| <!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."><meta name="keywords" content="rust, rustlang, rust-lang, Speculative"><title>Speculative in syn::parse::discouraged - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceCodePro-Semibold.ttf.woff2"><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="../../../ayu.css" disabled><link rel="stylesheet" type="text/css" href="../../../dark.css" disabled><link rel="stylesheet" type="text/css" href="../../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../../storage.js"></script><script src="../../../crates.js"></script><script defer src="../../../main.js"></script> |
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| </a><h2 class="location">Trait Speculative</h2><div class="sidebar-elems"><div class="block items"><h3 class="sidebar-title"><a href="#required-methods">Required Methods</a></h3><div class="sidebar-links"><a href="#tymethod.advance_to">advance_to</a></div><h3 class="sidebar-title"><a href="#implementors">Implementors</a></h3></div><h2 class="location">Other items in<br><a href="../../index.html">syn</a>::<wbr><a href="../index.html">parse</a>::<wbr><a href="index.html">discouraged</a></h2><div id="sidebar-vars" data-name="Speculative" data-ty="trait" data-relpath=""></div><script defer src="sidebar-items.js"></script></div></nav><main><div class="width-limiter"><div class="sub-container"><a class="sub-logo-container" href="../../../syn/index.html"><img class="rust-logo" src="../../../rust-logo.png" alt="logo"></a><nav class="sub"><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu" title="themes"><img width="18" height="18" alt="Pick another theme!" src="../../../brush.svg"></button><div id="theme-choices" role="menu"></div></div><form class="search-form"><div class="search-container"><div> |
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| <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)" title="Copy item path to clipboard"><img src="../../../clipboard.svg" width="19" height="18" alt="Copy item path"></button></span></h1><span class="out-of-band"> |
| <a class="srclink" href="../../../src/syn/discouraged.rs.html#6-158" title="goto source code">source</a> · |
| <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div> |
| <div class="docblock item-decl"><pre class="rust trait"><code>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); |
| }</code></pre></div><div class="item-info"><div class="stab portability">This is supported on <strong>crate feature <code>parsing</code></strong> only.</div></div><details class="rustdoc-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="small-section-header">Required methods<a href="#required-methods" class="anchor"></a></h2><div class="methods"><details class="rustdoc-toggle" open><summary><div id="tymethod.advance_to" class="method has-srclink"><div class="rightside"><a class="srclink" href="../../../src/syn/discouraged.rs.html#157" title="goto source code">source</a></div><h4 class="code-header">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)</h4></div></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"><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 |
| speculative parsing.</p> |
| <h5 id="example" class="section-header"><a href="#example">Example</a></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 |
| <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 |
| <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> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use</span> <span class="ident">syn::parse::discouraged::Speculative</span>; |
| |
| <span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">PathSegment</span> { |
| <span class="kw">pub</span> <span class="ident">ident</span>: <span class="ident">Ident</span>, |
| <span class="kw">pub</span> <span class="ident">arguments</span>: <span class="ident">PathArguments</span>, |
| } |
| |
| <span class="kw">impl</span> <span class="ident">Parse</span> <span class="kw">for</span> <span class="ident">PathSegment</span> { |
| <span class="kw">fn</span> <span class="ident">parse</span>(<span class="ident">input</span>: <span class="ident">ParseStream</span>) -> <span class="prelude-ty">Result</span><span class="op"><</span><span class="self">Self</span><span class="op">></span> { |
| <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>]) |
| <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>]) |
| <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>]) |
| { |
| <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> <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>, |
| <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>), |
| }); |
| } |
| <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>); |
| <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="prelude-val">Ok</span>(<span class="ident">PathSegment::from</span>(<span class="ident">ident</span>)) |
| } |
| } |
| </code></pre></div> |
| <h5 id="drawbacks" class="section-header"><a href="#drawbacks">Drawbacks</a></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> |
| <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> |
| <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></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> |
| <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></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> |
| <h5 id="performance" class="section-header"><a href="#performance">Performance</a></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> |
| <h5 id="panics" class="section-header"><a href="#panics">Panics</a></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></details></div><h2 id="implementors" class="small-section-header">Implementors<a href="#implementors" class="anchor"></a></h2><div class="item-list" id="implementors-list"><div id="impl-Speculative" class="impl has-srclink"><div class="rightside"><a class="srclink" href="../../../src/syn/discouraged.rs.html#160-194" title="goto source code">source</a></div><a href="#impl-Speculative" class="anchor"></a><h3 class="code-header 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></h3></div></div><script type="text/javascript" src="../../../implementors/syn/parse/discouraged/trait.Speculative.js" async></script></section><section id="search" class="content hidden"></section></div></main><div id="rustdoc-vars" data-root-path="../../../" data-current-crate="syn" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.60.0-nightly (1bd4fdc94 2022-01-12)" ></div> |
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