How do I choose between using structs, enums, and traits when modeling data and behavior in Rust?

So at the very high level...

A struct is a collection of data that typically represents something:

#![allow(unused)]
fn main() {
struct Drink {
    brand: String,
    size: i32,
}
}

An enum represents a possible set of things:

#![allow(unused)]
fn main() {
enum DrinkType { Coffee, Soda, Water }
}

Where enums get fun is that they can also carry data (like a tagged union from C, or a variant from C++):

#![allow(unused)]
fn main() {
enum DrinkType { 
    Coffee { roast: CoffeeRoast },
    Soda { brand: Brand },
    Water
}
}

This really shines when you want to only store things that are possible - make impossible things unrepresentable.

Either of these can use impl to define functions. Adding a trait applies an interface to them - so now you can represent the type as a trait, and know that certain methods are available without knowing the concrete type.

So:

• Typically a struct is the right place if an entry is going to have all (or most) of the same fields.
• An enum is the right choice if you want to restrict to a valid type.
• A trait extends these to offer something. For example, the Display trait makes them all work with format! without having to write a format for all of them. You can also use traits for dynamic dispatch if you need single layer polymorphism.