rustdoc: use the next solver for blanket impl synthesis

[fmease]

Furthermore use an ObligationCtxt instead of operating on an InferCtxt directly and don't drop the obligations generated by the type relating (fixes a FIXME).

Originally I just wanted to submit the infcx→ocx change. However, that regressed tests/rustdoc-ui/ice-blanket-impl-52873.rs (pass→overflow error) because with ocx.select_where_possible we would no longer suppress / defatalize (canonical) overflow errors. CC #54199 which introduced that special case. Obviously in the next solver overflows are non-fatal incl. ice-blanket-impl-52873.rs. Hence the switch now.

Note that I wanted to hold off on switching to the new solver since it makes #114891 go from long I-compiletime to I-compilemem + I-hang + eventual death by the OOM killer. So I don't know maybe we should block this PR on me finding a rustc reproducer for the rustdoc issue #114891 (this is on my agenda) to be able to properly investigate the underlying next-solver issue.

[fmease]

@bors try @rust-timer queue

[fmease]

I guess I'll also do a crater run 🤔? I don't want to accidentally make rustdoc hang or ice for half the ecosystem.

[fmease]

Let's try again. Bors, please respond this time around!

@bors try @rust-timer queue

[fmease]

@bors try

[bors]

⌛ Trying commit 737b8ac with merge dcbecf2...

[bors]

☀️ Try build successful - checks-actions
Build commit: dcbecf2 (dcbecf205cd3ecd75a91d7290ac77e62f9c4bad2)

[rust-timer]

Finished benchmarking commit (dcbecf2): comparison URL.

Overall result: ❌ regressions - 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.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@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)	988.1%	[0.6%, 18344.5%]	19
Regressions ❌ (secondary)	5.4%	[1.0%, 11.2%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	988.1%	[0.6%, 18344.5%]	19

Max RSS (memory usage)

Results (primary 23.3%, secondary 5.2%)

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)	23.3%	[3.5%, 54.7%]	11
Regressions ❌ (secondary)	5.2%	[4.4%, 6.1%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	23.3%	[3.5%, 54.7%]	11

Cycles

Results (primary 824.6%, secondary 7.0%)

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)	824.6%	[2.2%, 13651.3%]	17
Regressions ❌ (secondary)	11.5%	[10.8%, 12.2%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.0%	[-2.0%, -2.0%]	1
All ❌✅ (primary)	824.6%	[2.2%, 13651.3%]	17

Binary size

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

Bootstrap: 667.872s -> 669.107s (0.18%)
Artifact size: 318.92 MiB -> 318.92 MiB (0.00%)

[fmease]

okay lol, that is crazy