Unions discriminated by type could be useful in Rust, especially if you could use them without declaring the union type explicitly. It'd enable something like `

Unions discriminated by type could be useful in Rust, especially if you could use them without declaring the union type explicitly. It'd enable something like `Result<..., IoError | ParseError>` without having to manually create a union type, implement the appropriate traits for it, etc. Unfortunately it opens a can of worms related to type identity, so I don't expect to see such a feature soon.