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Pretty Assertions

When writing tests in Rust, you'll probably use assert_eq!(a, b) a lot.

If such a test fails, it will present all the details of a and b, but you have to spot, the differences yourself, which is not always straightforward, like here:

standard assertion

Wouldn't that task be much easier with a colorful diff?

pretty assertion

Yep — and you only need one line of code to make it happen:

#[macro_use] extern crate pretty_assertions;
Show the example behind the screenshots above.
// 1. add the `pretty_assertions` dependency to `Cargo.toml`.
// 2. insert this line at the top of your crate root or integration test
#[macro_use] extern crate pretty_assertions;

fn main() {
    #[derive(Debug, PartialEq)]
    struct Foo {
        lorem: &'static str,
        ipsum: u32,
        dolor: Result<String, String>,
    }

    let x = Some(Foo { lorem: "Hello World!", ipsum: 42, dolor: Ok("hey".to_string())});
    let y = Some(Foo { lorem: "Hello Wrold!", ipsum: 42, dolor: Ok("hey ho!".to_string())});

    assert_eq!(x, y);
}

Tip

Specify it as [dev-dependencies] and it will only be used for compiling tests, examples, and benchmarks. This way the compile time of cargo build won't be affected!

In your crate root, also add #[cfg(test)] to the crate import, like this:

#[cfg(test)] // <-- not needed in examples + integration tests
#[macro_use]
extern crate pretty_assertions;

Note

  • Each example and integration test also needs #[macro_use] extern crate pretty_assertions, if you want colorful diffs there.
  • The replacement is only effective in your own crate, not in other libraries you include.
  • assert_ne is also switched to multi-line presentation, but does not show a diff.
  • Under Windows, the terminal state is modified to properly handle VT100 escape sequences, which may break display for certain use cases.

License

Licensed under either of

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.