main.rs

use std::ptr::NonNull;

#[derive(Default)]
struct Heap {
    data: Data,
}
#[derive(Default)]
struct Data {
    a: u32,
}

impl Heap {
    #[cfg(not(mutable))]
    fn data_ptr(&self) -> NonNull<u8> {
        let a: *const _ = &self.data;
        NonNull::new(a.cast_mut()).unwrap().cast()
    }

    #[cfg(mutable)]
    fn data_ptr(&mut self) -> NonNull<u8> {
        NonNull::from(&mut self.data).cast()
    }

    fn mutate(&mut self) {
        self.data.a = 4;
    }
}

fn main() {
    let mut heap = Box::new(Heap::default());
    let ptr = heap.data_ptr();
    unsafe {
        ptr.cast::<u32>().write(4);
        heap.mutate();
        ptr.cast::<u32>().write(4);
    }
}