BTreeSet: avoid intermediate sorting when collecting sorted iterators

[ssomers]

As pointed out by droundy, an obvious optimization is to skip the first step introduced by #88448 (creation of a vector and sorting) and it's easy to do so for btree's own iterators. Also, exploit from in the examples.

[rust-highfive]

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? @joshtriplett

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[the8472]

LGTM

@bors r+ rollup=never

[bors]

📌 Commit 49ccb75 has been approved by the8472

[bors]

⌛ Testing commit 49ccb75 with merge be16c61...

[bors]

☀️ Test successful - checks-actions
Approved by: the8472
Pushing be16c61 to master...

[rust-timer]

Finished benchmarking commit (be16c61): comparison url.

Instruction count

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

Max RSS (memory usage)

Results

• Primary benchmarks: no relevant changes found
• Secondary benchmarks: 😿 relevant regression found

	mean1	max	count2
Regressions 😿 (primary)	N/A	N/A	0
Regressions 😿 (secondary)	2.5%	2.5%	1
Improvements 🎉 (primary)	N/A	N/A	0
Improvements 🎉 (secondary)	N/A	N/A	0
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. number of relevant changes ↩