E0004

The abstract type does not occur in public signature of current package, consider marking it as priv.

If an abstract type is not used elsewhere in the public interface of a package, then it is not possible for user to interact with the value of that type in any way. Therefore, it is highly probable that the type is part of the private implementation of the package. In this case, it is better to mark the type as priv to indicate that it is not part of the public interface of the package.

It is also possible that, this abstract type is part of the public interface of the package, but relevant functions, methods, or types are not marked as pub.

Erroneous example

type Code Int derive(Show)

fn Code::new(value : Int) -> Code {
  return value
}

test {
  inspect!(Code::new(1), content="Code(1)")
}

Suggestion

If the abstract type is not part of the public interface of the package, then it should be marked as priv:

priv type Code Int derive(Show)

// ...

Or, if the abstract type is part of the public interface of the package, then make relevant definitions pub:

type Code Int derive(Show)

pub fn Code::new(value : Int) -> Code {
  return value
}