Yeah, it's frustrating that there's no syntax for this. It could even be syntactic sugar; in this case if you had:

    type FooOrBarOrError = Foo | Bar | Error;

Then that could desugar to:

    enum FooOrBarOrError {
        Foo(Foo),
        Bar(Bar),
        Error(Error),
    }

And it could also implement From for you, so you can easily get a FooOrBarOrError from a Foo, Bar, or Error; as well as implementing Display, StdError, etc. if the components already implement them.

I actually wonder if you could implement this as a proc macro...