Not sure how to mock a Trait with a new with parameters

[stefanhp]

Hi,

I'm new at using mocks in rust. I'm trying to test a trait with a constructor that has parameters like so:

#[cfg(test)]
use mockall::{automock, predicate::*};

#[cfg_attr(test, automock)]
pub trait MyTrait {
    fn new<S>(param1: S, param2: S) -> Option<Self>
    where
        S: Into<String> + 'static,
        Self: std::marker::Sized;
}
#[cfg(test)]
mod tests {
    use crate::{MyTrait, MockMyTrait};

    #[test]
    fn create_test() {
        // how to setup expectation ???
        let mock = MockMyTrait::new("param 1", "param 2");
        assert!(mock.is_some());
    }
}

The test fails with:

thread 'tests::create_test' panicked at 'MockMyTrait::new(?, ?): No matching expectation found'

If I look at the generated code, I can see that there is a function for mocking the new function:

impl MockMyTrait {
    ///Create a [`Context`](__mock_MockMyTrait_MyTrait/__new/struct.Context.html) for mocking the `new` method
    pub fn new_context() -> __mock_MockMyTrait_MyTrait::__new::Context {
        __mock_MockMyTrait_MyTrait::__new::Context::default()
    }
}

But I'm not sure how to use it...

I would like to set it up so that it returns Some(MockMyTrait).

Later I'll add other functions to the trait that I can mock/test, but the standard use case is the create a new struct that implements the trait, verify that the Option<Self> is not None, then unwrap and call other functions:

#[cfg_attr(test, automock)]
pub trait MyTrait {
    fn new<S>(param1: S, param2: S) -> Option<Self>
    where
        S: Into<String> + 'static,
        Self: std::marker::Sized;

    fn operation(&self) -> Result<State, Error>;
}

#[cfg(test)]
mod tests {
    use crate::{MyTrait, MockMyTrait};

    #[test]
    fn create_test() {
        let mock = MockMyTrait::new("some param 1", "some param 2");
        assert!(mock.is_some());
        let mut mock = mock.unwrap();
        mock.expect_operation().returning(|| Ok(State::Connected));
        assert_eq!(mock.operation(), Ok(State::Connected));
    }
}

Any help would be appreciated.

[asomers]

Since your trait has a new method, Mockall doesn't generate the usual one. Instead, new works like a normal method and you must make an expectation for it. If you want to create a new mock object with no expectations, you'll have to use MyTrait:::default().

[stefanhp]

Instead, new works like a normal method and you must make an expectation for it.

Yes I got that from the docs, I'm not sure how though. Can you provide an example?
How can you set an expectation on an instance that is not created yet?
I want to mock different scenarios with different values of param1 and param2

[asomers]

Well, it isn't an instance method; it's a static method. So you can mock it like this:

let mut ctx = MockMyTrait::new_context();
ctx.expect::<SomeParameterType>()
    .with(...)
    .returning(|| Some(MockMyTrait::default()))

[stefanhp]

Got it, thanks!