Right, ultimately it will be enforced by the CLR type system, much the way most types are in .NET, but practically it will be enforced by the compiler. You can

Right, ultimately it will be enforced by the CLR type system, much the way most types are in .NET, but practically it will be enforced by the compiler. You can already see this in the "ref parameters" feature in C# today: they can be parameters to methods but cannot be stored in fields. This implies that they can only exist on the stack. Similarly, when we add support for ref-locals and ref-returns in C# 7 that will still disallow ref fields, so ref variables will still only be allowed on the stack.