rustdoc: use dyn Fn in run_test to avoid monomorphization bloat

[lolbinarycat]

I wrote a little program to evaluate monomorphization bloat, and it said this could save several kB alone, I'm curious what perf would look like on this.

[lolbinarycat]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: bd12758 (bd127588dbab06298daaefd7474fa435ed3fc2ce, parent: 4b1b6dde0c15243857165672a1c616a4523ef32a)

[rust-timer]

Finished benchmarking commit (bd12758): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

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

Max RSS (memory usage)

Results (primary -1.3%)

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

	mean	range	count
Regressions ❌ (primary)	3.1%	[3.1%, 3.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-5.7%	[-5.7%, -5.7%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.3%	[-5.7%, 3.1%]	2

Cycles

Results (primary 2.4%)

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

	mean	range	count
Regressions ❌ (primary)	2.4%	[2.1%, 2.8%]	6
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.4%	[2.1%, 2.8%]	6

Binary size

Results (primary 0.1%, secondary 0.1%)

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

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.1%]	51
Regressions ❌ (secondary)	0.1%	[0.0%, 0.1%]	23
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.0%, 0.1%]	51

Bootstrap: 474.164s -> 470.512s (-0.77%)
Artifact size: 386.17 MiB -> 386.18 MiB (0.00%)