Relax functor lint rules in ppx_version

[martyall]

Quoting from the original README

One linter rule is that stable-versioned modules cannot be contained in the result of a functor application. The reason is that the types might depend on the functor arguments, so that distinct functor applications yield different types with different serializations. Functors that take no module arguments are allowed to return stable-versioned modules.

This rule makes the following code illegal:

module Functor (X : sig type x val foo :int end) = struct
  type x = X.x
  
  let f = g X.foo
  
  [%%versioned
  module Stable = struct
    module V1 = struct
      type t = string
      let to_latest =  Fn.id
    end
  end]
end

This turns out to be overly conservative and can cause problems when writing/refactoring harmless code. Take for example zkapp_command.ml where you have versioned modules nested inside the top level. If you wanted to change Call_forest to a functor, you can't. Even if the functor's argument doesn't appear at all in the versioned module.

This PR changes the linting rule to enforce that it is only illegal if you access a functor's argument directly in types defined wia %%versioned extension if you are outside of an expression. I figure that it's allowed in an expression because AFAIK we are only using this as a method to version types for coherent serialization

[martyall]

!ci-build-me

[mrmr1993]

Shouldn't this be lident m acc? We don't care about the RHS of the path at all IMO

[mrmr1993]

Of course, this allows aliasing á la

module Make (A : S) = struct
  module B = A
  module C = struct
    module%versioned Stable = struct
      module V1 = struct
        type t = B.t * B.t
      end
    end
  end
end

so this really circumvents the protection. We should move all the versioning out of the main code anyway, so I don't really care at this point.

[martyall]

Of course, this allows aliasing á la ...

yeah I also thought of this 😞

We should move all the versioning out of the main code anyway, so I don't really care at this point.

I'm assuming here you mean if you want to defined versioned types, they need to be in a standalone module that contains only type definitions? This would be nice since this system doesn't always compose well with vanilla ocaml, maybe a lot of work.

In the meanwhile, what is your most preferred/least hated option:

1. Something like this PR and remain vigilant
2. Remove the in_functor check entirely and remain more vigilant
3. Separate out type definitions piecemeal, i.e. do it if I'm already working in there and it becomes an obstruction

[martyall]

!ci-build-me

[georgeee]

!ci-nix-me

[georgeee]

Nix Ci was successful: https://buildkite.com/o-1-labs-2/mina-nix-experimental/builds/34#0191ef23-0638-4f01-86a9-a9a1b11d10a4

This includes execution of ppx_version tests (not part of our regular CI).