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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="../../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 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'><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!"><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/spanned.rs.html#1-144' 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 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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<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="../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/std/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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<pre><code class="language-edition2018"># extern crate proc_macro;
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#
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use proc_macro::TokenStream;
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use proc_macro2::Span;
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use quote::quote_spanned;
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use syn::Type;
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use syn::spanned::Spanned;
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# const IGNORE_TOKENS: &str = stringify! {
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#[proc_macro_derive(MyMacro)]
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# };
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pub fn my_macro(input: TokenStream) -> TokenStream {
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# let ty = get_a_type();
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/* ... */
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let assert_sync = quote_spanned! {ty.span()=>
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struct _AssertSync where #ty: Sync;
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};
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/* ... */
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# input
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}
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#
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# fn get_a_type() -> Type {
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# unimplemented!()
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# }
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</code></pre>
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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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</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><aside id="help" class="hidden"><div><h1 class="hidden">Help</h1><div class="shortcuts"><h2>Keyboard Shortcuts</h2><dl><dt><kbd>?</kbd></dt><dd>Show this help dialog</dd><dt><kbd>S</kbd></dt><dd>Focus the search field</dd><dt><kbd>↑</kbd></dt><dd>Move up in search results</dd><dt><kbd>↓</kbd></dt><dd>Move down in search results</dd><dt><kbd>↹</kbd></dt><dd>Switch tab</dd><dt><kbd>⏎</kbd></dt><dd>Go to active search result</dd><dt><kbd>+</kbd></dt><dd>Expand all sections</dd><dt><kbd>-</kbd></dt><dd>Collapse all sections</dd></dl></div><div class="infos"><h2>Search Tricks</h2><p>Prefix searches with a type followed by a colon (e.g., <code>fn:</code>) to restrict the search to a given type.</p><p>Accepted types are: <code>fn</code>, <code>mod</code>, <code>struct</code>, <code>enum</code>, <code>trait</code>, <code>type</code>, <code>macro</code>, and <code>const</code>.</p><p>Search functions by type signature (e.g., <code>vec -> usize</code> or <code>* -> vec</code>)</p><p>Search multiple things at once by splitting your query with comma (e.g., <code>str,u8</code> or <code>String,struct:Vec,test</code>)</p></div></div></aside><script>window.rootPath = "../../";window.currentCrate = "syn";</script><script src="../../aliases.js"></script><script src="../../main.js"></script><script defer src="../../search-index.js"></script></body></html>
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