Debugging a struct in Rust

In writing this short Rust program:

struct Person {
    name: String,
    age_in_years: u32,
}

fn main() {
    let prince = Person {
        name: String::from("Prince"),
        age_in_years: 100,
    };
    dbg!(prince);
}

I came across an error I hadn’t seen in a long time so I forgot why it came up and also how to fix it:

11 |     dbg!(prince);
   |     ^^^^^^^^^^^^^ `Person` cannot be formatted using `{:?}`

Fortunately, Rust gives us hints on how to fix this:

= help: the trait `std::fmt::Debug` is not implemented for `Person`
= note: add `#[derive(Debug)]` or manually implement `std::fmt::Debug`
= note: required because of the requirements on the impl of `std::fmt::Debug` for `&Person`

So, the short answer to fix this is we need to add #[derive(Debug)] onto our struct Person, resulting in:

#[derive(Debug)]
struct Person {
    name: String,
    age_in_years: u32,
}

Alternatively, we could also do their other suggestion and implement the trait for struct our struct manually. That means we’d need to implement fmt::Debug for Person. And one of its requirements is that we have a function fmt that describes how to output it to the terminal:

use std::fmt;

struct Person {
  name: String,
  age_in_years: u32
}

impl fmt::Debug for Person {
  fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    f.debug_struct("Point")
      .field("name", &self.name)
      .field("age_in_years", &self.age_in_years)
      .finish()
  }
}

If we wanted to do any customization to how to output it for our user, we can also do that but this is what gets generated for us using #[derive(Debug)].