Tracking Issue for `arc_raw_get_strong`

[loreball]

Feature gate: #![feature(arc_raw_get_strong)]

This is a tracking issue for adding strong_count_from_raw to Arc and Rc for getting their strong count from a raw pointer.

Public API

impl<T: ?Sized, A: Allocator> Arc<T, A> {
    pub unsafe fn strong_count_from_raw(ptr: *const T) -> usize {
        ...
    }
}

impl<T: ?Sized, A: Allocator> Rc<T, A> {
    pub unsafe fn strong_count_from_raw(ptr: *const T) -> usize {
        ...
    }
}

Steps / History

(Remember to update the S-tracking-* label when checking boxes.)

• ACP: ACP: Add methods to get reference counts from raw pointers to Arc\Rc libs-team#792
• Implementation: #...
• Final comment period (FCP)¹
• Stabilization PR

Unresolved Questions

• None yet.

Footnotes

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

[valentynkit]

Opened a draft PR implementing this.

One thing I wasn't sure about: the ACP sketch puts the method on the impl<T: ?Sized, A: Allocator> block, but nothing in the signature ties A to anything (a raw pointer from into_raw has already erased the allocator type), so a bare Arc::strong_count_from_raw(ptr) can't infer A and callers would need a turbofish, I may be missing something though.

I put it in the A = Global block instead, next to increment_strong_count/decrement_strong_count. If anyone reviewing has a preference on that, I'd appreciate to change the implementation.