These are used by virtually every `ObjectKind`; I've been meaning to do this
for a while, but now that I'm about to introduce a new one (`Doc`), let's
just get it out of the way.
DEV-13708
This creates another trait and struct `ObjectIndexToTree` that assert a
stronger invariant than `ObjectIndexRelTo`---that not only does it uphold
the invariants of `ObjectIndexRelTo`, but also that it represents a _tree_
edge, which indicates _ownership_ rather than just a reference.
This will be used to statically infer what can serve as a scope boundary for
upcoming changes. Specifically, anything that can own an `Ident` introduces
a new level of scope.
DEV-13708
The previous commit demonstrated the amount of boilerplate necessary for
introducing new `ObjectKind`s; this abstracts away a lot of that
boilerplate, and allows for declarative relationship definition for the
ASG's ontology.
DEV-13708
There's quite a bit of boilerplate here that'll eventually need factoring
out. But it's also clear that it is somewhat onerous to add new object
types.
Note that a good chunk of this burden is _intentional_, via exhaustiveness
checks---adding a new type of object is an exceptional occurrence (well, in
principle, but we haven't added them all yet, so it'll be more common
initially), and we'd rather be safe to ensure that everything is properly
considering how that new type of object interacts with it.
Let's not confuse coupling with safety---the latter causes a burden because
of the former, not because of itself; it provides a service to us.
But, nonetheless, we'll want to reduce this burden somewhat since there are
a number more to add.
DEV-13708
This introduces the concept of ontological cross edges.
The term "cross edge" is most often seen in the context of graph traversals,
e.g. the trees formed by a depth-first search. This, however, refers to the
trees that are inherent in the ontology of the graph.
For example, an `ExprRef` will produce a cross edge to the referenced
`Ident`, that that is a different tree than the current expression. (Well,
I suppose technically it _could_ be a back edge, but then that'd be a cycle
which would fail the process once we get to preventing it. So let's ignore
that for now.)
DEV-13708
This causes a package definition to be rooted (so that it can be easily
accessed for a graph walk). This keeps consistent with the new
`ObjectIndex`-based API by introducing a unit `Root` `ObjectKind` and the
boilerplate that goes with it.
This boilerplate, now glaringly obvious, will be refactored at some point,
since its repetition is onerous and distracting.
DEV-13159
Mike Gerwitz