How to unit test one struct method while mocking other methods?

[rharish101]

Say I have a struct Foo with two methods method_1 and method_2 such that method_2 calls method_1:

struct Foo { ... };
struct Bar { ... };

impl Foo {
    fn method_1(&self) -> Bar {
        // Complex code
    }
    fn method_2(&self) {
        // Complex code
        let result = self.method_1();
        // Complex code
    }
}

How can I write a unit test for method_2 by mocking just method_1? I tried adding #[cfg(test, automock)] before method_1, but I got: message: automock does not support this item type.

[asomers]

The short answer is that you can't. Mockall replaces the entire object with a mock version. You might be able to get away with the following, if method_2 doesn't access any of the struct's data:

#[automock]
impl Foo {
    fn method_1(&self) -> Bar {
        ...
    }
}
impl Foo {
    fn method_2(&self) {
        ....
    }
}

However, that strategy will break down if method_2 tries to access any struct data. Instead, a better strategy is to separate the object into two. Not just in your test code, but in your product code as well. Arrange it in layers, such that you can mock the lower layer when testing the higher one. That's what I do. Like this:

mod foo {
    struct Foo {...}
    #[automock]
    impl Foo {...}
}
mod bar {
    #[double]
    use crate::foo::Foo;

    struct Bar{foo: Foo, ...}
    #[automock]
    impl Bar {...}
}
mod baz {
    #[double]
    use crate::bar::Baz;

    struct Baz {bar: Bar, ....}
}

[rharish101]

Thanks a lot! I went with the second method, since I needed to mock method_2 as well elsewhere in the codebase.