Improve error reporting when serializing non-Unicode strings to JSON

* src/coding.c (utf8_string_p): New helper function.
(syms_of_coding) <utf-8-unix>: Move from json.c.

* src/json.c (json_check_utf8): New helper function.
(lisp_to_json_toplevel_1, lisp_to_json): Use it.  To save a bit of
time, check for invalid UTF-8 strings only after encountering an
error, since Jansson already rejects them.

* test/src/json-tests.el (json-serialize/invalid-unicode): Adapt
expected error symbol.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/coding.h b/src/coding.h
index 66d125b07e6..bc4ef52e1ed 100644
--- a/src/coding.h
+++ b/src/coding.h
@@ -665,6 +665,7 @@ struct coding_system
 /* Extern declarations.  */
 extern Lisp_Object code_conversion_save (bool, bool);
 extern bool encode_coding_utf_8 (struct coding_system *);
+extern bool utf8_string_p (Lisp_Object);
 extern void setup_coding_system (Lisp_Object, struct coding_system *);
 extern Lisp_Object coding_charset_list (struct coding_system *);
 extern Lisp_Object coding_system_charset_list (Lisp_Object);