Add Box::clone_from_ref and similar under feature(clone_from_ref)

[zachs18]

Tracking issue: #149075

Accepted ACP: rust-lang/libs-team#483

This PR implements clone_from_ref (and try_* and _*in variants), to get a Box<T>, Arc<T>, or Rc<T> by cloning from a &T where T: CloneToUninit.

The "Implement..." commits replace some ad-hoc conversions with clone_from_ref variants, which can be split out to a separate PR if desired.

This PR will conflict with #148769 due to usage of Layout::dangling (which that PR is renaming to dangling_ptr), so they should not be rolled up together, and the one which merges later will need to be amended.

[rustbot]

r? @Mark-Simulacrum

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[Mark-Simulacrum]

@bors r+ rollup=iffy

[bors]

📌 Commit 2eac4db has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[bors]

⌛ Testing commit 2eac4db with merge 1be6b13...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 1be6b13 to main...

[github-actions]

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 99ca3fc (parent) -> 1be6b13 (this PR)

Test differences

Show 1264 test diffs

1264 doctest diffs were found. These are ignored, as they are noisy.

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 1be6b13be73dc12e98e51b403add4c41a0b77759 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

1. pr-check-1: 2058.4s -> 1641.8s (-20.2%)
2. x86_64-rust-for-linux: 3066.3s -> 2461.1s (-19.7%)
3. x86_64-gnu-tools: 3673.6s -> 3077.5s (-16.2%)
4. dist-x86_64-apple: 7537.1s -> 8653.2s (+14.8%)
5. aarch64-gnu-llvm-20-2: 2528.3s -> 2156.7s (-14.7%)
6. x86_64-msvc-2: 6575.8s -> 7303.4s (+11.1%)
7. dist-aarch64-llvm-mingw: 5344.3s -> 5933.8s (+11.0%)
8. x86_64-gnu-stable: 7716.5s -> 6875.3s (-10.9%)
9. aarch64-gnu-debug: 4289.0s -> 3827.1s (-10.8%)
10. dist-arm-linux-gnueabi: 5298.6s -> 4729.2s (-10.7%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

[rust-timer]

Finished benchmarking commit (1be6b13): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -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)	-0.6%	[-0.6%, -0.6%]	1
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	1
All ❌✅ (primary)	-0.6%	[-0.6%, -0.6%]	1

Max RSS (memory usage)

Results (primary 1.1%, secondary 4.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	3.2%	[3.2%, 3.2%]	1
Regressions ❌ (secondary)	4.2%	[4.2%, 4.2%]	1
Improvements ✅ (primary)	-1.0%	[-1.0%, -1.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.1%	[-1.0%, 3.2%]	2

Cycles

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

Binary size

Results (primary -0.1%, secondary -0.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.0%]	4
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.2%	[-0.6%, -0.1%]	5
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	38
All ❌✅ (primary)	-0.1%	[-0.6%, 0.0%]	9

Bootstrap: 475.097s -> 468.01s (-1.49%)
Artifact size: 386.96 MiB -> 386.86 MiB (-0.02%)