stabilize {Rc, Arc}::is_unique

[malezjaa]

Stabilization report:

Closes #138938

{Arc,Rc}::is_unique, is a function that allows determining whether there is exactly one strong count and zero weak counts to an Arc/Rc.

Implementation history

Implementation: #138939

Public api

// alloc::rc
impl<T> Rc<T> {
    pub fn is_unique(this: &Self) -> bool;
}

// alloc::sync
impl<T> Arc<T> {
    pub fn is_unique(this: &Self) -> bool;
}

@rustbot modify labels: +T-libs

This is a pretty small feature, so opening this to hopefully get a FCP started.

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[malezjaa]

r? libs

[clarfonthey]

It was already assigned to libs, but, sure.

Is there a reason why this API isn't offered on Rc too? That seems like an important detail to determine before stabilisation.

[malezjaa]

I don't think there was any. The ACP only proposed it for Arc, and Rc::is_unique was proposed by @tgross35 in the tracking issue, but never really implemented.

I can add it, but I'm not sure if it belongs in this PR.

[clarfonthey]

I mean, I don't like the precedent of stabilising asymmetric impls on Arc that could go on Rc too, which is maybe a sign we dont want to stabilise this yet.

[malezjaa]

I assume that Rc variant should be on its own tracking issue and ACP?

[clarfonthey]

I would say that unless there's significant reason to make things different on Arc and Rc then new features should apply to both. We don't want to stabilise separately.

[clarfonthey]

Once #163489 merges and this is rebased on top of that, I think it's okay to FCP merge, since Rc's functionality already existed, just wasn't public. If you could update the PR description also, that would help.

Thank you!

[clarfonthey]

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[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.

[malezjaa]

@rustbot ready

Rebased and updated :D

[clarfonthey]

@rfcbot merge libs

[rust-rfcbot]

@clarfonthey has proposed to merge this. The next step is review by the rest of the tagged team members:

• @ChrisDenton
• @Mark-Simulacrum
• @cuviper
• @dtolnay
• @joshtriplett
• @nia-e
• @the8472
• @thomcc

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/libs-ping: FCP proposed for libs, please feel free to register concerns.
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