The reason why I am doing this is so that I can create an adaptor class
(templated on Runtime) for reading protocol info from ReflectionInfo without
having to make ReflectionInfo itself generic. If ReflectionInfo becomes generic
on Runtime, it will cause a cascading need to mark classes in Reflection as
generic as well.
Now we can discern the types of values in heap boxes at runtime!
Closure reference captures are a common way of creating reference
cycles, so this provides some basic infrastructure for detecting those
someday.
A closure capture descriptor has the following:
- The number of captures.
- The number of sources of metadata reachable from the closure.
This is important for substituting generics at runtime since we
can't know precisely what will get captured until we observe a
closure.
- The number of types in the NecessaryBindings structure.
This is a holding tank in a closure for sources of metadata that
can't be gotten from the captured values themselves.
- The metadata source map, a list of pairs, for each
source of metadata for every generic argument needed to perform
substitution at runtime.
Key: The typeref for the generic parameter visible from the closure
in the Swift source.
Value: The metadata source, which describes how to crawl the heap from
the closure to get to the metadata for that generic argument.
- A list of typerefs for the captured values themselves.
Follow-up: IRGen tests for various capture scenarios, which will include
MetadataSource encoding tests.
rdar://problem/24989531
Create a builder divorced from the ReflectionContext so that
MetadataSources can be created in other contexts, such as emitting
private heap metadata during IRGen, where we'll have to record the
layout of captures and how to get metadata for generic arguments in
order to construct typerefs of the captures, etc.
Add Parent, Metadata capture, and Impossible metadata sources.
