disable yield var

src/parser/parser.yy

@@ -1448,9 +1448,26 @@ yield_column
         delete $3;
     }
     | expression {
+        auto vars = graph::ExpressionUtils::collectAll($1, {Expression::Kind::kVar});
+        if (!vars.empty()) {
+            for (auto* var : vars) {
+                if (!static_cast<const VariableExpression*>(var)->isInner()) {
+                    throw nebula::GraphParser::syntax_error(@1, "Direct output of variable is prohibited");
+                }
+            }
+        }
         $$ = new YieldColumn($1);
     }
     | expression KW_AS name_label {
+        auto vars = graph::ExpressionUtils::collectAll($1, {Expression::Kind::kVar});
+        if (!vars.empty()) {
+            for (auto* var : vars) {
+                if (!static_cast<const VariableExpression*>(var)->isInner()) {
+                    delete $3;
+                    throw nebula::GraphParser::syntax_error(@1, "Direct output of variable is prohibited");
+                }
+            }
+        }
         $$ = new YieldColumn($1, *$3);
         delete $3;
     }

tests/tck/features/bugfix/VariableExpression.feature

@@ -0,0 +1,26 @@
+# Copyright (c) 2021 vesoft inc. All rights reserved.
+#
+# This source code is licensed under Apache 2.0 License,
+# attached with Common Clause Condition 1.0, found in the LICENSES directory.
+Feature: Variable usage
+
+  Background:
+    Given a graph with space named "nba"
+
+  Scenario: disable yield $var
+    When executing query:
+      """
+      $var = yield 1;$var2 = yield 3;yield $var1 + $var2
+      """
+    Then a SyntaxError should be raised at runtime: Direct output of variable is prohibited near `$var1 + $var2'
+    When executing query:
+      """
+      $var=go from "Tim Duncan" over like yield like._dst as dst;yield $var
+      """
+    Then a SyntaxError should be raised at runtime: Direct output of variable is prohibited near `$var'
+    Then drop the used space
+    When executing query:
+      """
+      $var=go from "Tim Duncan" over like yield like._dst as dst;yield $var[0][0]
+      """
+    Then a SyntaxError should be raised at runtime: Direct output of variable is prohibited near `$var[0][0]'

tests/tck/features/go/GroupbyLimit.IntVid.feature

@@ -15,7 +15,7 @@ Feature: Groupby & limit Sentence
       """
       GO FROM hash("Marco Belinelli") OVER serve YIELD $$.team.name AS name | GROUP BY $-.start_year YIELD COUNT($var)
       """
-    Then a SemanticError should be raised at runtime:
+    Then a SyntaxError should be raised at runtime:
 
   Scenario: Syntax test4
     When executing query:

tests/tck/features/go/GroupbyLimit.feature

@@ -22,7 +22,7 @@ Feature: Groupby & limit Sentence
       """
       GO FROM "Marco Belinelli" OVER serve YIELD $$.team.name AS name | GROUP BY $-.start_year YIELD COUNT($var)
       """
-    Then a SemanticError should be raised at runtime:
+    Then a SyntaxError should be raised at runtime:
 
   Scenario: Syntax test4
     When executing query: