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<main>
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<h1 id="算术溢出导致的-panic"><a class="header" href="#算术溢出导致的-panic">算术溢出导致的 panic</a></h1>
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<p>在 Rust 中，溢出后的数值被截断是很正常的:</p>
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<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>let x: u16 = 65535;
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let v = x as u8;
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println!("{}", v)
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<span class="boring">}</span></code></pre>
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<p>最终程序会输出<code>255</code>, 因此大家可能会猜测算术溢出在 Rust 中也会只导致截断。但是实际上，如果在 debug 模式下运行，算术溢出会引发 panic:</p>
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<pre class="playground"><code class="language-rust edition2021">#[allow(arithmetic_overflow)] // 阻止编译器在编译时发现问题
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fn main() {
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let x: u8 = 10;
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let v = x + u8::MAX;
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println!("{}", v)
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}</code></pre>
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<p>而在 release 模式下为了最大化性能，编译器不会插入算术溢出检查，所以不会导致 panic，而是变成<a href="https://stackoverflow.com/a/60238510/19171888">二补数</a>。</p>
|
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<p>如何理解这种拧巴的行为？为了处理算术溢出，Rust 的基本运算实则以下关联函数：</p>
|
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<ul>
|
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<li><code>wrapping_xxx</code>：正常时返回数值，溢出时回到值域的另一端（<code>MAX+1=MIN</code>, <code>MIN-1=MAX</code>）</li>
|
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<li><code>check_xxx</code>：正常时返回<code>Some(x)</code>，溢出时返回<code>None</code></li>
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<li><code>unchecked_xxx</code>：正常时返回数值，溢出时 UB，不安全</li>
|
|
|
<li><code>saturating_xxx</code>：正常时返回数值，溢出时定在值域边缘（<code>MAX+1=MAX</code>, <code>MIN-1=MIN</code>）</li>
|
|
|
<li><code>overflowing_xxx</code>：正常时返回<code>(x, false)</code>，溢出时返回<code>(wrapped(x), true)</code></li>
|
|
|
<li><code>strict_xxx</code>：正常时返回数值，溢出时 panic</li>
|
|
|
</ul>
|
|
|
<p>特别地，除以<code>0</code>在任何情况下都会被检查。</p>
|
|
|
<p>运算符的默认行为在 debug 模式下是<code>strict_xxx</code>，release 模式下是<code>wrapping</code>。想在 release 模式下也默认到<code>strict_xxx</code>，可以在<code>Cargo.toml</code>加上：</p>
|
|
|
<pre><code class="language-toml">[profile.release]
|
|
|
overflow-checks = true
|
|
|
</code></pre>
|
|
|
<p>也许你会觉得本章内容其实算不上什么陷阱，但是在实际项目快速迭代中，越是不起眼的地方越是容易出错：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
let v = production_rate_per_hour(5);
|
|
|
println!("{}", v);
|
|
|
}
|
|
|
|
|
|
pub fn production_rate_per_hour(speed: u8) -> f64 {
|
|
|
let cph: u8 = 221;
|
|
|
match speed {
|
|
|
1..=4 => (speed * cph) as f64,
|
|
|
5..=8 => (speed * cph) as f64 * 0.9,
|
|
|
9..=10 => (speed * cph) as f64 * 0.77,
|
|
|
_ => 0 as f64,
|
|
|
}
|
|
|
}
|
|
|
|
|
|
pub fn working_items_per_minute(speed: u8) -> u32 {
|
|
|
(production_rate_per_hour(speed) / 60 as f64) as u32
|
|
|
}</code></pre>
|
|
|
<p>上述代码在 debug 模式下<code>speed * cph</code>就会直接 panic:</p>
|
|
|
<pre><code class="language-console">thread 'main' panicked at 'attempt to multiply with overflow', src/main.rs:10:18
|
|
|
</code></pre>
|
|
|
<p>release 模式下则输出<code>72.9</code>。</p>
|
|
|
<p>是不是还藏的挺隐蔽的？因此大家在 Rust 中做数学运算时，要多留一个心眼，免得上了生产才发现问题。或者直接打开<code>overflow-checks</code>直接 panic。</p>
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