Simplify code to generate line numbers in highlight

[GuillaumeGomez]

Follow-up of #146992.

cc @yotamofek
r? @lolbinarycat

[GuillaumeGomez]

I don't think it'll impact performance but just in case:

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: b384be5 (b384be56b050599ec6a78499b3413da442fb9150, parent: 34a8c7368c84fc699fc83a8851a02f93fd655931)

[rust-timer]

Finished benchmarking commit (b384be5): comparison URL.

Overall result: ✅ improvements - 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

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.4%	[-0.5%, -0.2%]	7
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (secondary 4.1%)

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)	4.1%	[3.3%, 4.6%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

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

Binary size

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

Bootstrap: 473.665s -> 475.551s (0.40%)
Artifact size: 390.50 MiB -> 390.48 MiB (-0.00%)