#![feature(impl_trait_in_bindings)]
#![feature(maybe_uninit)]

pub trait Behaviour {}

pub struct System;
impl Behaviour for System {}

fn mk_system() -> impl Behaviour {
    System
}

static mut S: impl Behaviour = core::mem::uninitialized();

fn main() {
    let concrete_system = mk_system();
    core::ptr::write(&mut S, concrete_system);
}