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60 lines
7.8 KiB
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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 `spanned` mod in crate `syn`."><meta name="keywords" content="rust, rustlang, rust-lang, spanned"><title>syn::spanned - 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="../../light.css" id="themeStyle"><link rel="stylesheet" type="text/css" href="../../dark.css" disabled ><link rel="stylesheet" type="text/css" href="../../ayu.css" disabled ><script id="default-settings"></script><script src="../../storage.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="icon" type="image/svg+xml" href="../../favicon.svg">
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<link rel="alternate icon" type="image/png" href="../../favicon-16x16.png">
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<link rel="alternate icon" type="image/png" href="../../favicon-32x32.png"><style type="text/css">#crate-search{background-image:url("../../down-arrow.svg");}</style></head><body class="rustdoc mod"><!--[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 rust-logo'><img src='../../rust-logo.png' alt='logo'></div></a><p class="location">Module spanned</p><div class="sidebar-elems"><div class="block items"><ul><li><a href="#traits">Traits</a></li></ul></div><p class="location"><a href="../index.html">syn</a></p><script>window.sidebarCurrent = {name: "spanned", ty: "mod", relpath: "../"};</script><script defer src="../sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu"><img src="../../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices" role="menu"></div></div><script src="../../theme.js"></script><nav class="sub"><form class="search-form"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" disabled autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><span class="help-button">?</span>
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<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/spanned.rs.html#1-114" title="goto source code">[src]</a></span><span class="in-band">Module <a href="../index.html">syn</a>::<wbr><a class="mod" href="">spanned</a></span></h1><div class="docblock"><p>A trait that can provide the <code>Span</code> of the complete contents of a syntax
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tree node.</p>
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<p><em>This module is available only if Syn is built with both the <code>"parsing"</code> and
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<code>"printing"</code> features.</em></p>
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<br>
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<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
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<p>Suppose in a procedural macro we have a <a href="../../syn/enum.Type.html"><code>Type</code></a> that we want to assert
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implements the <a href="https://doc.rust-lang.org/nightly/core/marker/trait.Sync.html"><code>Sync</code></a> trait. Maybe this is the type of one of the fields
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of a struct for which we are deriving a trait implementation, and we need to
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be able to pass a reference to one of those fields across threads.</p>
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<p>If the field type does <em>not</em> implement <code>Sync</code> as required, we want the
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compiler to report an error pointing out exactly which type it was.</p>
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<p>The following macro code takes a variable <code>ty</code> of type <code>Type</code> and produces a
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static assertion that <code>Sync</code> is implemented for that type.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">proc_macro</span>::<span class="ident">TokenStream</span>;
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<span class="kw">use</span> <span class="ident">proc_macro2</span>::<span class="ident">Span</span>;
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<span class="kw">use</span> <span class="ident">quote</span>::<span class="ident">quote_spanned</span>;
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<span class="kw">use</span> <span class="ident">syn</span>::<span class="ident">Type</span>;
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<span class="kw">use</span> <span class="ident">syn</span>::<span class="ident">spanned</span>::<span class="ident">Spanned</span>;
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<span class="attribute">#[<span class="ident">proc_macro_derive</span>(<span class="ident">MyMacro</span>)]</span>
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<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">my_macro</span>(<span class="ident">input</span>: <span class="ident">TokenStream</span>) <span class="op">-</span><span class="op">></span> <span class="ident">TokenStream</span> {
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<span class="comment">/* ... */</span>
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<span class="kw">let</span> <span class="ident">assert_sync</span> <span class="op">=</span> <span class="macro">quote_spanned</span><span class="macro">!</span> {<span class="ident">ty</span>.<span class="ident">span</span>()<span class="op">=</span><span class="op">></span>
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<span class="kw">struct</span> <span class="ident">_AssertSync</span> <span class="kw">where</span> #<span class="ident">ty</span>: <span class="ident">Sync</span>;
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};
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<span class="comment">/* ... */</span>
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}</pre></div>
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<p>By inserting this <code>assert_sync</code> fragment into the output code generated by
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our macro, the user's code will fail to compile if <code>ty</code> does not implement
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<code>Sync</code>. The errors they would see look like the following.</p>
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<pre><code class="language-text">error[E0277]: the trait bound `*const i32: std::marker::Sync` is not satisfied
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--> src/main.rs:10:21
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10 | bad_field: *const i32,
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| ^^^^^^^^^^ `*const i32` cannot be shared between threads safely
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</code></pre>
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<p>In this technique, using the <code>Type</code>'s span for the error message makes the
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error appear in the correct place underlining the right type.</p>
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<br>
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<h1 id="limitations" class="section-header"><a href="#limitations">Limitations</a></h1>
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<p>The underlying <a href="https://doc.rust-lang.org/nightly/proc_macro/struct.Span.html#method.join" title="proc_macro::Span::join"><code>proc_macro::Span::join</code></a> method is nightly-only. When
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called from within a procedural macro in a nightly compiler, <code>Spanned</code> will
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use <code>join</code> to produce the intended span. When not using a nightly compiler,
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only the span of the <em>first token</em> of the syntax tree node is returned.</p>
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<p>In the common case of wanting to use the joined span as the span of a
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<code>syn::Error</code>, consider instead using <a href="../../syn/struct.Error.html#method.new_spanned"><code>syn::Error::new_spanned</code></a> which is
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able to span the error correctly under the complete syntax tree node without
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needing the unstable <code>join</code>.</p>
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</div><h2 id="traits" class="section-header"><a href="#traits">Traits</a></h2>
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<table><tr class="module-item"><td><a class="trait" href="trait.Spanned.html" title="syn::spanned::Spanned trait">Spanned</a></td><td class="docblock-short"><p>A trait that can provide the <code>Span</code> of the complete contents of a syntax
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tree node.</p>
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</td></tr></table></section><section id="search" class="content hidden"></section><section class="footer"></section><script>window.rootPath = "../../";window.currentCrate = "syn";</script><script src="../../main.js"></script><script defer src="../../search-index.js"></script></body></html>
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