code: func deprecateBackend(b backend.Backend, message string) backend.Backend {
  	// Since a Backend wrapped by deprecatedBackendShim can no longer be
  	// asserted as an Enhanced or Local backend, disallow those types here
  	// entirely.  If something other than a basic backend.Backend needs to be
  	// deprecated, we can add that functionality to schema.Backend or the
  	// backend itself.
  	if _, ok := b.(backend.Enhanced); ok {
  		panic("cannot use DeprecateBackend on an Enhanced Backend")
  	}
  
  	if _, ok := b.(backend.Local); ok {
  		panic("cannot use DeprecateBackend on a Local Backend")
  	}
  
  	return deprecatedBackendShim{
  		Backend: b,
  		Message: message,
  	}
  }
docs: // DeprecateBackend can be used to wrap a backend to retrun a deprecation
  // warning during validation.