patch 8.2.1313: Vim9 script: cannot assign to environment variable

Problem:    Vim9 script: cannot assign to environment variable.
Solution:   Recognize environment variable assignment. (closes #6548)
            Also options and registers.

src/ex_docmd.c

@@ -1710,7 +1710,7 @@ do_one_cmd(
     char_u	*cmd;
     int		starts_with_colon = FALSE;
 #ifdef FEAT_EVAL
-    int		starts_with_quote;
+    int		may_have_range;
     int		vim9script = in_vim9script();
 #endif
 
@@ -1773,8 +1773,9 @@ do_one_cmd(
  */
     cmd = ea.cmd;
 #ifdef FEAT_EVAL
-    starts_with_quote = vim9script && !starts_with_colon && *ea.cmd == '\'';
-    if (!starts_with_quote)
+    // In Vim9 script a colon is required before the range.
+    may_have_range = !vim9script || starts_with_colon;
+    if (may_have_range)
 #endif
 	ea.cmd = skip_range(ea.cmd, NULL);
     if (*ea.cmd == '*' && vim_strchr(p_cpo, CPO_STAR) == NULL)
@@ -1783,7 +1784,10 @@ do_one_cmd(
 #ifdef FEAT_EVAL
     if (vim9script && !starts_with_colon)
     {
-	if (ea.cmd > cmd)
+	if (ea.cmd == cmd + 1 && *cmd == '$')
+	    // should be "$VAR = val"
+	    --ea.cmd;
+	else if (ea.cmd > cmd)
 	{
 	    emsg(_(e_colon_required));
 	    goto doend;
@@ -1876,7 +1880,7 @@ do_one_cmd(
 
     ea.cmd = cmd;
 #ifdef FEAT_EVAL
-    if (!starts_with_quote)
+    if (may_have_range)
 #endif
 	if (parse_cmd_address(&ea, &errormsg, FALSE) == FAIL)
 	    goto doend;
@@ -3267,9 +3271,14 @@ find_ex_command(
      * "lvar = value", "lvar(arg)", "[1, 2 3]->Func()"
      */
     p = eap->cmd;
-    if (lookup != NULL && (vim_strchr((char_u *)"{('[", *p) != NULL
-	       || ((p = to_name_const_end(eap->cmd)) > eap->cmd
-		   && *p != NUL)))
+    if (lookup != NULL)
+    {
+	// Skip over first char for "&opt = val", "$ENV = val" and "@r = val".
+	char_u *pskip = (*eap->cmd == '&' || *eap->cmd == '$'
+				|| *eap->cmd == '@') ? eap->cmd + 1 : eap->cmd;
+
+	if (vim_strchr((char_u *)"{('[", *p) != NULL
+	       || ((p = to_name_const_end(pskip)) > eap->cmd && *p != NUL))
 	{
 	    int oplen;
 	    int heredoc;
@@ -3302,10 +3311,11 @@ find_ex_command(
 
 	    // "[...]->Method()" is a list expression, but "[a, b] = Func()" is
 	    // an assignment.
-	// If there is no line break inside the "[...]" then "p" is advanced to
-	// after the "]" by to_name_const_end(): check if a "=" follows.
-	// If "[...]" has a line break "p" still points at the "[" and it can't
-	// be an assignment.
+	    // If there is no line break inside the "[...]" then "p" is
+	    // advanced to after the "]" by to_name_const_end(): check if a "="
+	    // follows.
+	    // If "[...]" has a line break "p" still points at the "[" and it
+	    // can't be an assignment.
 	    if (*eap->cmd == '[')
 	    {
 		p = to_name_const_end(eap->cmd);
@@ -3328,6 +3338,9 @@ find_ex_command(
 	    if (oplen > 0)
 	    {
 		if (((p - eap->cmd) > 2 && eap->cmd[1] == ':')
+			|| *eap->cmd == '&'
+			|| *eap->cmd == '$'
+			|| *eap->cmd == '@'
 			|| lookup(eap->cmd, p - eap->cmd, cctx) != NULL)
 		{
 		    eap->cmdidx = CMD_let;
@@ -3343,6 +3356,7 @@ find_ex_command(
 		return eap->cmd;
 	    }
 	}
+    }
 #endif
 
     /*

src/testdir/test_vim9_script.vim

@@ -61,6 +61,14 @@ def Test_assignment()
   assert_equal('foobar', $ENVVAR)
   $ENVVAR = ''
 
+  let lines =<< trim END
+    vim9script
+    $ENVVAR = 'barfoo'
+    assert_equal('barfoo', $ENVVAR)
+    $ENVVAR = ''
+  END
+  call CheckScriptSuccess(lines)
+
   s:appendToMe ..= 'yyy'
   assert_equal('xxxyyy', s:appendToMe)
   s:addToMe += 222
@@ -80,6 +88,15 @@ def Test_assignment()
   set ts=10
   &ts %= 4
   assert_equal(2, &ts)
+
+  lines =<< trim END
+    vim9script
+    &ts = 6
+    &ts += 3
+    assert_equal(9, &ts)
+  END
+  call CheckScriptSuccess(lines)
+
   call CheckDefFailure(['&notex += 3'], 'E113:')
   call CheckDefFailure(['&ts ..= "xxx"'], 'E1019:')
   call CheckDefFailure(['&ts = [7]'], 'E1013:')
@@ -106,6 +123,14 @@ def Test_assignment()
   call CheckDefFailure(['@a += "more"'], 'E1013:')
   call CheckDefFailure(['@a += 123'], 'E1013:')
 
+  lines =<< trim END
+    vim9script
+    @c = 'areg'
+    @c ..= 'add'
+    assert_equal('aregadd', @c)
+  END
+  call CheckScriptSuccess(lines)
+
   v:errmsg = 'none'
   v:errmsg ..= 'again'
   assert_equal('noneagain', v:errmsg)

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1313,
 /**/
     1312,
 /**/