Okay, I've been thinking about this a bit lately. So the current ruling is that Summon first gains, then sets aside a card. This means it loses track of Death Cart, because the when-gain on Death Cart of gaining Ruins means that Death Cart gets covered up before it gets set aside.
However, this would, at first glance, seem to be not consistent with the No Visiting rule. When an effect gains a card to somewhere other than the discard, it doesn't first visit the discard. So if I Armory a Death Cart, the Death Cart goes immediately to the top of my deck, and the Ruins are dumped in my discard pile, and no track was lost.
So why doesn't Summon operate this way? I think it comes down to the fact that there is a period between "gain" and "set aside" on Summon. On all the other no visiting cards, it's either a semicolon (like Bureaucrat or Mine) or a comma (like Armory or Explorer). So it would say "... gain a blah card, putting it blah". In contrast, Summon says "Gain a blah card. Set it aside." This to me indicates that No Visiting does not come into play here - the Summoned card is not gained to set-aside land, it is still gained to the discard pile, then moved after it is gained, which means it can be lost track of.
Sorry if this seems rambling, and if I'm right it means nothing has changed in terms of rulings, but I wanted to check to see if my logic seems sound.