Variants can hold data

enum Status {
    ToDo,
    InProgress,
    Done,
}

Our Status enum is what's usually called a C-style enum.
Each variant is a simple label, a bit like a named constant. You can find this kind of enum in many programming languages, like C, C++, Java, C#, Python, etc.

Rust enums can go further though. We can attach data to each variant.

Variants

Let's say that we want to store the name of the person who's currently working on a ticket.
We would only have this information if the ticket is in progress. It wouldn't be there for a to-do ticket or a done ticket. We can model this by attaching a String field to the InProgress variant:

enum Status {
    ToDo,
    InProgress {
        assigned_to: String,
    },
    Done,
}

InProgress is now a struct-like variant.
The syntax mirrors, in fact, the one we used to define a struct—it's just "inlined" inside the enum, as a variant.

Accessing variant data

If we try to access assigned_to on a Status instance,

let status: Status = /* */;

// This won't compile
println!("Assigned to: {}", status.assigned_to);

the compiler will stop us:

error[E0609]: no field `assigned_to` on type `Status`
 --> src/main.rs:5:40
  |
5 |     println!("Assigned to: {}", status.assigned_to);
  |                                        ^^^^^^^^^^^ unknown field

assigned_to is variant-specific, it's not available on all Status instances.
To access assigned_to, we need to use pattern matching:

match status {
    Status::InProgress { assigned_to } => {
        println!("Assigned to: {}", assigned_to);
    },
    Status::ToDo | Status::Done => {
        println!("Done");
    }
}

Bindings

In the match pattern Status::InProgress { assigned_to }, assigned_to is a binding.
We're destructuring the Status::InProgress variant and binding the assigned_to field to a new variable, also named assigned_to.
If we wanted, we could bind the field to a different variable name:

match status {
    Status::InProgress { assigned_to: person } => {
        println!("Assigned to: {}", person);
    },
    Status::ToDo | Status::Done => {
        println!("Done");
    }
}

Exercise

The exercise for this section is located in 05_ticket_v2/03_variants_with_data