bug#31676: 27.0.50; More helpful error message for unescaped character literals

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 2 Jun 2018 12:00:26 +0200
> Cc: 31676@debbugs.gnu.org
> 
> Sounds reasonable, here's a patch. 
> 
> From 4bd8348753980be95dc4bcba47e52f7f79255fb6 Mon Sep 17 00:00:00 2001
> From: Philipp Stephani <phst@google.com>
> Date: Sat, 2 Jun 2018 11:59:02 +0200
> Subject: [PATCH] Make warning about unescaped character literals more helpful.
> 
> See Bug#31676.
> 
> * src/lread.c (Flread_unescaped_character_literals_warning): New
> defun.
> (load_warn_unescaped_character_literals): Use it.
> (syms_of_lread): Define new defun.  Unintern internal
> variable, which is not used any more outside of lread.c
> 
> * lisp/emacs-lisp/bytecomp.el (byte-compile-from-buffer): Use new
> defun.
> 
> * test/src/lread-tests.el (lread-tests--unescaped-char-literals):
> test/lisp/emacs-lisp/bytecomp-tests.el
> (bytecomp-tests--unescaped-char-literals): Adapt unit tests.

Sounds okay, but can you tell why you implemented
lread--unescaped-character-literals in C?  If that's because you need
to call it from load_warn_unescaped_character_literals, then C
functions can call Lisp functions with no problems, we have several
examples of that in the sources.  AFAICT, the C implementation is just
a "transliteration" of straightforward Lisp code, so it reads strange.