Interactions between bitfields and packed structs, continued

Reject bitfields in packed structs, like before.

If we let them through, the resulting CompCert C code implements a
strange layout (alignment of regular fields is reduced but alignment
of bitfields is unchanged) that is quite different from that of GCC.
It's better to reject.

Author: xavierleroy

cparser/PackedStructs.ml

@@ -64,7 +64,7 @@ let transf_field_decl mfa swapped loc env struct_id f =
   (* Register as byte-swapped if needed *)
   if swapped then begin
     if f.fld_bitfield <> None then
-      fatal_error loc "cannot byte-swap a bitfield";
+      error loc "byte-swapped bit fields are not supported";
     let (can_swap, must_swap) = can_byte_swap env f.fld_typ in
     if not can_swap then
       fatal_error loc "cannot byte-swap field of type '%a'"
@@ -74,6 +74,8 @@ let transf_field_decl mfa swapped loc env struct_id f =
   end;
   (* Reduce alignment if requested *)
   if mfa = 0 then f else begin
+    if f.fld_bitfield <> None then
+      error loc "bit fields in packed structs are not supported";
     let al = safe_alignof loc env f.fld_typ in
     { f with fld_typ =
          change_attributes_type env (set_alignas_attr (min mfa al)) f.fld_typ }