patch 8.1.1913: not easy to compute the space on the command line

Problem:    Not easy to compute the space on the command line.
Solution:   Add v:echospace. (Daniel Hahler, closes #4732)

src/eval.c

@@ -201,6 +201,7 @@ static struct vimvar
     {VV_NAME("termblinkresp",	 VAR_STRING), VV_RO},
     {VV_NAME("event",		 VAR_DICT), VV_RO},
     {VV_NAME("versionlong",	 VAR_NUMBER), VV_RO},
+    {VV_NAME("echospace",	 VAR_NUMBER), VV_RO},
 };
 
 /* shorthand */
@@ -389,6 +390,8 @@ eval_init(void)
     set_vim_var_nr(VV_TYPE_CHANNEL, VAR_TYPE_CHANNEL);
     set_vim_var_nr(VV_TYPE_BLOB,    VAR_TYPE_BLOB);
 
+    set_vim_var_nr(VV_ECHOSPACE,    sc_col - 1);
+
     set_reg_var(0);  /* default for v:register is not 0 but '"' */
 
 #ifdef EBCDIC

src/option.c

@@ -10881,6 +10881,9 @@ comp_col(void)
     sc_col = Columns;
     ru_col = Columns;
 #endif
+#ifdef FEAT_EVAL
+    set_vim_var_nr(VV_ECHOSPACE, sc_col - 1);
+#endif
 }
 
 #if defined(FEAT_PYTHON) || defined(FEAT_PYTHON3) || defined(PROTO)

src/testdir/test_messages.vim

@@ -155,3 +155,20 @@ func Test_mode_message_at_leaving_insert_with_esc_mapped()
   exe buf . 'bwipe!'
   call delete(testfile)
 endfunc
+
+func Test_echospace()
+  set noruler noshowcmd laststatus=1
+  call assert_equal(&columns - 1, v:echospace)
+  split
+  call assert_equal(&columns - 1, v:echospace)
+  set ruler
+  call assert_equal(&columns - 1, v:echospace)
+  close
+  call assert_equal(&columns - 19, v:echospace)
+  set showcmd noruler
+  call assert_equal(&columns - 12, v:echospace)
+  set showcmd ruler
+  call assert_equal(&columns - 29, v:echospace)
+
+  set ruler& showcmd&
+endfunc

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1913,
 /**/
     1912,
 /**/

src/vim.h

@@ -1981,7 +1981,8 @@ typedef int sock_T;
 #define VV_TERMBLINKRESP 89
 #define VV_EVENT	90
 #define VV_VERSIONLONG	91
-#define VV_LEN		92	// number of v: vars
+#define VV_ECHOSPACE	92
+#define VV_LEN		93	// number of v: vars
 
 // used for v_number in VAR_SPECIAL
 #define VVAL_FALSE	0L