Mocking in Rust with a working mockall example

main.rs

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

pub struct Visa {}

#[cfg_attr(test, automock)]
pub trait Chargable {
    fn charge_credit_card(&self) -> u32;
}

impl Chargable for Visa {
    fn charge_credit_card(&self) -> u32 {
        println!("I really actually charged.  You should never see this in dev.");
        println!("This is like a network request going out or something.");
        42
    }
}

fn charge(adapter: &dyn Chargable) -> u32 {
    adapter.charge_credit_card()
}

fn main() {
    let visa = Visa {};
    let amount = charge(&visa);
    println!("Amount charged: {}", amount);
}

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

    #[test]
    fn a_test() {
        let mut mock = MockChargable::new();
        mock.expect_charge_credit_card().times(1).returning(|| 42);

        let amount = charge(&mock);
        assert_eq!(amount, 42);
    }
}

// $ cargo run
// I really actually charged.  You should never see this in dev.
// This is like a network request going out or something.
// Amount charged: 42

// $ cargo test
// running 1 test
// test tests::a_test ... ok