Protect LTCG placeholders with Sys.opaque_identity

This ensures that they are not inlined when using flambda.
See discussion in #1930.

Signed-off-by: Etienne Millon <me@emillon.org>

Author: emillon

CHANGES.md

@@ -27,6 +27,9 @@ next
 - Allow the use of the `context_name` variable in the `enabled_if` fields of
   executable(s) and install stanzas. (#3568, fixes #3566, @voodoos)
 
+- Fix dune-build-info not working correctly when flambda is active (#3599,
+  @emillon, @jeremiedimino).
+
 2.6.0 (05/06/2020)
 ------------------
 

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

@@ -160,7 +160,7 @@ let build_info_code cctx ~libs ~api_version =
   pr buf "[@@inline never]";
   pr buf "";
   Path.Source.Map.iteri !placeholders ~f:(fun path var ->
-      pr buf "let %s = eval %S" var
+      pr buf "let %s = eval (Sys.opaque_identity %S)" 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;