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<meta name="description" content="API documentation for the Rust `spanned` mod in crate `syn`.">
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<meta name="keywords" content="rust, rustlang, rust-lang, spanned">
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<title>syn::spanned - Rust</title>
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<nav class="sidebar">
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<div class="sidebar-menu">☰</div>
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<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>
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<section id='main' class="content">
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<h1 class='fqn'><span class='in-band'>Module <a href='../index.html'>syn</a>::<wbr><a class="mod" href=''>spanned</a></span><span class='out-of-band'><span id='render-detail'>
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<a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">
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[<span class='inner'>−</span>]
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</a>
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</span><a class='srclink' href='../../src/syn/spanned.rs.html#9-142' title='goto source code'>[src]</a></span></h1>
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<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 class="rust rust-example-rendered">
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<span class="attribute">#[<span class="ident">macro_use</span>]</span>
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<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">quote</span>;
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<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">syn</span>;
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<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">proc_macro</span>;
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<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">proc_macro2</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="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="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="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>
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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>
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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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|
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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>
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<tr class=' module-item'>
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<td><a class="trait" href="trait.Spanned.html"
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title='trait syn::spanned::Spanned'>Spanned</a></td>
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<td class='docblock-short'>
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<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>
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