Add option to pv_uni_display for better tr/// output

tr// has a special malformed UTF-8 character as a sentinel;  Teach
pv_uni_display about that.

utf8.c

@@ -4748,7 +4748,13 @@ See also L</sv_uni_display>.
 =for apidoc Amnh||UNI_DISPLAY_QQ
 =for apidoc Amnh||UNI_DISPLAY_REGEX
 =cut
+
+Undocumented is UNI_DISPLAY_TR_ which is used internally to display an operand
+of the tr/// operation.  These operands have a peculiar, deliberate UTF-8
+malformation which this flag enables the proper handling of.  It turns on
+ISPRINT and BACKSLASH as well.
 */
+
 char *
 Perl_pv_uni_display(pTHX_ SV *dsv, const U8 *spv, STRLEN len, STRLEN pvlim,
                           UV flags)
@@ -4770,6 +4776,14 @@ Perl_pv_uni_display(pTHX_ SV *dsv, const U8 *spv, STRLEN len, STRLEN pvlim,
              break;
         }
 
+        /* The minus is unambiguously the range indicator within a UTF-8 tr///
+         * operand */
+        if (UNLIKELY(flags & UNI_DISPLAY_TR_ && *s == ILLEGAL_UTF8_BYTE)) {
+            sv_catpvs(dsv, "-");
+            next_len = 1;
+            continue;
+        }
+
         u = utf8_to_uvchr_buf(s, e, &next_len);
         assert(next_len > 0);
 

utf8.h

@@ -1318,6 +1318,9 @@ point's representation.
 #define UNI_DISPLAY_BACKSLASH	0x0002
 #define UNI_DISPLAY_BACKSPACE	0x0004  /* Allow \b when also
                                            UNI_DISPLAY_BACKSLASH */
+#define UNI_DISPLAY_TR_       ( 0x0008                              \
+                               |UNI_DISPLAY_ISPRINT                 \
+                               |UNI_DISPLAY_BACKSLASH)
 #define UNI_DISPLAY_QQ		(UNI_DISPLAY_ISPRINT                \
                                 |UNI_DISPLAY_BACKSLASH              \
                                 |UNI_DISPLAY_BACKSPACE)