patch 8.2.1756: Vim9: :let will soon be disallowed

Problem:    Vim9: :let will soon be disallowed.
Solution:   Add v:disallow_let temporarily.  Fix tests.

src/errors.h

@@ -274,4 +274,6 @@ EXTERN char e_str_cannot_be_used_in_legacy_vim_script[]
 	INIT(= N_("E1124: \"%s\" cannot be used in legacy Vim script"));
 EXTERN char e_final_requires_a_value[]
 	INIT(= N_("E1125: Final requires a value"));
+EXTERN char e_cannot_use_let_in_vim9_script[]
+	INIT(= N_("E1126: Cannot use :let in Vim9 script"));
 #endif

src/evalvars.c

@@ -146,6 +146,7 @@ static struct vimvar
     {VV_NAME("echospace",	 VAR_NUMBER), VV_RO},
     {VV_NAME("argv",		 VAR_LIST), VV_RO},
     {VV_NAME("collate",		 VAR_STRING), VV_RO},
+    {VV_NAME("disallow_let",	 VAR_NUMBER), 0}, // TODO: remove
 };
 
 // shorthand
@@ -734,6 +735,12 @@ ex_let(exarg_T *eap)
 	    ex_finally(eap);
 	    return;
     }
+    if (get_vim_var_nr(VV_DISALLOW_LET)
+				      && eap->cmdidx == CMD_let && vim9script)
+    {
+	emsg(_(e_cannot_use_let_in_vim9_script));
+	return;
+    }
     if (eap->cmdidx == CMD_const && !vim9script && !eap->forceit)
 	// In legacy Vim script ":const" works like ":final".
 	eap->cmdidx = CMD_final;

src/userfunc.c

@@ -3214,19 +3214,20 @@ def_function(exarg_T *eap, char_u *name_arg)
 		is_heredoc = TRUE;
 	    }
 
-	    // Check for ":let v =<< [trim] EOF"
-	    //       and ":let [a, b] =<< [trim] EOF"
+	    // Check for ":cmd v =<< [trim] EOF"
+	    //       and ":cmd [a, b] =<< [trim] EOF"
+	    // Where "cmd" can be "let", "var", "final" or "const".
 	    arg = skipwhite(skiptowhite(p));
 	    if (*arg == '[')
 		arg = vim_strchr(arg, ']');
 	    if (arg != NULL)
 	    {
 		arg = skipwhite(skiptowhite(arg));
-		if ( arg[0] == '=' && arg[1] == '<' && arg[2] =='<'
-			&& ((p[0] == 'l'
-				&& p[1] == 'e'
-				&& (!ASCII_ISALNUM(p[2])
-				    || (p[2] == 't' && !ASCII_ISALNUM(p[3]))))))
+		if (arg[0] == '=' && arg[1] == '<' && arg[2] =='<'
+			&& (checkforcmd(&p, "let", 2)
+			    || checkforcmd(&p, "var", 3)
+			    || checkforcmd(&p, "final", 5)
+			    || checkforcmd(&p, "const", 5)))
 		{
 		    p = skipwhite(arg + 3);
 		    if (STRNCMP(p, "trim", 4) == 0)

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1756,
 /**/
     1755,
 /**/

src/vim.h

@@ -1992,7 +1992,8 @@ typedef int sock_T;
 #define VV_ECHOSPACE	93
 #define VV_ARGV		94
 #define VV_COLLATE      95
-#define VV_LEN		96	// number of v: vars
+#define VV_DISALLOW_LET 96	// TODO: remove again
+#define VV_LEN		97	// number of v: vars
 
 // used for v_number in VAR_BOOL and VAR_SPECIAL
 #define VVAL_FALSE	0L	// VAR_BOOL

src/vim9compile.c

@@ -6979,6 +6979,12 @@ compile_def_function(ufunc_T *ufunc, int set_return_type, cctx_T *outer_cctx)
 		    break;
 
 	    case CMD_let:
+		    if (get_vim_var_nr(VV_DISALLOW_LET))
+		    {
+			emsg(_(e_cannot_use_let_in_vim9_script));
+			break;
+		    }
+		    // FALLTHROUGH
 	    case CMD_var:
 	    case CMD_final:
 	    case CMD_const: