feat: per-module [-I]/[-H] include flags via [kept_libs]

src/dune_rules/compilation_context.ml

@@ -340,6 +340,24 @@ let create
 
 let for_ t = t.for_
 
+let filtered_include_flags t ~cm_kind ~kept_libs =
+  let lib_mode = Lib_mode.of_cm_kind cm_kind in
+  let open Action_builder.O in
+  let* direct_requires = Resolve.Memo.read t.requires_compile in
+  let+ hidden_requires = Resolve.Memo.read t.requires_hidden in
+  let direct_filtered = List.filter direct_requires ~f:(Lib.Set.mem kept_libs) in
+  let hidden_filtered = List.filter hidden_requires ~f:(Lib.Set.mem kept_libs) in
+  let project = Scope.project t.scope in
+  let lib_config = t.ocaml.lib_config in
+  Lib_flags.L.include_flags
+    ~project
+    ~direct_libs:direct_filtered
+    ~hidden_libs:hidden_filtered
+    lib_mode
+    lib_config
+  |> Command.Args.memo
+;;
+
 let alias_and_root_module_flags =
   let extra = [ "-w"; "-49" ] in
   fun base -> Ocaml_flags.append_common base extra |> Ocaml_flags.append_nostdlib

src/dune_rules/compilation_context.mli

@@ -63,6 +63,17 @@ val requires_hidden : t -> Lib.t list Resolve.Memo.t
 val requires_compile : t -> Lib.t list Resolve.Memo.t
 val parameters : t -> Module_name.t list Resolve.Memo.t
 val includes : t -> Command.Args.without_targets Command.Args.t Lib_mode.Cm_kind.Map.t
+
+(** Include flags ([-I]/[-H]) for compiling a module against [kept_libs]. The
+    cctx's [requires_compile] and [requires_hidden] are each restricted to
+    libraries in [kept_libs]; the kept direct entries become [-I], the kept
+    hidden entries become [-H]. *)
+val filtered_include_flags
+  :  t
+  -> cm_kind:Lib_mode.Cm_kind.t
+  -> kept_libs:Lib.Set.t
+  -> Command.Args.without_targets Command.Args.t Action_builder.t
+
 val lib_index : t -> Lib_file_deps.Lib_index.t Resolve.Memo.t
 
 (** [true] iff any library in the compilation context's direct or hidden

src/dune_rules/module_compilation.ml

@@ -205,7 +205,7 @@ let lib_deps_for_module ~cctx ~obj_dir ~for_ ~dep_graph ~opaque ~cm_kind ~ml_kin
           lib_index
           ~referenced_modules:tight_set
       in
-      let tight_deps, glob_libs, _kept_libs =
+      let tight_deps, glob_libs, kept_libs =
         List.fold_left
           all_libs
           ~init:(Dep.Set.empty, [], Lib.Set.empty)
@@ -226,8 +226,10 @@ let lib_deps_for_module ~cctx ~obj_dir ~for_ ~dep_graph ~opaque ~cm_kind ~ml_kin
                 else td, lib :: gl, Lib.Set.add kl lib))
       in
       let glob_deps = Lib_file_deps.deps_of_entries ~opaque ~cm_kind glob_libs in
-      Action_builder.return
-        (cctx_includes_for_cm_kind (), Dep.Set.union tight_deps glob_deps)
+      let+ include_flags =
+        Compilation_context.filtered_include_flags cctx ~cm_kind ~kept_libs
+      in
+      include_flags, Dep.Set.union tight_deps glob_deps
 ;;
 
 let lib_cm_deps ~cctx ~cm_kind ~ml_kind ~mode m =

test/blackbox-tests/test-cases/per-module-lib-deps/add-unreferenced-sibling-lib.t

@@ -1,20 +1,17 @@
-Observational baseline: adding a library to a stanza's
-[(libraries ...)] list currently rebuilds every module in the
-stanza, including modules that reference nothing from the new
-library. The [-I]/[-H] flags on each module's compiler invocation
-are derived from the cctx-wide library list rather than from the
-module's own ocamldep reference set, so adding an unreferenced
-library still changes every consumer's compile-action hash and
-invalidates the rule cache.
+Adding a library to a stanza's [(libraries ...)] list does not
+rebuild stanza modules that reference nothing from the new
+library. The per-module [-I]/[-H] include filter narrows each
+module's compile-action hash to the libraries its ocamldep
+reference set actually reaches, so an unreferenced added lib
+leaves every other consumer's cache key unchanged.
 
