Add customizable to display raw bytes as hex

* src/xdisp.c (get_next_display_element): Dispatch used format string
for unprintables based on new display-raw-bytes-as-hex variable.
(display-raw-bytes-as-hex): New variable.  (Bug#27122)

* lisp/cus-start.el: Add defcustom form for display-raw-bytes-as-hex.

* doc/emacs/display.texi: Document the new variable.
* etc/NEWS: Mention display-raw-bytes-as-hex.

* test/manual/redisplay-testsuite.el (test-redisplay-5-toggle)
(test-redisplay-5): New tests.
(test-redisplay): Call test-redisplay-5.

1 files changed, 13 insertions, 3 deletions

diff --git a/src/xdisp.c b/src/xdisp.c
index eaa701e9cf1..53210e5be5b 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -7055,7 +7055,7 @@ get_next_display_element (struct it *it)
              translated too.
 
              Non-printable characters and raw-byte characters are also
-             translated to octal form.  */
+             translated to octal or hexadecimal form.  */
           if (((c < ' ' || c == 127) /* ASCII control chars.  */
                ? (it->area != TEXT_AREA
                   /* In mode line, treat \n, \t like other crl chars.  */
@@ -7162,9 +7162,12 @@ get_next_display_element (struct it *it)
                 int len, i;
 
                 if (CHAR_BYTE8_P (c))
-                  /* Display \200 instead of \17777600.  */
+                  /* Display \200 or \x80 instead of \17777600.  */
                   c = CHAR_TO_BYTE8 (c);
-                len = sprintf (str, "%03o", c + 0u);
+                const char *format_string = display_raw_bytes_as_hex
+                                            ? "x%02x"
+                                            : "%03o";
+                len = sprintf (str, format_string, c + 0u);
 
                 XSETINT (it->ctl_chars[0], escape_glyph);
                 for (i = 0; i < len; i++)
@@ -32231,6 +32234,13 @@ display table takes effect; in this case, Emacs does not consult
   /* Initialize to t, since we need to disable reordering until
      loadup.el successfully loads charprop.el.  */
   redisplay__inhibit_bidi = true;
+
+  DEFVAR_BOOL ("display-raw-bytes-as-hex", display_raw_bytes_as_hex,
+    doc: /* Non-nil means display raw bytes in hexadecimal format.
+The default is to use octal format (\200) whereas hexadecimal (\x80)
+may be more familar to users.  */);
+  display_raw_bytes_as_hex = false;
+
 }