Your statement contradicts itself. It was unusable hard before non-lexical lifetimes, but they vastly increased the complexity? Then maybe what’s complex for compiler writers to implement can make the user’s life easier by lowering the complexity of their code?

I'm not sure it shows that. Even basic features of Rust we take from granted come from concepts common users do not need to understand. Borrowing of lifetime draws from affine types, but nobody cares when writing Rust code. If in 2012 you read a similar article explaining borrow checking in academic terms you would have thought Rust would be unusably hard, which is not.

Also I do not think that adding features is always bad to the point of comparing with Scala. Most of the things the article mentions will be almost invisible to users. For example, the `!Forget` thing it mentions will just end up with users getting new errors for things that before would have caused memory leaks. What a disgrace!

Then, pattern types allow you to remove panics from code, which is super helpful in many critical contexts where Rust is used in production, even in the Linux kernel once they will bump the language version so far.

Well, Rust as well has been around for more than 10 years now. I don't imply Rust invented the approach. Surely academia knew about it decades before. I was rather commenting on how one's mental model of things can change by learning new languages.

I think the misunderstanding here is that the article was not intended to users but to other language designers.

As a user, using a feature such as pattern types will be natural if you know the rest of the language.

Do you have a function that accepts an enum `MyEnum` but has an `unreachable!()` for some variant that you know is impossible to have at that point?

Then you can accept a `MyEnum is MyEnum::Variant | MyEnum::OtherVariant` instead of `MyEnum` to tell which are the accepted variants, and the pattern match will not require that `unreachable!()` anymore.

The fact someone does not know this is called "refinement types" does not limit their ability to use the feature effectively.

It’s unbelievable how that 16th century book looks like it is written in LaTeX. Or plain TeX, probably, given its age XD