Introduce --incompatible_comprehension_variables_do_not_leak

When the flag is activated, variables in list comprehensions do not leak anymore.
Even if a variable was defined before the loop, it's not accessible after the loop.
This change allows us to detect any breakage and ensures that no user is accessing
loop variables after the loop.

This will make possible for us to change the behavior and follow Python 3 semantics
in the future.

RELNOTES: None.
PiperOrigin-RevId: 158895514

1 files changed, 30 insertions, 0 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 d00c1bd50a..81fa580ecb 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
@@ -1490,4 +1490,34 @@ public class SkylarkEvaluationTest extends EvaluationTest {
             // TODO(bazel-team): This should probably match the error above better.
             "struct has no method 'nonexistent_method'", "v = val.nonexistent_method()");
   }
+
+  @Test
+  public void testListComprehensionsDoNotLeakVariables() throws Exception {
+    env =
+        newEnvironmentWithSkylarkOptions("--incompatible_comprehension_variables_do_not_leak=true");
+    checkEvalErrorContains(
+        "name 'a' is not defined",
+        "def foo():",
+        "  a = 10",
+        "  b = [a for a in range(3)]",
+        "  return a",
+        "x = foo()");
+  }
+
+  @Test
+  public void testListComprehensionsShadowGlobalVariable() throws Exception {
+    env =
+        newEnvironmentWithSkylarkOptions("--incompatible_comprehension_variables_do_not_leak=true");
+    eval("a = 18", "def foo():", "  b = [a for a in range(3)]", "  return a", "x = foo()");
+    assertThat(lookup("x")).isEqualTo(18);
+  }
+
+  @Test
+  public void testListComprehensionsLeakVariables() throws Exception {
+    env =
+        newEnvironmentWithSkylarkOptions(
+            "--incompatible_comprehension_variables_do_not_leak=false");
+    eval("def foo():", "  a = 10", "  b = [a for a in range(3)]", "  return a", "x = foo()");
+    assertThat(lookup("x")).isEqualTo(2);
+  }
 }