Why on Earth would you choose to code in C++ ?
I&I understand, if the libs you need to use are C++ only, but having spent a few days working in Rust it feels like the C++ death warrant has been well and truly signed. Is there any downside later on to using Rust?
Game developer, 22 years experience of AAA development.
Rust is literally the most exciting thing I've EVER discovered in my career.
Welcome!
Of course everyone in this forum agrees that Rust is great 
We could probably also endlessly bash C++, but let's not do that.
Obviously the games industry is very tied to C++, due to existing codebases, engines, and people with enough confidence to write C++.
How do you think Rust could get into a AAA title?
It needs to be supported by 1st party platform holders. Partial migration by creating DLLs so we could start to rewrite parts of existing source bases, that would benefit, in Rust is the way in.
1st party support though a priority. I suspect, as more game devs discover Rust, there will be a large increase in requests for Rust support from game devs themselves.
On the flip side, Rust has a very good FFI story so it may not be necessary to RIIR. Existing code can mostly stay as-is, providing a shared or dynamically-loaded library; or vice-versa, create the library in Rust and use it from other languages over FFI.
I know a few game developers are also excited about Rust. If you haven't yet, be sure to read Catherine West's whitepaper.
Because an unbelievable amount of human-years was put into libraries written in C++ and other languages, and it's a herculean task to develop something of similar quality/coverage for many domains even by utilizing Rust strengths. For example I would be really-really-really happy to use Rust solutions in robotics, but libraries are simply not there yet. Also there is a matter of human inertia, many C++ developers are heavily invested in the language, and they do not want to change anything, or, even worse, they feel threatened by Rust advancements, since they feel they can go from experienced veterans to complete rookies. And if we'll take academia world from which many libraries/frameworks originate (at least in robotics), it's even worse, as Rust qualities often go against quick-and-dirty approach abundantly used in academia, so it's even harder to sell Rust here.