IDL43 — Ability to add annotations by reference

At present, annotation syntax must be placed in the IDL file, usually right before or after the syntax being annotated.

This works very well for annotations that define shared interface constraints. It does not work well for annotations that control internal implementation details, such as the selection of an API option. These internal details should be separated from the shared interface and so do not belong in the shared IDL file.

This proposal is to add a new syntax for annotating IDL structures. The basic idea is to define an annotation that takes two parameters, a "place" in the IDL and an annotation, and has the effect of inserting the annotation at the given place. For example @insert(Foo.x, @some(value)) would be equivalent to placing @some(value) by the definition of Foo.x.

With this mechanism, end users could create a custom IDL that includes the shared IDL and then adds the custom annotations.