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

1 files changed, 10 insertions, 0 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemanticsOptions.java b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemanticsOptions.java
index 0c83c25760..5909ea7155 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemanticsOptions.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemanticsOptions.java
@@ -58,4 +58,14 @@ public class SkylarkSemanticsOptions extends OptionsBase implements Serializable
     help = "If set to true, disables the keyword-only argument syntax in function definition."
   )
   public boolean incompatibleKeywordOnlySyntax;
+
+  @Option(
+    name = "incompatible_list_plus_equals",
+    defaultValue = "false",
+    category = "incompatible changes",
+    help =
+        "If set to true, `+=` on lists works like the `extend` method mutating the original "
+            + "list. Otherwise it copies the original list without mutating it."
+  )
+  public boolean incompatibleListPlusEquals;
 }