Code shared from the Rust Playground

playground.rs

trait StateSet {
    fn next(self) -> (u32, Self);
}

trait State { 
    type S: StateSet;
    fn next(self) -> (u32, Self::S);
}

macro_rules! delegate {
    ($fn:ident => enum $name:ident { $($variant:tt,)*}) => {
        #[derive(Debug)]
        enum $name {
            $($variant($variant),)+
        }
        
        impl StateSet for $name {
            fn next(self) -> (u32, Self) {
                match self {
                    $(
                    $name::$variant(e) => e.$fn(),
                    )+
                }
            }
        }
    }
}

#[derive(Debug)]
struct First(u32);
#[derive(Debug)]
struct Second;
#[derive(Debug)]
struct Third;

impl State for First {
    type S = SMTP;
    fn next(self) -> (u32, Self::S) {
        (self.0 + 1, SMTP::Second(Second))
    }
}

impl State for Second {
    type S = SMTP;
    fn next(self) -> (u32, Self::S) {
        (2, SMTP::Third(Third))
    }
}

impl State for Third {
    type S = SMTP;
    fn next(self) -> (u32, Self::S) {
        (3, SMTP::First(First(4)))
    }
}

delegate!{next =>
enum SMTP {
    First,
    Second,
    Third,
}
}

fn main() {
    let machine = SMTP::First(First(0));
    let (n, machine) = machine.next();
    println!("{:?}", (n, &machine));
    let (n, machine) = machine.next();
    println!("{:?}", (n, &machine));
    let (n, machine) = machine.next();
    println!("{:?}", (n, &machine));
}