11.2 断言(Assert)11.2.1. 使用assert!宏检查测试结果assert!宏来自标准库用于判断某个条件是否为true。它接收一个返回类型为布尔值的表达式- 当assert!内的值为true时测试通过assert!也不会做多余的操作。- 当assert!内的值为false时assert!会调用panic!测试失败。看个例子#[derive(Debug)] struct Rectangle { width: u32, height: u32, } impl Rectangle { fn can_hold(self, other: Rectangle) - bool { self.width other.width self.height other.height } }结构体Rectangle存储矩形的宽和高。它定义了can_hold方法用于判断一个矩形能否容纳另一个矩形不考虑斜着放。逻辑很好理解只要看当前矩形的宽和高是否都大于另一个矩形即可。该如何测试这个方法呢因为它的返回类型正好是bool所以用assert!再合适不过#[derive(Debug)] struct Rectangle { width: u32, height: u32, } impl Rectangle { fn can_hold(self, other: Rectangle) - bool { self.width other.width self.height 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(smaller)); } }由于test是一个模块所以test模块内如果想使用外部的内容就必须先导入到当前作用域。这里写的是use super::*;*会把外部模块的所有内容导入进test模块。有关这部分的详细内容可以看 7.2. 路径PathPt.1 和 7.3. 路径PathPt.2。然后看下面的测试函数。首先声明了两个矩形larger和smaller分别存储大矩形和小矩形的宽高这就是准备(Arrange)阶段。下面的assert!宏调用了can_hold这就是运行(Act)阶段。最后用assert!来判断测试是否成功。在这个例子中larger存储的宽高绝对可以容纳smaller所以结果一定是true测试通过。运行cargo test$ cargo test Compiling rectangle v0.1.0 (file:///projects/rectangle) Finished test profile [unoptimized debuginfo] target(s) in 0.12s Running unittests src/lib.rs (target/debug/deps/rectangle-2f89d610a9fe6c00) running 1 test test tests::larger_can_hold_smaller ... ok test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s Doc-tests rectangle running 0 tests test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s那如果小矩形容纳不了大矩形呢#[cfg(test)] mod tests { use super::*; #[test] fn larger_can_hold_smaller() { //... } #[test] fn smaller_cannot_hold_larger() { let larger Rectangle { width: 8, height: 7, }; let smaller Rectangle { width: 5, height: 1, }; assert!(!smaller.can_hold(larger)); } }又声明了另一个测试函数smaller_cannot_hold_larger。smaller.can_hold(larger)一定返回false但前面加了取反运算符!所以最终assert!收到的仍然是true测试通过$ cargo test Compiling rectangle v0.1.0 (file:///projects/rectangle) Finished test profile [unoptimized debuginfo] target(s) in 0.08s Running unittests src/lib.rs (target/debug/deps/rectangle-2f89d610a9fe6c00) running 2 tests test tests::larger_can_hold_smaller ... ok test tests::smaller_cannot_hold_larger ... ok test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s Doc-tests rectangle running 0 tests test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s两个测试都能通过说明can_hold方法大概没问题。现在改一下这个方法把can_hold中的宽度比较从改成#[derive(Debug)] struct Rectangle { width: u32, height: u32, } impl Rectangle { fn can_hold(self, other: Rectangle) - bool { self.width other.width self.height other.height } }逻辑现在就错了。再运行同样的测试函数$ cargo test Compiling rectangle v0.1.0 (file:///projects/rectangle) Finished test profile [unoptimized debuginfo] target(s) in 0.07s Running unittests src/lib.rs (target/debug/deps/rectangle-2f89d610a9fe6c00) running 2 tests test tests::smaller_cannot_hold_larger ... ok test tests::larger_can_hold_smaller ... FAILED failures: ---- tests::larger_can_hold_smaller stdout ---- thread tests::larger_can_hold_smaller (454276) panicked at src/lib.rs:28:9: assertion failed: larger.can_hold(smaller) note: run with RUST_BACKTRACE1 environment variable to display a backtrace failures: tests::larger_can_hold_smaller test result: FAILED. 1 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s error: test failed, to rerun pass --lib有一个测试失败了说明错误被成功捕获了。这也是编写测试的目的尽早发现问题。11.2.2. 使用assert_eq!和assert_ne!测试相等性assert_eq!中的eq指的是equal相等assert_ne!中的ne指的是not equal不相等。这两者都来自标准库。这两个宏接收两个参数并判断这两个值是否相等。通常把被测试代码的结果作为一个参数把期待的结果作为另一个参数然后宏就会检查这两个结果是否相等。实际上这两个宏的用法很像和!运算符。不同之处在于如果失败它们会自动打印出两个参数的值从而帮助开发者理解测试失败的原因。使用这两个宏有一定要求。它们用debug格式打印值所以参数必须实现PartialEq和Debug这两个trait。所有基本类型和大部分标准库类型都已经实现了但自定义结构体和枚举必须自行实现这些trait。这两个trait都是可派生的所以对自定义类型通常只要这样写即可#[derive(PartialEq, Debug)] struct Point { x: i32, y: i32, }加上这个标注后就可以用assert_eq!/assert_ne!比较Point值并且断言失败时能够打印出这些值。下面是一个使用assert_eq!的例子pub fn add_two(a: usize) - usize { a 2 } #[cfg(test)] mod tests { use super::*; #[test] fn it_adds_two() { let result add_two(2); assert_eq!(result, 4); } }add_two函数会给参数加2。测试函数it_adds_two调用了add_two因为2 2 4所以期待的add_two(2)的值是4把4和函数调用放进宏里即可。其实在Rust中期待的值和函数调用的位置是可以互换的。有些语言对顺序有明确要求但Rust没有。放在左边第一个参数的值只是叫做左值另一个叫做右值。输出$ cargo test Compiling adder v0.1.0 (file:///projects/adder) Finished test profile [unoptimized debuginfo] target(s) in 0.07s Running unittests src/lib.rs (target/debug/deps/adder-302521ba8d0f0bdf) running 1 test test tests::it_adds_two ... ok test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s Doc-tests adder running 0 tests test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s接下来引入一个逻辑错误把add_two的a 2改成a 3其余不变看看会发生什么pub fn add_two(a: usize) - usize { a 3 } #[cfg(test)] mod tests { use super::*; #[test] fn it_adds_two() { let result add_two(2); assert_eq!(result, 4); } }输出$ cargo test Compiling adder v0.1.0 (file:///projects/adder) Finished test profile [unoptimized debuginfo] target(s) in 0.07s Running unittests src/lib.rs (target/debug/deps/adder-302521ba8d0f0bdf) running 1 test test tests::it_adds_two ... FAILED failures: ---- tests::it_adds_two stdout ---- thread tests::it_adds_two (455023) panicked at src/lib.rs:12:9: assertion left right failed left: 5 right: 4 note: run with RUST_BACKTRACE1 environment variable to display a backtrace failures: tests::it_adds_two test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s error: test failed, to rerun pass --lib测试抓住了这个bug。失败信息显示left是5也就是add_two(2)的结果right是4。另外还有assert_ne!两个值不相等时通过相等时失败。它最适合用在你不确定具体会得到什么值、但知道它绝对不该是某个值的场合。