Replace pointer casting in hashmap_random_keys with safe code

[BlackHoleFox]

The old code was unnecessarily unsafe and relied on the layout of tuples always being the same as an array of the same size (which might be bad with -Z randomize-layout)?

The replacement has identical codegen, so it seems like a reasonable change.

[rust-highfive]

r? @thomcc

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[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[Mark-Simulacrum]

@bors r+ rollup=never in case there's perf implications, though I don't expect any.

[bors]

📌 Commit 0cf9503 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[bors]

⌛ Testing commit 0cf9503 with merge 1876544...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 1876544 to master...

[rust-timer]

Finished benchmarking commit (1876544): comparison url.

Instruction count

• Primary benchmarks: ✅ relevant improvements found
• Secondary benchmarks: ✅ relevant improvement found

	mean1	max	count2
Regressions ❌ (primary)	N/A	N/A	0
Regressions ❌ (secondary)	N/A	N/A	0
Improvements ✅ (primary)	-0.2%	-0.2%	5
Improvements ✅ (secondary)	-0.2%	-0.2%	1
All ❌✅ (primary)	-0.2%	-0.2%	5

Max RSS (memory usage)

Results

• Primary benchmarks: no relevant changes found
• Secondary benchmarks: mixed results

	mean1	max	count2
Regressions ❌ (primary)	N/A	N/A	0
Regressions ❌ (secondary)	4.0%	4.0%	1
Improvements ✅ (primary)	N/A	N/A	0
Improvements ✅ (secondary)	-3.3%	-3.3%	1
All ❌✅ (primary)	N/A	N/A	0

Cycles

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

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression

Footnotes

1. the arithmetic mean of the percent change ↩ ↩²

2. number of relevant changes ↩ ↩²