Allow list expansion in preprocessor arguments

src/dune_rules/preprocessing.ml

@@ -420,10 +420,21 @@ let get_cookies ~loc ~expander ~lib_name libs =
          [ "--cookie"; sprintf "%s=%S" name value ])
   |> List.concat
 
-let ppx_driver_and_flags_internal sctx ~loc ~expander ~lib_name ~flags libs =
+let ppx_driver_and_flags_internal sctx ~scope ~loc ~expander ~lib_name ~flags
+    libs =
   let open Action_builder.O in
+  let dune_version = Scope.project scope |> Dune_project.dune_version in
   let* flags =
-    Action_builder.List.map ~f:(Expander.expand_str expander) flags
+    if dune_version <= (3, 2) then
+      Action_builder.List.map ~f:(Expander.expand_str expander) flags
+    else
+      (* Allow list expansion *)
+      Action_builder.List.concat_map flags
+        ~f:(Expander.expand ~mode:Many expander)
+      |> Action_builder.map
+           ~f:
+             (List.map
+                ~f:(Value.to_string ~dir:(Path.build @@ Expander.dir expander)))
   in
   let+ cookies =
     Action_builder.of_memo (get_cookies ~loc ~lib_name ~expander libs)
@@ -435,7 +446,8 @@ let ppx_driver_and_flags sctx ~lib_name ~expander ~scope ~loc ~flags pps =
   let open Action_builder.O in
   let* libs = Resolve.Memo.read (Lib.DB.resolve_pps (Scope.libs scope) pps) in
   let* exe, flags =
-    ppx_driver_and_flags_internal sctx ~loc ~expander ~lib_name ~flags libs
+    ppx_driver_and_flags_internal sctx ~loc ~expander ~scope ~lib_name ~flags
+      libs
   in
   let* libs = Resolve.Memo.read (Lib.closure libs ~linking:true) in
   let+ driver =
@@ -698,7 +710,7 @@ let make sctx ~dir ~expander ~lint ~preprocess ~preprocessor_deps
 let get_ppx_driver sctx ~loc ~expander ~scope ~lib_name ~flags pps =
   let open Action_builder.O in
   let* libs = Resolve.Memo.read (Lib.DB.resolve_pps (Scope.libs scope) pps) in
-  ppx_driver_and_flags_internal sctx ~loc ~expander ~lib_name ~flags libs
+  ppx_driver_and_flags_internal sctx ~loc ~expander ~scope ~lib_name ~flags libs
 
 let ppx_exe sctx ~scope pp =
   let open Resolve.Memo.O in

test/blackbox-tests/test-cases/dune-ppx-driver-system.t/driver-tests-list-args/dune

@@ -0,0 +1,45 @@
+(library
+ (name driver_print_args)
+ (modules ())
+ (ppx.driver (main "(fun () -> Array.iter print_endline Sys.argv)")))
+
+(rule (with-stdout-to test_ppx_args.ml (echo "")))
+
+(library
+ (name ppx_with_cookies_print_args)
+ (kind (ppx_rewriter
+   (cookies (italy "%{env:ITALY=undefined}")
+            (france "%{env:FRANCE=undefined}"))))
+ (modules ())
+ (libraries driver_print_args))
+
+(env (_ (env-vars (ITALY "Biscotti") (FRANCE "Petit Beurre") (AMERICA "Oreo") (ENGLAND "Snickerdoodle"))))
+
+(library
+ (name test_ppx_args)
+ (modules test_ppx_args)
+ (preprocess
+   (pps -arg1 driver_print_args ppx_with_cookies_print_args -arg2 -arg3=%{env:AMERICA=undefined} --
+    -foo bar %{env:ENGLAND=undefined} %{read-lines:ppx-args})))
+
+(library
+ (name driver_print_tool)
+ (public_name foo.driver_print_tool)
+ (kind ppx_rewriter)
+ (modules ())
+ (libraries compiler-libs.common)
+ (ppx.driver (main "\| (fun () ->
+                   "\|    Ast_mapper.run_main (fun argv ->
+                   "\|      Printf.eprintf "tool name: %s\nargs:%s\n"
+                   "\|        (Ast_mapper.tool_name ())
+                   "\|        (String.concat " " argv);
+                   "\|      Ast_mapper.default_mapper))
+                   )))
+
+(library
+ (name ppx_with_cookies_print_tool)
+ (kind (ppx_rewriter
+   (cookies (italy "%{env:ITALY=undefined}")
+            (france "%{env:FRANCE=undefined}"))))
+ (modules ())
+ (libraries driver_print_tool))

test/blackbox-tests/test-cases/dune-ppx-driver-system.t/driver-tests-list-args/dune-project

@@ -0,0 +1 @@
+(lang dune 3.3)

test/blackbox-tests/test-cases/dune-ppx-driver-system.t/driver-tests-list-args/ppx-args

@@ -0,0 +1,4 @@
+Args
+From
+A
+File

test/blackbox-tests/test-cases/dune-ppx-driver-system.t/run.t

@@ -92,6 +92,20 @@ Test the argument syntax
   - test_ppx_args.pp.ml
   [1]
 
+Test the argument syntax with list expansion allowed (dune > 3.2)
+
+  $ dune build --root driver-tests-list-args
+  Entering directory 'driver-tests-list-args'
+  File "dune-project", line 1, characters 11-14:
+  1 | (lang dune 3.3)
+                 ^^^
+  Error: Version 3.3 of the dune language is not supported.
+  Supported versions of this extension in version 3.3 of the dune language:
+  - 1.0 to 1.12
+  - 2.0 to 2.9
+  - 3.0 to 3.2
+  [1]
+
 Test that going through the -ppx option of the compiler works
 
   $ dune build --root driver-tests test_ppx_staged.cma