[pjmlp]

This seems to have become quite prevalent.

Apparently the majority of CS degrees don't teach history of programming languages.

On my language design lectures in the mid-90's we had to learn all major ones, all the way back to Fortran.

[Manishearth]

This really doesn't seem like a reinventing to me (from either the Rust or C++ side). "Reinventing" implies ignoring history in the way you mention.

Tagged unions are prevalent in so many languages that the designers of both C++17 and Rust are bound to know about them. This is "borrowing" (or "stealing" :p).

The original Rust was ML-like and had an OCaml compiler, so Rust's enums definitely descended from those. I can't talk for C++ for sure, but like I said it's common in so many languages that they're bound to have derived inspiration from them.

[pjmlp]

I guess I was ranting a bit out of place....

[nickpsecurity]

I agree with Manishearth. It looks like they're learning more than forgetting. The Rust history page indicated it cherry-picked good things from a number of languages. Looks like this is just another one they've evolved into a type-safe default. A Good Thing.