Release of Visual Studio 2015 Preview includes new version of C# and VB.NET compilers called "Roslyn".
Roslyn is a complete rewrite of C# and VB.NET compilers; if previously they were written in C++, now C# compiler is written in C#, and VB.NET compiler - in VB.NET respectively. Also Roslyn is fully open sourced and available on GitHub.
When using the previous compiler, developer couldn't affect the compilation process. New compiler follows the idea of Compiler-as-a-Service: now it's a platform with API that allows the developers taking part in the compilation process.