graphql_union from tagged union of structs

[arlyon]

Describe the solution you'd like

It would be cool to be able to define GraphQL unions like so:

#[derive(GraphQLObject)]
struct Square {
    /// length of one side
    pub length: i32,
}

#[derive(GraphQLObject)]
struct Circle {
    /// radius of the circle
    pub radius: i32,
}

#[juniper::graphql_union]
enum Shape {
    Square(Square),
    Circle(Circle),
}

I understand the current implementation is quite similar already, however it would be cool if we could pull the boilerplate into the macro somehouw..

Describe alternatives you've considered

This would be an alternative (and more ergonomic) alternative to the current implementation. Here is it in use (imagine a query that returns a FieldResult<Shape>):

query {
    getShapes {
        ...on Square { length }
        ...on Circle { radius }
    }
}

[LegNeato]

Oooh, I like it!