bug#50752: 28.0.50; easy-menu-define lowers the menu-bar key

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Mon, 18 Oct 2021 20:22:18 -0700
> Cc: Eli Zaretskii <eliz@gnu.org>, Shuguang Sun <shuguang79@qq.com>, 50752@debbugs.gnu.org
> 
> If anyone has any preferences or further ideas here, that would be much
> appreciated, otherwise I'll keep investigating.
> 
> The attached patch is what I have so far.  It's obviously not yet
> finished, but all tests pass except for the one for "I->i" conversion in
> the Turkish language environment.

Don't give up, you are close.

> +	      memcpy (dst, SSDATA (lc_key), SBYTES (lc_key));
> +	      for (int i = 0; i < SBYTES (lc_key); ++i)
> +		{
> +		  if (*(dst + i) == ' ')
> +		    *(dst + i) = '-';
> +		}

If you want to use an index to walk the string data, as you did above,
please use dst[i] instead of *(dst + i); the latter is correct, but
ugly and un-C-ish.  Or you could use a pointer to walk, like this:

        unsigned char *p = dst, *dst_end = dst + SBYTES (lc_key);
	for ( ; p < dst_end; p++)
	  {
	    if (*p == ' ')
	      *p = '-';
	  }

> > We could use the equivalent of
> >
> >   (get-char-code-property ?I 'lowercase)
> >
> > If the above returns nil, it means the lower-case variant is the
> > character itself.
> >
> > In C, this means to use uniprop_table, like bidi.c and casefiddle.c
> > do.  This accesses the database generated from UnicodeData.txt.
> 
> I didn't try this approach, mostly because it sounds more difficult to
> implement than what Lars said.  I think?  Wouldn't it amount to
> basically re-implementing Fdowncase?  Sorry, I didn't look too closely
> at this.  Perhaps this would be the better approach.

It shouldn't be hard.  You need to call uniprop_table to get a
char-table:

   Lisp_Object unicode_case_table = uniprop_table (intern ("lowercase"));

which you then reference with

   int low_ch = XFIXNUM (CHAR_TABLE_REF (unicode_case_table, ch));

to get the codepoint of the lower-case character that corresponds to
the (possibly upper-case) character whose codepoint is CH.  Then
downcasing a string boils down to a loop that fetches characters one
by one with fetch_string_char_advance and then stores the lower-case
characters, obtained as above, with CHAR_STRING.  (It's a bit more
complicated than that, because CHAR_TABLE_REF can return nil for the
characters that are either already lower-case or don't have case
variants.  And the uniprop_table call should be done once, at startup
time, or upon first usage, and stored in a staticpro'd variable, see
bidi_initialize for an example.)