How do I mock an external dependency in Rust

In Rust, you can mock an external dependency using various techniques, depending on the nature of the dependency. Here are a couple of commonly used approaches:

1. Dependency injection: This involves structuring your code in a way that allows you to inject different implementations of the dependency. By doing so, you can provide a mock implementation during testing. Here’s an example:

// Define a trait for the external dependency
trait ExternalDependency {
    fn do_something(&self);
}

// Implement the trait for the real dependency
struct RealDependency;

impl ExternalDependency for RealDependency {
    fn do_something(&self) {
        // Implementation for the real dependency
    }
}

// Create a struct that depends on ExternalDependency
struct MyStruct {
    dependency: T,
}

impl MyStruct {
    fn new(dependency: T) -> Self {
        Self { dependency }
    }

    fn perform_action(&self) {
        self.dependency.do_something();
    }
}

// In your test, create a mock implementation of ExternalDependency
struct MockDependency;

impl ExternalDependency for MockDependency {
    fn do_something(&self) {
        // Mock implementation for testing
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_action() {
        let my_struct = MyStruct::new(MockDependency);
        my_struct.perform_action();
        // Assert the desired behavior
    }
}

In this example, the MyStruct struct depends on the ExternalDependency trait, which has two implementations: RealDependency (the real implementation) and MockDependency (a mock implementation for testing). The MyStruct struct can be created with either implementation, allowing you to switch between the real and mock dependencies based on the context (e.g., production code or tests).

1. Using a testing framework or library: Rust provides various testing frameworks and libraries that offer built-in capabilities for mocking dependencies. For example, the mockall crate provides macros and utilities specifically for mocking in Rust. Here’s an example using mockall:

use mockall::predicate::*;
use mockall::Sequence;

// Define the trait for the external dependency
trait ExternalDependency {
    fn do_something(&self);
}

// Create a mock implementation using `mockall`
mock! {
    MyDependency {}

    trait ExternalDependency {
        fn do_something(&self);
    }
}

// Create a function that uses the external dependency
fn perform_action(dep: &dyn ExternalDependency) {
    dep.do_something();
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_action() {
        let mut dep = MockExternalDependency::new();
        dep.expect_do_something()
            .times(1)
            .in_sequence(&mut Sequence::new())
            .returning(|| ());

        perform_action(&dep);
    }
}

In this example, we use the mockall crate to create a mock implementation of the ExternalDependency trait. We then define expectations on the mock object’s methods and use them in our test. The expect_do_something method sets an expectation for the do_something method to be called once.

These are just a couple of approaches to mocking external dependencies in Rust. The choice of technique depends on the specific requirements of your project and the testing framework or libraries you are using.