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For mocking, Mockall is a widely used library. You need to refactor your code to use traits, which you can then quickly mock:

use std::time::Duration;

#[mockall::automock]
pub trait Pet {
    fn is_hungry(&self, since_last_meal: Duration) -> bool;
}

#[test]
fn test_robot_dog() {
    let mut mock_dog = MockPet::new();
    mock_dog.expect_is_hungry().return_const(true);
    assert_eq!(mock_dog.is_hungry(Duration::from_secs(10)), true);
}

This slide should take about 5 minutes.

The advice here is for Android (AOSP) where Mockall is the recommended mocking library. There are other mocking libraries available on crates.io, in particular in the area of mocking HTTP services. The other mocking libraries work in a similar fashion as Mockall, meaning that they make it easy to get a mock implementation of a given trait.
Note that mocking is somewhat controversial: mocks allow you to completely isolate a test from its dependencies. The immediate result is faster and more stable test execution. On the other hand, the mocks can be configured wrongly and return output different from what the real dependencies would do.

If at all possible, it is recommended that you use the real dependencies. As an example, many databases allow you to configure an in-memory backend. This means that you get the correct behavior in your tests, plus they are fast and will automatically clean up after themselves.

Similarly, many web frameworks allow you to start an in-process server which binds to a random port on localhost. Always prefer this over mocking away the framework since it helps you test your code in the real environment.
Mockall is not part of the Rust Playground, so you need to run this example in a local environment. Use cargo add mockall to quickly add Mockall to an existing Cargo project.
Mockall has a lot more functionality. In particular, you can set up expectations which depend on the arguments passed. Here we use this to mock a cat which becomes hungry 3 hours after the last time it was fed:

#[test]
fn test_robot_cat() {
    let mut mock_cat = MockPet::new();
    mock_cat
        .expect_is_hungry()
        .with(mockall::predicate::gt(Duration::from_secs(3 * 3600)))
        .return_const(true);
    mock_cat.expect_is_hungry().return_const(false);
    assert_eq!(mock_cat.is_hungry(Duration::from_secs(1 * 3600)), false);
    assert_eq!(mock_cat.is_hungry(Duration::from_secs(5 * 3600)), true);
}

You can use .times(n) to limit the number of times a mock method can be called to n --- the mock will automatically panic when dropped if this isn't satisfied.

Comprehensive Rust 🦀

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