feat(ocamldep): add read_immediate_deps_raw_of for raw module names (issue #4572)

Add a new function `read_immediate_deps_raw_of` to the ocamldep module that
returns raw module names (as Module_name.Set.t) from ocamldep output, without
resolving them to Module.t values.

This function will be used to determine which external library modules a
module actually references, enabling finer-grained dependency tracking where
modules are only recompiled when libraries they actually use change.

Signed-off-by: Robin Bate Boerop <me@robinbb.com>

Author: robinbb

src/dune_rules/ocamldep.ml

@@ -199,3 +199,18 @@ let read_immediate_deps_of ~obj_dir ~modules ~ml_kind ~for_ unit =
       |> parse_module_names ~dir:(Obj_dir.dir obj_dir) ~unit ~modules)
     |> Action_builder.memoize (Path.Build.to_string ocamldep_output)
 ;;
+
+let read_immediate_deps_raw_of ~obj_dir ~ml_kind ~for_ unit =
+  match Module.source ~ml_kind unit with
+  | None -> Action_builder.return Module_name.Set.empty
+  | Some source ->
+    let ocamldep_output =
+      Obj_dir.Module.dep obj_dir ~for_ (Immediate (unit, ml_kind)) |> Option.value_exn
+    in
+    Action_builder.lines_of (Path.build ocamldep_output)
+    |> Action_builder.map ~f:(fun lines ->
+      parse_deps_exn ~file:(Module.File.path source) lines
+      |> List.map ~f:Module_name.of_checked_string
+      |> Module_name.Set.of_list)
+    |> Action_builder.memoize (Path.Build.to_string ocamldep_output ^ "-raw")
+;;

src/dune_rules/ocamldep.mli

@@ -32,3 +32,15 @@ val read_immediate_deps_of
   -> for_:Compilation_mode.t
   -> Module.t
   -> Module.t list Action_builder.t
+
+(** [read_immediate_deps_raw_of ~obj_dir ~ml_kind ~for_ unit] returns the raw
+    module names (unresolved) from ocamldep output for the file with kind
+    [ml_kind] of the module [unit]. This includes ALL module references, both
+    intra-stanza and external library modules. If there is no such file with
+    kind [ml_kind], an empty set is returned. *)
+val read_immediate_deps_raw_of
+  :  obj_dir:Path.Build.t Obj_dir.t
+  -> ml_kind:Ml_kind.t
+  -> for_:Compilation_mode.t
+  -> Module.t
+  -> Module_name.Set.t Action_builder.t