vim-patch:8.1.0759: showing two characters for tab is limited

Problem:    Showing two characters for tab is limited.
Solution:   Allow for a third character for "tab:" in 'listchars'. (Nathaniel
            Braun, Ken Takata, closes vim/vim#3810)

https://github.com/vim/vim/commit/83a52171ba00b2b9fd2d1d22a07e38fc9fc69c1e

6 files changed, 115 insertions, 25 deletions

diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 61482282f6..732305b6aa 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -3662,11 +3662,26 @@ A jump table for the options with a short description can be found at |Q_op|.
 			omitted, there is no extra character at the end of the
 			line.
 							*lcs-tab*
-	  tab:xy	Two characters to be used to show a tab.  The first
-			char is used once.  The second char is repeated to
-			fill the space that the tab normally occupies.
-			"tab:>-" will show a tab that takes four spaces as
-			">---".  When omitted, a tab is show as ^I.
+	  tab:xy[z]	Two or three characters to be used to show a tab.
+			The third character is optional.
+
+	  tab:xy	The 'x' is always used, then 'y' as many times as will
+			fit.  Thus "tab:>-" displays:
+				>
+				>-
+				>--
+				etc.
+
+	  tab:xyz	The 'z' is always used, then 'x' is prepended, and
+			then 'y' is used as many times as will fit.  Thus
+			"tab:<->" displays:
+				>
+				<>
+				<->
+				<-->
+				etc.
+
+			When "tab:" is omitted, a tab is shown as ^I.
 							*lcs-space*
 	  space:c	Character to show for a space.  When omitted, spaces
 			are left blank.
diff --git a/src/nvim/buffer_defs.h b/src/nvim/buffer_defs.h
index a284ed5292..09d22c4233 100644
--- a/src/nvim/buffer_defs.h
+++ b/src/nvim/buffer_defs.h
@@ -1016,6 +1016,7 @@ struct window_S {
     int space;
     int tab1;                       ///< first tab character
     int tab2;                       ///< second tab character
+    int tab3;                       ///< third tab character
     int trail;
     int conceal;
   } w_p_lcs_chars;
diff --git a/src/nvim/message.c b/src/nvim/message.c
index c83c013225..1330460867 100644
--- a/src/nvim/message.c
+++ b/src/nvim/message.c
@@ -1517,10 +1517,11 @@ void msg_prt_line(char_u *s, int list)
   int col = 0;
   int n_extra = 0;
   int c_extra = 0;
-  char_u      *p_extra = NULL;              /* init to make SASC shut up */
+  int c_final = 0;
+  char_u *p_extra = NULL;  // init to make SASC shut up
   int n;
   int attr = 0;
-  char_u      *trail = NULL;
+  char_u *trail = NULL;
   int l;
 
   if (curwin->w_p_list) {
@@ -1543,7 +1544,9 @@ void msg_prt_line(char_u *s, int list)
   while (!got_int) {
     if (n_extra > 0) {
       n_extra--;
-      if (c_extra) {
+      if (n_extra == 0 && c_final) {
+        c = c_final;
+      } else if (c_extra) {
         c = c_extra;
       } else {
         assert(p_extra != NULL);
@@ -1572,9 +1575,13 @@ void msg_prt_line(char_u *s, int list)
         if (!list) {
           c = ' ';
           c_extra = ' ';
+          c_final = NUL;
         } else {
-          c = curwin->w_p_lcs_chars.tab1;
+          c = (n_extra == 0 && curwin->w_p_lcs_chars.tab3)
+              ? curwin->w_p_lcs_chars.tab3
+              : curwin->w_p_lcs_chars.tab1;
           c_extra = curwin->w_p_lcs_chars.tab2;
+          c_final = curwin->w_p_lcs_chars.tab3;
           attr = HL_ATTR(HLF_8);
         }
       } else if (c == 160 && list && curwin->w_p_lcs_chars.nbsp != NUL) {
@@ -1583,6 +1590,7 @@ void msg_prt_line(char_u *s, int list)
       } else if (c == NUL && list && curwin->w_p_lcs_chars.eol != NUL) {
         p_extra = (char_u *)"";
         c_extra = NUL;
+        c_final = NUL;
         n_extra = 1;
         c = curwin->w_p_lcs_chars.eol;
         attr = HL_ATTR(HLF_AT);
@@ -1591,6 +1599,7 @@ void msg_prt_line(char_u *s, int list)
         n_extra = n - 1;
         p_extra = transchar_byte(c);
         c_extra = NUL;
+        c_final = NUL;
         c = *p_extra++;
         /* Use special coloring to be able to distinguish <hex> from
          * the same in plain text. */
diff --git a/src/nvim/option.c b/src/nvim/option.c
index 77728a16e8..d8e5b78e80 100644
--- a/src/nvim/option.c
+++ b/src/nvim/option.c
@@ -3405,7 +3405,7 @@ static char_u *set_chars_option(win_T *wp, char_u **varp)
 {
   int round, i, len, entries;
   char_u *p, *s;
-  int c1, c2 = 0;
+  int c1 = 0, c2 = 0, c3 = 0;
 
