patch 9.2.0538: Cannot keep leading whitespace in %{} statusline expr

Problem:  A leading space in the result of a %{} item is sometimes
          stripped, and an all-digit result is converted to a number.
Solution: Add %0{} atom which inserts the expression result verbatim
          (glepnir)

fixes:  #3898
closes: #20315

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/options.txt

@@ -8670,7 +8670,8 @@ A jump table for the options with a short description can be found at |Q_op|.
 	{ NF  Evaluate expression between '%{' and '}' and substitute result.
 	      Note that there is no '%' before the closing '}'.  The
 	      expression cannot contain a '}' character, call a function to
-	      work around that.  See |stl-%{| below.
+	      work around that.  See |stl-%{| below.  Use '%0{' to insert the
+	      result verbatim.
 	{% -  This is almost same as { except the result of the expression is
 	      re-evaluated as a statusline format string.  Thus if the
 	      return value of expr contains % items they will get expanded.
@@ -8797,6 +8798,8 @@ A jump table for the options with a short description can be found at |Q_op|.
 	A result of all digits is regarded a number for display purposes.
 	Otherwise the result is taken as flag text and applied to the rules
 	described above.
+								*stl-%0{*
+	With %0{ neither applies: the result is inserted as a literal string.
 
 	Watch out for errors in expressions.  They may render Vim unusable!
 	If you are stuck, hold down ':' or 'Q' to get a prompt, then quit and

runtime/doc/tags

@@ -10562,6 +10562,7 @@ static-tag	tagsrch.txt	/*static-tag*
 status-line	windows.txt	/*status-line*
 statusmsg-variable	eval.txt	/*statusmsg-variable*
 stl-%!	options.txt	/*stl-%!*
+stl-%0{	options.txt	/*stl-%0{*
 stl-%@	options.txt	/*stl-%@*
 stl-%[FuncName]	options.txt	/*stl-%[FuncName]*
 stl-%{	options.txt	/*stl-%{*

runtime/doc/version9.txt

@@ -52640,6 +52640,8 @@ Other ~
   well.
 - New argument handling for user commands |:command-nargs| using the "-nars=_"
   attribute to handle completion of single arguments with spaces as expected.
+- Support %0{} in 'statusline' to insert the expression result verbatim and
+  not drop leading spaces |stl-%0{|.
 
 Platform specific ~
 -----------------

src/buffer.c

@@ -5041,7 +5041,8 @@ build_stl_str_hl_local(
 
 	    if (reevaluate)
 		s++;
-	    itemisflag = TRUE;
+	    // %0{} keeps the result verbatim
+	    itemisflag = zeropad ? FALSE : TRUE;
 	    t = p;
 	    while ((*s != '}' || (reevaluate && s[-1] != '%'))
 					  && *s != NUL && p + 1 < out + outlen)
@@ -5078,7 +5079,7 @@ build_stl_str_hl_local(
 	    do_unlet((char_u *)"g:actual_curbuf", TRUE);
 	    do_unlet((char_u *)"g:actual_curwin", TRUE);
 
-	    if (str != NULL && *str != NUL)
+	    if (!zeropad && str != NULL && *str != NUL)
 	    {
 		if (*skipdigits(str) == NUL)
 		{

src/testdir/test_statusline.vim

@@ -282,6 +282,16 @@ func Test_statusline()
   call assert_match('^vimLineComment\s*$', s:get_statusline())
   syntax off
 
+  " %0{: result of expression is inserted verbatim
+  set statusline=%{'\ x'}
+  call assert_match('^x\s*$', s:get_statusline())
+  set statusline=%0{'\ x'}
+  call assert_match('^ x\s*$', s:get_statusline())
+  set statusline=%{'000'}
+  call assert_match('^0\s*$', s:get_statusline())
+  set statusline=%0{'000'}
+  call assert_match('^000\s*$', s:get_statusline())
+
   "%{%expr%}: evaluates expressions present in result of expr
   func! Inner_eval()
     return '%n some other text'

src/version.c

@@ -729,6 +729,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    538,
 /**/
     537,
 /**/