Macro Enum 

Enum!() { /* proc-macro */ }

Enum!( Type0, Type1, ..., TypeM ) denotes a predefined enum, the definition of which looks like:

pub enum EnumN {
    _0( Type0 ),
    _1( Type1 ),
    // ...
    _M( TypeM ),
}

where M+1 == N.

Especially, Enum!() denotes a never type Enum0:

pub enum Enum0 {}