Avoid copying submessage bytes on each lite field selection hop

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

@@ -138,7 +138,7 @@ private Object mergeOrReadMessageField(
       if (existingValue instanceof RawProtoMessageLiteValue) {
         bytes = ((RawProtoMessageLiteValue) existingValue).toByteString().concat(bytes);
       }
-      return RawProtoMessageLiteValue.create(bytes, fieldProtoTypeName, this);
+      return RawProtoMessageLiteValue.create(bytes, fieldProtoTypeName, this, /* isView= */ true);
     }
     WellKnownProto wellKnownProto = WellKnownProto.getByTypeName(fieldProtoTypeName).orElse(null);
     if (isStructLike(wellKnownProto)) {
@@ -356,6 +356,8 @@ private Map.Entry<Object, Object> readSingleMapEntry(
       @Nullable FieldLiteDescriptor keyDescriptor,
       @Nullable FieldLiteDescriptor valueDescriptor) {
     CodedInputStream inputStream = bytes.newCodedInput();
+    // Submessages alias bytes; unwrapping parses or copies them, so no view outlives evaluation.
+    inputStream.enableAliasing(true);
     int targetFieldNumber = fieldDescriptor.getFieldNumber();
     Object fieldValue = null;
     try {

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

@@ -50,14 +50,23 @@ abstract class RawProtoMessageLiteValue extends StructValue<String, WireMessageL
 
   abstract ProtoLiteCelValueConverter protoLiteCelValueConverter();
 
+  // Set when toByteString() may alias a parent message's buffer.
+  abstract boolean isView();
+
   @Override
   public String protoTypeName() {
     return celType().name();
   }
 
   @Override
   public WireMessageLite value() {
-    return this;
+    // A view escaping evaluation would pin the parent's whole buffer, so it leaves as a copy.
+    return isView()
+        ? create(
+            ByteString.copyFrom(toByteString().asReadOnlyByteBuffer()),
+            protoTypeName(),
+            protoLiteCelValueConverter())
+        : this;
   }
 
   @Override
@@ -136,14 +145,23 @@ static RawProtoMessageLiteValue create(
       ByteString rawWireBytes,
       String protoTypeName,
       ProtoLiteCelValueConverter protoLiteCelValueConverter) {
+    return create(rawWireBytes, protoTypeName, protoLiteCelValueConverter, /* isView= */ false);
+  }
+
+  static RawProtoMessageLiteValue create(
+      ByteString rawWireBytes,
+      String protoTypeName,
+      ProtoLiteCelValueConverter protoLiteCelValueConverter,
+      boolean isView) {
     checkNotNull(rawWireBytes);
     checkNotNull(protoTypeName);
     checkNotNull(protoLiteCelValueConverter);
     return new AutoValue_RawProtoMessageLiteValue(
         rawWireBytes,
         StructTypeReference.create(
             protoTypeName.isEmpty() ? UNKNOWN_MESSAGE_TYPE_NAME : protoTypeName),
-        protoLiteCelValueConverter);
+        protoLiteCelValueConverter,
+        isView);
   }
 
   RawProtoMessageLiteValue() {}

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

@@ -62,6 +62,22 @@ public void create_accessorsAndType() {
     assertThat(value.celType().name()).isEqualTo("custom.Message");
   }
 
+  @Test
+  public void value_selectedSubmessage_returnsCopy() throws Exception {
+    ByteString submessage = encode(out -> out.writeInt32(1, 42));
+    RawProtoMessageLiteValue raw =
+        RawProtoMessageLiteValue.create(
+            encode(out -> out.writeBytes(21, submessage)), EMPTY_CONVERTER);
+    SelectField field =
+        SelectField.create(21L, "single_nested_message", SelectField.MESSAGE_TYPE_CODE);
+    RawProtoMessageLiteValue selected = (RawProtoMessageLiteValue) raw.selectByFieldNumber(field);
+
+    WireMessageLite value = selected.value();
+
+    assertThat(value).isNotSameInstanceAs(selected);
+    assertThat(value.toByteString()).isEqualTo(submessage);
+  }
+
   @Test
   public void create_defaultsUnknownMessageTypeName() {
     ByteString bytes = ByteString.copyFromUtf8("test");