feat(single-context): add conditional Melange preprocessor deps

src/dune_lang/preprocess.ml

@@ -369,20 +369,30 @@ module Per_module = struct
   ;;
 end
 
-let preprocess_fields =
+let preprocess_fields_with_prefix ~prefix:field_prefix =
   let open Decoder in
-  let+ preprocess = field "preprocess" Per_module.decode ~default:(Per_module.default ())
+  let prefix =
+    let prefix x = x ^ "." in
+    Option.map field_prefix ~f:prefix |> Option.value ~default:""
+  in
+  let+ preprocess =
+    match field_prefix with
+    | None ->
+      let+ v = field "preprocess" Per_module.decode ~default:(Per_module.default ()) in
+      Some v
+    | Some _ -> field_o (prefix ^ "preprocess") Per_module.decode
   and+ preprocessor_deps =
-    field_o
-      "preprocessor_deps"
-      (let+ loc = loc
-       and+ l = repeat Dep_conf.decode in
-       loc, l)
+    let decode =
+      let+ loc = loc
+      and+ l = repeat Dep_conf.decode in
+      loc, l
+    in
+    field_o (prefix ^ "preprocessor_deps") decode
   and+ syntax = Syntax.get_exn Stanza.syntax in
   let preprocessor_deps =
-    match preprocessor_deps with
-    | None -> []
-    | Some (loc, deps) ->
+    match preprocessor_deps, preprocess with
+    | Some _, None | None, _ -> []
+    | Some (loc, deps), Some preprocess ->
       let deps_might_be_used =
         Module_name.Per_item.exists preprocess ~f:(fun p ->
           match p with
@@ -394,15 +404,22 @@ let preprocess_fields =
         User_warning.emit
           ~loc
           ~is_error:(syntax >= (2, 0))
-          [ Pp.text
-              "This preprocessor_deps field will be ignored because no preprocessor that \
-               might use them is configured."
+          [ Pp.textf
+              "This %spreprocessor_deps field will be ignored because no preprocessor \
+               that might use them is configured."
+              prefix
           ];
       deps
   in
   preprocess, preprocessor_deps
 ;;
 
+let preprocess_fields =
+  let open Decoder in
+  let+ preprocess, deps = preprocess_fields_with_prefix ~prefix:None in
+  Option.value_exn preprocess, deps
+;;
+
 type preprocess =
   { config : With_instrumentation.t Per_module.t
   ; preprocessor_deps : Dep_conf.t list

src/dune_rules/stanzas/buildable.ml

@@ -54,6 +54,40 @@ let decode_melange_pps =
     (Dune_lang.Syntax.since Stanza.syntax (3, 24) >>> Preprocess.Pps.decode)
 ;;
 
+let decode_melange_preprocess =
+  let+ preprocess = field_o "melange.preprocess" Preprocess.Per_module.decode
+  and+ preprocessor_deps =
+    field_o
+      "melange.preprocessor_deps"
+      (Dune_lang.Syntax.since Stanza.syntax (3, 24)
+       >>> let+ loc = loc
+           and+ l = repeat Dep_conf.decode in
+           loc, l)
+  and+ syntax = Dune_lang.Syntax.get_exn Stanza.syntax in
+  let preprocessor_deps =
+    match preprocessor_deps, preprocess with
+    | Some _, None | None, _ -> []
+    | Some (loc, deps), Some preprocess ->
+      let deps_might_be_used =
+        Module_name.Per_item.exists preprocess ~f:(fun p ->
+          match p with
+          | Preprocess.Action _ | Preprocess.Pps _ -> true
+          | Preprocess.No_preprocessing | Preprocess.Future_syntax _ -> false)
+      in
+      if not deps_might_be_used
+      then
+        User_warning.emit
+          ~loc
+          ~is_error:(syntax >= (2, 0))
+          [ Pp.text
+              "This melange.preprocessor_deps field will be ignored because no \
+               preprocessor that might use them is configured."
+          ];
+      deps
+  in
+  preprocess, preprocessor_deps
+;;
+
 let decode (for_ : for_) =
   let use_foreign =
     Dune_lang.Syntax.deleted_in
@@ -78,6 +112,7 @@ let decode (for_ : for_) =
   let+ loc = loc
   and+ instrumentation = Preprocess.Instrumentation.instrumentation
   and+ preprocess, preprocessor_deps = Preprocess.preprocess_fields
+  and+ melange_preprocess, melange_preprocessor_deps = decode_melange_preprocess
   and+ melange_pps = decode_melange_pps
   and+ lint = decode_lint
   and+ foreign_stubs =
@@ -153,11 +188,20 @@ let decode (for_ : for_) =
     Preprocess.preprocess_config ~preprocess ~instrumentation ~preprocessor_deps
   in
   let melange_preprocess =
-    match melange_pps with
-    | None -> preprocess
-    | Some pps ->
+    match melange_preprocess, melange_pps with
+    | None, None -> preprocess
+    | Some preprocess, None ->
+      Preprocess.preprocess_config
+        ~preprocess
+        ~instrumentation
+        ~preprocessor_deps:melange_preprocessor_deps
+    | None, Some pps ->
       let preprocess = Module_name.Per_item.for_all (Preprocess.Pps pps) in
       Preprocess.preprocess_config ~preprocess ~instrumentation ~preprocessor_deps:[]
+    | Some _, Some pps ->
+      User_error.raise
+        ~loc:pps.loc
+        [ Pp.text "Cannot use both melange.pps and melange.preprocess." ]
   in
   let foreign_stubs =
     foreign_stubs

