fix: sandboxing of ppx for dialect

Signed-off-by: Ali Caglayan <alizter@gmail.com>

CHANGES.md

@@ -12,6 +12,9 @@ Unreleased
 - Add `dune build --dump-gc-stats FILE` argument to dump garbage collection
   stats to a named file. (#8072, @Alizter)
 
+- Fix bug with ppx and Reason syntax due to missing dependency in sandboxed
+  action (#7932, fixes #7930, @Alizter)
+
 3.9.1 (2023-07-06)
 ------------------
 

src/dune_rules/preprocessing.ml

@@ -699,6 +699,10 @@ let pp_one_module sctx ~lib_name ~scope ~preprocessor_deps
                            ; Path (Path.build dst)
                            ; Command.Ml_kind.ppx_driver_flag ml_kind
                            ; Dep (Path.build src)
+                           ; Hidden_deps
+                               (Module.source m ~ml_kind |> Option.value_exn
+                              |> Module.File.path |> Dep.file
+                              |> Dep.Set.singleton)
                            ; As flags
                            ]
                          >>| Action.Full.add_sandbox sandbox))))

test/blackbox-tests/test-cases/dune

@@ -142,5 +142,4 @@
  (applies_to reason-expect)
  (deps
   %{bin:refmt}
-  ;(package ppx_expect)
-  ))
+  (package ppx_expect)))

test/blackbox-tests/test-cases/reason-expect.t/run.t

@@ -15,43 +15,14 @@ Testing the interaction of reason syntax and ppx expect
   > EOF
 
 In https://github.com/ocaml/dune/issues/7930 there is an issue where reason
-syntax and ppx_expect break at dune lang 3.3.
-
-When dune lang is 3.2 everything should work as expected:
+syntax and ppx_expect break at dune lang 3.3 due to the sandboxing of ppx.
 
   $ cat > dune-project << EOF
-  > (lang dune 3.2)
+  > (lang dune 3.9)
   > EOF
 
   $ dune test
   File "test.re", line 1, characters 0-0:
   Error: Files _build/default/test.re and _build/default/test.re.corrected
   differ.
   [1]
-
-However on dune lang 3.3 this appears to break:
-
-  $ cat > dune-project << EOF
-  > (lang dune 3.3)
-  > EOF
-
-  $ dune test
-  File "test.re", line 1:
-  Error: I/O error: test.re: No such file or directory
-  [1]
-
-This appears to only happen for .re files:
-
-  $ rm test.re
-
-  $ cat > test.ml << EOF
-  > let%expect_test _ =
-  >   print_endline "Hello, world!";
-  >   [%expect {| |}]
-  > EOF
-
-  $ dune test
-  File "test.ml", line 1, characters 0-0:
-  Error: Files _build/default/test.ml and _build/default/test.ml.corrected
-  differ.
-  [1]