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<h1 class='fqn'><span class='in-band'>Macro <a href='index.html'>std</a>::<wbr><a class="macro" href=''>unreachable</a></span><span class='out-of-band'><span id='render-detail'>
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<pre class="rust macro">
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">unreachable</span> {
( ) <span class="op">=></span> { ... };
(
$ <span class="ident">msg</span> : <span class="ident">expr</span> ) <span class="op">=></span> { ... };
(
$ <span class="ident">fmt</span> : <span class="ident">expr</span> , $ ( $ <span class="ident">arg</span> : <span class="ident">tt</span> ) <span class="op">*</span> ) <span class="op">=></span> { ... };
}</pre>
<div class='docblock'><p>A utility macro for indicating unreachable code.</p>
<p>This is useful any time that the compiler can't determine that some code is unreachable. For
example:</p>
<ul>
<li>Match arms with guard conditions.</li>
<li>Loops that dynamically terminate.</li>
<li>Iterators that dynamically terminate.</li>
</ul>
<h1 id='panics' class='section-header'><a href='#panics'>Panics</a></h1>
<p>This will always panic.</p>
<h1 id='examples' class='section-header'><a href='#examples'>Examples</a></h1>
<p>Match arms:</p>
<pre class="rust rust-example-rendered">
<span class="kw">fn</span> <span class="ident">foo</span>(<span class="ident">x</span>: <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">i32</span><span class="op">></span>) {
<span class="kw">match</span> <span class="ident">x</span> {
<span class="prelude-val">Some</span>(<span class="ident">n</span>) <span class="kw">if</span> <span class="ident">n</span> <span class="op">>=</span> <span class="number">0</span> <span class="op">=></span> <span class="macro">println</span><span class="macro">!</span>(<span class="string">"Some(Non-negative)"</span>),
<span class="prelude-val">Some</span>(<span class="ident">n</span>) <span class="kw">if</span> <span class="ident">n</span> <span class="op"><</span> <span class="number">0</span> <span class="op">=></span> <span class="macro">println</span><span class="macro">!</span>(<span class="string">"Some(Negative)"</span>),
<span class="prelude-val">Some</span>(_) <span class="op">=></span> <span class="macro">unreachable</span><span class="macro">!</span>(), <span class="comment">// compile error if commented out</span>
<span class="prelude-val">None</span> <span class="op">=></span> <span class="macro">println</span><span class="macro">!</span>(<span class="string">"None"</span>)
}
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=fn%20main()%20%7B%0A%23%5Ballow(dead_code)%5D%0Afn%20foo(x%3A%20Option%3Ci32%3E)%20%7B%0A%20%20%20%20match%20x%20%7B%0A%20%20%20%20%20%20%20%20Some(n)%20if%20n%20%3E%3D%200%20%3D%3E%20println!(%22Some(Non-negative)%22)%2C%0A%20%20%20%20%20%20%20%20Some(n)%20if%20n%20%3C%20%200%20%3D%3E%20println!(%22Some(Negative)%22)%2C%0A%20%20%20%20%20%20%20%20Some(_)%20%20%20%20%20%20%20%20%20%20%20%3D%3E%20unreachable!()%2C%20%2F%2F%20compile%20error%20if%20commented%20out%0A%20%20%20%20%20%20%20%20None%20%20%20%20%20%20%20%20%20%20%20%20%20%20%3D%3E%20println!(%22None%22)%0A%20%20%20%20%7D%0A%7D%0A%7D">Run</a></pre>
<p>Iterators:</p>
<pre class="rust rust-example-rendered">
<span class="kw">fn</span> <span class="ident">divide_by_three</span>(<span class="ident">x</span>: <span class="ident">u32</span>) <span class="op">-></span> <span class="ident">u32</span> { <span class="comment">// one of the poorest implementations of x/3</span>
<span class="kw">for</span> <span class="ident">i</span> <span class="kw">in</span> <span class="number">0</span>.. {
<span class="kw">if</span> <span class="number">3</span><span class="kw-2">*</span><span class="ident">i</span> <span class="op"><</span> <span class="ident">i</span> { <span class="macro">panic</span><span class="macro">!</span>(<span class="string">"u32 overflow"</span>); }
<span class="kw">if</span> <span class="ident">x</span> <span class="op"><</span> <span class="number">3</span><span class="kw-2">*</span><span class="ident">i</span> { <span class="kw">return</span> <span class="ident">i</span><span class="op">-</span><span class="number">1</span>; }
}
<span class="macro">unreachable</span><span class="macro">!</span>();
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=fn%20main()%20%7B%0A%23%5Ballow(dead_code)%5D%0Afn%20divide_by_three(x%3A%20u32)%20-%3E%20u32%20%7B%20%2F%2F%20one%20of%20the%20poorest%20implementations%20of%20x%2F3%0A%20%20%20%20for%20i%20in%200..%20%7B%0A%20%20%20%20%20%20%20%20if%203*i%20%3C%20i%20%7B%20panic!(%22u32%20overflow%22)%3B%20%7D%0A%20%20%20%20%20%20%20%20if%20x%20%3C%203*i%20%7B%20return%20i-1%3B%20%7D%0A%20%20%20%20%7D%0A%20%20%20%20unreachable!()%3B%0A%7D%0A%7D">Run</a></pre>
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