Rust's 2019 Roadmap(blog.rust-lang.org)

  #[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)
    }
  }

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