rustdoc: skip blanket impls on recursive types

[im-lunex]

When a type contains itself through its fields (e.g. struct A(B<A>)),
evaluating blanket impl obligations on it can hang or take exponential
time. Skip blanket impl synthesis entirely for such types.

Adds a TypeVisitor that walks field types with cycle detection and
returns early from synthesize_blanket_impls when a recursive ADT
is found.

fixes #160105.

[rustbot]

Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @notriddle (or someone else) some time within the next two weeks.

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[notriddle]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 1701156 (1701156946ac94f71ac6fe1ac6110e8762eb2841)
Base parent: be3d26d (be3d26db984c6f96335faca1f254dc04873cb1c1)

[rust-timer]

Finished benchmarking commit (1701156): comparison URL.

Overall result: ✅ improvements - no action needed

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.

@bors rollup=never rustc-perf
@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
Improvements ✅ (primary)	-1.0%	[-3.5%, -0.2%]	9
Improvements ✅ (secondary)	-1.4%	[-2.4%, -0.3%]	3
All ❌✅ (primary)	-1.0%	[-3.5%, -0.2%]	9

Max RSS (memory usage)

Results (primary 0.5%, secondary -1.6%)

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

	mean	range	count
Regressions ❌ (primary)	2.7%	[0.4%, 4.1%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.2%	[-2.1%, -0.4%]	4
Improvements ✅ (secondary)	-1.6%	[-1.6%, -1.6%]	1
All ❌✅ (primary)	0.5%	[-2.1%, 4.1%]	7

Cycles

Results (primary -0.6%, secondary -2.6%)

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

	mean	range	count
Regressions ❌ (primary)	0.7%	[0.4%, 1.0%]	2
Regressions ❌ (secondary)	1.2%	[0.4%, 3.7%]	12
Improvements ✅ (primary)	-1.0%	[-3.2%, -0.4%]	7
Improvements ✅ (secondary)	-6.4%	[-21.5%, -0.4%]	12
All ❌✅ (primary)	-0.6%	[-3.2%, 1.0%]	9

Binary size

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

Bootstrap: 489.155s -> 489.631s (0.10%)
Artifact size: 390.64 MiB -> 390.69 MiB (0.01%)