Implement alloc::sync::UniqueArc

[frank-king]

This implements the alloc::sync::UniqueArc part of #112566.

[rustbot]

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[steffahn]

Hah! I’m just noticing that this, as well as UniqueRc, absolutely must be invariant (w.r.t. the parameter T).

Otherwise, you can

fn extend_lifetime<'a, 'b>(x: &'a str) -> &'b str {
    let r = UniqueRc::new(""); // UniqueRc<&'static str>
    let w = UniqueRc::downgrade(&r); // Weak<&'static str>
    let mut r = r; // COVARIANT: ==>> UniqueRc<&'a str>
    *r = x; // assign the &'a str
    let _r = UniqueRc::into_rc(r); // Rc<&'a str>, but we only care to activate the weak
    let r = w.upgrade().unwrap(); // upgrade succeeds: Rc<&'static str>
    *r // &'static str, coerces to &'b str
}

(playground)

[bors]

☔ The latest upstream changes (presumably #135357) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #138208) made this pull request unmergeable. Please resolve the merge conflicts.

[Amanieu]

Could you add a test that checks the behavior of weak pointers created from UniqueArc?

[Amanieu]

@bors r+

[bors]

📌 Commit 5004e10 has been approved by Amanieu

It is now in the queue for this repository.

[bors]

⌛ Testing commit 5004e10 with merge 1799887...

[bors]

☀️ Test successful - checks-actions
Approved by: Amanieu
Pushing 1799887 to master...

[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 d4812c8 (parent) -> 1799887 (this PR)

Test differences

Show 20 test diffs

Stage 0

• arc::test_unique_arc_weak: [missing] -> pass (J3)

Stage 1

• arc::test_unique_arc_weak: [missing] -> pass (J0)
• [ui] tests/ui/variance/variance-uniquearc.rs: [missing] -> pass (J2)

Stage 2

• [ui] tests/ui/variance/variance-uniquearc.rs: [missing] -> pass (J1)

Additionally, 16 doctest diffs were found. These are ignored, as they are noisy.

Job group index

• J0: aarch64-apple, aarch64-gnu, arm-android, armhf-gnu, dist-i586-gnu-i586-i686-musl, i686-gnu-1, i686-gnu-nopt-1, i686-msvc-1, test-various, x86_64-apple-1, x86_64-gnu, x86_64-gnu-aux, x86_64-gnu-llvm-18-1, x86_64-gnu-llvm-18-2, x86_64-gnu-llvm-18-3, x86_64-gnu-llvm-19-1, x86_64-gnu-llvm-19-2, x86_64-gnu-llvm-19-3, x86_64-gnu-nopt, x86_64-gnu-stable, x86_64-mingw-1, x86_64-msvc-1
• J1: aarch64-apple, aarch64-gnu, arm-android, armhf-gnu, dist-i586-gnu-i586-i686-musl, i686-gnu-1, i686-gnu-nopt-1, i686-msvc-1, test-various, x86_64-apple-2, x86_64-gnu, x86_64-gnu-llvm-18-1, x86_64-gnu-llvm-18-2, x86_64-gnu-llvm-19-1, x86_64-gnu-llvm-19-2, x86_64-gnu-nopt, x86_64-gnu-stable, x86_64-mingw-1, x86_64-msvc-1
• J2: x86_64-gnu-llvm-18-3, x86_64-gnu-llvm-19-3
• J3: mingw-check

Job duration changes

1. x86_64-apple-1: 6838.0s -> 8036.7s (17.5%)
2. x86_64-apple-2: 4457.6s -> 5236.7s (17.5%)
3. dist-aarch64-apple: 4601.0s -> 5331.5s (15.9%)
4. dist-various-2: 3234.8s -> 3384.0s (4.6%)
5. x86_64-gnu-nopt: 5428.0s -> 5664.4s (4.4%)
6. test-various: 4110.6s -> 4258.8s (3.6%)
7. aarch64-apple: 3796.0s -> 3926.6s (3.4%)
8. i686-mingw-2: 6494.3s -> 6715.7s (3.4%)
9. dist-powerpc64le-linux: 9252.0s -> 9518.7s (2.9%)
10. dist-x86_64-netbsd: 5043.5s -> 5182.7s (2.8%)

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 (1799887): 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 (primary 2.5%, secondary 2.5%)

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.

	mean	range	count
Regressions ❌ (primary)	2.5%	[2.3%, 2.7%]	2
Regressions ❌ (secondary)	2.5%	[2.5%, 2.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.5%	[2.3%, 2.7%]	2

Cycles

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

Binary size

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

Bootstrap: 777.207s -> 777.64s (0.06%)
Artifact size: 365.95 MiB -> 365.94 MiB (-0.00%)