The first thing to happen is that any liquids (saliva, tears, blood) will start to boil in the very low pressure, but your body won't explode like in some films. This boiling will pull heat from your body causing your nose and mouth to nearly freeze.
Another film trope is that you freeze over, but you'll often overheat first since you can't radiate your heat away quickly enough (depending on if you're in sunlight or not).
How did you find Leptos to work with? I never got further than the tutorial so I have yet to form a real opinion on it.