Can anybody explain the downvotes? It looks suspicious.

It's well known and long standing problem. Rust the Language allows proper enum, but Rust the Compiler cannot pack enum into single value, as in C.

Example:

  #[repr(C)]
  #[repr(i32)]
  #[derive(Debug)]
  #[allow(dead_code)]
  enum E {
    A,
    B,
    C,
    Unlabeled(i32),
  }

  fn main() {
    {
        let v: E = unsafe { std::mem::transmute::<(i32,i32), E>((0,0)) };
        println!("v = {:?}", v)
    }
    {
        let v: E = unsafe { std::mem::transmute::<(i32,i32), E>((1,1)) };
        println!("v = {:?}", v)
    }
    {
        let v: E = unsafe { std::mem::transmute::<(i32,i32), E>((2,2)) };
        println!("v = {:?}", v)
    }
    {
        let v: E = unsafe { std::mem::transmute::<(i32,i32), E>((3,3)) };
        println!("v = {:?}", v)
    }
    {
        let v: E = unsafe { std::mem::transmute::<(i32,i32), E>((3,55)) };
        println!("v = {:?}", v)
    }
    {
        let v: E = unsafe { std::mem::transmute::<(i32,i32), E>((55,55)) };
        println!("v = {:?}", v)
    }
  }

Output:

  v = A
  v = B
  v = C
  v = Unlabeled(3)
  v = Unlabeled(55)
  v = A