use std::cell::RefCell;

thread_local! {
    static GLOBAL: RefCell<Option<Vec<usize>>> = RefCell::new(Some(vec![1,2,3]));
}

fn main() {
    GLOBAL.with(|v| {
        // let mut x = v.borrow_mut();
        // let mut y = x.as_mut().unwrap();
        let mut y = v.borrow_mut().as_mut().unwrap();
        y.push(4);
    });
}