The Unity 3D engine is doing interesting things with this subset idea, where you can write most of your game in regular C#, then if you need a chunk of super fast stuff, you can use a subset of C# that can be easily vectorized, or parallelized, or GPU-ified. You can just do that all inline, no need for separate files or build processes. It is pretty neat and letting them get some crazy performance numbers.

https://unity3d.com/learn/tutorials/topics/scripting/using-b...