I think the Roslyn compiler does something similar to what you're suggesting.

http://www.dotnetcurry.com/csharp/1258/dotnet-platform-compi...

> Every time a developer changes a single character in any of the files, a new copy of all the data structures is created, leaving the previous version unchanged. This allows a high level of parallelism and concurrency in the Roslyn engine, as well as in its consumers, thereby preventing any race conditions to occur. Of course, in the interest of performance, these operations are highly optimized and reuse as much of the existing data structures as possible. Again, those being immutable makes this possible!