patch 8.1.2156: first character after Tab is not highlighted

Problem:    First character after Tab is not highlighted.
Solution:   Remember the syntax attribute for a column.

Author: brammool

src/drawline.c

@@ -289,6 +289,8 @@ win_line(
 #ifdef FEAT_SYN_HL
     int		vcol_save_attr = 0;	// saved attr for 'cursorcolumn'
     int		syntax_attr = 0;	// attributes desired by syntax
+    int		prev_syntax_col = -1;	// column of prev_syntax_attr
+    int		prev_syntax_attr = 0;	// syntax_attr at prev_syntax_col
     int		has_syntax = FALSE;	// this buffer has syntax highl.
     int		save_did_emsg;
     int		draw_color_col = FALSE;	// highlight colorcolumn
@@ -1414,12 +1416,20 @@ win_line(
 		    did_emsg = FALSE;
 
 		    v = (long)(ptr - line);
-		    can_spell = TRUE;
-		    syntax_attr = get_syntax_attr((colnr_T)v,
+		    if (v == prev_syntax_col)
+			// at same column again
+			syntax_attr = prev_syntax_attr;
+		    else
+		    {
+			can_spell = TRUE;
+			syntax_attr = get_syntax_attr((colnr_T)v,
 # ifdef FEAT_SPELL
 						has_spell ? &can_spell :
 # endif
 						NULL, FALSE);
+			prev_syntax_col = v;
+			prev_syntax_attr = syntax_attr;
+		    }
 
 		    // combine syntax attribute with 'wincolor'
 		    if (syntax_attr != 0 && win_attr != 0)

src/testdir/dumps/Test_syntax_c_01.dump

@@ -12,9 +12,9 @@
 |}| @73
 @3|s+0#00e0003&|t|a|t|i|c| +0#0000000&|v+0#00e0003&|o|i|d| +0#0000000&@60
 |m|y|F|u|n|c|t|i|o|n|(|c+0#00e0003&|o|n|s|t| +0#0000000&|d+0#00e0003&|o|u|b|l|e| +0#0000000&|c|o|u|n|t|,| |s+0#00e0003&|t|r|u|c|t| +0#0000000&|n|o|t|h|i|n|g|,| |l+0#00e0003&|o|n|g| +0#0000000&|t|h|e|r|e|)| |{| @14
-@2|/+0#0000e05&@1| |1+0#e000002&|2|3|:+0#0000e05&| |n|o|t|h|i|n|g| |t|o| |r|e|a|d| |h|e|r|e| +0#0000000&@44
-@2|f+0#af5f00255&|o|r| +0#0000000&|(|i+0#00e0003&|n|t| +0#0000000&|i| |=| |0+0#e000002&|;+0#0000000&| |i| |<| |c|o|u|n|t|;| |+@1|i|)| |{| @39
-@4|b+0#af5f00255&|r|e|a|k|;+0#0000000&| @64
-@2|}| @71
+@1| +0#0000e05&@6|/@1| |1+0#e000002&|2|3|:+0#0000e05&| |n|o|t|h|i|n|g| |t|o| |r|e|a|d| |h|e|r|e| +0#0000000&@38
+@1| +0#af5f00255&@6|f|o|r| +0#0000000&|(|i+0#00e0003&|n|t| +0#0000000&|i| |=| |0+0#e000002&|;+0#0000000&| |i| |<| |c|o|u|n|t|;| |+@1|i|)| |{| @33
+@11|b+0#af5f00255&|r|e|a|k|;+0#0000000&| @57
+@8|}| @65
 |}| @73
-|"|X|t|e|s|t|.|c|"| |1|9|L|,| |3|6|4|C| @37|1|,|1| @10|A|l@1| 
+|"|X|t|e|s|t|.|c|"| |1|9|L|,| |3|6|1|C| @37|1|,|1| @10|A|l@1| 

src/testdir/test_syntax.vim

@@ -551,10 +551,10 @@ func Test_syntax_c()
 	\ '}',
 	\ '   static void',
 	\ 'myFunction(const double count, struct nothing, long there) {',
-	\ '  // 123: nothing to read here',
-	\ '  for (int i = 0; i < count; ++i) {',
-	\ '    break;',
-	\ '  }',
+	\ "\t// 123: nothing to read here",
+	\ "\tfor (int i = 0; i < count; ++i) {",
+	\ "\t   break;",
+	\ "\t}",
 	\ '}',
 	\ ], 'Xtest.c')
 

src/version.c

@@ -753,6 +753,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2156,
 /**/
     2155,
 /**/