On the Rust side they require a compiler released 10 weeks ago, and it is actually reasonable, thanks to Rust's strong backwards compatibility and painless auto-update.
Yeah the problem isn't nearly as pronounced for rust, since rustup exists and rust itself isn't really available on any platforms you can't just update the toolchain yourself.
C++ has a lot more tangled dependencies going on especially when you're stuck with a vendor's compiler.
Make no mistake: the problem here isn't necessarily technological.
I don't understand what would be an environment where you could fetch the latest rust from internet but not the latest clang. I do my builds on centos:8 (well, rockylinux now) and it has access to the latest GCC 11. On Debian derivatives llvm itself provides up to date repos and on Windows, llvm-mingw works perfectly.
but this does not make sense. If you can't even change the C++ compiler you use you can't write non-C++ code (e.g. Rust) either.
Otherwise let me just fork Clang into the "Gnalc" compiler for the "Sulpsulpees" language which happens to be exactly compatible with the programming language that the clang++-13 binary supports. Takes me a whole 15 minutes, most of which will be taken by forking the LLVM repo on github, and I can guarantee than porting to it will be less effort than porting to Rust. I can even provide the backing of a proper non-profit foundation established on two continents for your legal team's needs.
The reason you can't use clang is not a technical one, but that you are /required/ to use the platform's provided C++ compiler.
And yes, you are making my point. We are in violent agreement.
You can't write Rust either, which is why C++ support would be initially exciting, until you learn you can't use just any C++ compiler and the one you can use is excluded.
C++ the language also has pretty strong backwards compatibility. It's also trivial to install a new Clang version on most systems.
But it certain spaces that's not what matters. Maybe you are only allowed to use That One Certified Compiler from 15 years ago and getting a certification for a newer compiler is not feasible. Or maybe you're on a system where the C++ standard library is part of the system and so you cannot use newer language features because that would require raising the minimum required OS version (I remember that some parts of the C++ standard library are not available if you compile for older macOS versions, for example).
C++ has a lot more tangled dependencies going on especially when you're stuck with a vendor's compiler.
Make no mistake: the problem here isn't necessarily technological.