error[E0308]: mismatched types
  --> src\client.rs:75:9
   |
75 |         tokio_executor::spawn(async move {
   |         ^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected type `std::ops::FnOnce<(client_messages::ClientMessage<std::borrow::Cow<'_, str>>,)>`
              found type `std::ops::FnOnce<(client_messages::ClientMessage<std::borrow::Cow<'_, str>>,)>`