Borrowed forms of `Rc` and `Arc`.

`ArcBorrow<'_, _>` is functionally equivalent to `&'_ Arc<_>`,
but it's represented as `&'_ T`, avoiding the extra indirection.

## Examples

```rust
let resource: Arc<Resource> = acquire_resource();
let borrowed: ArcBorrow<'_, Resource> = (&resource).into();
let reference: &Resource = ArcBorrow::downgrade(borrowed);
let cloned: Arc<Resource> = ArcBorrow::upgrade(borrowed);
fn use_resource(resource: &Resource) { /* ... */ }
use_resource(&borrowed);
```

## Related Crates

- [`erasable`](https://lib.rs/crates/erasable): Erase pointers of their concrete type.
- [`ptr-union`](https://lib.rs/crates/ptr-union): Pointer unions the size of a pointer.
- [`rc-box`](https://lib.rs/crates/rc-box): Known unique forms of `Rc` and `Arc`.
- [`slice-dst`](https://lib.rs/crates/slice-dst): Support for custom slice-based DSTs.

## Why not [triomphe](https://crates.io/crates/triomphe)?

Triomphe is a great atomic reference counting library!
The main difference between triomphe and these utilities is that
triomphe implements a new `Arc` type that doesn't support weak references
(and as such does not have to pay the cost of handling potential weak references),
whereas these pointer utilities use the standard library's reference counting types.
If you need to work with standard library `Arc`/`Rc`, triomphe won't work for you.

If you want a more battle-tested library by the servo developers, use triomphe.
If you want small, self-contained extensions to the standard library types,
use these pointer utilities.

Additionally, triomphe only supports atomic reference counting.
We provide support for both `Arc` and `Rc`.

## Minimum Supported Rust Version

We require a minimum Rust version of 1.41.0.
This is for an adjustment of local trait impl checking.

Minimum version support is only guaranteed with minimal version resolution
(`-Z minimal-versions`/`--minimal-versions`) due to how dependencies are handled.
The minimum version of Rust will only be incremented with minor version bumps,
not patch version bumps, and will be deliberate and clearly noted in change notes.

## License

Licensed under either of

 * Apache License, Version 2.0
   ([LICENSE/APACHE](../../LICENSE/APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
 * MIT license
   ([LICENSE/MIT](../../LICENSE/MIT) or http://opensource.org/licenses/MIT)

at your option.

If you are a highly paid worker at any company that prioritises profit over
people, you can still use this crate. I simply wish you will unionise and push
back against the obsession for growth, control, and power that is rampant in
your workplace. Please take a stand against the horrible working conditions
they inflict on your lesser paid colleagues, and more generally their
disrespect for the very human rights they claim to fight for.

## Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
dual licensed as above, without any additional terms or conditions.
