Yeah, I think there's four of five different proposals mixed up in there. In addition to the aforementioned enum impl Trait and try fn, some other proposals/features that I think are either buried somewhere in your original post or just tangentially relevant include:
- the Error trait reform tracked by Tracking issue for RFC 2504, "Fix the Error trait" · Issue #53487 · rust-lang/rust · GitHub covers (among other things) dealing with backtraces
- various anonymous enum proposals, most recently RFC for anonymous variant types, a minimal ad-hoc sum type by eaglgenes101 · Pull Request #2587 · rust-lang/rfcs · GitHub
- the trait upcasting issue was discussed in Where's the catch with Box<Read + Write>?, I feel like there's something more recent too
- and the postponed RFC for making enum variants into types: https://github.com/rust-lang/rfcs/pull/1450
The key thing is that there doesn't seem to be any need to smush all these features together into a single proposal, so we probably should keep them cleanly separated.