> Did they have to write the runtime in the same language?
No, they originally wrote the runtime in C++, but ended up re-implementing it in Rust because the C++ runtime had too many bugs.
> They could've used an ML with GC and it would've been better (for a compiler).
They originally wrote the compiler in Scala with the JVM GC, and they said it was much slower and much harder to embed.
> It doesn't really have any functional programming paradigms. Pattern matching is present in imperative languages like past versions of ATS
When choosing a language for such a project, there are many engineering trade-offs that must be evaluated.
The Weld project has dozens of developers that need onboarding, documentation, examples, tooling, etc. Rust has a lower barrier of entry than ATS. One of the main things Weld does is interfacing with LLVM: this is one of the main things the Rust compiler does and Rust has great libraries for this. Another thing Weld does is interfacing with many dynamic languages (Python, R, etc.). Rust does not just have "C FFI". It also has _a lot_ of great tooling for automatically generating and validating all the boilerplate automatically. Finally, performance and code size of Weld is one of the main advantages over the alternatives. Rust generates reasonable code with LLVM, ATS has its own code machine code generator, which while reasonable, isn't as good.
Finally, it is hard to find people who enjoy writing ATS code. They exist, but are not many. Even when you do find them, they often don't like collaborating with people (I only know one ATS user, vmchale on github, and they don't really like interacting with others). OTOH it is trivial to find lots of people that enjoy writing Rust with others. It doesn't matter if this is due to technical reasons, marketing, or hype, but it's a fact that you have to consider if you want a project to grow fast.
