There's no special magical way to mock things in Rust. What you're doing makes sense. Rust's (built-in) unit testing story is not very complex, you have 3 assertion macros and the #[test] attribute basically.

What additional functionality are you hoping to get through an alternative means of initializing the cat?

I'm not quite sure I fully understand the question, but would implementing the Default trait work?

Otherwise, I've used Arbitrary in tests before, maybe that's what you're looking for?

Just FYI -- your test isn't going to run, you need to mark it with #[test].

So if you're used to a language like JS or python, or even Java, you're going to be a bit frustrated at how to mock things in rust. In those languages everything is boxed. In JS or python, because they're dynamically typed, you don't have to do anything special to mock, and in Java you can either play nice and use interfacees everywhere, or else you can do some runtime magic to mock an object of a regular class.

You can do something similar in rust -- e.g. you can have a trait Cat and a struct RealCat and a (or possibly many) struct FakeCat. (There are crates that will help you with this). Then you need to either accept a Box<dyn Cat> or a &dyn Cat, or make your code under test generic (which can infect the rest of your code if you aren't careful), something like fn uses_a_cat<C: Cat>(cat: C) {}

So there's not quite as easy of an answer. You also have several more options, for example you can

pub struct FakeCat;

pub struct RealCat;

#[cfg(test)]
pub type Cat = FakeCat;

#[cfg(not(test))]
pub type Cat = RealCat;

and get a fake (or mock, or spy, whatever test double you'd like) in all test code in your same crate. This doesn't work well across crate boundaries though, and it only lets you provide one double, so it makes sense for that double to be very generic (there are crates to do this for you as well).

So there's not really a one-size-fits-all approach. You have to think about the tradeoffs.

However I think the best overall test strategy (and it doesn't always apply, but it should be preferred when it does), is the same one used for functional programming: just accept and return values. Pure functions don't need mocks, and even impure functions can easily be tested if they don't have other side effects that you need to prevent during tests. Obviously you still need to deal with side effects if your program is going to work, but if you have lots of pure unit tests that don't need any fancy test doubles you can do end-to-end testing for all of your I/O and other messy side effects. Which as I said, doesn't always apply (sometimes you really need test doubles), but it's good to use whenever possible.

I don't think you can mock a struct. I'm guessing a mock would be a different struct type with the same fields, or a subset of fields. But since Rust types are nominative, not structural, you can't substitute one struct type with another even if the data is identical.

When you need a struct for a test you create a value of the original type. Some problems that might come up are,

• you need to call a constructor to create a struct, and it has side-effects that you don't want to run in the test
• you need to call a constructor that is not visible to the test
• the struct requires data that is difficult or cumbersome to produce in the test

For the first two problems one option is to use conditional compilation to create test-only constructors that bypass code you don't want to run, or that have broader visibility.

For the last problem you might want to refactor the code to use traits which you can create mock/test implementations for. Or you might change the struct to make the problematic data optional, or lazily initialized.