I looks like snail damage. They also love basil. And, they don't like the hairs on the pumpkin stems and leaves. WHich is why I make my suggestion.

That frass tells me that it may be corn worms/borers type caterpillars. If they are still present you may find them hiding deep down inside the center of the leaf whorl. Wasps feed on these worms so they may be already gone.

Vgkg"s comment about checking the leaf whorl just reminded me of a time I've had similar damage. The cause that time was earwigs, which were hiding deep within the leaf whorl. They chewed on the young leaves, and when those leaves grew out, the damage closely resembled that in the photo.

Thanks so much, all. I went out this morning and looked down into the center of the leaf whorls, and sure enough, found a caterpillar-ish critter. So now, what to do? Do beneficial nematodes work?

If you have a small crop of corn I'd just inspect them daily and use tweezers to pluck them out. The corn should survive the damage ok as long as it's not too extensive. Some folks use BT on their plants for this caterpillar type pest.