[perf] rustdoc: Rewrite blanket impl synthesis

[fmease]

Alternative to #125907.
I'm pretty sure this is a lot worse and probably quite a bad idea. Anyways..

Fixes #114891.
Fixes #155759.

[fmease]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 1dae8fc (1dae8fc596adcf9f8bdec7de489bdc48590f8fa4, parent: 9838411cb723b60dc62b1625751075c4d933b992)

[rust-timer]

Finished benchmarking commit (1dae8fc): 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)	24.8%	[1.6%, 146.9%]	19
Regressions ❌ (secondary)	10.1%	[0.4%, 42.8%]	10
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.1%	[-3.1%, -3.1%]	1
All ❌✅ (primary)	24.8%	[1.6%, 146.9%]	19

Max RSS (memory usage)

Results (primary 22.0%, secondary 12.5%)

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

	mean	range	count
Regressions ❌ (primary)	22.0%	[1.1%, 61.9%]	17
Regressions ❌ (secondary)	14.9%	[5.6%, 30.9%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.0%	[-2.0%, -2.0%]	1
All ❌✅ (primary)	22.0%	[1.1%, 61.9%]	17

Cycles

Results (primary 22.6%, secondary 14.4%)

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

	mean	range	count
Regressions ❌ (primary)	22.6%	[2.9%, 90.5%]	17
Regressions ❌ (secondary)	17.4%	[2.9%, 41.6%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.7%	[-3.7%, -3.7%]	1
All ❌✅ (primary)	22.6%	[2.9%, 90.5%]	17

Binary size

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

Bootstrap: 486.61s -> 489.145s (0.52%)
Artifact size: 394.05 MiB -> 394.03 MiB (-0.00%)