Fornjot is written in Rust, so starting with Rust as the modeling language was the most straight-forward option. I do agree that Rust is not ideal for this use case.
Fornjot is architected to be language-agnostic. Long-term, I hope there will be a selection of interoperable options for the modeling language.
And yes, I do agree the world needs a code first open source CAD program :-)
I've used openjscad in the past. It's fairly basic but easy enough to do the modeling part.
Fornjot is written in Rust, so starting with Rust as the modeling language was the most straight-forward option. I do agree that Rust is not ideal for this use case.
Fornjot is architected to be language-agnostic. Long-term, I hope there will be a selection of interoperable options for the modeling language.