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<h1 id="断言-assertion"><a class="header" href="#断言-assertion">断言 assertion</a></h1>
<p>在编写测试函数时，断言决定了我们的测试是通过还是失败，它为结果代言。在前面，大家已经见识过 <code>assert_eq!</code> 的使用，下面一起来看看 Rust 为我们提供了哪些好用的断言。</p>
<h2 id="断言列表"><a class="header" href="#断言列表">断言列表</a></h2>
<p>在正式开始前，来看看常用的断言有哪些:</p>
<ul>
<li><code>assert!</code>, <code>assert_eq!</code>, <code>assert_ne!</code>, 它们会在所有模式下运行</li>
<li><code>debug_assert!</code>, <code>debug_assert_eq!</code>, <code>debug_assert_ne!</code>, 它们只会在 <code>Debug</code> 模式下运行</li>
</ul>
<h2 id="assert_eq"><a class="header" href="#assert_eq">assert_eq!</a></h2>
<p><code>assert_eq!</code> 宏可以用于判断两个表达式返回的值是否相等 :</p>
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
let a = 3;
let b = 1 + 2;
assert_eq!(a, b);
}</code></pre></pre>
<p>当不相等时，当前线程会直接 <code>panic</code>:</p>
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
let a = 3;
let b = 1 + 3;
assert_eq!(a, b, &quot;我们在测试两个数之和{} + {}，这是额外的错误信息&quot;, a, b);
}</code></pre></pre>
<p>运行后报错如下:</p>
<pre><code class="language-shell">$ cargo run
thread 'main' panicked at 'assertion failed: `(left == right)`
left: `3`,
right: `4`: 我们在测试两个数之和3 + 4，这是额外的错误信息', src/main.rs:4:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
</code></pre>
<p>可以看到，错误不仅按照预期发生了，我们还成功的定制了错误信息! 这种格式化输出的方式跟 <code>println!</code> 并无区别，具体参见 <a href="https://doc.rust-lang.org/std/fmt/index.html"><code>std::fmt</code></a>。</p>
<p>因为涉及到相等比较( <code>==</code> )和错误信息打印，因此两个表达式的值必须实现 <code>PartialEq</code> 和 <code>Debug</code> 特征，其中所有的原生类型和大多数标准库类型都实现了这些特征，而对于你自己定义的结构体、枚举，如果想要对其进行 <code>assert_eq!</code> 断言，则需要实现 <code>PartialEq</code> 和 <code>Debug</code> 特征:</p>
<ul>
<li>若希望实现个性化相等比较和错误打印，则需手动实现</li>
<li>否则可以为自定义的结构体、枚举添加 <code>#[derive(PartialEq, Debug)]</code> 注解，来<a href="https://course.rs/appendix/derive.html">自动派生</a>对应的特征</li>
</ul>
<p><strong>以上特征限制对于下面即将讲解的 <code>assert_ne!</code> 一样有效，</strong> 就不再重复讲述。</p>
<h2 id="assert_ne"><a class="header" href="#assert_ne">assert_ne!</a></h2>
<p><code>assert_ne!</code> 在使用和限制上与 <code>assert_eq!</code> 并无区别，唯一的区别就在于，前者判断的是两者的不相等性。</p>
<p>我们将之前报错的代码稍作修改：</p>
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
let a = 3;
let b = 1 + 3;
assert_ne!(a, b, &quot;我们在测试两个数之和{} + {}，这是额外的错误信息&quot;, a, b);
}</code></pre></pre>
<p>由于 <code>a</code> 和 <code>b</code> 不相等，因此 <code>assert_ne!</code> 会顺利通过，不再报错。</p>
<h2 id="assert"><a class="header" href="#assert">assert!</a></h2>
<p><code>assert!</code> 用于判断传入的布尔表达式是否为 <code>true</code>:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>// 以下断言的错误信息只包含给定表达式的返回值
assert!(true);
fn some_computation() -&gt; bool { true }
assert!(some_computation());
// 使用自定义报错信息
let x = true;
assert!(x, &quot;x wasn't true!&quot;);
// 使用格式化的自定义报错信息
let a = 3; let b = 27;
assert!(a + b == 30, &quot;a = {}, b = {}&quot;, a, b);
<span class="boring">}</span></code></pre></pre>
<p>来看看该如何使用 <code>assert!</code> 进行单元测试 :</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>#[derive(Debug)]
struct Rectangle {
width: u32,
height: u32,
}
impl Rectangle {
fn can_hold(&amp;self, other: &amp;Rectangle) -&gt; bool {
self.width &gt; other.width &amp;&amp; self.height &gt; other.height
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn larger_can_hold_smaller() {
let larger = Rectangle {
width: 8,
height: 7,
};
let smaller = Rectangle {
width: 5,
height: 1,
};
assert!(larger.can_hold(&amp;smaller));
}
}
<span class="boring">}</span></code></pre></pre>
<h2 id="debug_assert-系列"><a class="header" href="#debug_assert-系列"><code>debug_assert!</code> 系列</a></h2>
<p><code>debug_assert!</code>, <code>debug_assert_eq!</code>, <code>debug_assert_ne!</code> 这三个在功能上与之前讲解的版本并无区别，主要区别在于，<code>debug_assert!</code> 系列只能在 <code>Debug</code> 模式下输出，例如如下代码：</p>
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
let a = 3;
let b = 1 + 3;
debug_assert_eq!(a, b, &quot;我们在测试两个数之和{} + {}，这是额外的错误信息&quot;, a, b);
}</code></pre></pre>
<p>在 <code>Debug</code> 模式下运行输出错误信息：</p>
<pre><code class="language-shell">$ cargo run
thread 'main' panicked at 'assertion failed: `(left == right)`
left: `3`,
right: `4`: 我们在测试两个数之和3 + 4，这是额外的错误信息', src/main.rs:4:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
</code></pre>
<p>但是在 <code>Release</code> 模式下却没有任何输出:</p>
<pre><code class="language-shell">$ cargo run --release
</code></pre>
<p>若一些断言检查会影响发布版本的性能时，大家可以使用 <code>debug_assert!</code> 来避免这种情况的发生。</p>
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