The problem is that #[repr(C)] enum is not compatible with C, even with #[non_exhaustive] annotation, and will never be, because C enum can be described in Rust terms as:

  #[repr(C)]
  enum Foo {
    A = 1, 
    B = 2,
    C = 3,
    UNKNOWN(i32),
  }

which is not possible to define in current version of Rust.