rustdoc: Eliminate edition sensitivity around RPIT lifetime capturing

[fmease]

Fixes #135453.

TODO:

• proper PR description
• perf run
• avoid synth'ing redundant use-bounds if feasible wrt. maintainability & perf
• figure out how to deal with APITs (I'm probably "just" gonna go with use<…, /*all in-scope APITs*/>)
• write tests
• document this behavior in the rustdoc book
• issue an FCP-merge

[fmease]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 22299d7 (22299d71997d42c10d254f758953ef37d057b7b5, parent: 1ce45a011a3fe99ab08643d8eb7229bfc3b40bda)

[rust-timer]

Finished benchmarking commit (22299d7): comparison URL.

Overall result: no relevant changes - 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

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

Max RSS (memory usage)

Results (secondary 6.4%)

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)	6.4%	[6.4%, 6.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary 3.5%)

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)	3.5%	[2.1%, 5.3%]	8
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.3%, 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)	0.3%	[0.1%, 0.7%]	19
Regressions ❌ (secondary)	0.3%	[0.0%, 0.4%]	12
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[0.1%, 0.7%]	19

Bootstrap: 513.507s -> 509.419s (-0.80%)
Artifact size: 400.69 MiB -> 400.74 MiB (0.01%)