Implement alloc::sync::UniqueArc

[frank-king]

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

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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.