First, a little background. A couple of years ago, I was researching making my own candy, and I came across this page: Lego brick shaped gummy candies, describing how to use real Lego bricks as a positive to make silicone molds for Lego brick shaped candy. Now that I have a 3D printer, and inspired by the usual description of ABS filament ("It's the same plastic used to make Lego bricks") it occurs to me that I can now make any positive I want.
The question is, would that be safe? I know the filament I'm using is not food safe, but if I create positives for a food safe silicone mold, would toxins leech into the mold? And if so, is there a barrier I can use to prevent this, such as some kind of coating?
The plastic is not quite your main concern (though it still can be). You should worry about the cracks and crevices in FDM prints. Bacteria loves to hide there. For most people, this is the first concern when it comes to "Is X 3D printed food-safe?" If the end product is a hard material, you should sand or smooth your print to prevent the layer crevices from appearing in the end use product. Also consider food-safe epoxies for filling in gaps. If the end product is made of a flexible silicone, then this is less of a concern.
ABS is not food safe. PLA as a material is considered food safe by the 3D printing community, but I have not seen a scientific study on this. However, many filament makers do not extrude pure PLA. Therefore, the answer to your question is that it depends on the manufacturer. You will need to contact the company to know what is in the filament first. Beware of untrustworthy manufacturers that claim food safety without backing that up.