The identity of these giant slugs has been established!
It is a Giant Garden Slug, and yes, they’re an invasive species. They are native to Europe and northern Africa and have been wreaking havoc on gardens all around the world. I suppose it’s a good sign that nobody I showed this picture to had ever seen one before, but who knows what the future will hold.
Attack of the giant slugs, indeed.
Oh yeah, and tempting as it is, don’t eat them.