Macro emit_struct_definition 

macro_rules! emit_struct_definition {
    (
        found_exhaustive_attr: false,
        body: { $(#[$attr:meta])* $vis:vis $name:ident { $($field:tt)* } }
    ) => { ... };
    (
        found_exhaustive_attr: true,
        body: { $(#[$attr:meta])* $vis:vis $name:ident { $($field:tt)* } }
    ) => { ... };
}

Expands the definition of the record defined at [custom_struct], with either one of an additional private field or with its verbatim fields.