   struct chars_tab {
     int     *cp;    ///< char value
@@ -3462,6 +3462,7 @@ static char_u *set_chars_option(win_T *wp, char_u **varp)
       }
       if (varp == &wp->w_p_lcs) {
         wp->w_p_lcs_chars.tab1 = NUL;
+        wp->w_p_lcs_chars.tab3 = NUL;
       }
     }
     p = *varp;
@@ -3471,6 +3472,7 @@ static char_u *set_chars_option(win_T *wp, char_u **varp)
         if (STRNCMP(p, tab[i].name, len) == 0
             && p[len] == ':'
             && p[len + 1] != NUL) {
+          c1 = c2 = c3 = 0;
           s = p + len + 1;
 
           // TODO(bfredl): use schar_T representation and utfc_ptr2len
@@ -3488,12 +3490,20 @@ static char_u *set_chars_option(win_T *wp, char_u **varp)
             if (mb_char2cells(c2) > 1 || (c2len == 1 && c2 > 127)) {
               continue;
             }
+            if (!(*s == ',' || *s == NUL)) {
+              int c3len = utf_ptr2len(s);
+              c3 = mb_cptr2char_adv((const char_u **)&s);
+              if (mb_char2cells(c3) > 1 || (c3len == 1 && c3 > 127)) {
+                continue;
+              }
+            }
           }
           if (*s == ',' || *s == NUL) {
             if (round) {
               if (tab[i].cp == &wp->w_p_lcs_chars.tab2) {
                 wp->w_p_lcs_chars.tab1 = c1;
                 wp->w_p_lcs_chars.tab2 = c2;
+                wp->w_p_lcs_chars.tab3 = c3;
               } else if (tab[i].cp != NULL) {
                 *(tab[i].cp) = c1;
               }
diff --git a/src/nvim/screen.c b/src/nvim/screen.c
index c1fb001cc4..29a71fc0e1 100644
--- a/src/nvim/screen.c
+++ b/src/nvim/screen.c
@@ -2055,6 +2055,7 @@ win_line (
   char_u      *p_extra = NULL;        // string of extra chars, plus NUL
   char_u      *p_extra_free = NULL;   // p_extra needs to be freed
   int c_extra = NUL;                  // extra chars, all the same
+  int c_final = NUL;                  // final char, mandatory if set
   int extra_attr = 0;                 // attributes when n_extra != 0
   static char_u *at_end_str = (char_u *)"";  // used for p_extra when displaying
                                              // curwin->w_p_lcs_chars.eol at
@@ -2066,6 +2067,7 @@ win_line (
   int saved_n_extra = 0;
   char_u      *saved_p_extra = NULL;
   int saved_c_extra = 0;
+  int saved_c_final = 0;
   int saved_char_attr = 0;
 
   int n_attr = 0;                       /* chars with special attr */
@@ -2644,6 +2646,7 @@ win_line (
           /* Draw the cmdline character. */
           n_extra = 1;
           c_extra = cmdwin_type;
+          c_final = NUL;
           char_attr = win_hl_attr(wp, HLF_AT);
         }
       }
@@ -2662,6 +2665,7 @@ win_line (
           p_extra_free[n_extra] = NUL;
           p_extra = p_extra_free;
           c_extra = NUL;
+          c_final = NUL;
           char_attr = win_hl_attr(wp, HLF_FC);
         }
       }
@@ -2675,6 +2679,7 @@ win_line (
               int text_sign;
               // Draw cells with the sign value or blank.
               c_extra = ' ';
+              c_final = NUL;
               char_attr = win_hl_attr(wp, HLF_SC);
               n_extra = win_signcol_width(wp);
 
