messages: support shortmess-=S in ext_messages

runtime/doc/ui.txt

@@ -706,6 +706,7 @@ events, which the UI must handle.
 		"rpc_error"	Error response from |rpcrequest()|
 		"return_prompt"	|press-enter| prompt after a multiple messages
 		"quickfix"	Quickfix navigation message
+		"search_count"	Search count message ("S" flag of 'shortmess')
 		"wmsg"		Warning ("search hit BOTTOM", |W10|, …)
 	    New kinds may be added in the future; clients should treat unknown
 	    kinds as the empty kind.

src/nvim/message.c

@@ -3018,7 +3018,10 @@ void give_warning(char_u *message, bool hl) FUNC_ATTR_NONNULL_ARG(1)
   } else {
     keep_msg_attr = 0;
   }
+
+  if (msg_ext_kind == NULL) {
     msg_ext_set_kind("wmsg");
+  }
 
   if (msg_attr((const char *)message, keep_msg_attr) && msg_scrolled == 0) {
     set_keep_msg(message, keep_msg_attr);

src/nvim/search.c

@@ -1168,7 +1168,9 @@ int do_search(
         // search stat.  Use all the space available, so that the
         // search state is right aligned.  If there is not enough space
         // msg_strtrunc() will shorten in the middle.
-        if (msg_scrolled != 0) {
+        if (ui_has(kUIMessages)) {
+          len = 0;  // adjusted below
+        } else if (msg_scrolled != 0) {
           // Use all the columns.
           len = (int)(Rows - msg_row) * Columns - 1;
         } else {
@@ -4328,6 +4330,7 @@ static void search_stat(int dirc, pos_T *pos,
 
       // keep the message even after redraw, but don't put in history
       msg_hist_off = true;
+      msg_ext_set_kind("search_count");
       give_warning(msgbuf, false);
       msg_hist_off = false;
     }

test/functional/ui/messages_spec.lua

@@ -21,6 +21,8 @@ describe('ui/ext_messages', function()
       [4] = {bold = true, foreground = Screen.colors.SeaGreen4},
       [5] = {foreground = Screen.colors.Blue1},
       [6] = {bold = true, reverse = true},
+      [7] = {background = Screen.colors.Yellow},
+      [8] = {foreground = Screen.colors.Red},
     })
   end)
   after_each(function()
@@ -303,6 +305,33 @@ describe('ui/ext_messages', function()
     }}
   end)
 
+  it('shortmess-=S', function()
+    command('set shortmess-=S')
+    feed('iline 1\nline 2<esc>')
+
+    feed('/line<cr>')
+    screen:expect{grid=[[
+      {7:^line} 1                   |
+      {7:line} 2                   |
+      {1:~                        }|
+      {1:~                        }|
+      {1:~                        }|
+    ]], messages={
+      {content = {{"/line      [1/2] W"}}, kind = "search_count"}
+    }}
+
+    feed('n')
+    screen:expect{grid=[[
+      {7:line} 1                   |
+      {7:^line} 2                   |
+      {1:~                        }|
+      {1:~                        }|
+      {1:~                        }|
+    ]], messages={
+      {content = {{"/line        [2/2]"}}, kind = "search_count"}
+    }}
+  end)
+
   it('&showmode', function()
     command('imap <f2> <cmd>echomsg "stuff"<cr>')
     feed('i')