PPX output flags warning 53 misplaced-attribute in OCaml 5.2

[samwgoldman]

Given the following contents in test.ml:

type t = string
[@@deriving show]

let () =
  print_endline (show "foo")

And a ppx binary to do the transforms:

$ ocamlfind opt -predicates ppx_driver -package ppx_deriving.show -package ppxlib.runner -linkpkg -linkall -o ppx_driver

We get the following output from ./ppx_driver test.ml

type t = string[@@deriving show]
let rec pp
  : Ppx_deriving_runtime.Format.formatter -> t -> Ppx_deriving_runtime.unit =
  ((let open! ((Ppx_deriving_runtime)[@ocaml.warning "-A"]) in
      fun fmt -> Ppx_deriving_runtime.Format.fprintf fmt "%S")
  [@ocaml.warning "-A"])
and show : t -> Ppx_deriving_runtime.string =
  fun x -> Ppx_deriving_runtime.Format.asprintf "%a" pp x[@@ocaml.warning
                                                           "-32"]
include struct let _ = fun (_ : t) -> () end[@@ocaml.doc "@inline"][@@merlin.hide
                                                                    ]
let () = print_endline (show "foo")

If we then try to compile that code:

$ ocamlopt test.pp.ml 
File "test.pp.ml", line 3, characters 4-41:
3 |   : Ppx_deriving_runtime.Format.formatter -> t -> Ppx_deriving_runtime.unit =
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: Unbound module Ppx_deriving_runtime
[[email protected] ~/local/ocaml5]$ ocamlfind ocamlopt -package ppx_deriving.runtime test.pp.ml 
File "test.pp.ml", line 4, characters 39-52:
4 |   ((let open! ((Ppx_deriving_runtime)[@ocaml.warning "-A"]) in
                                           ^^^^^^^^^^^^^
Warning 53 [misplaced-attribute]: the "ocaml.warning" attribute cannot appear in this context

This is from a clean switch with ocaml.5.2~alpha1 and ppx_deriving

$ opam list
# Packages matching: installed
# Name                      # Installed          # Synopsis
base-bigarray               base
base-domains                base
base-nnp                    base                 Naked pointers prohibited in the OCaml heap
base-threads                base
base-unix                   base
cppo                        1.6.9                Code preprocessor like cpp for OCaml
dune                        3.14.2               Fast, portable, and opinionated build system
ocaml                       5.2.0                The OCaml compiler (virtual package)
ocaml-compiler-libs         v0.17.0              OCaml compiler libraries repackaged
ocaml-config                3                    OCaml Switch Configuration
ocaml-option-flambda        1                    Set OCaml to be compiled with flambda activated
ocaml-option-fp             1                    Set OCaml to be compiled with frame-pointers enabled
ocaml-option-no-compression 1                    Set OCaml to be compiled with --without-zstd
ocaml-variants              5.2.0~alpha1+options First alpha release of OCaml 5.2.0
ocamlfind                   1.9.6                A library manager for OCaml
ppx_derivers                1.2.1                Shared [@@deriving] plugin registry
ppx_deriving                5.2.1                Type-driven code generation for OCaml
ppxlib                      0.32.1~5.2preview    Standard infrastructure for ppx rewriters
result                      1.5                  Compatibility Result module
sexplib0                    v0.16.0              Library containing the definition of S-expressions and some base converters
stdlib-shims                0.3.0                Backport some of the new stdlib features to older compiler

[NathanReb]

You got this error when using one of the plugins right? Could you test if you still get this error when pinning ppx_deriving to the master branch?

We recently merged a port of all plugins to ppxlib and I believe this should fix the issue as they are not using the ppx_deriving API anymore and if I understand your fix in #283 correctly, this attribute was attached for all plugins by the ppx_deriving API.