vim-patch:8.2.2454: leading space can not be made visible (#14138)

Problem:    Leading space can not be made visible.
Solution:   Add "lead:" to 'listchars'. (closes vim/vim#7772)
https://github.com/vim/vim/commit/91478ae49a1b2dc1de63821db731a343e855dcc0

6 files changed, 78 insertions, 7 deletions

diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index decf226b12..04310ca8d4 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -3716,6 +3716,10 @@ A jump table for the options with a short description can be found at |Q_op|.
 							*lcs-space*
 	  space:c	Character to show for a space.  When omitted, spaces
 			are left blank.
+							*lcs-lead*
+	  lead:c	Character to show for leading spaces.  When omitted,
+			leading spaces are blank.  Overrides the "space"
+			setting for leading spaces.
 							*lcs-trail*
 	  trail:c	Character to show for trailing spaces.  When omitted,
 			trailing spaces are blank.  Overrides the "space"
diff --git a/src/nvim/buffer_defs.h b/src/nvim/buffer_defs.h
index a4105e2796..b36b7beab8 100644
--- a/src/nvim/buffer_defs.h
+++ b/src/nvim/buffer_defs.h
@@ -1201,6 +1201,7 @@ struct window_S {
     int tab1;                       ///< first tab character
     int tab2;                       ///< second tab character
     int tab3;                       ///< third tab character
+    int lead;
     int trail;
     int conceal;
   } w_p_lcs_chars;
diff --git a/src/nvim/message.c b/src/nvim/message.c
index cdac4b602a..dea6696f55 100644
--- a/src/nvim/message.c
+++ b/src/nvim/message.c
@@ -1705,6 +1705,7 @@ void msg_prt_line(char_u *s, int list)
   char_u *p_extra = NULL;  // init to make SASC shut up
   int n;
   int attr = 0;
+  char_u *lead = NULL;
   char_u *trail = NULL;
   int l;
 
@@ -1712,11 +1713,24 @@ void msg_prt_line(char_u *s, int list)
     list = true;
   }
 
-  // find start of trailing whitespace
-  if (list && curwin->w_p_lcs_chars.trail) {
-    trail = s + STRLEN(s);
-    while (trail > s && ascii_iswhite(trail[-1])) {
-      trail--;
+  if (list) {
+    // find start of trailing whitespace
+    if (curwin->w_p_lcs_chars.trail) {
+      trail = s + STRLEN(s);
+      while (trail > s && ascii_iswhite(trail[-1])) {
+        trail--;
+      }
+    }
+    // find end of leading whitespace
+    if (curwin->w_p_lcs_chars.lead) {
+      lead = s;
+      while (ascii_iswhite(lead[0])) {
+        lead++;
+      }
+      // in a line full of spaces all of them are treated as trailing
+      if (*lead == NUL) {
+        lead = NULL;
+      }
     }
   }
 
