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<h1 class='fqn'><span class='in-band'>Macro <a href='index.html'>synom</a>::<wbr><a class="macro" href=''>opt_vec</a></span><span class='out-of-band'><span id='render-detail'>
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                       [<span class='inner'>&#x2212;</span>]
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               </span><a class='srclink' href='../src/synom/helper.rs.html#173-180' title='goto source code'>[src]</a></span></h1>
<pre class="rust macro">
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">opt_vec</span> {
    (<span class="macro-nonterminal">$</span><span class="macro-nonterminal">i</span>:<span class="ident">expr</span>, <span class="macro-nonterminal">$</span><span class="macro-nonterminal">submac</span>:<span class="macro">ident</span><span class="macro">!</span>( $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">args</span>:<span class="ident">tt</span>)<span class="op">*</span> )) <span class="op">=&gt;</span> { ... };
}</pre>
<div class='docblock'><p>Turn a failed parse into an empty vector. The argument parser must itself
return a vector.</p>
<p>This is often more convenient than <code>option!(...)</code> when the argument produces
a vector.</p>
<ul>
<li><strong>Syntax:</strong> <code>opt_vec!(THING)</code></li>
<li><strong>Output:</strong> <code>THING</code>, which must be <code>Vec&lt;T&gt;</code></li>
</ul>

<pre class="rust rust-example-rendered">
<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">syn</span>;
<span class="attribute">#[<span class="ident">macro_use</span>]</span> <span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">synom</span>;

<span class="kw">use</span> <span class="ident">syn</span>::{<span class="ident">Lifetime</span>, <span class="ident">Ty</span>};
<span class="kw">use</span> <span class="ident">syn</span>::<span class="ident">parse</span>::{<span class="ident">lifetime</span>, <span class="ident">ty</span>};

<span class="macro">named</span><span class="macro">!</span>(<span class="ident">bound_lifetimes</span> <span class="op">-&gt;</span> (<span class="ident">Vec</span><span class="op">&lt;</span><span class="ident">Lifetime</span><span class="op">&gt;</span>, <span class="ident">Ty</span>), <span class="macro">tuple</span><span class="macro">!</span>(
    <span class="macro">opt_vec</span><span class="macro">!</span>(<span class="macro">do_parse</span><span class="macro">!</span>(
        <span class="macro">keyword</span><span class="macro">!</span>(<span class="string">&quot;for&quot;</span>) <span class="op">&gt;&gt;</span>
        <span class="macro">punct</span><span class="macro">!</span>(<span class="string">&quot;&lt;&quot;</span>) <span class="op">&gt;&gt;</span>
        <span class="ident">lifetimes</span>: <span class="macro">terminated_list</span><span class="macro">!</span>(<span class="macro">punct</span><span class="macro">!</span>(<span class="string">&quot;,&quot;</span>), <span class="ident">lifetime</span>) <span class="op">&gt;&gt;</span>
        <span class="macro">punct</span><span class="macro">!</span>(<span class="string">&quot;&gt;&quot;</span>) <span class="op">&gt;&gt;</span>
        (<span class="ident">lifetimes</span>)
    )),
    <span class="ident">ty</span>
));

<span class="kw">fn</span> <span class="ident">main</span>() {
    <span class="kw">let</span> <span class="ident">input</span> <span class="op">=</span> <span class="string">&quot;for&lt;&#39;a, &#39;b&gt; fn(&amp;&#39;a A) -&gt; &amp;&#39;b B&quot;</span>;
    <span class="kw">let</span> <span class="ident">parsed</span> <span class="op">=</span> <span class="ident">bound_lifetimes</span>(<span class="ident">input</span>).<span class="ident">expect</span>(<span class="string">&quot;bound lifetimes&quot;</span>);
    <span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">parsed</span>.<span class="number">0</span>, [<span class="ident">Lifetime</span>::<span class="ident">new</span>(<span class="string">&quot;&#39;a&quot;</span>), <span class="ident">Lifetime</span>::<span class="ident">new</span>(<span class="string">&quot;&#39;b&quot;</span>)]);
    <span class="macro">println</span><span class="macro">!</span>(<span class="string">&quot;{:?}&quot;</span>, <span class="ident">parsed</span>);

    <span class="kw">let</span> <span class="ident">input</span> <span class="op">=</span> <span class="string">&quot;From&lt;String&gt;&quot;</span>;
    <span class="kw">let</span> <span class="ident">parsed</span> <span class="op">=</span> <span class="ident">bound_lifetimes</span>(<span class="ident">input</span>).<span class="ident">expect</span>(<span class="string">&quot;bound lifetimes&quot;</span>);
    <span class="macro">assert</span><span class="macro">!</span>(<span class="ident">parsed</span>.<span class="number">0</span>.<span class="ident">is_empty</span>());
    <span class="macro">println</span><span class="macro">!</span>(<span class="string">&quot;{:?}&quot;</span>, <span class="ident">parsed</span>);
}</pre>
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