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<h1 class='fqn'><span class='in-band'>Macro <a href='index.html'>synstructure</a>::<wbr><a class="macro" href=''>quote_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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</span></span></h1>
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<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="ident">span</span> : <span class="ident">expr</span> <span class="op">=></span> $ ( $ <span class="ident">tt</span> : <span class="ident">tt</span> ) <span class="op">*</span> ) <span class="op">=></span> { ... };
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}</pre>
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<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 class="rust rust-example-rendered">
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<span class="kw">let</span> <span class="ident">span</span> <span class="op">=</span> <span class="comment">/* ... */</span>;
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<span class="comment">// On one line, use parentheses.</span>
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<span class="kw">let</span> <span class="ident">tokens</span> <span class="op">=</span> <span class="macro">quote_spanned</span><span class="macro">!</span>(<span class="ident">span</span><span class="op">=></span> <span class="ident">Box</span>::<span class="ident">into_raw</span>(<span class="ident">Box</span>::<span class="ident">new</span>(#<span class="ident">init</span>)));
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<span class="comment">// On multiple lines, place the span at the top and use braces.</span>
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<span class="kw">let</span> <span class="ident">tokens</span> <span class="op">=</span> <span class="macro">quote_spanned</span><span class="macro">!</span> {<span class="ident">span</span><span class="op">=></span>
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<span class="ident">Box</span>::<span class="ident">into_raw</span>(<span class="ident">Box</span>::<span class="ident">new</span>(#<span class="ident">init</span>))
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};</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 class="rust rust-example-rendered">
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<span class="kw">let</span> <span class="ident">ty_span</span> <span class="op">=</span> <span class="ident">ty</span>.<span class="ident">span</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><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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};</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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