`ArcBorrow`

[Tamschi]

Is your feature request related to a problem? Please describe.

While some APIs can be constrained to allow cloning Pin<Arc<T>> from &T, this isn't universally true and especially won't work if T isn't pinned. However, it's still possible to efficiently borrow the value with a newtype that remembers its status: struct ArcBorrow<'a, T>(&'a T), where the reference is guaranteed to point inside an Arc<T>.

Describe the solution you'd like

ArcBorrow<T> must be Deref<T> and so on. It should overall behave like the plain reference would.

There should only be two exposed associated functions:

• fn clone_arc(this: Self) -> Arc<T> and
• fn clone_pin(this: Pin<Self>) -> Pin<Arc<T>>

Describe alternatives you've considered

None, though the type name is somewhat up in the air.

Additional context

This is essentially the same API that triomphe has, which in part inspired this crate.
It's slightly more convenient to implement it with an intrusive counter, but otherwise should behave just about the same.