Macro emit_struct_default_body 

macro_rules! emit_struct_default_body {
    (
        found_exhaustive_attr: false,
        body: { $($field_default:tt)* }
    ) => { ... };
    (
        found_exhaustive_attr: true,
        body: { $($field_default:tt)* }
    ) => { ... };
}

Expands the Default implementation of the record defined at [custom_struct], with either one of an additional private field initialized to the unit value, or the record’s fields verbatim.