Borrow non-'static data in future with wasm_bindgen_futures

Question

I am launching multiple threads from incoming tcp connections which need to read from a file or anything that implements the std::io::Read and std::io::Seek trait.

I have the following working implementation (1):

fn upload<R: Read + Seek + Send>(file: &mut R, listen_addr: SocketAddr) -> IoResult<()> {
  let listener = TcpListener::bind(listen_addr)?;
  let file: Arc<Mutex<&mut R>> = Arc::new(Mutex::new(file));

  std::thread::scope(|scope| {
    for socket in listener.incoming() {
      let socket = socket.unwrap() // error handling omitted ..
      let f = file.clone();
      scope.spawn(move || {
        start_upload_task(socket, f).unwrap();
      });
    }
  }
  Ok(())
}

fn start_upload_task<R: Read + Seek>(conn: TcpStream, file: Arc<Mutex<&mut R>>) -> IoResult<()>{
  // do something wtih file ..
  let mut buf = vec![0u8; 42];
  {
    let mut file = file.lock().unwrap();
    file.seek(SeekFrom::Start(42))?;
    file.read_exact(&mut buf)?;
  }
  Ok(())
}

However, since I want to use wasm_bindgen_futures::spawn_local, the future must be 'static and there is no equivalent to std::thread::scope in rust webassembly that I know of.

What I end up with is the following (2):

fn upload<R: Read + Seek + Send>(file: &mut R, listen_addr: SocketAddr) -> IoResult<()> {
  let listener = TcpListener::bind(listen_addr)?;
  let file: Arc<Mutex<&mut R>> = Arc::new(Mutex::new(file));

  for socket in listener.incoming() {
    let socket = socket.unwrap() // error handling omitted ..
    let f = file.clone();
    std::thread::spawn(move || {
      start_upload_task(socket, f).unwrap();
    });
  }
  Ok(())
}

But this gives me the following compile error:

borrowed data escapes outside of associated function requirement occurs because of the type Mutex<&mut R>, which makes the generic argument &mut R invariant the struct Mutex<T> is invariant over the parameter T see https://doc.rust-lang.org/nomicon/subtyping.html for more information about variance rustcE0521 lib.rs(258, 47): file is a reference that is only valid in the associated function body

with the following suggestion:

   --> src/lib.rs:273:25
    |
258 |       fn upload<R: Read + Seek + Send>(&self, file: &mut R, listen_addr: SocketAddr) -> IoResult<()> {
    |                                                 ----  - let's call the lifetime of this reference `'1`
    |                                                 |
    |                                                 `file` is a reference that is only valid in the associated function body

How do I get implementation (2) to work?