> far away from what you could achieve more easily by using e.g. Carbon, or just better Rust/C++ interop (see creusot).
At that point, why not just upgrade to C++20? Carbon, in particular, is left behind in the C++17 world, so it's outdated by C++20 (not to mention future developments).
Google is basically stuck in the C++17 world, so I wouldn't expect Carbon to move forward if they end up investing heavily in it.
In particular Google hasn't even started to adopt C++ coroutines, threading, ranges, or modules (they have their own module implementation which is actually better in most ways, but different)
At that point, why not just upgrade to C++20? Carbon, in particular, is left behind in the C++17 world, so it's outdated by C++20 (not to mention future developments).