perf: Use 1 bit to track splatting in FnDecl

[teor2345]

This PR is based on #153697, and is waiting for it to merge.

This PR reduces the perf impact of #153697, resulting in an average 0.24% primary benchmark improvement, and no primary regressions.

This experiment stores one bit for "any argument is splatted" in FnDecl, then searches the arguments' attributes for the actual splatted index (if needed).

It also inlines some hot functions, which likely contributes to the improvement.

[makai410]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 95d605f (95d605f06120a8779456a7f48b4edbc65b46f1b4, parent: ca9a134e0985765ded9cfdde4030a5df4db7e2bd)

[rust-timer]

Finished benchmarking commit (95d605f): comparison URL.

Overall result: ❌ regressions - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.1%, 0.2%]	7
Regressions ❌ (secondary)	0.3%	[0.2%, 0.4%]	11
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	1
All ❌✅ (primary)	0.2%	[0.1%, 0.2%]	7

Max RSS (memory usage)

Results (primary 1.0%, secondary -5.5%)

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

	mean	range	count
Regressions ❌ (primary)	1.0%	[1.0%, 1.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-5.5%	[-5.5%, -5.5%]	1
All ❌✅ (primary)	1.0%	[1.0%, 1.0%]	1

Cycles

Results (primary 2.3%, secondary 2.3%)

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

	mean	range	count
Regressions ❌ (primary)	2.3%	[2.3%, 2.3%]	2
Regressions ❌ (secondary)	2.3%	[2.3%, 2.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.3%	[2.3%, 2.3%]	2

Binary size

Results (primary 0.1%, secondary 0.2%)

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

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.2%]	84
Regressions ❌ (secondary)	0.2%	[0.0%, 0.7%]	58
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.0%, 0.2%]	84

Bootstrap: 488.117s -> 488.584s (0.10%)
Artifact size: 393.52 MiB -> 393.62 MiB (0.03%)

[teor2345]

Perf between this PR and its parent #153697 is an 0.2% overall average improvement, with a few primary benchmarks regressing.

Perf between this PR and its grandparent (rollup) #155687 is an 0.24% average improvement, and no primary benchmarks regress.

So this is a win, new feature and better perf from the associated refactors.

Edit: secondary regressions (worst first) are issue #46449, then coercions, then wg-grammar/externs.

[jieyouxu]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 72799b4 (72799b4e5e6830b7189ffbb40f0362bd1b0d896e, parent: 4933094f25ba1edf2de0a203665976832fc85bc9)

[rust-timer]

Finished benchmarking commit (72799b4): comparison URL.

Overall result: ❌ regressions - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.1%, 0.2%]	11
Regressions ❌ (secondary)	0.3%	[0.2%, 0.4%]	13
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[0.1%, 0.2%]	11

Max RSS (memory usage)

Results (primary 3.5%, secondary 3.5%)

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

	mean	range	count
Regressions ❌ (primary)	3.5%	[3.5%, 3.5%]	1
Regressions ❌ (secondary)	3.5%	[1.1%, 5.8%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.5%	[3.5%, 3.5%]	1

Cycles

Results (secondary -2.2%)

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)	-2.2%	[-2.2%, -2.2%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.1%, secondary 0.2%)

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

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.2%]	84
Regressions ❌ (secondary)	0.2%	[0.0%, 0.7%]	58
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.0%, 0.2%]	84

Bootstrap: 488.817s -> 487.887s (-0.19%)
Artifact size: 393.48 MiB -> 393.56 MiB (0.02%)