The Roslyn Compiler’s Service APIs, which are registered with MEF, can be used to create Visual Studio extensions. By implementing specific interfaces and using MEF-style attributes that match, Visual Studio developers can take advantage of Roslyn.
A developer can use the Code Issue extension, for instance, to write their own compiler warning and error messages. A sample project is included with the CTP that demonstrates this principle by displaying compiler warnings any time the letter A shows up in the syntax tree.