Throw a static error when lvalue of an augmented assignment is a list.

RELNOTES:
  When the lvalue of an augmented assignment is a list, we now throw an error
  before evaluating the code (e.g. `a, b += 2, 3`).
PiperOrigin-RevId: 165906611

src/main/java/com/google/devtools/build/lib/syntax/ValidationEnvironment.java

@@ -158,6 +158,16 @@ public void visit(IfStatement node) {
     super.visit(node);
   }
 
+  @Override
+  public void visit(AugmentedAssignmentStatement node) {
+    if (node.getLValue().getExpression() instanceof ListLiteral) {
+      throw new ValidationException(
+          node.getLocation(), "cannot perform augmented assignment on a list or tuple expression");
+    }
+    // Other bad cases are handled when visiting the LValue node.
+    super.visit(node);
+  }
+
   /** Returns true if the current block is the top level i.e. has no parent. */
   private boolean isTopLevel() {
     return block.parent == null;

src/test/java/com/google/devtools/build/lib/syntax/SkylarkEvaluationTest.java

@@ -964,7 +964,8 @@ public void testAugmentedAssignmentHasNoSideEffects() throws Exception {
 
   @Test
   public void testInvalidAugmentedAssignment_ListLiteral() throws Exception {
-    new SkylarkTest().testIfErrorContains("cannot perform augmented assignment on a list literal",
+    new SkylarkTest().testIfErrorContains(
+        "cannot perform augmented assignment on a list or tuple expression",
         "def f(a, b):",
         "  [a, b] += []",
         "f(1, 2)");

src/test/java/com/google/devtools/build/lib/syntax/ValidationTest.java

@@ -38,6 +38,11 @@ public void testAssignmentNotValidLValue() {
     checkError("cannot assign to '\"a\"'", "'a' = 1");
   }
 
+  @Test
+  public void testAugmentedAssignmentWithMultipleLValues() {
+    checkError("cannot perform augmented assignment on a list or tuple expression", "a, b += 2, 3");
+  }
+
   @Test
   public void testReturnOutsideFunction() throws Exception {
     checkError("Return statements must be inside a function", "return 2\n");