refactor: simplify map_rules (#9147)

Signed-off-by: Rudi Grinberg <[email protected]>

src/dune_rules/gen_rules.ml

@@ -500,8 +500,8 @@ let gen_rules_regular_directory sctx ~components ~dir =
       | Some dir -> Memo.return (Some dir)
       | None -> Source_tree.find_dir (Path.Source.parent_exn src_dir)
     in
-    let* rules =
-      let* rules =
+    let+ rules =
+      let+ rules =
         match st_dir with
         | None -> gen_rules_build_dir sctx ~nearest_src_dir ~dir ~src_dir
         | Some source_dir ->
@@ -533,13 +533,12 @@ let gen_rules_regular_directory sctx ~components ~dir =
             rules.build_dir_only_sub_dirs
             allowed_subdirs
         in
-        Memo.return { rules with build_dir_only_sub_dirs })
+        { rules with build_dir_only_sub_dirs })
     in
     match Opam_create.gen_rules sctx ~dir ~nearest_src_dir ~src_dir with
-    | None -> Memo.return rules
+    | None -> rules
     | Some opam_rules ->
-      Gen_rules.map_rules rules ~f:(fun rules ->
-        Memo.return (Gen_rules.Rules.combine_exn opam_rules rules))
+      Gen_rules.map_rules rules ~f:(Gen_rules.Rules.combine_exn opam_rules)
   and+ melange_rules = Melange_rules.setup_emit_js_rules sctx ~dir in
   Gen_rules.combine melange_rules rules
 ;;

src/dune_rules/import.ml

@@ -125,15 +125,10 @@ module Build_config = struct
     ;;
 
     let map_rules t ~f =
-      let open Memo.O in
       match t with
-      | Unknown_context -> Memo.return Unknown_context
-      | Rules rules ->
-        let+ rules = f rules in
-        Rules rules
-      | Redirect_to_parent rules ->
-        let+ rules = f rules in
-        Redirect_to_parent rules
+      | Unknown_context -> Unknown_context
+      | Rules rules -> Rules (f rules)
+      | Redirect_to_parent rules -> Redirect_to_parent (f rules)
     ;;
 
     let combine x y =