如何让Rust中的assert_eq!显示多行字符串

方法一：使用pretty_assertions crate #

这是最简便的方案，该 crate 会自动格式化多行字符串的对比输出，让差异一目了然。

1. 添加依赖
   在Cargo.toml的dev-dependencies中加入：

[dev-dependencies]
pretty_assertions = "1.4.0" # 可替换为最新稳定版本

2. 修改测试代码
   引入 crate 中的assert_eq并替换默认断言：

use pretty_assertions::assert_eq;

#[test]
fn test_multiline_strings_are_equal() {
    let expected = "\
one
two
three
four
";
    let actual = "\
one
two
four
";
    assert_eq!(actual, expected);
}

测试失败时的输出会自动分行展示：

---- printer::tests::test_multiline_strings_are_equal stdout ----
thread 'printer::tests::test_multiline_strings_are_equal' panicked at 'assertion failed: `(left == right)`
  left: `
one
two
four
`
 right: `
one
two
three
four
`', src\printer.rs:600:9

方法二：自定义多行断言函数 #

如果不想引入额外依赖，可以手动实现一个断言函数，直接打印分行的字符串内容：

fn assert_multiline_eq(actual: &str, expected: &str) {
    if actual != expected {
        panic!(
            "多行字符串不匹配:\n\n实际内容:\n{}\n\n预期内容:\n{}",
            actual, expected
        );
    }
}

#[test]
fn test_multiline_strings_are_equal() {
    let expected = "\
one
two
three
four
";
    let actual = "\
one
two
four
";
    assert_multiline_eq(actual, expected);
}

失败时的输出示例：

---- printer::tests::test_multiline_strings_are_equal stdout ----
thread 'printer::tests::test_multiline_strings_are_equal' panicked at '多行字符串不匹配:

实际内容:
one
two
four

预期内容:
one
two
three
four
', src\printer.rs:600:9

内容的提问来源于stack exchange，提问作者Huw Walters