Add AllowInternalUnstable enum to avoid Arc clones for static desugaring feature lists

[spastorino]

This is exploratory followup of #162747

There are upsides and downsides of this. In particular, the bad part is that we grow ExpnData size by 8 bytes. The rest I think is way better this way.
Maybe we can do a perf run and see if it make things worse and decide.

r? @nnethercote

[spastorino]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: f9f1546 (f9f1546379ba808fc075d149550371b9613a0314)
Base parent: 420ed2a (420ed2a0c3d7225b1744266fd884d431b4d8cfe0)

[rust-timer]

Finished benchmarking commit (f9f1546): comparison URL.

Overall result: ❌✅ regressions and improvements - 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

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.2%	[0.2%, 0.2%]	1
Improvements ✅ (primary)	-0.2%	[-0.2%, -0.2%]	1
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.1%]	3
All ❌✅ (primary)	-0.2%	[-0.2%, -0.2%]	1

Max RSS (memory usage)

Results (primary -0.0%, secondary -0.9%)

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

	mean	range	count
Regressions ❌ (primary)	1.1%	[0.8%, 1.4%]	2
Regressions ❌ (secondary)	2.2%	[2.2%, 2.2%]	1
Improvements ✅ (primary)	-2.2%	[-2.2%, -2.2%]	1
Improvements ✅ (secondary)	-2.4%	[-3.6%, -1.2%]	2
All ❌✅ (primary)	-0.0%	[-2.2%, 1.4%]	3

Cycles

Results (primary 3.2%, secondary -0.5%)

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

	mean	range	count
Regressions ❌ (primary)	3.2%	[3.2%, 3.2%]	1
Regressions ❌ (secondary)	2.6%	[2.6%, 2.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.6%	[-3.6%, -3.6%]	1
All ❌✅ (primary)	3.2%	[3.2%, 3.2%]	1

Binary size

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

Bootstrap: 498.333s -> 500.939s (0.52%)
Artifact size: 408.93 MiB -> 408.97 MiB (0.01%)

[nnethercote]

TBH I don't think this is worth it? If it improved performance, then yes. But without a perf improvement it's just a bit more complexity and code for no real gain.

[spastorino]

This was originally an experiment, not very important and we can close it. My reasoning was different I feel the code is better in the sense that we were using an Arc in a very artificial way when what we really needed was a way to split between statically created lists of strs and dynamic ones. What the new enum does is just introducing this idea. I was more on the side of, if this doesn’t make perf worse is probably worth merging.