Optimize empty token streams

[nnethercote]

Empty token streams are common. Every one causes an allocation for the Arc::new(). We can instead have a single shared empty TokenStream and AttrTokenStream.

r? @Kobzol

[nnethercote]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 11dd43e (11dd43efe8e73314a31dde8b820722be36da3904)
Base parent: 5321a4f (5321a4f40c957cf3587c055e77461febc2ebc865)

[rust-timer]

Finished benchmarking commit (11dd43e): 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)

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

Cycles

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

Binary size

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

Bootstrap: missing data
Artifact size: 402.71 MiB -> 403.53 MiB (0.20%)

[Kobzol]

(Sorry, a rustc-perf failure, looking into it)

[Kobzol]

@rust-timer build 11dd43e

[rust-timer]

Finished benchmarking commit (11dd43e): comparison URL.

Overall result: ✅ improvements - no action needed

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.

@bors rollup=never rustc-perf
@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%	[0.2%, 0.2%]	1
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	2
Improvements ✅ (primary)	-0.2%	[-0.3%, -0.1%]	18
Improvements ✅ (secondary)	-0.3%	[-0.6%, -0.1%]	27
All ❌✅ (primary)	-0.2%	[-0.3%, 0.2%]	19

Max RSS (memory usage)

Results (primary -0.8%, secondary -0.6%)

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

	mean	range	count
Regressions ❌ (primary)	1.5%	[1.0%, 1.9%]	2
Regressions ❌ (secondary)	4.2%	[2.7%, 8.3%]	7
Improvements ✅ (primary)	-1.4%	[-2.3%, -0.6%]	9
Improvements ✅ (secondary)	-2.4%	[-5.0%, -0.6%]	18
All ❌✅ (primary)	-0.8%	[-2.3%, 1.9%]	11

Cycles

Results (primary -2.6%, secondary 9.4%)

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)	9.4%	[6.6%, 12.6%]	4
Improvements ✅ (primary)	-2.6%	[-2.6%, -2.6%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.6%	[-2.6%, -2.6%]	1

Binary size

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

Bootstrap: 477.635s -> 475.679s (-0.41%)
Artifact size: 402.71 MiB -> 403.53 MiB (0.20%)

[Kobzol]

Makes sense. You can r=me.

[rustbot]

Kobzol is not on the review rotation at the moment.
They may take a while to respond.

[nnethercote]

@bors r=Kobzol

[rust-bors]

📌 Commit 5f96a1c has been approved by Kobzol

It is now in the queue for this repository.