[Opam] Don't generate the dune-site build part when not needed

       Fixes #4849

Signed-off-by: François Bobot <francois.bobot@cea.fr>

Author: bobot

otherlibs/site/test/run.t

@@ -150,11 +150,10 @@ Test with an opam like installation
       name
       "-j"
       jobs
-      "--promote-install-files"
-      "false"
       "@install"
       "@runtest" {with-test}
       "@doc" {with-doc}
+      "--promote-install-files=false"
     ]
     ["dune" "install" "-p" name "--create-install-files" name]
   ]

src/dune_engine/dune_project.ml

@@ -293,12 +293,15 @@ module Extension = struct
   let register_deleted ~name ~deleted_in =
     Table.add_exn extensions name (Deleted_in deleted_in)
 
-  let register_simple syntax stanzas =
+  let register_unit syntax stanzas =
     let unit_stanzas =
       let+ r = stanzas in
       ((), r)
     in
-    let (_ : unit t) = register syntax unit_stanzas Unit.to_dyn in
+    register syntax unit_stanzas Unit.to_dyn
+
+  let register_simple syntax stanzas =
+    let (_ : unit t) = register_unit syntax stanzas in
     ()
 
   let instantiate ~dune_lang_ver ~loc ~parse_args (name_loc, name) (ver_loc, ver)
@@ -841,10 +844,10 @@ let executables_implicit_empty_intf t = t.executables_implicit_empty_intf
 
 let accept_alternative_dune_file_name t = t.accept_alternative_dune_file_name
 
-let () =
-  let open Dune_lang.Decoder in
-  Extension.register_simple Action_plugin.syntax (return []);
-  Extension.register_simple Section.dune_site_syntax (return [])
+let () = Extension.register_simple Action_plugin.syntax (return [])
+
+let dune_site_extension =
+  Extension.register_unit Section.dune_site_syntax (return [])
 
 let strict_package_deps t = t.strict_package_deps
 

src/dune_engine/dune_project.mli

@@ -175,3 +175,5 @@ val info : t -> Package.Info.t
     [dune-project] file. *)
 val update_execution_parameters :
   t -> Execution_parameters.t -> Execution_parameters.t
+
+val dune_site_extension : unit Extension.t

src/dune_rules/opam_create.ml

@@ -43,31 +43,50 @@ let default_build_command =
 ]
 |}))
   and from_2_9 =
-    lazy
-      (Opam_file.parse_value
-         (Lexbuf.from_string ~fname:"<internal>"
-            {|
-[
-  [ "dune" "subst" ] {dev}
-  [ "dune" "build" "-p" name "-j" jobs "--promote-install-files" "false"
+    let fname = "<internal>" in
+    let parse s = lazy (Opam_file.parse_value (Lexbuf.from_string ~fname s)) in
+    let subst = parse {| [ "dune" "subst" ] {dev} |} in
+    let build =
+      parse
+        {|
+  [ "dune" "build" "-p" name "-j" jobs
       "@install"
       "@runtest" {with-test}
       "@doc" {with-doc}
   ]
-  [ "dune" "install" "-p" name "--create-install-files" name ]
-]
-|}))
+|}
+    in
+    let install =
+      parse {| [ "dune" "install" "-p" name "--create-install-files" name ] |}
+    in
+    fun ~with_sites ->
+      let dumb_pos = (fname, 0, 0) in
+      if with_sites then
+        let build =
+          match Lazy.force build with
+          | OpamParserTypes.List (op, l) ->
+            OpamParserTypes.List
+              (op, l @ [ String (op, "--promote-install-files=false") ])
+          | _ -> assert false
+        in
+        OpamParserTypes.List
+          (dumb_pos, [ Lazy.force subst; build; Lazy.force install ])
+      else
+        OpamParserTypes.List (dumb_pos, [ Lazy.force subst; Lazy.force build ])
   in
   fun project ->
-    Lazy.force
-      (if Dune_project.dune_version project < (1, 11) then
-        before_1_11
-      else if Dune_project.dune_version project < (2, 7) then
-        from_1_11_before_2_7
-      else if Dune_project.dune_version project < (2, 9) then
-        from_2_7
-      else
-        from_2_9)
+    if Dune_project.dune_version project < (1, 11) then
+      Lazy.force before_1_11
+    else if Dune_project.dune_version project < (2, 7) then
+      Lazy.force from_1_11_before_2_7
+    else if Dune_project.dune_version project < (2, 9) then
+      Lazy.force from_2_7
+    else
+      from_2_9
+        ~with_sites:
+          (Option.is_some
+             (Dune_project.find_extension_args project
+                Dune_project.dune_site_extension))
 
 let package_fields
     { Package.synopsis

test/blackbox-tests/test-cases/dune-project-meta/main.t/run.t

@@ -517,3 +517,13 @@ the doc dependencies:
     "something"
     "odoc" {with-doc}
   ]
+
+  $ cat > dune-project <<EOF
+  > (lang dune 2.9)
+  > (name foo)
+  > (generate_opam_files true)
+  > (package (name foo))
+  > EOF
+
+  $ dune build foo.opam
+  $ grep -A15 ^build: foo.opam