The implementation took me a little while to get, but despite being a little hacky (I've certainly done worse), it's a relatively small change overall, and does not impact code structure in a major fashion.
Made it a PR because there could've been something that non-mixin templates are supposed to be able to do that this implementation disallows, but I couldn't think of anything off of the top of my head. And also because I wanted to make sure the implementation didn't rub anyone the wrong way.
bhelyer
commented
6 years ago 
The implementation took me a little while to get, but despite being a little hacky (I've certainly done worse), it's a relatively small change overall, and does not impact code structure in a major fashion.
Made it a PR because there could've been something that non-mixin templates are supposed to be able to do that this implementation disallows, but I couldn't think of anything off of the top of my head. And also because I wanted to make sure the implementation didn't rub anyone the wrong way.