Traits that can be derived

For some traits, the compiler itself knows how to implement them for a type. If we want them, we only have to tell it that we do, and it will take care of the rest for us.

Telling the compiler that we want a data type to have a derivable trait is easy.

Here, we're telling it that we want our CopyExample enumeration to implement Copy and Clone:

#[derive(Copy, Clone)]
pub enum CopyExample {
    Good,
    Bad,
}

A trait can only be derived if the people who created the trait were able to write a program to generate the trait implementation. When we write #[derive(Copy, Clone)], we're telling the compiler to go find those programs for deriving Copy and Clone in the source code of the packages where the traits were defined, and run those...