Fix validation for messages with required fields

Currently, validation fails for proto 2 messages with required fields.
Update the code which loads constraints for a message to use
buildPartial() instead of parseFrom() to allow messages with required
fields to work.

Fixes #41.

src/main/java/build/buf/protovalidate/internal/evaluator/EvaluatorBuilder.java

@@ -102,7 +102,7 @@ private Evaluator build(Descriptor desc) throws CompilationException {
   private void buildMessage(Descriptor desc, MessageEvaluator msgEval) throws CompilationException {
     try {
       DynamicMessage defaultInstance =
-          DynamicMessage.parseFrom(desc, new byte[0], EXTENSION_REGISTRY);
+          DynamicMessage.newBuilder(desc).mergeFrom(new byte[0], EXTENSION_REGISTRY).buildPartial();
       Descriptor descriptor = defaultInstance.getDescriptorForType();
       MessageConstraints msgConstraints =
           resolver.resolveMessageConstraints(descriptor, EXTENSION_REGISTRY);

src/test/java/build/buf/protovalidate/ValidatorDynamicMessageTest.java

@@ -20,6 +20,7 @@
 import com.example.noimports.validationtest.ExampleFieldConstraints;
 import com.example.noimports.validationtest.ExampleMessageConstraints;
 import com.example.noimports.validationtest.ExampleOneofConstraints;
+import com.example.noimports.validationtest.ExampleRequiredFieldConstraints;
 import com.google.protobuf.DescriptorProtos;
 import com.google.protobuf.Descriptors;
 import com.google.protobuf.DynamicMessage;
@@ -86,6 +87,31 @@ public void testMessageConstraintDynamicMessage() throws Exception {
         .containsExactly(expectedViolation);
   }
 
+  @Test
+  public void testRequiredFieldConstraintDynamicMessage() throws Exception {
+    DynamicMessage.Builder messageBuilder =
+        createMessageWithUnknownOptions(ExampleRequiredFieldConstraints.getDefaultInstance());
+    messageBuilder.setField(
+        messageBuilder.getDescriptorForType().findFieldByName("regex_string_field"), "abc123");
+    assertThat(new Validator().validate(messageBuilder.build()).getViolations()).isEmpty();
+  }
+
+  @Test
+  public void testRequiredFieldConstraintDynamicMessageInvalid() throws Exception {
+    DynamicMessage.Builder messageBuilder =
+        createMessageWithUnknownOptions(ExampleRequiredFieldConstraints.getDefaultInstance());
+    messageBuilder.setField(
+        messageBuilder.getDescriptorForType().findFieldByName("regex_string_field"), "0123456789");
+    Violation expectedViolation =
+        Violation.newBuilder()
+            .setConstraintId("string.pattern")
+            .setFieldPath("regex_string_field")
+            .setMessage("value does not match regex pattern `^[a-z0-9]{1,9}$`")
+            .build();
+    assertThat(new Validator().validate(messageBuilder.build()).getViolations())
+        .containsExactly(expectedViolation);
+  }
+
   private static void gatherDependencies(
       Descriptors.FileDescriptor fd, Set<DescriptorProtos.FileDescriptorProto> dependencies) {
     dependencies.add(fd.toProto());

src/test/resources/proto/validationtest/required.proto

@@ -0,0 +1,24 @@
+// Copyright 2023 Buf Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+syntax = "proto2";
+
+package validationtest;
+
+import "buf/validate/validate.proto";
+
+message ExampleRequiredFieldConstraints {
+  required string regex_string_field = 1 [(buf.validate.field).string.pattern = "^[a-z0-9]{1,9}$"];
+  optional string unconstrained = 2;
+}