fix(melange): add .cmj include dirs for installed private libs

doc/changes/10416.md

@@ -0,0 +1,3 @@
+- melange: add include paths for private library `.cmj` files during JS
+  emission. (#10416, @anmonteiro)
+

src/dune_rules/lib_flags.ml

@@ -98,18 +98,30 @@ module L = struct
     | lib :: _ -> Path.Set.remove dirs (Lib.lib_config lib).stdlib_dir
   ;;
 
+  type mode =
+    { lib_mode : Lib_mode.t
+    ; melange_emit : bool
+    }
+
   let include_paths =
     let add_public_dir ~visible_cmi obj_dir acc mode =
       match visible_cmi with
       | false -> acc
       | true ->
-        let public_cmi_dir =
-          (match mode with
-           | `Byte -> Obj_dir.public_cmi_ocaml_dir
-           | `Melange -> Obj_dir.public_cmi_melange_dir)
-            obj_dir
+        let public_cmi_dirs =
+          List.map
+            ~f:(fun f -> f obj_dir)
+            (match mode with
+             | { lib_mode = Ocaml _; _ } -> [ Obj_dir.public_cmi_ocaml_dir ]
+             | { lib_mode = Melange; melange_emit = false } ->
+               [ Obj_dir.public_cmi_melange_dir ]
+             | { lib_mode = Melange; melange_emit = true } ->
+               (* Add the dir where `.cmj` files exist, even for installed
+                  private libraries. Melange needs to query `.cmj` files for
+                  `import` information *)
+               [ Obj_dir.melange_dir; Obj_dir.public_cmi_melange_dir ])
         in
-        Path.Set.add acc public_cmi_dir
+        List.fold_left public_cmi_dirs ~init:acc ~f:Path.Set.add
     in
     fun ?project ts mode ->
       let visible_cmi =
@@ -130,11 +142,11 @@ module L = struct
         List.fold_left ts ~init:Path.Set.empty ~f:(fun acc t ->
           let obj_dir = Lib_info.obj_dir (Lib.info t) in
           let visible_cmi = visible_cmi t in
-          match mode with
-          | Lib_mode.Melange -> add_public_dir ~visible_cmi obj_dir acc `Melange
-          | Ocaml mode ->
-            let acc = add_public_dir ~visible_cmi obj_dir acc `Byte in
-            (match mode with
+          match mode.lib_mode with
+          | Melange -> add_public_dir ~visible_cmi obj_dir acc mode
+          | Ocaml ocaml_mode ->
+            let acc = add_public_dir ~visible_cmi obj_dir acc mode in
+            (match ocaml_mode with
              | Byte -> acc
              | Native ->
                let native_dir = Obj_dir.native_dir obj_dir in
@@ -143,7 +155,17 @@ module L = struct
       remove_stdlib dirs ts
   ;;
 
-  let include_flags ?project ts mode = to_iflags (include_paths ?project ts mode)
+  let include_flags ?project ts mode =
+    to_iflags (include_paths ?project ts { lib_mode = mode; melange_emit = false })
+  ;;
+
+  let melange_emission_include_flags ?project ts =
+    to_iflags (include_paths ?project ts { lib_mode = Melange; melange_emit = true })
+  ;;
+
+  let include_paths ?project ts mode =
+    include_paths ?project ts { lib_mode = mode; melange_emit = false }
+  ;;
 
   let c_include_paths ts =
     let dirs =

src/dune_rules/lib_flags.mli

@@ -25,6 +25,7 @@ module L : sig
   val to_iflags : Path.Set.t -> _ Command.Args.t
   val include_paths : ?project:Dune_project.t -> t -> Lib_mode.t -> Path.Set.t
   val include_flags : ?project:Dune_project.t -> t -> Lib_mode.t -> _ Command.Args.t
+  val melange_emission_include_flags : ?project:Dune_project.t -> t -> _ Command.Args.t
   val c_include_flags : t -> Super_context.t -> _ Command.Args.t
   val toplevel_ld_paths : t -> Path.Set.t
   val toplevel_include_paths : t -> Path.Set.t

src/dune_rules/melange/melange_rules.ml

@@ -116,7 +116,9 @@ let cmj_includes ~(requires_link : Lib.t list Resolve.t) ~scope =
   let+ requires_link = requires_link in
   let deps = List.map requires_link ~f:deps_of_lib |> Dep.Set.of_list in
   Command.Args.S
-    [ Lib_flags.L.include_flags ~project requires_link Melange; Hidden_deps deps ]
+    [ Lib_flags.L.melange_emission_include_flags ~project requires_link
+    ; Hidden_deps deps
+    ]
 ;;
 
 let compile_info ~scope (mel : Melange_stanzas.Emit.t) =

test/blackbox-tests/test-cases/melange/installed-private-lib-dep.t

@@ -60,16 +60,9 @@ Melange (installed) library depends on private library
   > let () = Js.log Foo.x
   > EOF
 
-An issue similar to #7104 still present because the `.cmj` is not visible.
+Build the app that depends on `foo`, which in turn depends on a private lib
 
-  $ OCAMLPATH=$PWD/prefix/lib/:$OCAMLPATH dune build @melange --root app --display=short
+  $ OCAMLPATH=$PWD/prefix/lib/:$OCAMLPATH dune build @melange --root app
   Entering directory 'app'
-          melc output/node_modules/foo.__private__.priv/priv.js
-          melc .output.mobjs/melange/melange__Entry.{cmi,cmj,cmt}
-          melc output/entry.js
-          melc output/node_modules/foo/foo.js (exit 2)
-  File "_none_", line 1:
-  Error: Priv not found, it means either the module does not exist or it is a namespace
   Leaving directory 'app'
-  [1]