I think what you'd most likely do here is something like:

    const HUGE: usize = 10_000_000;
    let mut values = Box::<[i32]>::new_uninit_slice(HUGE);
    for k in 0..HUGE {
         values[k].write(42);
    }

    let values = values.assume_init();

Edited to fix early enter oops, typo in loop length that'd be caught if I'd tried this.