Events `.send_batch` makes a `trace!` entry even for empty insertions

[CGMossa]

Bevy version

main

What you did

When using RUST_LOG=bevy_ecs=trace to figure out what is going on, there is a lot of entries:

2022-06-15T07:20:53.034538Z TRACE bevy_ecs::event: Events::extend() -> ids: (254..254)
2022-06-15T07:20:53.034966Z TRACE bevy_ecs::event: Events::extend() -> ids: (159..159)
2022-06-15T07:20:53.035293Z TRACE bevy_ecs::event: Events::extend() -> ids: (159..159)
2022-06-15T07:20:53.036467Z TRACE bevy_ecs::event: Events::extend() -> ids: (254..254)
2022-06-15T07:20:53.036863Z TRACE bevy_ecs::event: Events::extend() -> ids: (159..159)
2022-06-15T07:20:53.037252Z TRACE bevy_ecs::event: Events::extend() -> ids: (159..159)
2022-06-15T07:20:53.039673Z TRACE bevy_ecs::event: Events::extend() -> ids: (254..254)

These denotes empty insertions of events.

This is a minimal snippet that showcases this issue:

//! Run with
//! ```text
//! RUST_LOG="bevy_ecs=trace" cargo run --bin bevy_ecs_events_extend
//! ```
use bevy_app::prelude::*;
use bevy_ecs::prelude::*;
use blofeld_asf::bevy_app_toolbox::BevyDiagnosticPlugins;
use blofeld_asf::bevy_app_toolbox::DefaultPlugins;
use blofeld_asf::weekly_model::runner::run_until_app_exit;

#[derive(Debug)]
struct AEvent;

fn main() {
    let mut app = App::new();
    app.add_plugins(DefaultPlugins);
    app.add_plugins(BevyDiagnosticPlugins);

    app.add_event::<AEvent>();

    app.add_startup_system(send_one_event);
    // app.add_system(send_one_event);

    app.set_runner(run_until_app_exit);
    app.run();
}

fn send_one_event(mut a_event_wrt: EventWriter<AEvent>) {
    // a_event_wrt.send_batch(std::iter::once(AEvent));
    a_event_wrt.send_batch(std::iter::once(AEvent).skip(1)); // empty iterator of events
}

What went wrong

I'm not sure it is a good idea to log insertions of nothing.
Nor do I want to check for this every time I want to send off events, because this requires an allocation:

    // this causes a call to extend even if `aging_events_tick` is empty.
    // thus we are just going to allocate here:
    let aging_events_tick = aging_events_tick.collect_vec();
    if !aging_events_tick.is_empty() {
        aging_events.send_batch(aging_events_tick.into_iter());
    }

This is not a bug but I don't want to fill logging with empty insertions, and re-allocating every time
I want to send events seems wasteful.
I tried to post this on the discord first, but didn't get any traction there.

Additional information

The responsible chunk is this:

impl<E: Event> std::iter::Extend<E> for Events<E> {
    fn extend<I>(&mut self, iter: I)
    where
        I: IntoIterator<Item = E>,
    {
        let mut event_count = self.event_count;
        let events = iter.into_iter().map(|event| {
            let event_id = EventId {
                id: event_count,
                _marker: PhantomData,
            };
            event_count += 1;
            EventInstance { event_id, event }
        });

        self.events_b.extend(events);

        trace!(
            "Events::extend() -> ids: ({}..{})",
            self.event_count,
            event_count
        );
        self.event_count = event_count;
    }
}