Build ProtoMessageValue's CelType on demand

common/src/main/java/dev/cel/common/values/ProtoMessageValue.java

@@ -35,20 +35,29 @@ public abstract class ProtoMessageValue extends StructValue<String, Message>
   @Override
   public abstract Message value();
 
-  @Override
-  public abstract CelType celType();
-
   abstract CelDescriptorPool celDescriptorPool();
 
   abstract ProtoCelValueConverter protoCelValueConverter();
 
   abstract boolean enableJsonFieldNames();
 
+  @Override
+  public CelType celType() {
+    // Built on demand, since one of these values is created on every hop but rarely asked its type.
+    return StructTypeReference.create(value().getDescriptorForType().getFullName());
+  }
+
   @Override
   public boolean isZeroValue() {
     return value().getDefaultInstanceForType().equals(value());
   }
 
+  @Override
+  public final String toString() {
+    // AutoValue's would print the message's contents, which may be sensitive.
+    return "ProtoMessageValue{protoTypeName=" + value().getDescriptorForType().getFullName() + "}";
+  }
+
   @Override
   public Object select(String field) {
     FieldDescriptor fieldDescriptor =
@@ -95,11 +104,7 @@ public static ProtoMessageValue create(
     checkNotNull(celDescriptorPool);
     checkNotNull(protoCelValueConverter);
     return new AutoValue_ProtoMessageValue(
-        value,
-        StructTypeReference.create(value.getDescriptorForType().getFullName()),
-        celDescriptorPool,
-        protoCelValueConverter,
-        enableJsonFieldNames);
+        value, celDescriptorPool, protoCelValueConverter, enableJsonFieldNames);
   }
 
   private Optional<Object> findFieldValue(FieldDescriptor fieldDescriptor) {

common/src/test/java/dev/cel/common/values/ProtoMessageValueTest.java

@@ -426,6 +426,19 @@ public void celTypeTest() {
         .isEqualTo(StructTypeReference.create(TestAllTypes.getDescriptor().getFullName()));
   }
 
+  @Test
+  public void toString_returnsTypeNameOnly() {
+    ProtoMessageValue protoMessageValue =
+        ProtoMessageValue.create(
+            TestAllTypes.newBuilder().setSingleString("secret").build(),
+            DefaultDescriptorPool.INSTANCE,
+            PROTO_CEL_VALUE_CONVERTER,
+            /* enableJsonFieldNames= */ false);
+
+    assertThat(protoMessageValue.toString())
+        .isEqualTo("ProtoMessageValue{protoTypeName=cel.expr.conformance.proto2.TestAllTypes}");
+  }
+
   @Test
   public void findField_jsonName_success() {
     TestAllTypes testAllTypes = TestAllTypes.newBuilder().setSingleInt32(42).build();