Don't serialize empty predicates to metadata

[Mark-Simulacrum]

This keeps empty predicates out of the metadata. In practice, it seems like it
is quite common to have a default predicate set, so this should be a win.
Diesel, for example, has 40% of decoded predicates being default/empty.

I believe this is an attempt to implement this suggestion, and hopefully will help with that regression.

[rust-highfive]

r? @oli-obk

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[Mark-Simulacrum]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit 7cb427e with merge 485510f732a05f117b3daa440af2d0413fff7cb4...

[bors]

☀️ Try build successful - checks-actions
Build commit: 485510f732a05f117b3daa440af2d0413fff7cb4 (485510f732a05f117b3daa440af2d0413fff7cb4)

[rust-timer]

Queued 485510f732a05f117b3daa440af2d0413fff7cb4 with parent 5ecc8ad, future comparison URL.

[rust-timer]

Finished benchmarking commit (485510f732a05f117b3daa440af2d0413fff7cb4): comparison url.

Summary: This change led to small relevant mixed results 🤷 in compiler performance.

• Very small improvement in instruction counts (up to -0.3% on incr-full builds of ctfe-stress-4)
• Small regression in instruction counts (up to 0.4% on full builds of coercions)

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR led to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf +perf-regression

[Mark-Simulacrum]

Doesn't seem like this really has an effect. Closing.