Implement a flag for extend-like behavior of the `+=` operator for lists

Usage: --incompatible_list_plus_equals=true (the default value is false).
PiperOrigin-RevId: 155084916

2 files changed, 80 insertions, 8 deletions

diff --git a/src/test/java/com/google/devtools/build/lib/syntax/SkylarkEvaluationTest.java b/src/test/java/com/google/devtools/build/lib/syntax/SkylarkEvaluationTest.java
index 848498b229..a71f656070 100644
--- a/src/test/java/com/google/devtools/build/lib/syntax/SkylarkEvaluationTest.java
+++ b/src/test/java/com/google/devtools/build/lib/syntax/SkylarkEvaluationTest.java
@@ -987,6 +987,73 @@ public class SkylarkEvaluationTest extends EvaluationTest {
   }
 
   @Test
+  public void testPlusEqualsOnListCopying() throws Exception {
+    new SkylarkTest("--incompatible_list_plus_equals=false")
+        .setUp(
+            "def func():",
+            "  l1 = [1, 2]",
+            "  l2 = l1",
+            "  l2 += [3, 4]",
+            "  return l1, l2",
+            "lists = str(func())")
+        .testLookup("lists", "([1, 2], [1, 2, 3, 4])");
+  }
+
+  @Test
+  public void testPlusEqualsOnListMutating() throws Exception {
+    new SkylarkTest("--incompatible_list_plus_equals=true")
+        .setUp(
+            "def func():",
+            "  l1 = [1, 2]",
+            "  l2 = l1",
+            "  l2 += [3, 4]",
+            "  return l1, l2",
+            "lists = str(func())")
+        .testLookup("lists", "([1, 2, 3, 4], [1, 2, 3, 4])");
+
+    // The same but with += after an IndexExpression
+    new SkylarkTest("--incompatible_list_plus_equals=true")
+        .setUp(
+            "def func():",
+            "  l = [1, 2]",
+            "  d = {0: l}",
+            "  d[0] += [3, 4]",
+            "  return l, d[0]",
+            "lists = str(func())")
+        .testLookup("lists", "([1, 2, 3, 4], [1, 2, 3, 4])");
+  }
+
+  @Test
+  public void testPlusEqualsOnTuple() throws Exception {
+    new SkylarkTest("--incompatible_list_plus_equals=false")
+        .setUp(
+            "def func():",
+            "  t1 = (1, 2)",
+            "  t2 = t1",
+            "  t2 += (3, 4)",
+            "  return t1, t2",
+            "tuples = func()")
+        .testLookup("tuples", SkylarkList.Tuple.of(
+            SkylarkList.Tuple.of(1, 2),
+            SkylarkList.Tuple.of(1, 2, 3, 4)
+        ));
+
+    // This behavior should remain the same regardless of the incompatible_list_plus_equals flag
+    new SkylarkTest("--incompatible_list_plus_equals=true")
+        .setUp(
+            "def func():",
+            "  t1 = (1, 2)",
+            "  t2 = t1",
+            "  t2 += (3, 4)",
+            "  return t1, t2",
+            "tuples = func()")
+        .testLookup("tuples", SkylarkList.Tuple.of(
+            SkylarkList.Tuple.of(1, 2),
+            SkylarkList.Tuple.of(1, 2, 3, 4)
+        ));
+  }
+
+  @Test
   public void testPlusEqualsOnDict() throws Exception {
     new SkylarkTest().setUp("def func():",
         "  d = {'a' : 1}",
diff --git a/src/test/java/com/google/devtools/build/lib/syntax/util/EvaluationTestCase.java b/src/test/java/com/google/devtools/build/lib/syntax/util/EvaluationTestCase.java
index 0aa9b0a1e4..33ce2d8d09 100644
--- a/src/test/java/com/google/devtools/build/lib/syntax/util/EvaluationTestCase.java
+++ b/src/test/java/com/google/devtools/build/lib/syntax/util/EvaluationTestCase.java
@@ -115,20 +115,21 @@ public class EvaluationTestCase {
 
   /**
    * Sets the specified {@code TestMode} and tries to create the appropriate {@code Environment}
+   *
    * @param testMode
    * @throws Exception
    */
-  protected void setMode(TestMode testMode) throws Exception {
+  protected void setMode(TestMode testMode, String... skylarkOptions) throws Exception {
     this.testMode = testMode;
-    env = newEnvironment();
+    env = newEnvironmentWithSkylarkOptions(skylarkOptions);
   }
 
-  protected void enableSkylarkMode() throws Exception {
-    setMode(TestMode.SKYLARK);
+  protected void enableSkylarkMode(String... skylarkOptions) throws Exception {
+    setMode(TestMode.SKYLARK, skylarkOptions);
   }
 
-  protected void enableBuildMode() throws Exception {
-    setMode(TestMode.BUILD);
+  protected void enableBuildMode(String... skylarkOptions) throws Exception {
+    setMode(TestMode.BUILD, skylarkOptions);
   }
 
   public EventHandler getEventHandler() {
@@ -560,11 +561,15 @@ public class EvaluationTestCase {
    * A class that runs all tests in Skylark mode
    */
   protected class SkylarkTest extends ModalTestCase {
-    public SkylarkTest() {}
+    private final String[] skylarkOptions;
+
+    public SkylarkTest(String... skylarkOptions) {
+      this.skylarkOptions = skylarkOptions;
+    }
 
     @Override
     protected void run(Testable testable) throws Exception {
-      enableSkylarkMode();
+      enableSkylarkMode(skylarkOptions);
       testable.run();
     }
   }