Filter inter-library dependencies per module

doc/changes/added/14492.md

@@ -0,0 +1,7 @@
+- Filter inter-library dependencies per-module using `ocamldep` output, and
+  match the `-I`/`-H` include flags to that filter. When a dependency
+  library's cmi changes, only consumer modules that reference it get rebuilt
+  — eliminating spurious recompilations of unrelated sibling modules.
+  Applies to unwrapped dependency libraries; wrapped libraries continue to
+  use a glob over the whole library's objdir. (#14492, fixes #4572,
+  @robinbb)

src/dune_lang/lib_mode.ml

@@ -11,6 +11,12 @@ let equal x y =
   | Melange, Melange -> true
 ;;
 
+let hash = function
+  | Ocaml Byte -> 0
+  | Ocaml Native -> 1
+  | Melange -> 2
+;;
+
 let decode =
   let open Decoder in
   enum [ "byte", Ocaml Byte; "native", Ocaml Native; "melange", Melange ]

src/dune_lang/lib_mode.mli

@@ -6,6 +6,7 @@ type t =
 
 val decode : t Decoder.t
 val equal : t -> t -> bool
+val hash : t -> int
 val to_dyn : t -> Dyn.t
 
 module Cm_kind : sig

src/dune_rules/cinaps.ml

@@ -207,6 +207,11 @@ let gen_rules sctx t ~dir ~scope =
       in
       let requires_compile = Lib.Compile.direct_requires compile_info ~for_ in
       let requires_link = Lib.Compile.requires_link compile_info ~for_ in
+      let pps_runtime_libs =
+        let open Resolve.Memo.O in
+        let* pps = Lib.Compile.pps compile_info ~for_ in
+        Resolve.Memo.List.concat_map pps ~f:(Lib.ppx_runtime_deps ~for_)
+      in
       let obj_dir = Obj_dir.make_exe ~dir:cinaps_dir ~name in
       Compilation_context.create
         for_
@@ -217,6 +222,7 @@ let gen_rules sctx t ~dir ~scope =
         ~opaque:(Explicit false)
         ~requires_compile
         ~requires_link
+        ~pps_runtime_libs
         ~flags
         ~js_of_ocaml:(Js_of_ocaml.Mode.Pair.make None)
         ~melange_package_name:None