use std::sync::{Arc, Mutex};

thread_local! {
    static APP_STATE: Arc<Mutex<Box<Trait>>> = Arc::new(Mutex::new(Box::new(Foo {})));
}

fn main() {
    println!("Hello, world!");

    APP_STATE.with(|state_mutex| {
        let mut foo_or_bar = state_mutex.lock().unwrap();
        *foo_or_bar = foo_or_bar.consume();
    });
}

trait Trait {
    fn consume(self: Box<Self>) -> Box<Trait>;
    fn get_name(self: Box<Self>) -> &'static str;
}

#[derive(Debug)]
struct Foo {}

impl Trait for Foo {
    fn consume(self: Box<Self>) -> Box<Trait> { Box::new(Bar {}) }
    fn get_name(self: Box<Self>) -> &'static str { "foo" }
}

#[derive(Debug)]
struct Bar {}

impl Trait for Bar {
    fn consume(self: Box<Self>) -> Box<Trait> { self as Box<Trait> }
    fn get_name(self: Box<Self>) -> &'static str { "bar" }
}