Mockall

A powerful mock object library for Rust.

Overview

Mock objects are a powerful technique for unit testing software. A mock object is an object with the same interface as a real object, but whose responses are all manually controlled by test code. They can be used to test the upper and middle layers of an application without instantiating the lower ones, or to inject edge and error cases that would be difficult or impossible to create when using the full stack.

Statically typed languages are inherently more difficult to mock than dynamically typed languages. Since Rust is a statically typed language, previous attempts at creating a mock object library have had mixed results. Mockall incorporates the best elements of previous designs, resulting in it having a rich feature set with a terse and ergonomic interface. Mockall is written in 100% safe and stable Rust.

Usage

Typically mockall is only used by unit tests. To use it this way, add this to your Cargo.toml:

[dev-dependencies]
mockall = "0.15.0"

Then use it like this:

#[cfg(test)]
use mockall::{automock, mock, predicate::*};
#[cfg_attr(test, automock)]
trait MyTrait {
    fn foo(&self, x: u32) -> u32;
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn mytest() {
        let mut mock = MockMyTrait::new();
        mock.expect_foo()
            .with(eq(4))
            .times(1)
            .returning(|x| x + 1);
        assert_eq!(5, mock.foo(4));
    }
}

See the API docs for more information.

Minimum Supported Rust Version (MSRV)

Mockall is supported on Rust 1.77.0 and higher. Mockall's MSRV will not be changed in the future without bumping the major or minor version.

License

mockall is primarily distributed under the terms of both the MIT license and the Apache License (Version 2.0).

See LICENSE-APACHE, and LICENSE-MIT for details

Acknowledgements

Mockall was not built in a day. JMock was probably the first popular mock object library. Many ports and imitations have been made, including GoogleMock for C++. Mockers, inspired by GoogleMock, was the first attempt to bring the concept to Rust. The now-defunct Mock_derive was the first library to generate mock objects with procedural macros, greatly reducing the user's workload. Mockall also uses proc macros, and copies many of Mockers' features and conventions. Mockall also takes inspiration from Simulacrum's internal design, and its technique for mocking generic methods.