Not just interoperability with Objective C but with C (full) and C++ (increasingly better but not full) as well.
replySwift is also interoperable with different versions of itself courtesy of the Swift stable ABI (Application Binary Interface)[0], which they invested a significant amount of time into at the expense of adding other new features to the language, which have come along later.
Rust offers a different approach: recompile everything and static linking.
C compatibility comes via Objective-C, because contrary to C++, Objective-C extends C, instead of being based on a C subset.
replyYou missed Java as well.