Designing Compile-Time Mocking Interfaces for Unit Testing

pub trait Database {
    fn fetch_user_role(&self, user_id: u32) -> Result<String, &'static str>;
}
 
pub struct UserService<'a, D: Database> {
    pub db: &'a D,
}
 
impl<'a, D: Database> UserService<'a, D> {
    pub fn is_admin(&self, user_id: u32) -> bool {
        self.db.fetch_user_role(user_id)
            .map(|role| role == "Admin")
            .unwrap_or(false)
    } 
}
 
#[cfg(test)]
mod tests {
    use super::*;
 
    struct MockDatabase {
        expected_id: u32,
        returned_role: Result<String, &'static str>,
    }
 
    impl Database for MockDatabase {
        fn fetch_user_role(&self, user_id: u32) -> Result<String, &'static str> {
            assert_eq!(user_id, self.expected_id);
            self.returned_role.clone()
        }
    }
 
    #[test]
    fn test_admin_verification() {
        let mock = MockDatabase {
            expected_id: 42,
            returned_role: Ok(String::from("Admin")),
        };
        let service = UserService { db: &mock };
        assert!(service.is_admin(42));
    }
}