A modern programming language that makes developers happier.

kotlinlang

What about making frozen-ness a part of type, much like nullability is?
Maybe a `#` at the end to indicate that you expect a mutable thing rather than a frozen thing.
I haven’t thought this all the way through yet.  There might be some big problems with that.
I just see immutability issues being a huge problem for Kotlin Native in the future unless we have some language features to prevent some common issues.

I agree. I don't know the most about it, but when I first heard about the design choices with regards to multithreading in kotlin native I... understood the argument, but I found it a strange choice to enforce no shared mutable state.
And the weirder part is the strange select cases where it just works differently such as top level variables.
I believe a lot of it is driven by the implementation of the VM.

It's definitely also unfortunate in multiplatform projects, where common code is restricted to the same set of rules. 