feat: default string for empty auto sign segment

Resolve #145

README.md

@@ -109,8 +109,8 @@ Each segment can contain the following elements:
     -- below values list the default when omitted:
     maxwidth = 1,        -- maximum number of signs that will be displayed in this segment
     colwidth = 2,        -- number of display cells per sign in this segment
-    auto = false,        -- when true, the segment will not be drawn if no signs matching
-                         -- the pattern are currently placed in the buffer.
+    auto = false,        -- boolean or string indicating what will be drawn when no signs
+                         -- matching the pattern are currently placed in the buffer.
     wrap = false,        -- when true, signs in this segment will also be drawn on the
                          -- virtual or wrapped part of a line (when v:virtnum != 0).
     fillchar = " ",      -- character used to fill a segment with less signs than maxwidth

lua/statuscol/builtin.lua

@@ -68,6 +68,7 @@ function M.signfunc(args, segment)
   if args.virtnum ~= 0 and not ss.wrap then return wss.empty.."%*" end
   local sss = wss.signs[args.lnum]
   local nonhl = ss.fillcharhl or (args.cul and args.relnum == 0 and "%#CursorLineSign#") or "%#SignColumn#"
+  if type(ss.auto) == "string" and wss.padwidth == 0 then return nonhl..ss.auto.."%*" end
   if not sss then return nonhl..wss.empty.."%*" end
   local text = ""
   local signcount = #sss