Expand description

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:

use pretty_assertions::{assert_eq, assert_ne};
Show the example behind the screenshots above.
// 1. add the `pretty_assertions` dependency to `Cargo.toml`.
// 2. insert this line at the top of each module, as needed
use pretty_assertions::{assert_eq, assert_ne};

#[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!

Also add #[cfg(test)] to your use statements, like this:

#[cfg(test)]
use pretty_assertions::{assert_eq, assert_ne};

Note

  • Since Rust 2018 edition, you need to declare use pretty_assertions::{assert_eq, assert_ne}; per module. Before you would write #[macro_use] extern crate pretty_assertions;.
  • 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.

Features

Features provided by the crate are:

  • std: Use the Rust standard library. Enabled by default. Exactly one of std and alloc is required.
  • alloc: Use the alloc crate. Exactly one of std and alloc is required.
  • unstable: opt-in to unstable features that may not follow Semantic Versioning. Implmenetion behind this feature is subject to change without warning between patch versions.

Macros

Structs

  • A comparison of two values.
  • A comparison of two strings.
  • A style is a collection of properties that can format a string using ANSI escape codes.