Macro define_runtime_struct 

macro_rules! define_runtime_struct {
    (
        $registry:expr
        =>
        $(mod $module_name:ident)?
        struct $name:ident {
            $( $field_name:ident : $field_type:ty ),*
        }
    ) => { ... };
}

Builds a Struct that no Rust type backs, laying its fields out.

define_runtime_struct! {
    registry => mod lib struct Foo {
        a: bool,
        b: i32
    }
}