it sounds like they're still generating an AST, but replacing AST nodes directly through the code string. It's definitely trickier but I think it's possible to

it sounds like they're still generating an AST, but replacing AST nodes directly through the code string. It's definitely trickier but I think it's possible to do this in a clean way if the changes are just "replace this AST node with this other AST node". You know the text boundaries, and the output is context free so...