How to mock dependencies

Create a MockMessenger struct that implements the Messenger trait and wraps a Vec<String> in std::cell::RefCell to allow mutation through an immutable reference.

use std::cell::RefCell;

struct MockMessenger {
    sent_messages: RefCell<Vec<String>>,
}

impl MockMessenger {
    fn new() -> MockMessenger {
        MockMessenger {
            sent_messages: RefCell::new(vec![]),
        }
    }
}

impl Messenger for MockMessenger {
    fn send(&self, message: &str) {
        self.sent_messages.borrow_mut().push(String::from(message));
    }
}

In your test, instantiate MockMessenger::new(), pass it to LimitTracker::new, and assert against mock_messenger.sent_messages.borrow().

Mocking replaces a real dependency with a fake one that records actions instead of performing them. This lets you test your code in isolation without needing external systems like email servers or databases. Think of it as using a stunt double in a movie to safely capture a specific scene without risking the main actor.