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