refactor: link time code gen cleanups (#7559)

* tighten up the scopes of variables
* make the code generation a little lazier by delaying it until the
  rules are executed

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

Author: rgrinberg

src/dune_rules/link_time_code_gen.ml

@@ -138,17 +138,11 @@ let build_info_code cctx ~libs ~api_version =
             | Installed_private | Installed -> Memo.return ("None", placeholders)
             | Public (_, p) -> version_of_package placeholders p
             | Private _ ->
-              let p =
-                Lib.info lib |> Lib_info.obj_dir |> Obj_dir.dir
-                |> Path.drop_build_context_exn
-              in
-              placeholder placeholders p)
+              Lib.info lib |> Lib_info.obj_dir |> Obj_dir.dir
+              |> Path.drop_build_context_exn |> placeholder placeholders)
         in
         ((Lib.name lib, v) :: libs, placeholders))
   in
-  let libs = List.rev libs in
-  let context = Compilation_context.context cctx in
-  let ocaml_version = Ocaml.Version.of_ocaml_config context.ocaml_config in
   let buf = Buffer.create 1024 in
   (* Parse the replacement format described in [artifact_substitution.ml]. *)
   pr buf
@@ -167,6 +161,8 @@ let build_info_code cctx ~libs ~api_version =
 [@@inline never]
 |ocaml};
   let fmt_eval : _ format6 =
+    let context = Compilation_context.context cctx in
+    let ocaml_version = Ocaml.Version.of_ocaml_config context.ocaml_config in
     if Ocaml.Version.has_sys_opaque_identity ocaml_version then
       "let %s = eval (Sys.opaque_identity %S)"
     else "let %s = eval %S"
@@ -177,7 +173,7 @@ let build_info_code cctx ~libs ~api_version =
   if not (Path.Source.Map.is_empty placeholders) then pr buf "";
   pr buf "let version = %s" version;
   pr buf "";
-  prlist buf "statically_linked_libraries" libs ~f:(fun (name, v) ->
+  prlist buf "statically_linked_libraries" (List.rev libs) ~f:(fun (name, v) ->
       pr buf "%S, %s" (Lib_name.to_string name) v);
   Buffer.contents buf
 
@@ -222,8 +218,10 @@ let handle_special_libs cctx =
   let ( let& ) m f = Resolve.Memo.bind m ~f in
   let& all_libs = Compilation_context.requires_link cctx in
   let obj_dir = Compilation_context.obj_dir cctx |> Obj_dir.of_local in
-  let sctx = Compilation_context.super_context cctx in
-  let ctx = Super_context.context sctx in
+  let ctx =
+    let sctx = Compilation_context.super_context cctx in
+    Super_context.context sctx
+  in
   let open Memo.O in
   let* builtins =
     let+ findlib =
@@ -247,7 +245,8 @@ let handle_special_libs cctx =
               ~obj_name:None
               ~code:
                 (Action_builder.of_memo
-                   (build_info_code cctx ~libs:all_libs ~api_version))
+                   (let* () = Memo.return () in
+                    build_info_code cctx ~libs:all_libs ~api_version))
               ~requires:(Resolve.Memo.return [ lib ])
               ~precompiled_cmi:true
           in
@@ -276,10 +275,10 @@ let handle_special_libs cctx =
             in
             generate_and_compile_module ~obj_name cctx ~lib ~name
               ~code:
-                (Action_builder.return
-                   (findlib_init_code
-                      ~preds:Findlib.findlib_predicates_set_by_dune
-                      ~libs:all_libs))
+                (Action_builder.delayed (fun () ->
+                     findlib_init_code
+                       ~preds:Findlib.findlib_predicates_set_by_dune
+                       ~libs:all_libs))
               ~requires ~precompiled_cmi:false
           in
           process_libs libs
@@ -290,10 +289,9 @@ let handle_special_libs cctx =
           process_libs libs ~to_link_rev:(Lib lib :: to_link_rev) ~force_linkall
         | Dune_site { data_module; plugins } ->
           let code =
-            if plugins then
-              Action_builder.return
-                (dune_site_plugins_code ~libs:all_libs ~builtins)
-            else Action_builder.return (dune_site_code ())
+            Action_builder.delayed @@ fun () ->
+            if plugins then dune_site_plugins_code ~libs:all_libs ~builtins
+            else dune_site_code ()
           in
           let& module_ =
             generate_and_compile_module cctx ~obj_name:None ~name:data_module