Recent 12% perf regression in unused-warnings

[alexcrichton]

A recent perf regression showed up somewhere in this range. I don't think perf has enough data to narrow it down further yet to a particular PR, but of the PRs in that list #51895 looks the most suspect.

@nikomatsakis or @tmandry y'all may be interested in this!

[alexcrichton]

The wall time measurements look a bit more severe :(

also cc @nnethercote

[nnethercote]

perf.rust-lang.org now clearly points to PR #51895:
a739c51...8dd715e

[pnkfelix]

visiting for triage. Assigning to @nikomatsakis for now, under assumption that he is best person to investigate this in the very short term.

[pnkfelix]

also making P-high for now. (I'm not 100% clear on how important the efficiency of this particular benchmark is, but it is probably worth treating it as high until someone can provide an argument for deprioritization.)

[nikomatsakis]

I can investigate, but I'm not all that worried about this particular benchmark. It'd be good to do a quick profile though. I can't really imagine how this would make things slower apart from the fact that I did add a bit of data to metadata -- so maybe it's just the extra encoding costs?

[nikomatsakis]

I've not done any investigation this week. I lost access to my linux machine I use for profiling, so it will have to wait until 2 weeks from now when I'm back home. (Unless someone else wants to take a look.)

[pnkfelix]

visiting for triage, assigning to self in hopes I find time to look at this during next week.