use std::cell::LazyCell;

enum Cycle<'c, F> {
    There(Box<Cycle<'c, F>>),
    Later(Box<LazyCell<Cycle<'c, F>, F>>),
}

trait Recursive<'c>: FnOnce() -> Cycle<'c, Self> + Sized {}

impl<'c, F> Recursive<'c> for F where F: FnOnce() -> Cycle<'c, F> {}

fn recursive<'c>() -> Cycle<'c, impl Recursive<'c>> {
    let make = || Cycle::There(Box::new(recursive()));
    Cycle::Later(LazyCell::new(make))
}