rustdoc-json: Add tests for unsafe/safe extern blocks (RFC 3484)

[aDotInTheVoid]

Closes #126786, turns out this all Just Works (TM)

Tracking issue: #123743

r? @GuillaumeGomez

[GuillaumeGomez]

Nice, thanks! r=me once CI pass.

[aDotInTheVoid]

@bors r=GuillaumeGomez rollup

[bors]

📌 Commit 8a9b670 has been approved by GuillaumeGomez

It is now in the queue for this repository.

[bors]

⌛ Testing commit 8a9b670 with merge 0321e73...

[bors]

☀️ Test successful - checks-actions
Approved by: GuillaumeGomez
Pushing 0321e73 to master...

[rust-timer]

Finished benchmarking commit (0321e73): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.3%	[-0.3%, -0.3%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (secondary -2.1%)

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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.1%	[-2.1%, -2.1%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (primary 0.9%)

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)	0.9%	[0.9%, 0.9%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.9%	[0.9%, 0.9%]	1

Binary size

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

Bootstrap: 780.411s -> 780.89s (0.06%)
Artifact size: 332.15 MiB -> 332.06 MiB (-0.03%)