Change fallback check to use ?check param

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

src/jbuild.ml

@@ -1129,11 +1129,9 @@ module Rule = struct
     field "deps"    (list Dep_conf.t) ~default:[] >>= fun deps ->
     field "locks"   (list String_with_vars.t) ~default:[] >>= fun locks ->
     map_validate
-      (field_b "fallback" >>= fun fallback ->
-       (if fallback then
-          Syntax.renamed_in Stanza.syntax (1, 0) ~to_:"(mode fallback)"
-        else
-          return ()) >>= fun () ->
+      (field_b
+         ~check:(Syntax.renamed_in Stanza.syntax (1, 0) ~to_:"(mode fallback)")
+         "fallback" >>= fun fallback ->
        field_o "mode" Mode.t >>= fun mode ->
        return (fallback, mode))
       ~f:(function

src/stdune/sexp.ml

@@ -481,9 +481,10 @@ module Of_sexp = struct
     | None ->
       (None, add_known name state)
 
-  let field_b name =
+  let field_b ?check name =
     field name ~default:false
-      (eos >>= function
+      (Option.value check ~default:(return ()) >>= fun () ->
+        eos >>= function
        | true -> return true
        | _ -> bool)
 

src/stdune/sexp.mli

@@ -216,7 +216,7 @@ module Of_sexp : sig
     -> 'a t
     -> 'a option fields_parser
 
-  val field_b : string -> bool fields_parser
+  val field_b : ?check:(unit t) -> string -> bool fields_parser
 
   (** A field that can appear multiple times *)
   val multi_field