Don't transmute &List<GenericArg> <-> &List<Ty>

[WaffleLapkin]

In #93505 we allowed safely transmuting between &List<GenericArg<'_>> and &List<Ty<'_>>. This was possible because GenericArg is a tagged pointer and the tag for types is 0b00, such that a GenericArg with a type inside has the same layout as Ty.

While this was meant as an optimization, it doesn't look like it was actually any perf or max-rss win (see #94799 (comment), #94841, #110496 (comment)).

Additionally the way it was done is quite fragile — unsafe code was not properly documented or contained in a module, types were not marked as repr(C) (making the transmutes possibly unsound). All of this makes the code maintenance harder and blocks other possible optimizations (as an example I've found out about these transmutes when my change caused them to sigsegv compiler).

Thus, I think we can safely (pun intended) remove those transmutes, making maintenance easier, optimizations possible, code less cursed, etc.

r? @compiler-errors

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[WaffleLapkin]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 10ec03c with merge 6156b93350a5f3c622e7eec405ce70b7f72da35f...

[compiler-errors]

(sorry, i like the title but it has nothing to do with what the pr's actually doing, so i changed it 😅)

[bors]

☀️ Try build successful - checks-actions
Build commit: 6156b93350a5f3c622e7eec405ce70b7f72da35f (6156b93350a5f3c622e7eec405ce70b7f72da35f)

[rust-timer]

Finished benchmarking commit (6156b93350a5f3c622e7eec405ce70b7f72da35f): comparison URL.

Overall result: ✅ improvements - no action needed

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 may lead to changes in compiler perf.

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

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.0%	[-2.8%, -0.7%]	3
Improvements ✅ (secondary)	-5.2%	[-7.4%, -0.3%]	5
All ❌✅ (primary)	-2.0%	[-2.8%, -0.7%]	3

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.9%	[1.9%, 1.9%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.3%	[-3.3%, -0.5%]	4
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.7%	[-3.3%, 1.9%]	5

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.2%	[2.2%, 2.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-5.2%	[-9.0%, -3.1%]	3
All ❌✅ (primary)	-	-	0

[compiler-errors]

cool, r=me

Seems from the PR that seldom in practice do we ever need to interconvert between SubstsRef and List<Ty>, and it seems not worth the complication or additional unsafe transmutes in the compiler.

[WaffleLapkin]

what

[WaffleLapkin]

Well, that's cool with me, I'll cleanup & r=errs tomorrow.

[compiler-errors]

cc #94799 (comment)
cc #93505
cc #94841

Seems like it may have already been understood that this transmute wasn't really saving much perf in reality. Seems like max-rss isn't affected either. Lgtm.

[WaffleLapkin]

@bors r=compiler-errors rollup-

[bors]

📌 Commit 10ec03c has been approved by compiler-errors

It is now in the queue for this repository.

[WaffleLapkin]

@bors rollup=never

[bors]

⌛ Testing commit 10ec03c with merge df0d9b4...

[bors]

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing df0d9b4 to master...

[rust-timer]

Finished benchmarking commit (df0d9b4): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.5%	[3.5%, 3.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.5%	[-1.5%, -1.5%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.9%	[-3.9%, -3.9%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	4.4%	[4.4%, 4.4%]	2
Improvements ✅ (primary)	-2.8%	[-2.8%, -2.8%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.8%	[-2.8%, -2.8%]	1