fix(duild-info): reapply flambda fix from #3599 removed in #5049

With flambda, Sys.opaque_identity is needed around the placeholder
string or else flambda is able to compile it away. This prevents the
rewriting done by e.g. `dune install` from working.

Signed-off-by: Josh Berdine <josh@berdine.net>

Author: jberdine

otherlibs/build-info/test/run.t

@@ -100,9 +100,9 @@ Check what the generated build info module looks like:
       None
   [@@inline never]
 
-  let p1 = eval "%%DUNE_PLACEHOLDER:64:vcs-describe:1:a%%%%%%%%%%%%%%%%%%%%%%%%%%"
-  let p2 = eval "%%DUNE_PLACEHOLDER:64:vcs-describe:1:b%%%%%%%%%%%%%%%%%%%%%%%%%%"
-  let p0 = eval "%%DUNE_PLACEHOLDER:64:vcs-describe:1:c%%%%%%%%%%%%%%%%%%%%%%%%%%"
+  let p1 = eval (Sys.opaque_identity "%%DUNE_PLACEHOLDER:64:vcs-describe:1:a%%%%%%%%%%%%%%%%%%%%%%%%%%")
+  let p2 = eval (Sys.opaque_identity "%%DUNE_PLACEHOLDER:64:vcs-describe:1:b%%%%%%%%%%%%%%%%%%%%%%%%%%")
+  let p0 = eval (Sys.opaque_identity "%%DUNE_PLACEHOLDER:64:vcs-describe:1:c%%%%%%%%%%%%%%%%%%%%%%%%%%")
 
   let version = p0
 

src/dune_rules/link_time_code_gen.ml

@@ -150,6 +150,8 @@ let build_info_code cctx ~libs ~api_version =
         ((Lib.name lib, v) :: libs, placeholders))
   in
   let libs = List.rev libs in
+  let context = CC.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
@@ -167,7 +169,12 @@ let build_info_code cctx ~libs ~api_version =
     None
 [@@inline never]
 |ocaml};
-  let fmt_eval : _ format6 = "let %s = eval %S" in
+  let fmt_eval : _ format6 =
+    if Ocaml.Version.has_sys_opaque_identity ocaml_version then
+      "let %s = eval (Sys.opaque_identity %S)"
+    else
+      "let %s = eval %S"
+  in
   Path.Source.Map.iteri placeholders ~f:(fun path var ->
       pr buf fmt_eval var
         (Artifact_substitution.encode ~min_len:64 (Vcs_describe path)));