Macro __internal__offset_of_enum__ 

macro_rules! __internal__offset_of_enum__ {
    ($type:tt :: $variant:ident [ $( $field:ident ),* ] => $used_field:ident => $discriminant:literal) => { ... };
    ($type:tt :: $variant:ident ( $index:tt ) => $discriminant:literal) => { ... };
    ($type:tt :: $variant:ident { $field:ident } => $discriminant:literal) => { ... };
}

Returns the byte offset of a field inside one variant of a repr(u8) enum.

Used by define_native_enum!. Only sound for repr(u8) enums, whose discriminant sits at offset zero.