Add Java FileDescriptor.copyHeadingTo() which copies file-level settings (e.g. syntax, package, file options) to FileDescriptorProto.Builder

PiperOrigin-RevId: 516635134

Author: zhangskz

java/core/src/main/java/com/google/protobuf/Descriptors.java

@@ -182,6 +182,17 @@ public Syntax getSyntax() {
       return Syntax.PROTO2;
     }
 
+    public void copyHeadingTo(FileDescriptorProto.Builder protoBuilder) {
+      protoBuilder.setName(getName()).setSyntax(getSyntax().name);
+      if (!getPackage().isEmpty()) {
+        protoBuilder.setPackage(getPackage());
+      }
+
+      if (!getOptions().equals(FileOptions.getDefaultInstance())) {
+        protoBuilder.setOptions(getOptions());
+      }
+    }
+
     /**
      * Find a message type in the file by name. Does not find nested types.
      *

java/core/src/test/java/com/google/protobuf/DescriptorsTest.java

@@ -159,6 +159,40 @@ public void testFileDescriptorGetSyntax() throws Exception {
     assertThat(file3.getSyntax()).isEqualTo(Descriptors.FileDescriptor.Syntax.PROTO3);
   }
 
+  @Test
+  public void testFileDescriptorCopyHeadingTo() throws Exception {
+    FileDescriptorProto.Builder protoBuilder =
+        FileDescriptorProto.newBuilder()
+            .setName("foo.proto")
+            .setPackage("foo.bar.baz")
+            .setSyntax("proto2")
+            .setOptions(FileOptions.newBuilder().setJavaPackage("foo.bar.baz").build())
+            // Won't be copied.
+            .addMessageType(DescriptorProto.newBuilder().setName("Foo").build());
+    FileDescriptor file2 =
+        Descriptors.FileDescriptor.buildFrom(protoBuilder.build(), new FileDescriptor[0]);
+    FileDescriptorProto.Builder protoBuilder2 = FileDescriptorProto.newBuilder();
+    file2.copyHeadingTo(protoBuilder2);
+    FileDescriptorProto toProto2 = protoBuilder2.build();
+    assertThat(toProto2.getName()).isEqualTo("foo.proto");
+    assertThat(toProto2.getPackage()).isEqualTo("foo.bar.baz");
+    assertThat(toProto2.getSyntax()).isEqualTo("proto2");
+    assertThat(toProto2.getOptions().getJavaPackage()).isEqualTo("foo.bar.baz");
+    assertThat(toProto2.getMessageTypeList()).isEmpty();
+
+    protoBuilder.setSyntax("proto3");
+    FileDescriptor file3 =
+        Descriptors.FileDescriptor.buildFrom(protoBuilder.build(), new FileDescriptor[0]);
+    FileDescriptorProto.Builder protoBuilder3 = FileDescriptorProto.newBuilder();
+    file3.copyHeadingTo(protoBuilder3);
+    FileDescriptorProto toProto3 = protoBuilder3.build();
+    assertThat(toProto3.getName()).isEqualTo("foo.proto");
+    assertThat(toProto3.getPackage()).isEqualTo("foo.bar.baz");
+    assertThat(toProto3.getSyntax()).isEqualTo("proto3");
+    assertThat(toProto2.getOptions().getJavaPackage()).isEqualTo("foo.bar.baz");
+    assertThat(toProto3.getMessageTypeList()).isEmpty();
+  }
+
   @Test
   public void testDescriptor() throws Exception {
     Descriptor messageType = TestAllTypes.getDescriptor();