It was a goal to not “put more other language support stuff” into the Swift compiler. Swift’s C++ interop is deeply embedded in the Swift compiler but that is primarily because of Clang. It was a goal to see if we can pull it off without having to tie the language interop with the specific compiler. If necessary, we can do changes in the compiler to support the interop, but it would be better of those are generally useful features for all kinds of languages, rather than every language hacking in their ad-hoc interop pieces.
In other words: we’re trying to decouple the evolution and features offered by swift-java from being specifically tied to a specific compiler in order to evolve it more quickly and independently. If there are things we’d need the compiler’s help for it is possible to make changes, but if we do, hopefully we’d manifest them as “generally useful” features that many languages could benefit from.