It can try to; but it will fail to parse, usually (both macros and new-style syntax extensions go through a sort of re-parse step) Old-style (AST-based, unstab

It can try to; but it will fail to parse, usually (both macros and new-style syntax extensions go through a sort of re-parse step) Old-style (AST-based, unstable, will go away) syntax extensions can do this. But this isn't relevant, the compiler can define custom syntax extensions that do whatever it wants :)