@@ -1793,6 +1807,9 @@ void msg_prt_line(char_u *s, int list)
         /* Use special coloring to be able to distinguish <hex> from
          * the same in plain text. */
         attr = HL_ATTR(HLF_8);
+      } else if (c == ' ' && lead != NULL && s <= lead) {
+        c = curwin->w_p_lcs_chars.lead;
+        attr = HL_ATTR(HLF_8);
       } else if (c == ' ' && trail != NULL && s > trail) {
         c = curwin->w_p_lcs_chars.trail;
         attr = HL_ATTR(HLF_8);
diff --git a/src/nvim/option.c b/src/nvim/option.c
index 9dba70b4ee..a4736656cd 100644
--- a/src/nvim/option.c
+++ b/src/nvim/option.c
@@ -3477,6 +3477,7 @@ static char_u *set_chars_option(win_T *wp, char_u **varp, bool set)
     { &wp->w_p_lcs_chars.prec,    "precedes", NUL  },
     { &wp->w_p_lcs_chars.space,   "space",    NUL  },
     { &wp->w_p_lcs_chars.tab2,    "tab",      NUL  },
+    { &wp->w_p_lcs_chars.lead,    "lead",     NUL  },
     { &wp->w_p_lcs_chars.trail,   "trail",    NUL  },
     { &wp->w_p_lcs_chars.conceal, "conceal",  NUL  },
   };
diff --git a/src/nvim/screen.c b/src/nvim/screen.c
index 7c71a440af..1e20b77c5c 100644
--- a/src/nvim/screen.c
+++ b/src/nvim/screen.c
@@ -2082,6 +2082,7 @@ static int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow,
   int change_start = MAXCOL;            // first col of changed area
   int change_end = -1;                  // last col of changed area
   colnr_T trailcol = MAXCOL;            // start of trailing spaces
+  colnr_T leadcol = 0;                  // start of leading spaces
   bool need_showbreak = false;          // overlong line, skip first x chars
   int line_attr = 0;                    // attribute for the whole line
   int line_attr_lowprio = 0;            // low-priority attribute for the line
@@ -2427,6 +2428,7 @@ static int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow,
   if (wp->w_p_list && !has_fold) {
     if (wp->w_p_lcs_chars.space
         || wp->w_p_lcs_chars.trail
+        || wp->w_p_lcs_chars.lead
         || wp->w_p_lcs_chars.nbsp) {
       extra_check = true;
     }
@@ -2438,6 +2440,20 @@ static int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow,
       }
       trailcol += (colnr_T) (ptr - line);
     }
+    // find end of leading whitespace
+    if (wp->w_p_lcs_chars.lead) {
+      leadcol = 0;
+      while (ascii_iswhite(ptr[leadcol])) {
+        leadcol++;
+      }
+      if (ptr[leadcol] == NUL) {
+        // in a line full of spaces all of them are treated as trailing
+        leadcol = (colnr_T)0;
+      } else {
+        // keep track of the first column not filled with spaces
+        leadcol += (colnr_T)(ptr - line) + 1;
+      }
+    }
   }
 
   /*
@@ -3462,6 +3478,7 @@ static int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow,
                   || (mb_utf8 && (mb_c == 160 || mb_c == 0x202f)))
                  && curwin->w_p_lcs_chars.nbsp)
                 || (c == ' ' && curwin->w_p_lcs_chars.space
+                    && ptr - line >= leadcol
                     && ptr - line <= trailcol))) {
           c = (c == ' ') ? wp->w_p_lcs_chars.space : wp->w_p_lcs_chars.nbsp;
           n_attr = 1;
@@ -3477,8 +3494,10 @@ static int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow,
           }
         }
 
-        if (trailcol != MAXCOL && ptr > line + trailcol && c == ' ') {
-          c = wp->w_p_lcs_chars.trail;
+        if ((trailcol != MAXCOL && ptr > line + trailcol && c == ' ')
+            || (leadcol != 0 && ptr < line + leadcol && c == ' ')) {
+          c = (ptr > line + trailcol) ? wp->w_p_lcs_chars.trail
+                                      : wp->w_p_lcs_chars.lead;
           n_attr = 1;
           extra_attr = win_hl_attr(wp, HLF_0);
           saved_attr2 = char_attr;  // save current attr
diff --git a/src/nvim/testdir/test_listchars.vim b/src/nvim/testdir/test_listchars.vim
index dcc588120c..4cb609aaf0 100644
--- a/src/nvim/testdir/test_listchars.vim
+++ b/src/nvim/testdir/test_listchars.vim
@@ -110,6 +110,35 @@ func Test_listchars()
 	      \ '.....h>-$',
 	      \ 'iii<<<<><<$', '$'], l)
 
+  " Test lead and trail
+  normal ggdG
+  set listchars=eol:$
+  set listchars+=lead:>,trail:<,space:x
+  set list
+
+  call append(0, [
+	      \ '    ffff    ',
+	      \ '          gg',
+	      \ 'h           ',
+	      \ '            ',
+	      \ '    0  0    ',
+	      \ ])
+
+  let expected = [
+	      \ '>>>>ffff<<<<$',
+	      \ '>>>>>>>>>>gg$',
+	      \ 'h<<<<<<<<<<<$',
+	      \ '<<<<<<<<<<<<$',
+	      \ '>>>>0xx0<<<<$',
+              \ '$'
+	      \ ]
+  redraw!
+  for i in range(1, 5)
+    call cursor(i, 1)
+    call assert_equal([expected[i - 1]], ScreenLines(i, virtcol('$')))
+  endfor
+
+  call assert_equal(expected, split(execute("%list"), "\n"))
 
   " test nbsp
   normal ggdG