(PathSegment {
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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>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();
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<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,
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arguments: PathArguments::AngleBracketed(arguments),
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});
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}
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}
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<span class="prelude-val">Ok</span>(PathSegment::from(ident))
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}
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}
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</code></pre></div>
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<h5 id="drawbacks"><a class="doc-anchor" href="#drawbacks">§</a>Drawbacks</h5>
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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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<div class="example-wrap"><pre class="language-text"><code>let _ = f<&'a fn(), for<'a> serde::>();
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</code></pre></div>
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<p>If this is parsed as generic arguments, we can provide the error message</p>
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<div class="example-wrap"><pre class="language-text"><code>error: expected identifier
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--> src.rs:L:C
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L | let _ = f<&'a fn(), for<'a> serde::>();
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| ^
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</code></pre></div>
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<p>but if parsed using the above speculative parsing, it falls back to
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assuming that the <code><</code> is a less-than when it fails to parse the generic
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arguments, and tries to interpret the <code>&'a</code> as the start of a labelled
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loop, resulting in the much less helpful error</p>
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<div class="example-wrap"><pre class="language-text"><code>error: expected `:`
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--> src.rs:L:C
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L | let _ = f<&'a fn(), for<'a> serde::>();
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| ^^
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</code></pre></div>
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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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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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<h5 id="performance"><a class="doc-anchor" href="#performance">§</a>Performance</h5>
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<p>This method performs a cheap fixed amount of work that does not depend
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on how far apart the two streams are positioned.</p>
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<h5 id="panics"><a class="doc-anchor" href="#panics">§</a>Panics</h5>
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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></details></div><h2 id="object-safety" class="section-header">Object Safety<a href="#object-safety" class="anchor">§</a></h2><div class="object-safety-info">This trait is <b>not</b> <a href="https://doc.rust-lang.org/1.82.0/reference/items/traits.html#object-safety">object safe</a>.</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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