T4 barely counts, as it's string- and not AST-based. For meta-programming to be useful, IMHO it really needs to be part of the language and base tool-chain. I certainly don't want to write code in Java, f'rinstance, that needs random compiler plug-in to weave in AOP calls, etc. But in Lisp, Prolog (and not Elixer, etc.) macros are part of "how stuff works".