use get_unchecked_mut for hotspot

src/future_moves.rs

@@ -253,7 +253,7 @@ impl FutureMoves {
                 // TODO! impl dynamic sorting based on children's states, maybe it propegates
                 // upwards using the `parent` field
                 // SAFETY! the sort_by_key function should not modify anything
-                unsafe { &mut *(self as *mut Self) }.arena[idx]
+                unsafe { (&mut *(self as *mut Self)).arena.get_unchecked_mut(idx) }
                     .children
                     // negative because we want the largest value in the first index
                     .sort_by_key(|&x| -self.arena[x].value);