Hello everyone
I have an idea for a new language feature to solve the problem of types that depend on runtime context to be meaningful. I also made a post about this on Zulip [1], but it isn't getting much traction so I want to share the idea here as well to get more feedback, and check if there was something obvious (or subtle) that I missed.
To avoid restating the problem and bloating this post, check out @tmandry excellent explainer on the issues that arrise when using types that depend on runtime context, and his solutions to them [2].
There is seemingly quite a lot of prior disscussion around this issue, but everyone seems to have differing ideas about just how exactly runtime variable data should influence how types and functions work, if at all [3], [4].
Of cause, I have my own solution. I have thought about it a lot, so to explain its intricacies properly, I needed more words then is allowed for a single post. It's not complicated, just a tough concept to explain without introducing it in small peices seperated by examples. You can read the full explanation (as a pdf) 
here, what follows is a high level summery of the idea.
Summary
My solution to this problem is to extend the type system to include a new kind of generic, generic variables. Generic variables would allow types to depend on runtime variables without duplicating references to those variables across every instance of the type or explicitly passing references to them as arguments to all its functions/methods. References to generic variables are implicitly passed to all methods on types that use those variables, making them in scope for the method body. Generic variables are simple in their construction, and work seemlessly with methods, traits, lifetimes, and generic code.
To support this I also propose introducing a new kind of variable whose name can't be shadowed so it can be used in type signature as a concrete subsitution for a generic variable.