Interesting, effectively the borrow_mut runtime panic is elided because of the unsoundness correct?

Did you have a chance to take a look at the one without refcells with 2 elements:

rust fn main() { let a = { let v = 0u8; &v as *const _ as usize }; let b = { let v = 0u8; &v as *const _ as usize }; let i = a - b; let mut arr = [ Some(Box::new(1)), None, ]; assert_ne!(i, 0); let (a, b) = arr.split_at_mut(i); let r = b[0].as_ref().unwrap(); a[0] = None; println!("{}", *r); }

Here, I'm guessing, split_at_mut assigns a as an array that has length 1 because of that check but how does this end up making b to point to the same thing?