pub fn result_type(&self) -> Type {
        use self::BuilderKind::*;
        use self::Type::*;
        match *self {
            Appender(ref elem) => Vector(elem.clone()),
            Merger(ref elem, _) => *elem.clone(),
            DictMerger(ref key, ref value, _) => Dict(key.clone(), value.clone()),
            GroupMerger(ref key, ref value) => Dict(key.clone(), Box::new(Vector(value.clone()))),
            VecMerger(ref elem, _) => Vector(elem.clone()),
        }
    }
Returns the type produced by this `BuilderKind`.
weld/src/ast/ast.rs