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: https://github.com/peterjamesmatthews/rust-mock/blob/8e6303efc4035f87d4bfd7204484634838393ce9/src/lib.rs#L74-L83

And implement it for external service's client ExternalI32Calculator: https://github.com/peterjamesmatthews/rust-mock/blob/8e6303efc4035f87d4bfd7204484634838393ce9/src/lib.rs#L89-L105

We then declare an i32_calculator field in our Application that is I32Calculator: https://github.com/peterjamesmatthews/rust-mock/blob/8e6303efc4035f87d4bfd7204484634838393ce9/src/lib.rs#L109-L111

To enable mocking of I32Calculator, we add the #[cfg_attr(test, mockall::automock)] attribute to the trait's definition: https://github.com/peterjamesmatthews/rust-mock/blob/8e6303efc4035f87d4bfd7204484634838393ce9/src/lib.rs#L73-L74

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: https://github.com/peterjamesmatthews/rust-mock/blob/8e6303efc4035f87d4bfd7204484634838393ce9/src/lib.rs#L129-L174