src/dune_rules/vimpl.ml

@@ -38,7 +38,9 @@ let make ~sctx ~scope ~(lib : Library.t) ~info ~vlib ~for_ =
         let* preprocess =
           (* TODO wrong, this should be delayed *)
           Instrumentation.with_instrumentation
-            lib.buildable.preprocess.config
+            (match for_ with
+             | Ocaml -> lib.buildable.preprocess.config
+             | Melange -> lib.buildable.melange_preprocess.config)
             ~instrumentation_backend:(Lib.DB.instrumentation_backend db)
           |> Resolve.Memo.read_memo
         in

test/blackbox-tests/test-cases/melange/conditional-pps.t

@@ -57,3 +57,56 @@ The field is available starting in Dune 3.24.
   $ find ./app/_build/default -name '*.pp.ml' | censor | sort
   ./app/_build/default/.melange_src/foo.pp.ml
   ./app/_build/default/.melange_src/melange_only.pp.ml
+
+`melange.preprocessor_deps` is available starting in Dune 3.24.
+
+  $ mkdir old-preprocessor-deps
+  $ cat > old-preprocessor-deps/dune-project <<EOF
+  > (lang dune 3.23)
+  > (using melange 0.1)
+  > EOF
+  $ cat > old-preprocessor-deps/dune <<EOF
+  > (library
+  >  (name old_preprocessor_deps)
+  >  (modes melange)
+  >  (melange.preprocessor_deps dep.txt))
+  > EOF
+  $ dune build --root old-preprocessor-deps
+  Entering directory 'old-preprocessor-deps'
+  File "dune", line 4, characters 1-36:
+  4 |  (melange.preprocessor_deps dep.txt))
+       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  Error: 'melange.preprocessor_deps' is only available since version 3.24 of
+  the dune language. Please update your dune-project file to have (lang dune
+  3.24).
+  Leaving directory 'old-preprocessor-deps'
+  [1]
+
+`melange.preprocessor_deps` applies only to Melange preprocessing.
+
+  $ mkdir deps
+  $ cat > deps/dune-project <<EOF
+  > (lang dune 3.24)
+  > (using melange 0.1)
+  > EOF
+  $ cat > deps/dune <<EOF
+  > (library
+  >  (modes melange :standard)
+  >  (name deps)
+  >  (melange.preprocess (action (run ./pp.sh %{input-file})))
+  >  (melange.preprocessor_deps pp.sh))
+  > EOF
+
+  $ cat > deps/foo.ml <<EOF
+  > let x = "foo"
+  > EOF
+  $ cat > deps/pp.sh <<EOF
+  > #!/bin/sh
+  > cat "\$1"
+  > echo 'let y = "from melange preprocessor"'
+  > EOF
+  $ chmod +x deps/pp.sh
+
+  $ dune build --root deps
+  $ grep y deps/_build/default/.melange_src/foo.pp.ml
+  let y = "from melange preprocessor"