Add Read, Write and Seek impls for Arc<T> where appropriate

[tbu-]

If &T implements these traits, Arc<T> has no reason not to do so
either. This is useful for operating system handles like File or
TcpStream which don't need a mutable reference to implement these
traits.

Fix #53835.

[rust-highfive]

r? @dtolnay

(rust-highfive has picked a reviewer for you, use r? to override)

[SNCPlay42]

This is incorrect, or at least has very surprising behaviour, if R = [u8]. impl Read for &[u8] expects mutable borrows of the same [u8] to be passed to read each time it is called, and updates the mutably borrowed slice to track the position in the buffer.

But this impl will create a new &[u8] pointing to the whole buffer each time read is called, with the effect that repeated calls to read will each read from the beginning of the buffer each time.

(playground demonstration)

[dtolnay]

I was about to comment the same as @SNCPlay42. I don't think any of these impls behave correctly in general. Mutations of the &mut &R by the underlying trait impls are just thrown away.

FYI @rust-lang/libs-api @jonhoo in case anyone sees a way to rescue this.

Impls for specific types like Arc<File> can work but I'm not sure which ones make sense / are needed.

[jonhoo]

Oh, interesting, I hadn't though of that — good catch! In that case I'd be in favor of at least adding the impls for the concrete types we know where this would be really useful (like File and TcpStream).