Rust Mock

An example rust library that easily mocks an interface for unit testing with the mockall crate.

An important Application method cool_algorithm needs to be unit tested.

cool_algorithm relies on an external service and for whatever reason, that external service cannot be used during unit testing.

To remedy this, we define the I32Calculator trait:

rust-mock/src/lib.rs

Lines 74 to 83 in 8e6303e

	pub trait I32Calculator {
	/// Returns the sum of `x` and `y`.
	fn add(&self, x: i32, y: i32) -> i32;
	/// Returns the difference of `x` and `y`.
	fn subtract(&self, x: i32, y: i32) -> i32;
	/// Returns the product of `x` and `y`.
	fn multiply(&self, x: i32, y: i32) -> i32;
	/// Returns the quotient of `x` and `y`.
	fn divide(&self, x: i32, y: i32) -> i32;
	}

And implement it for external service's client ExternalI32Calculator:

rust-mock/src/lib.rs

Lines 89 to 105 in 8e6303e

	impl I32Calculator for ExternalI32Calculator {
	fn add(&self, _x: i32, _y: i32) -> i32 {
	panic!("Can't call this in unit tests!")
	}
	fn subtract(&self, _x: i32, _y: i32) -> i32 {
	panic!("Can't call this in unit tests!")
	}
	fn multiply(&self, _x: i32, _y: i32) -> i32 {
	panic!("Can't call this in unit tests!")
	}
	fn divide(&self, _x: i32, _y: i32) -> i32 {
	panic!("Can't call this in unit tests!")
	}
	}

We then declare an i32_calculator field in our Application that is I32Calculator:

rust-mock/src/lib.rs

Lines 109 to 111 in 8e6303e

	pub struct Application {
	pub i32_calculator: Box<dyn I32Calculator>,
	}

To enable mocking of I32Calculator, we add the #[cfg_attr(test, mockall::automock)] attribute to the trait's definition:

rust-mock/src/lib.rs

Lines 73 to 74 in 8e6303e

	#[cfg_attr(test, mockall::automock)]
	pub trait I32Calculator {

In our unit test, we then create a mock object that is I32Calculator and set up expectations and return values for the methods will be called during the cool_algorithm call:

rust-mock/src/lib.rs

Lines 129 to 174 in 8e6303e

	#[cfg(test)]
	mod tests {
	use super::*;
	use mockall::predicate;
	#[test]
	fn cool_algorithm_does_nothing() {
	let number = 100;
	// mock object that is I32Calculator
	let mut mock_i32_calculator = MockI32Calculator::new();
	// set up our expectations
	mock_i32_calculator
	.expect_add()
	.times(1)
	.with(predicate::eq(number), predicate::eq(0))
	.return_const(number);
	mock_i32_calculator
	.expect_subtract()
	.times(1)
	.with(predicate::eq(number), predicate::eq(0))
	.return_const(number);
	mock_i32_calculator
	.expect_multiply()
	.times(1)
	.with(predicate::eq(number), predicate::eq(1))
	.return_const(number);
	mock_i32_calculator
	.expect_divide()
	.times(1)
	.with(predicate::eq(number), predicate::eq(1))
	.return_const(number);
	// create our application with our mock calculator
	let app = Application {
	i32_calculator: Box::new(mock_i32_calculator),
	};
	// run our unit test of the cool_algorithm
	assert_eq!(app.cool_algorithm(number), number);
	}
	}