 [consumer_lib] starts with [(libraries existing_dep)] and two
 modules: [consumes_dep] references [Existing_dep_module];
 [unrelated_module] references nothing from [existing_dep] or any
 other library. After the initial build, [added_lib] is added to
 the stanza's [(libraries ...)] list. Neither [consumes_dep] nor
-[unrelated_module] uses anything from [added_lib], yet both
-rebuild. Records the rebuild counts on each so a future change
-can flip them to 0.
+[unrelated_module] uses anything from [added_lib], so neither
+rebuilds — the trace shows empty target lists for both.
 
   $ cat > dune-project <<EOF
   > (lang dune 3.23)
@@ -72,22 +69,6 @@ Add [added_lib] to [consumer_lib]'s [(libraries ...)]. Neither
 
   $ dune build @check
   $ dune trace cat | jq -s 'include "dune"; [.[] | targetsMatchingFilter(test("consumes_dep"))]'
-  [
-    {
-      "target_files": [
-        "_build/default/.consumer_lib.objs/byte/consumes_dep.cmi",
-        "_build/default/.consumer_lib.objs/byte/consumes_dep.cmo",
-        "_build/default/.consumer_lib.objs/byte/consumes_dep.cmt"
-      ]
-    }
-  ]
+  []
   $ dune trace cat | jq -s 'include "dune"; [.[] | targetsMatchingFilter(test("unrelated_module"))]'
-  [
-    {
-      "target_files": [
-        "_build/default/.consumer_lib.objs/byte/unrelated_module.cmi",
-        "_build/default/.consumer_lib.objs/byte/unrelated_module.cmo",
-        "_build/default/.consumer_lib.objs/byte/unrelated_module.cmt"
-      ]
-    }
-  ]
+  []

test/blackbox-tests/test-cases/per-module-lib-deps/per-module-include-flags.t

@@ -1,15 +1,12 @@
-Observational baseline: a consumer module's compile command
-currently carries [-I] flags for every library in the cctx-wide
-[(libraries ...)] closure, regardless of which libraries the
-module's source actually references.
+A consumer module's compile command carries [-I] flags only for
+the libraries its ocamldep reference set actually reaches, not
+for every library in the cctx-wide [(libraries ...)] closure.
 
 [consumer_lib] declares two library dependencies, [dep_lib] and
 [unrelated_lib], but its only module references just [Dep_module].
-A future per-module filter could observe via ocamldep that the
-module references nothing from [unrelated_lib] and drop that
-library's objdir from the [-I] path. This test records today's
-[-I] contents so that a future filter improvement can flip the
-asserted array to a single entry.
+The per-module filter observes via ocamldep that the module
+references nothing from [unrelated_lib] and drops that library's
+objdir from the [-I] path; only [dep_lib]'s objdir survives.
 
   $ cat > dune-project <<EOF
   > (lang dune 3.0)
@@ -40,12 +37,10 @@ asserted array to a single entry.
 Inspect the [-I] flags on [consumer_module]'s [.cmo] compile rule.
 Filter to objdir-shaped paths so we don't print the consumer's
 own [.consumer_lib.objs/byte] entry — that's always present and
-not what this test is about. Today both [dep_lib]'s objdir and
-[unrelated_lib]'s objdir appear:
+not what this test is about. Only [dep_lib]'s objdir appears:
 
   $ dune rules --root . --format=json _build/default/.consumer_lib.objs/byte/consumer_module.cmo \
   > | jq 'include "dune"; .[] | [ruleActionFlagValues("-I") | select(test("\\.dep_lib\\.objs|\\.unrelated_lib\\.objs"))]'
   [
-    ".dep_lib.objs/byte",
-    ".unrelated_lib.objs/byte"
+    ".dep_lib.objs/byte"
   ]