Crystal's implementation wouldn't work for something like C#, though, because C# supports dynamic assembly loading at runtime and must provide reflective facilities for operating on types loaded from them. (I once wrote an extensibility system[1] and on-the-fly compiler for a game engine that took advantage of this, it was neat.)

Macros don't help you here and limiting yourself to "I can compile the whole blob at once" cuts off a lot of avenues of extensibility.

[1] - https://github.com/eropple/exor