Derive Macro StateMachine 

#[derive(StateMachine)]
{
    // Attributes available to this derive:
    #[state_machine]
    #[only_accepts]
}

Derive StateMachine for an enum of states.

#[derive(StateMachine, Clone, Debug, PartialEq)]
#[state_machine(initial = Draft, terminal = [Published, Archived])]
enum Article { Draft, Review, Published, Archived }

Data-carrying fields must implement Default

analyze! and test! walk every state variant, and the derive builds one representative per variant to walk. Analysis is variant-level, so any data-carrying fields are filled with Default::default() — the payload types must therefore implement Default, or you can hand-write the StateMachine impl. Fieldless enums (including the aggregate tier’s phase machines) need nothing extra.