Raise an error when type nonrec is encountered

This feature has two different implementations in the AST:

- in 4.02.3, it is an annotation on a type,
- in >= 4.03.0 it is straight in the constructor.

Note that this can break existing programs; however relying on
`ppx_deriving` to do the right thing seems dangerous.
Maybe an escape hatch could be added so that the recursive version is
derived nonetheless.

It is an intermediate solution to #116. Solving it is tricky because it
would be necessary to expose this to the plugins, breaking the API. It
is probably easier to do so once 4.02.3 support is dropped, since the
per-type semantics are difficult to handle.

src/ppx_deriving.cppo.ml

@@ -462,6 +462,21 @@ let module_from_input_name () =
   | "//toplevel//" -> []
   | filename -> [String.capitalize (Filename.(basename (chop_suffix filename ".ml")))]
 
+let pstr_desc_rec_flag pstr =
+  match pstr with
+  | Pstr_type(rec_flag, typ_decls) ->
+#if OCAML_VERSION < (4, 03, 0)
+    begin
+      if List.exists (fun ty -> has_attr "nonrec" ty.ptype_attributes) typ_decls then
+        Nonrecursive
+      else
+        Recursive
+    end
+#else
+    rec_flag
+#endif
+  | _ -> assert false
+
 let mapper =
   let module_nesting = ref [] in
   let with_module name f =
@@ -496,6 +511,10 @@ let mapper =
   in
   let structure mapper items =
     match items with
+    | { pstr_desc = Pstr_type(_, typ_decls) as pstr_desc ; pstr_loc } :: rest when
+        List.exists (fun ty -> has_attr "deriving" ty.ptype_attributes) typ_decls
+        && pstr_desc_rec_flag pstr_desc = Nonrecursive ->
+      raise_errorf ~loc:pstr_loc "The nonrec flag is not supported by ppx_deriving"
     | { pstr_desc = Pstr_type(_, typ_decls); pstr_loc } as item :: rest when
         List.exists (fun ty -> has_attr "deriving" ty.ptype_attributes) typ_decls ->
       let derived =