Stop creating block scope for synthetic block nodes

A BLOCK marked as synthetic is not a real JS block with {} around it in the JS code. It just represents a span of statements that need to be kept together. Synthetic blocks do not exist in the source AST and hence do not create their own scope.

This CL impacts the NodeTraversal class and several JSCompiler passes.

PiperOrigin-RevId: 618383865

Author: rishipal

src/com/google/javascript/jscomp/CodeGenerator.java

@@ -769,6 +769,8 @@ protected void add(Node node, Context context, boolean printComments) {
               add("static");
             }
           }
+          // A BLOCK marked as synthetic is not a real JS block with {} around it in the JS code.
+          // It just represents a span of statements that need to be kept together.
           boolean preserveBlock = node.isBlock() && !node.isSyntheticBlock();
           if (preserveBlock) {
             cc.beginBlock();

src/com/google/javascript/jscomp/NodeUtil.java

@@ -2451,6 +2451,10 @@ public static boolean isStatementBlock(Node n) {
   static boolean createsBlockScope(Node n) {
     switch (n.getToken()) {
       case BLOCK:
+        if (n.isSyntheticBlock()) {
+          // Don't create block scope for synthetic blocks.
+          return false;
+        }
         Node parent = n.getParent();
         // Don't create block scope for switch cases or catch blocks.
         return parent != null && !isSwitchCase(parent) && !parent.isCatch();

test/com/google/javascript/jscomp/CreateSyntheticBlocksTest.java

@@ -15,6 +15,9 @@
  */
 package com.google.javascript.jscomp;
 
+import static com.google.common.truth.Truth.assertThat;
+import static com.google.javascript.rhino.testing.NodeSubject.assertNode;
+
 import com.google.javascript.jscomp.parsing.parser.FeatureSet;
 import com.google.javascript.rhino.Node;
 import org.junit.Before;
@@ -161,4 +164,16 @@ public void testVariableDeclaration1() {
     testSame("startMarker();let x=1;endMarker()");
     testSame("startMarker();const x=1;endMarker()");
   }
+
+  @Test
+  public void testSyntheticBlock_doesNotCreateNewScope() {
+    testSame("startMarker();var x=1;endMarker()");
+    Node script = this.getLastCompiler().getJsRoot().getOnlyChild();
+    assertNode(script).isScript();
+    Node synctheticBlock = script.getFirstChild();
+    assertNode(synctheticBlock).isBlock();
+    assertThat(synctheticBlock.isSyntheticBlock()).isTrue();
+    // confirm that synthetic block does not create a new block scope
+    assertThat(NodeUtil.createsBlockScope(synctheticBlock)).isFalse();
+  }
 }