Question on per-subscriber filter internals

[xuorig]

Hey folks, I've been trying to understand tracing's internals better and there's a question that's been bugging me and I haven't been able to figure it out.

Let's imagine we've got something like this, ignoring the fact both subscribers are FmtSubscribers here:

fn main() {
    let reg = Registry::default()
        .with(FmtSubscriber::default().with_filter(EnvFilter::new("info")))
     .with(FmtSubscriber::default().with_filter(EnvFilter::new("tokio=trace,runtime=trace")));
    let _ = set_global_default(reg);

    info!("test");
}

We've got two subscribers with a per-subscriber filter each. None of the filters have "dynamics", so the static callsite should be enough to say that the first one is an Always and the second one is a Never.

When the callsites are registered through a Filtered, it looks like they are combined into a Sometimes in the FILTERING thread local.

This then ends up calling enabled on the collector for every single event. From my perspective, in this specific example, it looks like enabled would always return true for this callsite. When it comes to per-subscriber filters, only if all of them return false would the overall enabled return false.

I get that enabled is used by each filter to evaluate their filter and store it in FILTERED. What I'm wondering is why can't we skip enabled by returning Always here? Then the filtering could still be done in event_enabled? ?

Note: This is not a problem, just seeking to understand internals a bit better.

Thanks!