Protect link-time code generation placeholders with Sys.opaque_identity

CHANGES.md

@@ -22,6 +22,8 @@ next
 - `$ dune utop` no longer tries to load optional libraries that are unavailable
   (#3612, fixes #3188, @anuragsoni)
 
+- Fix dune-build-info on 4.10.0+flambda (#3599, @emillon, @jeremiedimino).
+
 2.6.1 (02/07/2020)
 ------------------
 

otherlibs/build-info/test/run.t

@@ -99,9 +99,9 @@ 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/link_time_code_gen.ml

@@ -144,6 +144,8 @@ let build_info_code cctx ~libs ~api_version =
               in
               placeholder p ) ))
   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 "let eval s =";
@@ -159,8 +161,14 @@ let build_info_code cctx ~libs ~api_version =
   pr buf "    None";
   pr buf "[@@inline never]";
   pr buf "";
+  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 "let %s = eval %S" var
+      pr buf fmt_eval var
         (Artifact_substitution.encode ~min_len:64 (Vcs_describe path)));
   if not (Path.Source.Map.is_empty !placeholders) then pr buf "";
   pr buf "let version = %s" version;

src/dune/ocaml_version.ml

@@ -45,3 +45,5 @@ let custom_or_output_complete_exe version =
     "-custom"
 
 let ocamlopt_always_calls_library_linker version = version < (4, 12, 0)
+
+let has_sys_opaque_identity version = version >= (4, 3, 0)

src/dune/ocaml_version.mli

@@ -63,3 +63,6 @@ val custom_or_output_complete_exe : t -> string
 
 (** ocamlopt -a always calls the native C linker, even for empty archives *)
 val ocamlopt_always_calls_library_linker : t -> bool
+
+(** Whether [Sys.opaque_identity] is in the standard library *)
+val has_sys_opaque_identity : t -> bool