Find a better way to hide interfaces from content

[kmcallister]

@Ms2ger is unhappy with [NoInterfaceObject] (could you elaborate on why?)

[Ms2ger]

It's kind of an antipattern, and there's about one place in the platform where it's used (excluding mixins, which might get dedicated syntax), so it's possible we'll remove NoInterfaceObject from the platform, and in that case it'd be nice not to have to support it for our own implementation details.

[ChrisParis]

I count two instances where NoInterfaceObject is used in Servo outside of a mixin context: DOMRectList and ServoHTMLParser. In these two instances, Servo's codegen does generate code to create interface objects, despite the NoInterfaceObject attribute in the IDL files (DefineDOMInterface is not generated, but CreateInterfaceObjects is generated and used).

This presents a challenge to one proposal to replace an assert not isCallback() with an assert hasInterfaceObject(). The problem is that the interface objects method is created unconditionally. If that call is made conditional on hasInterfaceObject(), then the definition of CreateInterfaceObjects is omitted, but the call to it is still present in GetProtoObject. GetProtoObject is still used by Wrap.

I'm not proposing a solution at this time, but rather elaborating on one instance of where the current situation is a problem.