[Bug][http-client-js]: the mfd part serialization is problematic

[qiaozha]

Describe the bug

in todoApp, we have mfd operations

model FileAttachmentMultipartRequest {
  contents: HttpPart<File>;
}
    @post
    op createFileAttachment(
      @header contentType: "multipart/form-data",
      @path itemId: TodoItem.id,
      @multipartBody body: FileAttachmentMultipartRequest,
    ): WithStandardErrors<NoContentResponse | NotFoundErrorResponse>;

which the operation inline serialization logic should be enough

export async function createFileAttachment(
  client: AttachmentsClientContext,
  itemId: number,
  body: FileAttachmentMultipartRequest,
  options?: CreateFileAttachmentOptions,): Promise<void> {
  const path = parse("/items/{itemId}/attachments").expand({
    "itemId": itemId
  });
  const httpRequestOptions = {
    headers: {
      "content-type": options?.contentType ?? "multipart/form-data"
    },body: [createFilePartDescriptor("contents", body.contents)], // this should be enough
  };
  const response = await client.pathUnchecked(path).post(httpRequestOptions)

  if (typeof options?.operationOptions?.onResponse === "function") {
    options?.operationOptions?.onResponse(response);
  }
  ;if (+response.status === 204 && !response.body) {
    return;
  }throw createRestError(response);
};

but we also generate the serialization for the models as the following, which is wrong.

export function jsonFileAttachmentMultipartRequestToApplicationTransform(
  input_?: any,): FileAttachmentMultipartRequest {
  if(!input_) {
    return input_ as any;
  }
    return {
     "contents": jsonFileToApplicationTransform(input_.contents)
  }!;
}

Reproduction

https://github.com/allenjzhang/typespec-e2e-demo/tree/main/todoApp/spec

Checklist

• Follow our Code of Conduct
• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion.
• The provided reproduction is a minimal reproducible example of the bug.

[joheredi]

This shouldn't really impact the client, I agree we probably don't want to generate jsonFileAttachmentMultipartRequestToApplicationTransform if it is not used but the operations doesn't seem to use it

[qiaozha]

@timovv the generated serializer is failing the compile in this case, therefore, we still need to fix it.

[timovv]

I tested this on my local with the todo spec and TypeSpec 1.0.0-rc.1 the other day. This particular serializer appears to be compiling (although there seems to be an unrelated issue to do with visibility in another serializer...). What's the exact error that's showing up here? I agree that we shouldn't generate the helper if it's not actually being used, but don't think that this is a super high priority if it's not affecting compilation or the public API surface.

[qiaozha]

Currently the generation is blocked by this #6847 I can't repro this issue. But it's affecting the compilation.

[timovv]

Those errors should be fixed by #7194. But even so, there's still compiled output despite the error diagnostics and when I looked at that the helper didn't have any error.