Deriving for a module with the same name as a standard library module

[khooyp]

In ppx_deriving 2.2., it was possible to apply @@deriving on a module that happens to have the same name as a standard library module, e.g.:

module List = struct
    type 'a t = [`Cons of 'a | `Nil] [@@deriving show]
end
type 'a t = 'a List.t option [@@deriving show]

However, this results in an error in ppx_deriving 3.0:

ocamlfind ocamlc -c -package 'ppx_deriving.std' test.ml
File "test.ml", line 4, characters 0-46:
Error: Unbound value List.pp

Looking at the output of ppx_tools/rewriter, it appears that Ppx_deriving_runtime.List is shadowing the user-defined List module:

module List = struct ... end
let rec pp : ... =
  ((let open! Ppx_deriving_runtime in
      fun poly_a  ->
        fun fmt  ->
          function
          | None  -> Format.pp_print_string fmt "None"
          | Some x ->
              (Format.pp_print_string fmt "(Some ";
               (List.pp (fun fmt  -> poly_a fmt) fmt) x;
               Format.pp_print_string fmt ")"))[@ocaml.warning "-A"])
and ...

[whitequark]

Oh, this was an unintended result of the hygiene improvements. (Note that in the past, it would not be possible to use List.t and the standard library list in the same data structure.)

Let me take a stab at this...