Problem-first approach

[mre]

Add screenshots of error messages from other languages

Focus on a "problem-first narrative":

- Debug trait first
- You can use derive for Debug
- This way, they already know what Debug is (a trait)
- Then we can go from there and introduce other traits like Display

Story arch:

"I cannot print my type!"

(because you didn't derive Debug):

```rust
fn main() {
    let first_name = FirstName("John".to_string());
    println!("My name is {:?}", first_name); // doesn't compile because FirstName doesn't implement Debug
}

"But you can implement Debug yourself:"

impl Debug for FirstName {
    // ...
}

"That's a lot of code!"
-> "you can derive Debug and it will do that for you!"

#[derive(Debug)]
struct FirstName(String);

fn main() {
    let first_name = FirstName("John".to_string());
    println!("My name is {:?}", first_name); // now it works!
}

Sometimes it's useful to implement manually.

// deriving Debug here is dangerous, because it would leak the password string
struct Password(String);

// instead...
impl Debug for Password {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "Password(****)")
    }
}

"My code is ugly!"

E.g. missing Display impl:

fn main() {
    // newtype
    let first_name = FirstName("John".to_string());
    // now you can't easily print that anymore!
    // so you have to use .0:
    let name = first_name.0;
    println!("My name is {}", name);
}

"Here's a trait. If you implement that, your code will be better."

impl std::fmt::Display for FirstName {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}", self.0)
    }
}

fn main() {
    let first_name = FirstName("John".to_string());
    // now you can easily print that!
    println!("My name is {}", first_name);
}