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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="../../../dark.css"><link rel="stylesheet" type="text/css" href="../../../light.css" id="themeStyle"><script src="../../../storage.js"></script><noscript><link rel="stylesheet" href="../../../noscript.css"></noscript><link rel="shortcut icon" href="../../../favicon.ico"><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'><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!"><img src="../../../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../../../theme.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><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#6-159' 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>
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<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 id='advance_to.v'>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></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="../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.</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 its 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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<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
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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"><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"><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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<pre><code class="language-edition2018"># use syn::ext::IdentExt;
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use syn::parse::discouraged::Speculative;
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# use syn::parse::{Parse, ParseStream};
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# use syn::{Ident, PathArguments, Result, Token};
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pub struct PathSegment {
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pub ident: Ident,
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pub arguments: PathArguments,
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}
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#
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# impl<T> From<T> for PathSegment
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# where
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# T: Into<Ident>,
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# {
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# fn from(ident: T) -> Self {
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# PathSegment {
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# ident: ident.into(),
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# arguments: PathArguments::None,
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# }
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# }
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# }
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impl Parse for PathSegment {
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fn parse(input: ParseStream) -> Result<Self> {
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if input.peek(Token![super])
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|| input.peek(Token![self])
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|| input.peek(Token![Self])
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|| input.peek(Token![crate])
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|| input.peek(Token![extern])
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{
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let ident = input.call(Ident::parse_any)?;
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return Ok(PathSegment::from(ident));
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}
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let ident = input.parse()?;
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if input.peek(Token![::]) && input.peek3(Token![<]) {
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return Ok(PathSegment {
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ident: ident,
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arguments: PathArguments::AngleBracketed(input.parse()?),
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});
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}
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if input.peek(Token![<]) && !input.peek(Token![<=]) {
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let fork = input.fork();
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if let Ok(arguments) = fork.parse() {
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input.advance_to(&fork);
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return Ok(PathSegment {
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ident: 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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Ok(PathSegment::from(ident))
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}
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}
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# syn::parse_str::<PathSegment>("a<b,c>").unwrap();
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</code></pre>
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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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<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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<pre><code class="language-text">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>
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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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<pre><code class="language-text">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>
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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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<h1 id="performance" class="section-header"><a href="#performance">Performance</a></h1>
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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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<h1 id="panics" class="section-header"><a href="#panics">Panics</a></h1>
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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>
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<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#161-171' title='goto source code'>[src]</a></h3><div class='impl-items'><h4 id='method.advance_to' class="method hidden"><code id='advance_to.v-1'>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#162-170' title='goto source code'>[src]</a></h4></div></div><span class='loading-content'>Loading content...</span><script type="text/javascript">window.inlined_types=new Set([]);</script><script type="text/javascript" async
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src="../../../implementors/syn/parse/discouraged/trait.Speculative.js">
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