I'm building a memory safe programming language that's like Rust, with a declarative concurrency model, which makes it much safer, simpler, and easier and nearly as fast & efficient

https://github.com/cuzzo/clear

The goal is to make Rust code nearly as easy to write as Ruby, but it almost always does the absolute best strategy.

You can write somewhat slow untyped code, and the internal tooling can guide you to adding all the types and optimizations and concurrency strategies that will make your code as fast as possible.

Sounds a lot like Crystal, which is also similar to Ruby and features a green fiber runtime: https://crystal-lang.org/#concurrency