Micro-optimize rustdoc search index parsing

[yotamofek]

This should pre-allocate the correct size for the slot vec. Not sure how much of a difference it'll make, but let's see.

[rustbot]

r? @GuillaumeGomez

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[yotamofek]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 68a9c0e (68a9c0e6ecf9481452ec6e82f8b0675cd643fd63, parent: 8e0b68e63cd2b7b6d18474fe6f49df6fb1570c25)

[rust-timer]

Finished benchmarking commit (68a9c0e): 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 4.5%)

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)	4.5%	[4.5%, 4.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -2.9%, secondary 3.4%)

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)	6.1%	[5.2%, 7.0%]	2
Improvements ✅ (primary)	-2.9%	[-2.9%, -2.9%]	1
Improvements ✅ (secondary)	-2.1%	[-2.1%, -2.1%]	1
All ❌✅ (primary)	-2.9%	[-2.9%, -2.9%]	1

Binary size

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

Bootstrap: 475.57s -> 474.667s (-0.19%)
Artifact size: 390.76 MiB -> 390.71 MiB (-0.01%)