@@ -2685,6 +2690,7 @@ win_line (
                       int symbol_blen = (int)STRLEN(p_extra);
                       if (p_extra != NULL) {
                           c_extra = NUL;
+                          c_final = NUL;
                           // symbol(s) bytes + (filling spaces) (one byte each)
                           n_extra = symbol_blen +
                             (win_signcol_width(wp) - mb_string2cells(p_extra));
@@ -2736,8 +2742,11 @@ win_line (
               rl_mirror(extra);
             p_extra = extra;
             c_extra = NUL;
-          } else
+            c_final = NUL;
+          } else {
             c_extra = ' ';
+            c_final = NUL;
+          }
           n_extra = number_width(wp) + 1;
           char_attr = win_hl_attr(wp, HLF_N);
 
@@ -2795,8 +2804,10 @@ win_line (
           // draw "deleted" diff line(s)
           if (char2cells(wp->w_p_fcs_chars.diff) > 1) {
             c_extra = '-';
+            c_final = NUL;
           } else {
             c_extra = wp->w_p_fcs_chars.diff;
+            c_final = NUL;
           }
           if (wp->w_p_rl) {
             n_extra = col + 1;
@@ -2809,6 +2820,7 @@ win_line (
           /* Draw 'showbreak' at the start of each broken line. */
           p_extra = p_sbr;
           c_extra = NUL;
+          c_final = NUL;
           n_extra = (int)STRLEN(p_sbr);
           char_attr = win_hl_attr(wp, HLF_AT);
           need_showbreak = false;
@@ -2830,6 +2842,7 @@ win_line (
           /* Continue item from end of wrapped line. */
           n_extra = saved_n_extra;
           c_extra = saved_c_extra;
+          c_final = saved_c_final;
           p_extra = saved_p_extra;
           char_attr = saved_char_attr;
         } else {
@@ -3027,20 +3040,18 @@ win_line (
       }
     }
 
-    /*
-     * Get the next character to put on the screen.
-     */
-    /*
-     * The "p_extra" points to the extra stuff that is inserted to
-     * represent special characters (non-printable stuff) and other
-     * things.  When all characters are the same, c_extra is used.
-     * "p_extra" must end in a NUL to avoid mb_ptr2len() reads past
-     * "p_extra[n_extra]".
-     * For the '$' of the 'list' option, n_extra == 1, p_extra == "".
-     */
+    // Get the next character to put on the screen.
+    //
+    // The "p_extra" points to the extra stuff that is inserted to
+    // represent special characters (non-printable stuff) and other
+    // things.  When all characters are the same, c_extra is used.
+    // If c_final is set, it will compulsorily be used at the end.
+    // "p_extra" must end in a NUL to avoid mb_ptr2len() reads past
+    // "p_extra[n_extra]".
+    // For the '$' of the 'list' option, n_extra == 1, p_extra == "".
     if (n_extra > 0) {
-      if (c_extra != NUL) {
-        c = c_extra;
+      if (c_extra != NUL || (n_extra == 1 && c_final != NUL)) {
+        c = (n_extra == 1 && c_final != NUL) ? c_final : c_extra;
         mb_c = c;               // doesn't handle non-utf-8 multi-byte!
         if (enc_utf8 && utf_char2len(c) > 1) {
           mb_utf8 = true;
@@ -3147,6 +3158,7 @@ win_line (
             mb_utf8 = (c >= 0x80);
             n_extra = (int)STRLEN(p_extra);
             c_extra = NUL;
+            c_final = NUL;
             if (area_attr == 0 && search_attr == 0) {
               n_attr = n_extra + 1;
               extra_attr = win_hl_attr(wp, HLF_8);
@@ -3199,6 +3211,7 @@ win_line (
               p_extra = extra;
               n_extra = (int)STRLEN(extra) - 1;
               c_extra = NUL;
+              c_final = NUL;
               c = *p_extra++;
               if (area_attr == 0 && search_attr == 0) {
                 n_attr = n_extra + 1;
@@ -3233,6 +3246,7 @@ win_line (
         if (n_skip > 0 && mb_l > 1 && n_extra == 0) {
           n_extra = 1;
           c_extra = MB_FILLER_CHAR;
+          c_final = NUL;
           c = ' ';
           if (area_attr == 0 && search_attr == 0) {
             n_attr = n_extra + 1;
@@ -3392,6 +3406,7 @@ win_line (
                       - vcol % (int)wp->w_buffer->b_p_ts - 1;
           }
           c_extra = mb_off > 0 ? MB_FILLER_CHAR : ' ';
+          c_final = NUL;
           if (ascii_iswhite(c)) {
             if (c == TAB)
               /* See "Tab alignment" below. */
@@ -3523,12 +3538,15 @@ win_line (
 
