Indexing rules: correctly declare indexes dependencies (ocaml#10623)

Signed-off-by: Ulysse Gérard <thevoodoos@gmail.com>

doc/changes/10623.md

@@ -0,0 +1,2 @@
+- Correctly declare dependencies of indexes so that they are rebuilt when
+  needed. (#10623, @voodoos)

src/dune_rules/merlin/ocaml_index.ml

@@ -4,9 +4,11 @@ let ocaml_index sctx ~dir =
   Super_context.resolve_program ~loc:None ~dir sctx "ocaml-index"
 ;;
 
+let index_file_name = "cctx.ocaml-index"
+
 let index_path_in_obj_dir obj_dir =
   let dir = Obj_dir.obj_dir obj_dir in
-  Path.Build.relative dir "cctx.ocaml-index"
+  Path.Build.relative dir index_file_name
 ;;
 
 let project_index ~build_dir = Path.Build.relative build_dir "project.ocaml-index"
@@ -36,12 +38,27 @@ let cctx_rules cctx =
       in
       Lib_flags.L.include_flags non_compile_libs (Lib_mode.Ocaml Byte)
     in
+    (* Indexing depends (recursively) on [required_compile] libs:
+       - These libs's cmt files should be built before indexing starts
+       - If these libs are rebuilt a re-indexation is needed *)
+    let other_indexes_deps =
+      let open Resolve.Memo.O in
+      let+ requires_compile = Compilation_context.requires_compile cctx in
+      let deps =
+        List.filter_map requires_compile ~f:(fun l ->
+          match Lib.info l |> Lib_info.obj_dir |> Obj_dir.obj_dir with
+          | In_build_dir dir -> Some (Path.relative (Path.build dir) index_file_name)
+          | _ -> None)
+      in
+      Command.Args.Hidden_deps (Dep.Set.of_files deps)
+    in
     let context_dir =
       Compilation_context.context cctx
       |> Context.name
       |> Context_name.build_dir
       |> Path.build
     in
+    (* Indexation also depends on the current stanza's modules *)
     let modules_deps =
       let cm_kind = Lib_mode.Cm_kind.(Ocaml Cmi) in
       (* We only index occurrences in user-written modules *)
@@ -64,6 +81,7 @@ let cctx_rules cctx =
       ; Target fn
       ; Deps modules_deps
       ; Dyn (Resolve.Memo.read additional_libs)
+      ; Dyn (Resolve.Memo.read other_indexes_deps)
       ]
   in
   Super_context.add_rule sctx ~dir aggregate

test/blackbox-tests/utils/ocaml_index.ml

@@ -29,7 +29,7 @@ module Common = struct
 end
 
 module Aggregate = struct
-  let from_files _ _ output_file _ _ () = touch output_file
+  let from_files _ _ output_file _ _ _ () = touch output_file
 
   let root =
     let doc = "if provided all locations will be appended to that path" in
@@ -51,13 +51,19 @@ module Aggregate = struct
     Arg.(value & flag & info [ "store-shapes" ] ~doc)
   ;;
 
+  let no_cmt =
+    let doc = "" in
+    Arg.(value & flag & info [ "no-cmt-load-path" ] ~doc)
+  ;;
+
   let term =
     Term.(
       const from_files
       $ store_shapes
       $ root
       $ Common.output_file
       $ build_path
+      $ no_cmt
       $ files
       $ Common.with_log)
   ;;