Code shared from the Rust Playground

playground.rs

trait TSociable{ 
    fn say(&self);
    fn instance() -> Box<dyn TSociable> {
        if true { Hello::new() } else { GoodBye::new() }
    }
}

struct Hello;

impl Hello {
    fn new() -> Box<Hello> {
        Box::new(Hello)
    }
}

struct GoodBye;

impl GoodBye {
    fn new() -> Box<GoodBye> {
        Box::new(GoodBye) 
    }
}

impl TSociable for Hello { fn say(&self) { println!("Hello"); } }
impl TSociable for GoodBye { fn say(&self) { println!("GoodBye"); } }

fn main() {
    let mut somenthing = TSociable::instance();
    let mut somenthing: Box<TSociable>;
    
    somenthing = Hello::new();
    somenthing.say();
    
    somenthing = GoodBye::new();
    somenthing.say();
}