fix(module_compilation): add dep to original source file

src/dune_rules/module.ml

@@ -336,6 +336,15 @@ let sources_without_pp t =
     ~f:(Option.map ~f:(fun (x : File.t) -> x.original_path))
 ;;
 
+let source_without_pp ~ml_kind t =
+  let source =
+    match (ml_kind : Ml_kind.t) with
+    | Impl -> t.source.files.impl
+    | Intf -> t.source.files.intf
+  in
+  Option.map source ~f:(fun (x : File.t) -> x.original_path)
+;;
+
 module Obj_map = struct
   include Map.Make (struct
       type nonrec t = t

src/dune_rules/module.mli

@@ -88,6 +88,7 @@ end
 
 val sources : t -> Path.t list
 val sources_without_pp : t -> Path.t list
+val source_without_pp : ml_kind:Ml_kind.t -> t -> Path.t option
 val visibility : t -> Visibility.t
 val encode : t -> src_dir:Path.t -> Dune_lang.t list
 val decode : src_dir:Path.t -> t Dune_lang.Decoder.t

src/dune_rules/module_compilation.ml

@@ -158,6 +158,7 @@ let build_cm
    let* compiler = compiler in
    let ml_kind = Lib_mode.Cm_kind.source cm_kind in
    let+ src = Module.file m ~ml_kind in
+   let original = Module.source_without_pp m ~ml_kind in
    let dst = Obj_dir.Module.cm_file_exn obj_dir m ~kind:cm_kind in
    let obj =
      Obj_dir.Module.obj_file obj_dir m ~kind:(Ocaml Cmx) ~ext:ocaml.lib_config.ext_obj
@@ -324,6 +325,7 @@ let build_cm
             ; A "-c"
             ; Command.Ml_kind.flag ml_kind
             ; Dep src
+            ; Hidden_deps (Dep.Set.of_files (Option.to_list original))
             ; other_targets
             ]
       >>| Action.Full.add_sandbox sandbox))

test/blackbox-tests/test-cases/melange/ppx-preview.t

@@ -25,6 +25,8 @@ Show PPX snippet preview is shown in Dune
   $ export DUNE_SANDBOX=symlink
   $ dune build @melange
   File "lib/the_lib.ml", line 1, characters 7-11:
+  1 | let x: nope = "hello"
+             ^^^^
   Error: Unbound type constructor nope
   [1]