Roslyn: Proposal: Add attributes which run custom functions to and throw compiler errors on fail.

[henrikklarup]

Add some way of putting attributes on different classes/enums and run custom functions for validation on compile time.
E.g.

A custom NoDublicatesAttribute could be added to enums to prevent multiple in the enum.
A custom NoImplicitValuesAttribute could be added to enums to only allow explicit value assignment the enum.

[alrz]

These are examples of what analyzers can do.