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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 `quote_spanned` macro in crate `quote`."><meta name="keywords" content="rust, rustlang, rust-lang, quote_spanned"><title>quote::quote_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 macro"><!--[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='../quote/index.html'><div class='logo-container'><img src='../rust-logo.png' alt='logo'></div></a><div class="sidebar-elems"><p class='location'><a href='index.html'>quote</a></p><script>window.sidebarCurrent = {name: 'quote_spanned', ty: 'macro', 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/quote/lib.rs.html#435-442' title='goto source code'>[src]</a></span><span class='in-band'>Macro <a href='index.html'>quote</a>::<wbr><a class="macro" href=''>quote_spanned</a></span></h1><div class="docblock type-decl hidden-by-usual-hider"><div class="example-wrap"><pre class="rust macro">
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<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">quote_spanned</span> {
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(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">span</span>:<span class="ident">expr</span><span class="op">=</span><span class="op">></span> $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">tt</span>:<span class="ident">tt</span>)<span class="kw-2">*</span>) <span class="op">=</span><span class="op">></span> { ... };
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}</pre></div>
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</div><div class='docblock'><p>Same as <code>quote!</code>, but applies a given span to all tokens originating within
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the macro invocation.</p>
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<h1 id="syntax" class="section-header"><a href="#syntax">Syntax</a></h1>
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<p>A span expression of type <a href="https://docs.rs/proc-macro2/0.4/proc_macro2/struct.Span.html"><code>Span</code></a>, followed by <code>=></code>, followed by the tokens
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to quote. The span expression should be brief -- use a variable for anything
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more than a few characters. There should be no space before the <code>=></code> token.</p>
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<pre><code class="language-edition2018"># use proc_macro2::Span;
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# use quote::quote_spanned;
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#
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# const IGNORE_TOKENS: &'static str = stringify! {
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let span = /* ... */;
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# };
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# let span = Span::call_site();
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# let init = 0;
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// On one line, use parentheses.
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let tokens = quote_spanned!(span=> Box::into_raw(Box::new(#init)));
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// On multiple lines, place the span at the top and use braces.
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let tokens = quote_spanned! {span=>
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Box::into_raw(Box::new(#init))
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};
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</code></pre>
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<p>The lack of space before the <code>=></code> should look jarring to Rust programmers
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and this is intentional. The formatting is designed to be visibly
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off-balance and draw the eye a particular way, due to the span expression
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being evaluated in the context of the procedural macro and the remaining
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tokens being evaluated in the generated code.</p>
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<h1 id="hygiene" class="section-header"><a href="#hygiene">Hygiene</a></h1>
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<p>Any interpolated tokens preserve the <code>Span</code> information provided by their
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<code>ToTokens</code> implementation. Tokens that originate within the <code>quote_spanned!</code>
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invocation are spanned with the given span argument.</p>
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<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
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<p>The following procedural macro code uses <code>quote_spanned!</code> to assert that a
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particular Rust type implements the <a href="https://doc.rust-lang.org/std/marker/trait.Sync.html"><code>Sync</code></a> trait so that references can be
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safely shared between threads.</p>
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<pre><code class="language-edition2018"># use quote::{quote_spanned, TokenStreamExt, ToTokens};
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# use proc_macro2::{Span, TokenStream};
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#
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# struct Type;
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#
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# impl Type {
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# fn span(&self) -> Span {
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# Span::call_site()
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# }
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# }
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#
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# impl ToTokens for Type {
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# fn to_tokens(&self, _tokens: &mut TokenStream) {}
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# }
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#
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# let ty = Type;
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# let call_site = Span::call_site();
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#
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let ty_span = ty.span();
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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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</code></pre>
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<p>If the assertion fails, the user will see an error like the following. The
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input span of their type is hightlighted in the error.</p>
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<pre><code class="language-text">error[E0277]: the trait bound `*const (): std::marker::Sync` is not satisfied
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--> src/main.rs:10:21
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10 | static ref PTR: *const () = &();
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| ^^^^^^^^^ `*const ()` cannot be shared between threads safely
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</code></pre>
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<p>In this example it is important for the where-clause to be spanned with the
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line/column information of the user's input type so that error messages are
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placed appropriately by the compiler. But it is also incredibly important
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that <code>Sync</code> resolves at the macro definition site and not the macro call
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site. If we resolve <code>Sync</code> at the same span that the user's type is going to
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be resolved, then they could bypass our check by defining their own trait
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named <code>Sync</code> that is implemented for their type.</p>
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