Look up span for event

[nightkr]

Feature Request

Crates

• tracing-subscriber

Motivation

Currently you need to handle a lot of cases yourself when writing a Layer that tries to map an Event back to its Span stack. To handle both implicit and explicit span association you more or less need the following logic:

if event.is_root() {
  None
} else if event.is_contextual() {
  ctx.lookup_current()
} else {
  event.parent().and_then(|id| ctx.span(id))
}

Otherwise you might get bugs like the following event being associated with the wrong Span!:

let span_a = info_span!("a");
let span_b = info_span!("b").entered();
info!(parent: span_a, "hey");

Aside from being tedious and encoding a lot of logic about how Events work internally, even seasoned Tracing users get this wrong, both informally and in published libraries (note: this is not a dig against @davidbarsky, just an example of the confusion that this can cause).

Motivated by some discussion on Discord, starting at: https://canary.discord.com/channels/500028886025895936/627649734592561152/852143116244221973

Proposal

Add a Context::event_span method that encodes this logic. Add documentation to Context::current_span and Event::parent referring to it.

Alternatives

Another option would be to have tracing::event!("message") (with no explicit parent) roughly equivalent to tracing::event!(parent: tracing::dispatch::get_current().current_span(), "message"), just like tracing::span! does. That would mean that Event::parent always Does The Right Thing, and no new API is required. @hawkw seemed to be against this path, but to be honest I never quite got what the problem is here.