Consider some form of extensible enums

[zwarich]

There are a number of different approaches to making enums more extensible, e.g. extensible sum types, polymorphic variants, and union types. We should survey the landscape of ideas and find one that works best in the context of Rust.

Any concrete proposal should include details of the type system changes (including interaction with other type system features) and in-memory data representation.

[reem]

My take 2 of the original RFC (#402) which addresses many of the issues of the original by stripping most crazy features and adding detailed memory representation and compilation strategy sections: https://github.com/reem/rfcs/blob/join-types/text/0000-join-types.md

Comments desired and very welcome.

[xfoxfu]

Four years has passed and now Rust has reached 1.0 by long ago. Is there any possibility to discuss union types again? If RFC passed and possible, I'd like to work on the implementation.

[jonas-schievink]

Status update: We have unions now!

(also see RFC 2008, which introduces the #[non_exhaustive] attribute, also referred to as "extensible enums")