test(kernel): cover Lang's sexp validation path

test_lang exercised only of_string. Add a sexp round-trip plus a
check_raises proving a malformed-but-plausible code ("e1": right length,
fails the lowercase rule) is rejected by t_of_sexp rather than silently
accepted -- mirrors test_slug_sexp's guard against a future regression to
[@@deriving sexp].

1 files changed, 14 insertions, 1 deletions

diff --git a/test/test_slug.ml b/test/test_slug.ml
index af70e9a..3c17e4a 100644
--- a/test/test_slug.ml
+++ b/test/test_slug.ml
@@ -31,9 +31,22 @@ let test_lang () =
       Alcotest.(check bool) (Printf.sprintf "reject %S" bad) true (Result.is_error (L.of_string bad)))
     [ ""; "e"; "eng"; "EN"; "e1" ]
 
+let test_lang_sexp () =
+  let l = match L.of_string "la" with Ok t -> t | Error e -> Alcotest.failf "%s" e in
+  Alcotest.(check bool) "roundtrip" true (L.equal (L.t_of_sexp (L.sexp_of_t l)) l);
+  (* A malformed lang code in a data file must be rejected, not silently
+     accepted. "e1" is shaped like a lang code (2 chars) but fails the
+     lowercase-a-z rule, so this pins the actual validation, not just the
+     sexp shape. *)
+  Alcotest.check_raises "bad lang in sexp"
+    (Sexplib0.Sexp_conv_error.Of_sexp_error
+       (Failure "lang \"e1\": must be lowercase a-z", Sexplib0.Sexp.Atom "e1"))
+    (fun () -> ignore (L.t_of_sexp (Sexplib0.Sexp.Atom "e1")))
+
 let suite =
   ( "Slug/Lang",
     [ Alcotest.test_case "slug accepts" `Quick test_slug_accepts;
       Alcotest.test_case "slug rejects" `Quick test_slug_rejects;
       Alcotest.test_case "slug sexp validates" `Quick test_slug_sexp;
-      Alcotest.test_case "lang" `Quick test_lang ] )
+      Alcotest.test_case "lang" `Quick test_lang;
+      Alcotest.test_case "lang sexp validates" `Quick test_lang_sexp ] )