[perf] delete no-op caching optimization from (old) trait solver

[lukas-code]

This code path is only taken if the caller bounds are already empty. Found in #107688 (comment).

r? ghost

[the8472]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 5c6deaf with merge 3f8bf38ccb0c12ffdfc99edf4db18cea885fc33a...

[bors]

☀️ Try build successful - checks-actions
Build commit: 3f8bf38ccb0c12ffdfc99edf4db18cea885fc33a (3f8bf38ccb0c12ffdfc99edf4db18cea885fc33a)

[bors]

☀️ Try build successful - checks-actions
Build commit: 3f8bf38ccb0c12ffdfc99edf4db18cea885fc33a (3f8bf38ccb0c12ffdfc99edf4db18cea885fc33a)

[rust-timer]

Finished benchmarking commit (3f8bf38ccb0c12ffdfc99edf4db18cea885fc33a): 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)	-1.3%	[-1.5%, -1.2%]	2
Improvements ✅ (secondary)	-3.6%	[-4.2%, -3.2%]	6
All ❌✅ (primary)	-1.3%	[-1.5%, -1.2%]	2

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)	-3.2%	[-3.3%, -3.1%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.2%	[-3.3%, -3.1%]	2

Cycles

This benchmark run did not return any relevant results for this metric.

[lqd]

Note that cranelift-codegen and keccak are currently noisy and should (hopefully) stabilize soon, so that a new future perf run has more trustworthy results (or do a rustc-perf run locally to see if they're similar).

[lukas-code]

Yeah all improvements here are just the inverse of #107778 (comment), so I'm no convinced that this is worth it.