must_use attribute not used properly

[de-sh]

Currently, the following warning is shown by the compiler, as the must_use attribute is being applied to a Future and not the actual function that is supposed to be called. A solution had been trialed in #342 but has been deemed not right.

warning: `must_use` attribute on `async` functions applies to the anonymous `Future` returned by the function, not the value within
Warning:    --> rumqttc/src/eventloop.rs:124:5
    |
124 |       #[must_use = "Eventloop should be iterated over a loop to make progress"]
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
125 | /     pub async fn poll(&mut self) -> Result<Event, ConnectionError> {
126 | |         if self.network.is_none() {
127 | |             let (network, connack) = connect_or_cancel(&self.options, &self.cancel_rx).await?;
128 | |             self.network = Some(network);
...   |
143 | |         }
144 | |     }
    | |_____- this attribute does nothing, the `Future`s returned by async functions are already `must_use`
    |
    = note: `#[warn(unused_attributes)]` on by default

[jplatte]

Both Futures and Results are already #[must_use]. This attribute should simply be removed, there's (unfortunately) no way to show this custom error instead of the ones for Future / Result when the user ignores the future, or its yielded value.

I suppose you could consider making the Event type #[must_use], with a somewhat different message.