Build each descriptorless field's descriptor once, not on every hop

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

@@ -464,20 +464,29 @@ boolean hasFieldByNumber(ByteString bytes, String protoTypeName, SelectField fie
     return false;
   }
 
-  /** Describes {@code field} by the type information it carries. */
+  /** Describes {@code field} by the type information it carries, built once and kept on it. */
   private static FieldLiteDescriptor newFieldDescriptor(SelectField field) {
+    Object cached = field.liteFieldDescriptor();
+    if (cached != null) {
+      return (FieldLiteDescriptor) cached;
+    }
     // Only presence tests omit the type code: their presence check doesn't depend on the type, and
     // the fields they navigate through are messages, as are map entries.
     boolean isMap = field.mapEntrySpec() != null;
     boolean isMessage = isMap || field.typeCode() == SelectField.NO_TYPE_CODE;
-    return newFieldDescriptor(
-        field.fieldNumber(),
-        field.fieldName(),
-        isMap
-            ? EncodingType.MAP
-            : (field.defaultValue() instanceof List ? EncodingType.LIST : EncodingType.SINGULAR),
-        isMessage ? SelectField.MESSAGE_TYPE_CODE : field.typeCode(),
-        field.protoTypeName());
+    FieldLiteDescriptor fieldDescriptor =
+        newFieldDescriptor(
+            field.fieldNumber(),
+            field.fieldName(),
+            isMap
+                ? EncodingType.MAP
+                : (field.defaultValue() instanceof List
+                    ? EncodingType.LIST
+                    : EncodingType.SINGULAR),
+            isMessage ? SelectField.MESSAGE_TYPE_CODE : field.typeCode(),
+            field.protoTypeName());
+    field.setLiteFieldDescriptor(fieldDescriptor);
+    return fieldDescriptor;
   }
 
   private static FieldLiteDescriptor newFieldDescriptor(

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

@@ -46,16 +46,17 @@ abstract class RawProtoMessageLiteValue extends StructValue<String, WireMessageL
   public abstract ByteString toByteString();
 
   @Override
-  public abstract CelType celType();
+  public abstract String protoTypeName();
 
   abstract ProtoLiteCelValueConverter protoLiteCelValueConverter();
 
   // Set when toByteString() may alias a parent message's buffer.
   abstract boolean isView();
 
   @Override
-  public String protoTypeName() {
-    return celType().name();
+  public CelType celType() {
+    // Built on demand, since one of these values is created on every hop but rarely asked its type.
+    return StructTypeReference.create(protoTypeName());
   }
 
   @Override
@@ -117,23 +118,22 @@ private CelAttributeNotFoundException newUnoptimizedFieldResolutionException(Str
         String.format(
             "Error resolving field '%s' on '%s'. Field selection by name is not supported on raw"
                 + " proto wire bytes; register its CelLiteDescriptor or enable SelectOptimizer.",
-            field, celType().name()));
+            field, protoTypeName()));
   }
 
   @Override
   public Object selectByFieldNumber(SelectField field) {
-    return protoLiteCelValueConverter()
-        .selectByFieldNumber(toByteString(), celType().name(), field);
+    return protoLiteCelValueConverter().selectByFieldNumber(toByteString(), protoTypeName(), field);
   }
 
   @Override
   public boolean hasFieldByNumber(SelectField field) {
-    return protoLiteCelValueConverter().hasFieldByNumber(toByteString(), celType().name(), field);
+    return protoLiteCelValueConverter().hasFieldByNumber(toByteString(), protoTypeName(), field);
   }
 
   @Override
   public Optional<Object> findByFieldNumber(SelectField field) {
-    return protoLiteCelValueConverter().findByFieldNumber(toByteString(), celType().name(), field);
+    return protoLiteCelValueConverter().findByFieldNumber(toByteString(), protoTypeName(), field);
   }
 
   static RawProtoMessageLiteValue create(
@@ -158,8 +158,7 @@ static RawProtoMessageLiteValue create(
     checkNotNull(protoLiteCelValueConverter);
     return new AutoValue_RawProtoMessageLiteValue(
         rawWireBytes,
-        StructTypeReference.create(
-            protoTypeName.isEmpty() ? UNKNOWN_MESSAGE_TYPE_NAME : protoTypeName),
+        protoTypeName.isEmpty() ? UNKNOWN_MESSAGE_TYPE_NAME : protoTypeName,
         protoLiteCelValueConverter,
         isView);
   }

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

@@ -32,7 +32,8 @@
 @AutoValue
 @AutoValue.CopyAnnotations
 @Immutable
-@SuppressWarnings("Immutable") // Default value is an immutable CEL literal or null
+// Default value is an immutable CEL literal or null; liteFieldDescriptor is a memo of this field.
+@SuppressWarnings("Immutable")
 public abstract class SelectField {
 
   private static final int MAX_FIELD_NUMBER = 536870911;
@@ -83,6 +84,19 @@ public abstract class SelectField {
    */
   abstract @Nullable MapEntrySpec mapEntrySpec();
 
+  // The lite runtime's FieldLiteDescriptor for this field, built from this field alone on first
+  // use so descriptorless hops don't rebuild it. Typed Object to keep this target free of protobuf
+  // deps. A race at most rebuilds it, as the descriptor's fields are all final.
+  private @Nullable Object liteFieldDescriptor;
+
+  @Nullable Object liteFieldDescriptor() {
+    return liteFieldDescriptor;
+  }
+
+  void setLiteFieldDescriptor(Object liteFieldDescriptor) {
+    this.liteFieldDescriptor = liteFieldDescriptor;
+  }
+
   /**
    * Creates a presence-test qualifier hop.
    *

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

@@ -277,6 +277,20 @@ public void selectByFieldNumber_presentOnWire_decoded() throws Exception {
     assertThat(val).isEqualTo("hello");
   }
 
+  @Test
+  public void selectByFieldNumber_twice_reusesFieldDescriptor() throws Exception {
+    RawProtoMessageLiteValue raw =
+        RawProtoMessageLiteValue.create(encode(out -> out.writeInt64(1, 42L)), EMPTY_CONVERTER);
+    SelectField field = SelectField.create(1L, "single_int64", 3, 0L);
+    raw.selectByFieldNumber(field);
+    Object fieldDescriptor = field.liteFieldDescriptor();
+
+    raw.selectByFieldNumber(field);
+
+    assertThat(fieldDescriptor).isNotNull();
+    assertThat(field.liteFieldDescriptor()).isSameInstanceAs(fieldDescriptor);
+  }
+
   @Test
   public void selectByFieldNumber_absentWithDefaultValue_returnsDefault() {
     RawProtoMessageLiteValue raw =