Lazy-ify some markdown rendering

[yotamofek]

Seems to have a positive effect in my local perf runs 😍

r? @GuillaumeGomez if you're interested, otherwise feel free to reassign
(would also love a perf run)

[GuillaumeGomez]

Code changes look good to me. Curious to see the impact on performance. =D

@bors try @rust-timer queue

[bors]

⌛ Trying commit d6cfa0b with merge c4d5641...

[yotamofek]

@GuillaumeGomez I had a unit test that failed to compile. Would love to re-trigger the perf run (maybe we can just perf the commit that failed the CI but passed the try build?)

[Kobzol]

@rust-timer build c4d5641

[rust-timer]

Finished benchmarking commit (c4d5641): 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 (secondary -2.4%)

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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.4%	[-2.5%, -2.4%]	2
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary -7.0%)

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)	-	-	0
Improvements ✅ (secondary)	-7.0%	[-8.7%, -4.4%]	3
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 690.034s -> 690.459s (0.06%)
Artifact size: 371.99 MiB -> 372.00 MiB (0.00%)

[GuillaumeGomez]

No impact on performance. As is, I don't think it's really worth merging (more code with more nightly features for no perf gain). Letting it open in case you want to tweak some more things to try to improve performance results. :)