The Rust documentation has a get_mut() method for OnceLock that returns a mutable reference. But the following code does not compile. What am I missing ?

use std::sync::{OnceLock};
#[derive(Clone)]
struct System {comp: Vec<i32>}

static GLOB : OnceLock<System> = OnceLock::new();

impl System {
    fn new() -> Self {Self{comp:Vec::new()}}
}
fn modify(global: &mut System) {
    global.comp.push(42);
}

fn main() {
    let sys:Box<System> = Box::new(System::new());
    GLOB.get_or_init(|| *sys);
    let global : &mut System = GLOB.get_mut().unwrap();
    println!("global size is {}", global.comp.len());
    modify(global);
    println!("global size is {}", global.comp.len());
}

Is it really possible to get a mutable reference? How am I supposed to use get_mut()?