Add ability to shallow-freeze individual objects

Clarify that the IMMUTABLE Mutability should only be used for deeply immutable things, not tuples. Created a new SHALLOW_IMMUTABLE Mutability for them.

Note that the new shallow-freezing functionality marks things as deeply IMMUTABLE without traversing its contents. I.e., it lies, and it is up to the caller to ensure this doesn't cause problems.

RELNOTES: NONE
PiperOrigin-RevId: 189767422

2 files changed, 36 insertions, 0 deletions

diff --git a/src/test/java/com/google/devtools/build/lib/syntax/MutabilityTest.java b/src/test/java/com/google/devtools/build/lib/syntax/MutabilityTest.java
index 67c3e0ec8b..b9b896f2bc 100644
--- a/src/test/java/com/google/devtools/build/lib/syntax/MutabilityTest.java
+++ b/src/test/java/com/google/devtools/build/lib/syntax/MutabilityTest.java
@@ -233,4 +233,26 @@ public final class MutabilityTest {
     assertThat(expected).hasMessageThat().contains(
         "trying to check the lock of an object from a different context");
   }
+
+  @Test
+  public void checkUnsafeShallowFreezePrecondition_FailsWhenAlreadyFrozen() throws Exception {
+    Mutability mutability = Mutability.create("test").freeze();
+    assertThrows(
+        IllegalArgumentException.class,
+        () -> Freezable.checkUnsafeShallowFreezePrecondition(new DummyFreezable(mutability)));
+  }
+
+  @Test
+  public void checkUnsafeShallowFreezePrecondition_FailsWhenDisallowed() throws Exception {
+    Mutability mutability = Mutability.create("test");
+    assertThrows(
+        IllegalArgumentException.class,
+        () -> Freezable.checkUnsafeShallowFreezePrecondition(new DummyFreezable(mutability)));
+  }
+
+  @Test
+  public void checkUnsafeShallowFreezePrecondition_SucceedsWhenAllowed() throws Exception {
+    Mutability mutability = Mutability.createAllowingShallowFreeze("test");
+    Freezable.checkUnsafeShallowFreezePrecondition(new DummyFreezable(mutability));
+  }
 }
diff --git a/src/test/java/com/google/devtools/build/lib/syntax/SkylarkListTest.java b/src/test/java/com/google/devtools/build/lib/syntax/SkylarkListTest.java
index b2768390da..016c30f85a 100644
--- a/src/test/java/com/google/devtools/build/lib/syntax/SkylarkListTest.java
+++ b/src/test/java/com/google/devtools/build/lib/syntax/SkylarkListTest.java
@@ -279,6 +279,20 @@ public class SkylarkListTest extends EvaluationTestCase {
   }
 
   @Test
+  public void testCannotMutateAfterShallowFreeze() throws Exception {
+    Mutability mutability = Mutability.createAllowingShallowFreeze("test");
+    MutableList<Object> list = MutableList.copyOf(mutability, ImmutableList.of(1, 2, 3));
+    list.unsafeShallowFreeze();
+
+    try {
+      list.add(4, null, mutability);
+      fail("expected exception");
+    } catch (EvalException e) {
+      assertThat(e).hasMessage("trying to mutate a frozen object");
+    }
+  }
+
+  @Test
   public void testCopyOfTakesCopy() throws EvalException {
     ArrayList<String> copyFrom = Lists.newArrayList("hi");
     Mutability mutability = Mutability.create("test");