Tracking Issue for `{Arc, Rc}::into_inner`

[steffahn]

Feature gates: #![feature(arc_into_inner)], #![feature(rc_into_inner)]

This is a tracking issue for the Arc::into_inner method, and (for API-consistency) also an Rc::into_inner analogue.

The function Arc::into_inner(x) serves as an atomic race-condition-free alternative to Arc::try_unwrap(x).ok(). See the ACP for more details.

Public API

// alloc::sync

impl<T> Arc<T> {
    /// Returns the inner value, if the `Arc` has exactly one strong reference.
    /// Otherwise, `None` is returned and the `Arc` is dropped.
    pub fn into_inner(this: Arc<T>) -> Option<T>;
}

and the extension to Rc

// alloc::rc

impl<T> Rc<T> {
    /// Returns the inner value, if the `Rc` has exactly one strong reference.
    /// Otherwise, `None` is returned and the `Rc` is dropped.
    pub fn into_inner(this: Rc<T>) -> Option<T>;
}

Steps / History

• ACP: Add Arc::into_inner for safely discarding Arcs without calling the destructor on the inner type. libs-team#162
• Implementation:
  • Arc::into_inner Add Arc::into_inner for safely discarding Arcs without calling the destructor on the inner type. #106854
  • Rc::into_inner Introduce Rc::into_inner, as a parallel to Arc::into_inner #109026
• Final comment period (FCP)¹
• Stabilization PR: Stabilize arc_into_inner and rc_into_inner. #109504

Unresolved Questions

• None yet.

Footnotes

1. https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩

[ryoqun]

Hey, I've stumbled on this. I wonder how soon this can be stabilized? My usecase is complex. but in a nut shell, I want this to conditionally and cleanly restart bunch of interconnected threads of mixed roles, all sharing Arc<GlobalContext>, depending on which thread did actually dropped the GlobalContext.

if sharing helps, i can write the usecase in detail...

Of course, I can resort to sync::Weak or tls. but it incurs overhead and/or further complexities. or mem::transmute like this to forcibly bring ::into_inner into stable rustc: https://rust.godbolt.org/z/r5rbsYYcT, in which i have to use unsafe... Also, due to already-existing wide-spread use of Arc<GlobalContext> in our codebase, switching to different Arc impl altogether isn't viable as well. That precondition also precludes more saner cleaning code arrangement in the first place. ;)

[joshtriplett]

Submitted the Rc::into_inner implementation as #109026

[joshtriplett]

I think Arc::into_inner makes sense to stabilize at this point. And Rc::into_inner is trivial.

@rust-lang/libs-api, shall we stabilize these?

@rfcbot merge

[rfcbot]

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

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se

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!

See this document for info about what commands tagged team members can give me.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