#![feature(maybe_uninit)]

use std::marker::PhantomData;
use std::mem::MaybeUninit;

struct Invariant<'a>(PhantomData<&'a mut &'a mut ()>);

struct MustInit<'a, T> {
    marker: Invariant<'a>,
    maybe_uninit: &'a mut MaybeUninit<T>,
}

struct DidInit<'a, T> {
    marker: Invariant<'a>,
    value: &'a mut T,
}

impl<'a, T> MustInit<'a, T> {
    fn set(self, val: T) -> DidInit<'a, T> {
        DidInit {
            marker: Invariant(PhantomData),
            value: self.maybe_uninit.set(val),
        }
    }
}

fn require_init<T>(
    f: impl for<'a> FnOnce(MustInit<'a, T>) -> DidInit<'a, T>,
) {
    let mut value = MaybeUninit::uninitialized();
    let _ = f(MustInit { marker: Invariant(PhantomData), maybe_uninit: &mut value });
}

fn main() {
    require_init(|x| {
        require_init(|y| {
            y.set(5u32)
        });
        x.set(5u32)
    });
}