perf: Store 1 bit for splatting in FnDeclFlags

[teor2345]

This PR tries to fix a perf regression in 1 primary benchmark introduced by #157699, by reducing the extra data in FnDecl to 1 bit (packed into existing free bits).

Regression details: #157699 (comment)

[panstromek]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: a9e2a2a (a9e2a2a9a4f966e632b0252ea4a224cf87f73c7c, parent: 9d862dd2c17b2dc90e9f623bb9279a8c4090b311)

[rust-timer]

Finished benchmarking commit (a9e2a2a): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

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

Instruction count

This perf run didn't have relevant results for this metric.

Max RSS (memory usage)

Results (secondary -2.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.0%	[-2.0%, -2.0%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -2.7%, secondary 0.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.5%	[2.5%, 2.5%]	1
Improvements ✅ (primary)	-2.7%	[-2.7%, -2.7%]	1
Improvements ✅ (secondary)	-2.3%	[-2.3%, -2.3%]	1
All ❌✅ (primary)	-2.7%	[-2.7%, -2.7%]	1

Binary size

Results (primary -0.1%, secondary -0.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.0%]	53
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.0%]	23
All ❌✅ (primary)	-0.1%	[-0.1%, -0.0%]	53

Bootstrap: 519.36s -> 519.482s (0.02%)
Artifact size: 400.90 MiB -> 401.36 MiB (0.11%)

[teor2345]

Well that's good I guess, the complicated fix has no perf impact, so we don't need to do it.

[panstromek]

The result can also be interpreted as slightly positive. If you tick show non relevant results, most of them are green. That can be misleading, because each one got marked as insignificant on its own, but if they are collectively largely green, then that's usually a good sign.

unicode-normalization incr-unchanged (the one you're trying to address) also shows improvements in metadata related queries in detailed results. That table uses cpu-clock which is a bit noisy, so it's also not such a strong signal (and there are bigger changes on other queries or variants of this benchmark).

We have to take all of this with a massive grain of salt but I'd be cautiously optimistic about the result :D.

[teor2345]

I would expect most results to be positive because we're reducing the data size of FnDef, so there's less data being copied around.

Let's merge this as a partial mitigation of the regression in #157699 (comment)

[rustbot]

HIR ty lowering was modified

cc @fmease

[rustbot]

r? @davidtwco

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[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

...............................F.................. (50/144)
.................................................. (100/144)
............................................       (144/144)

======== tests/rustdoc-gui/go-to-collapsed-elem.goml ========

[ERROR] line 40
    at `tests/rustdoc-gui/go-to-collapsed-elem.goml` line 21: Error: Node is detached from document: for command `click: "//*[@id='search']//a[@href='../test_docs/struct.Foo.html#method.must_use']"`
    at <file:///checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/doc/test_docs/struct.Foo.html?search=t_use>


<= doc-ui tests done: 143 succeeded, 1 failed, 0 filtered out

Error: ()