           mb_utf8 = false;  // don't draw as UTF-8
           if (wp->w_p_list) {
-            c = wp->w_p_lcs_chars.tab1;
+            c = (n_extra == 0 && wp->w_p_lcs_chars.tab3)
+                 ? wp->w_p_lcs_chars.tab3
+                 : wp->w_p_lcs_chars.tab1;
             if (wp->w_p_lbr) {
               c_extra = NUL; /* using p_extra from above */
             } else {
               c_extra = wp->w_p_lcs_chars.tab2;
             }
+            c_final = wp->w_p_lcs_chars.tab3;
             n_attr = tab_len + 1;
             extra_attr = win_hl_attr(wp, HLF_0);
             saved_attr2 = char_attr;  // save current attr
@@ -3539,6 +3557,7 @@ win_line (
               c = 0xc0;
             }
           } else {
+            c_final = NUL;
             c_extra = ' ';
             c = ' ';
           }
@@ -3566,6 +3585,7 @@ win_line (
               p_extra = at_end_str;
               n_extra = 1;
               c_extra = NUL;
+              c_final = NUL;
             }
           }
           if (wp->w_p_list && wp->w_p_lcs_chars.eol > 0) {
@@ -3593,6 +3613,7 @@ win_line (
           if ((dy_flags & DY_UHEX) && wp->w_p_rl)
             rl_mirror(p_extra);                 /* reverse "<12>" */
           c_extra = NUL;
+          c_final = NUL;
           if (wp->w_p_lbr) {
             char_u *p;
 
@@ -3716,6 +3737,7 @@ win_line (
         /* Double-width character being overwritten by the "precedes"
          * character, need to fill up half the character. */
         c_extra = MB_FILLER_CHAR;
+        c_final = NUL;
         n_extra = 1;
         n_attr = 2;
         extra_attr = win_hl_attr(wp, HLF_AT);
@@ -4231,6 +4253,7 @@ win_line (
       saved_n_extra = n_extra;
       saved_p_extra = p_extra;
       saved_c_extra = c_extra;
+      saved_c_final = c_final;
       saved_char_attr = char_attr;
       n_extra = 0;
       lcs_prec_todo = wp->w_p_lcs_chars.prec;
diff --git a/src/nvim/testdir/test_listchars.vim b/src/nvim/testdir/test_listchars.vim
index 57ea7ca5a9..4899f59910 100644
--- a/src/nvim/testdir/test_listchars.vim
+++ b/src/nvim/testdir/test_listchars.vim
@@ -42,6 +42,38 @@ func Test_listchars()
     call assert_equal([expected[i - 1]], ScreenLines(i, virtcol('$')))
   endfor
 
+  " tab with 3rd character.
+  set listchars-=tab:>-
+  set listchars+=tab:<=>,trail:-
+  let expected = [
+	      \ '<======>aa<====>$',
+	      \ '..bb<==>--$',
+	      \ '...cccc>-$',
+	      \ 'dd........ee--<>$',
+	      \ '-$'
+	      \ ]
+  redraw!
+  for i in range(1, 5)
+    call cursor(i, 1)
+    call assert_equal([expected[i - 1]], ScreenLines(i, virtcol('$')))
+  endfor
+
+  set listchars-=trail:-
+  let expected = [
+	      \ '<======>aa<====>$',
+	      \ '..bb<==>..$',
+	      \ '...cccc>.$',
+	      \ 'dd........ee..<>$',
+	      \ '.$'
+	      \ ]
+  redraw!
+  for i in range(1, 5)
+    call cursor(i, 1)
+    call assert_equal([expected[i - 1]], ScreenLines(i, virtcol('$')))
+  endfor
+
+  set listchars-=tab:<=>
+  set listchars+=tab:>-
   set listchars+=trail:<
   set nolist
   normal ggdG