> Alas, counting in macro by example is possible, but not trivial.

It's not stabilized yet, but RFC3086[1] introduces macro metavariable expressions, which make counting trivial!

The example macro becomes:

    macro_rules! define_error {
      ($($err:ident,)*) => {
        #[derive(Debug, Clone, Copy, PartialEq, Eq)]
        pub enum Error {
          $($err,)*
        }
    
        impl Error {
          pub fn from_code(code: u32) -> Option<Error> {
            match code {
              $( ${index()} => Some(Error::$err), )*
              _ => None,
            }
          }
        }
      };
    }

You can see this in practice on the playground[2].

[1]: https://github.com/rust-lang/rfcs/blob/master/text/3086-macr... [2]: https://play.rust-lang.org/?version=nightly&mode=debug&editi...