In the only compiler I'm somewhat familiar with (OCaml), a decent amount of the work is done in AST->AST transformations. In the AST, every node is annotated w

In the only compiler I'm somewhat familiar with (OCaml), a decent amount of the work is done in AST->AST transformations. In the AST, every node is annotated with a location, which is a tuple of filename, line number, char start, char end. As long as these are propagated in the right way, you get good location information in errors.