Disambiguate identical type names in impl headers

[qaijuang]

Render impl header paths with enough qualification when the same visible path segment refers to different DefIds in that header.

Fixes #154960.

[rustbot]

r? @GuillaumeGomez

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The reviewer was selected based on:

• Owners of files modified in this PR: rustdoc
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• Random selection from GuillaumeGomez, fmease, lolbinarycat, notriddle

[GuillaumeGomez]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: bec2324 (bec23244bd1d9bc9b23421589d1085e218900a59, parent: 82bee965077a631d6fbdee4014f2ec535535aaa3)

[rust-timer]

Finished benchmarking commit (bec2324): comparison URL.

Overall result: ❌ regressions - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@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)	0.2%	[0.2%, 0.2%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[0.2%, 0.2%]	1

Max RSS (memory usage)

Results (primary -2.2%)

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)	-2.2%	[-2.7%, -1.5%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.2%	[-2.7%, -1.5%]	3

Cycles

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

Binary size

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

Bootstrap: 497.725s -> 508.058s (2.08%)
Artifact size: 397.13 MiB -> 397.10 MiB (-0.01%)

[GuillaumeGomez]

Small perf impact, nice. Although, we are currently discussing to do something similar (disambiguate item's name) but everywhere. Not too sure if we should merge this as is or if we should do the whole work at once...

An opinion @rust-lang/rustdoc-frontend ?

[rust-bors]

☔ The latest upstream changes (presumably #157586) made this pull request unmergeable. Please resolve the merge conflicts.