> Meanwhile clang crawls along on its support for newer ISO C++ features, as the biggest contributors now have their focus elsewhere.
What did you mean by this? I am not an LLVM expert or anything, but I find more-often-than-not most of the new features I read about that make its way into LLVM are requirements needed due to clang adding things from new C++ standards. These improvements then benefit other frontends like the one for Rust.
Well, for Apple, C++ is relevant only in the context of Metal shaders (a C++14 subset) and IO/DriverKit (an embedded C++ like subset), everything else is about Objective-C and Swift, with C++17 being good enough for whatever else they need.
Likewise on Google style, it is all about C++ guidelines at Googleplex, where even not all C++17 features are welcomed.
Everyone else seems more interested in C++ features for their own platforms rather than contributing to upstream (thanks license), so there you go.
I have no need to read hype about Rust, regardless of where it might run. And, I have no desire to build Android apps, in any language.