Ferris Talk #15: Bedingte Kompilierung in Rust

fn main() {
  // Note that you can combine configuration 
  // predicates with all, any, and not.
  #[cfg(not(any(traditional, vegetarian, vegan)))]
  {
    println!(r"You haven't specified whether the
recipe should be traditional, vegetarian, or vegan.");

    panic!();
  }

  #[cfg(any(all(traditional, vegetarian), 
        all(traditional, vegan), 
        all(vegetarian, vegan)))]
  {
    println!(r"You've specified more than one
recipe type. Please specify only one.");

    panic!();
  }

  print_ingredients();
  print_preparation();
}

fn print_ingredients() {
  println!("Ingredients for Caesar Salad:");
    
  println!("- Romaine lettuce");
  println!("- Croutons");
  println!("- Lemon juice");
  println!("- Olive oil");
  println!("- Salt");
  println!("- Black pepper");

  // We combine config predicates again
  #[cfg(any(traditional, vegetarian))]
  println!("- Parmesan cheese");
  #[cfg(any(vegan))]
  println!("- Vegan Parmesan cheese substitute");

  // Note that the cfg attribute can be applied 
  // to any item, not just a line of code. In 
  // this case, we're using it to conditionally
  // include a block of code. You could also 
  // conditionally define a module, struct, 
  // trait, function, etc.
  #[cfg(traditional)]
  {
    println!("- Anchovies");
    println!("- Egg yolks");
  }

  #[cfg(vegetarian)]
  {
    println!("- Capers");
    println!("- Egg yolks");
  }

  #[cfg(vegan)]
  {
    println!("- Capers");
    println!("- Vegan mayonnaise");
  }
}

fn print_preparation() {
  println!("\nPreparation:");

  println!(r"1. Wash and tear the romaine
lettuce into bite-size pieces.");
  println!(r"2. In a bowl, combine lemon juice,
olive oil, salt, and pepper.");
    
  // Instead of the config attribute, we can also 
  // use the cfg! macro. Note that it does NOT 
  // remove any code. The macro only evaluates 
  // to true or false at compile time.
  // The following code might not be the best 
  // option in real life, but it's a good
  // example of how to use the cfg! macro.
  match (cfg!(traditional), 
         cfg!(vegetarian), 
         cfg!(vegan)) {
    (true, false, false) => 
      println!(r"3. Add grated Parmesan cheese,
anchovies, and egg yolks to the bowl."),
    (false, true, false) => 
       println!(r"3. Add grated Parmesan cheese,
capers, and egg yolks to the bowl."),
    (false, false, true) => 
       println!(r"3. Add vegan Parmesan cheese 
substitute, capers, and vegan mayonnaise to the bowl."),
    _ => panic!(r"More than one recipe type? 
This should never happen!"),
    };

    println!(r"4. Mix well until the ingredients
are well-combined.");
    println!(r"5. Add the croutons and romaine
lettuce to the bowl.");
    println!(r"6. Toss until the lettuce is 
well-coated with the dressing.");
    println!(r"7. Serve immediately and enjoy
your Caesar Salad!");
}