Re: Fill column indicator functionality

[Ergus]

Hi Eli:

On Fri, Apr 05, 2019 at 06:28:40PM +0300, Eli Zaretskii wrote:
> > Date: Fri, 05 Apr 2019 16:06:17 +0300
> > From: Eli Zaretskii <address@hidden>
> > Cc: address@hidden
> >
> > Once you can reproduce the problem, I suggest to use the macro
> > FACE_FOR_CHAR to find the ID of a face suitable for displaying the
> > indicator character, instead of using DEFAULT_FACE_ID.  You can find
> > examples of using this macro elsewhere in xdisp.c.
>
> On second thought, I think using FACE_FOR_CHAR would produce
> unpleasant results.  If the font required to display U+2502 is not the
> default font, it could be higher than the default font, which will
> cause the line height become larger on those screen lines where the
> indicator is displayed.  The result will cause complaints.
I have a version already implemented with this solution and I don't
like it for the reasons you expose.

> So I think we should insist on being able to display the indicator
> character by the default face's font.  Whether this is possible could
> be tested by comparing the font returned by internal-char-font and the
> default face's font returned by face-font.  Like this:
>
>  (if (eq (aref (car (internal-char-font nil ?│)) 0)
>          w(face-font 'default))
>       ;; can use the default font for U+2502
>
> (But we should use the actual value of
> display-fill-column-indicator-char instead of the literal character.)
>
> WDYT?

Actually this is what we have now:

(define-minor-mode display-fill-column-indicator-mode
 "Toggle display fill column indicator.
This uses `display-fill-column-indicator' internally.

To change the position of the column displayed by default,
customize `display-fill-column-indicator-column' you can change the
character for the indicator setting `display-fill-column-indicator-character'."
 :lighter nil
 (if display-fill-column-indicator-mode
     (progn
       (setq display-fill-column-indicator t)
       (unless display-fill-column-indicator-character
         (if (char-displayable-p ?\u2502)
             (setq display-fill-column-indicator-character ?\u2502)
           (setq display-fill-column-indicator-character ?|))))
   (setq display-fill-column-indicator nil)))


I think we only need to extend the inner if with your condition, but I
got an error because (internal-char-font nil ?│) always return nil in a
terminal and aref has issues with that.

Debugger entered--Lisp error: (wrong-type-argument arrayp nil)
 aref(nil 0)
 eval((aref (car (internal-char-font nil 9474)) 0) nil)
 elisp--eval-last-sexp(nil)
 #f(compiled-function (eval-last-sexp-arg-internal) "Evaluate sexp before point; print value in the 
echo area.\nInteractively, with a non `-' prefix argument, print output into\ncurrent 
buffer.\n\nNormally, this function truncates long output according to the\nvalue of the variables 
`eval-expression-print-length' and\n`eval-expression-print-level'.  With a prefix argument of 
zero,\nhowever, there is no such truncation.\nInteger values are printed in several formats (decimal, 
octal,\nand hexadecimal).  When the prefix argument is -1 or the value\ndoesn't exceed 
`eval-expression-print-maximum-character', an\ninteger value is also printed as a character of that 
codepoint.\n\nIf `eval-expression-debug-on-error' is non-nil, which is the default,\nthis command 
arranges for all errors to enter the debugger." (interactive "P") #<bytecode 
0x1fdfdd428807>)(nil)
 apply(#f(compiled-function (eval-last-sexp-arg-internal) "Evaluate sexp before point; print value 
in the echo area.\nInteractively, with a non `-' prefix argument, print output into\ncurrent 
buffer.\n\nNormally, this function truncates long output according to the\nvalue of the variables 
`eval-expression-print-length' and\n`eval-expression-print-level'.  With a prefix argument of 
zero,\nhowever, there is no such truncation.\nInteger values are printed in several formats (decimal, 
octal,\nand hexadecimal).  When the prefix argument is -1 or the value\ndoesn't exceed 
`eval-expression-print-maximum-character', an\ninteger value is also printed as a character of that 
codepoint.\n\nIf `eval-expression-debug-on-error' is non-nil, which is the default,\nthis command 
arranges for all errors to enter the debugger." (interactive "P") #<bytecode 
0x1fdfdd428807>) nil)
 eval-last-sexp(nil)
 funcall-interactively(eval-last-sexp nil)
 call-interactively(eval-last-sexp nil nil)
 command-execute(eval-last-sexp)

In a gui I get this error:

Debugger entered--Lisp error: (wrong-type-argument arrayp #<font-object 
"-1ASC-Liberation Mono-normal-normal-normal-*-15-*-*-*-*-0-iso10646-1">)
 aref(#<font-object "-1ASC-Liberation 
Mono-normal-normal-normal-*-15-*-*-*-*-0-iso10646-1"> 0)
 eval((aref (car (internal-char-font nil 9474)) 0) nil)
 elisp--eval-last-sexp(nil)
 eval-last-sexp(nil)
 funcall-interactively(eval-last-sexp nil)
 call-interactively(eval-last-sexp nil nil)
 command-execute(eval-last-sexp)

Could we use something else?