Generally, an environment where you can't use clang /at all/. There are many C++ compilers.
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.
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.
It does not make sense: the problem is even worse for rust in that case, since you cannot even use it ?
However, I realise the point is moot since there would be no way to compile the library. I've been living too long in "Compiles to C" languages this month I guess.