Another thought:

My first idea was to mix expression variables into ConstraintDescriptor attributes.

• Pros: No API change, transparent for constraint validation handling code.
• Cons: this is a bit hackish, ConstraintDescriptor attributes are supposed to be static things.

But after all CD attributes are overriden by expression variables during interpolation. This perfectly matches my use case, but it's a bit violent IMO.

What do you think about it ?