As far as I can tell the goal of the project isn't to target more platforms (Rust targets quite a few by way of LLVM), so I don't think I'd choose any other language, including C++.
Having a compiler and standard library written in the language that it compiles has some huge benefits for increasing the pool of possible contributors.
There appears to be an attempt to leverage this project to target ESP8266, which AFAIK LLVM does not support: https://github.com/emosenkis/esp-rs
But this isn't the official compiler, this is someone's personal project?
True, but compilers are complicated machines and Rust is still changing at a fairly frantic rate.
The author seems to be doing quite a good job of development today, but if it has any hope of staying current, it probably needs to think about how to increase its bus factor (something happens like changing jobs, starting a family, or they just become interested in something else, and a single person suddenly has less time to contribute).
I don't think there's any real plans for a source-based approach. But epochs can only change a limited amount of things for exactly this reason; they minimize the compiler burden of supporting them.