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

Asserts that two expressions are equal to each other (using PartialEq).
Asserts that two expressions are not equal to each other (using PartialEq).
Asserts that two expressions are equal to each other (using PartialEq).

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.