Privacy: fix effective visibility calculation for items reachable through RPIT

[Bryanskiy]

For example, if a trait is ReachableThroughImplTrait, then the defs reachable from the trait should also be marked as ReachableThroughImplTrait. However, this is not the case, since effective visibility is incorrectly limited by nominal visibility.
r? @petrochenkov

[petrochenkov]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: e9de5d9 (e9de5d9b70ebe43e1edce7a1ee56737608d099c3, parent: ba0949ab745985a442e274ba52e8fb348cb0c662)

[rust-timer]

Finished benchmarking commit (e9de5d9): 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)	0.5%	[0.2%, 5.6%]	25
Regressions ❌ (secondary)	0.4%	[0.2%, 1.3%]	21
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.5%	[0.2%, 5.6%]	25

Max RSS (memory usage)

Results (primary -3.7%, secondary -2.0%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.7%	[-4.7%, -2.9%]	3
Improvements ✅ (secondary)	-2.0%	[-2.0%, -2.0%]	1
All ❌✅ (primary)	-3.7%	[-4.7%, -2.9%]	3

Cycles

Results (primary 2.4%, secondary -0.3%)

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

	mean	range	count
Regressions ❌ (primary)	4.8%	[3.0%, 6.6%]	2
Regressions ❌ (secondary)	2.8%	[2.8%, 2.8%]	1
Improvements ✅ (primary)	-2.5%	[-2.5%, -2.5%]	1
Improvements ✅ (secondary)	-3.4%	[-3.4%, -3.4%]	1
All ❌✅ (primary)	2.4%	[-2.5%, 6.6%]	3

Binary size

Results (primary 0.1%)

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

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.2%, 0.2%]	4
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.4%, -0.4%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-0.4%, 0.2%]	5

Bootstrap: 511.658s -> 511.881s (0.04%)
Artifact size: 398.51 MiB -> 400.43 MiB (0.48%)

[Bryanskiy]

The fix is too conservative. Even if there is no RPIT in the crate, it will mark private items as ReachableThroughImplTrait.

[petrochenkov]

This needs an issue reproduction using impl Trait, but not involving associated type bounds, which are a known type privacy hole.
Otherwise it's not clear whether this change actually fixes anything.
@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[rust-bors]

☔ The latest upstream changes (presumably #156228) made this pull request unmergeable. Please resolve the merge conflicts.