Do the calloc optimization for Option<bool>

[scottmcm]

Inspired by https://old.reddit.com/r/rust/comments/xtiqj8/why_is_this_functional_version_faster_than_my_for/iqqy37b/.

[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.

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[rust-highfive]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

@bors r+ rollup=never

Seems OK. I do wonder if we should be adding a safe-transmute like intrinsic that core uses here, rather than producing impls for an ever-increasing number of types (and only ones in the standard library, so user-defined enums don't get the same benefit). It seems pretty easy for the compiler to say "yep" and likely much lighter on the size of libcore and simpler logic wise in core.

[bors]

📌 Commit 31cd0aa has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[bors]

⌛ Testing commit 31cd0aa with merge a6b7274...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing a6b7274 to master...

[rust-timer]

Finished benchmarking commit (a6b7274): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean1	range	count2
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.9%	[-3.9%, -3.9%]	1
Improvements ✅ (secondary)	-4.5%	[-4.5%, -4.5%]	1
All ❌✅ (primary)	-3.9%	[-3.9%, -3.9%]	1

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean1	range	count2
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.8%	[-3.8%, -3.8%]	1
All ❌✅ (primary)	-	-	0

Footnotes

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

2. number of relevant changes ↩ ↩²