It seems that it will CLR-enforced so I would guess that it's not going to be available for any type:

  The fast representation makes the type automatically stack-only, i.e. the constraint 
  will be enforced by CLR type loader. This restriction should also be enforced by 
  managed language compilers and/or analyzers for better developer experience. For 
  the slow span, language compiler checks and/or analyzers is the only option (as 
  the runtimes won't enforce the stack-only restriction).