Getting rid of `CNil`

[Centril]

This is a follow up issue to #93.

I think we should get rid of special uninhabited types unless we have a particularly good reason to keep them. The standard library is in the process of getting rid of Infallible and other uninhabited types.

So I think we should replace CNil and Here with just !.

Thoughts?

[ExpHP]

Please do not replace Here with ! because I plan to turn it into a unit struct. (for reified indices)

[Centril]

@ExpHP Ah right; sounds like a good idea =)

[lloydmeta]

Interesting; I guess the empty enum trick has fallen out of fashion :)

Here aside, should we wait until ! as a type is stabilised before going ahead with getting rid of CNil? I'm not fully sure if it's done yet because these are still open

• Tracking issue for promoting ! to a type (RFC 1216) rust-lang/rust#35121 (comment)
• Summary issue - stabilization of ! rust-lang/rust#48950

[Centril]

Yeah, I think we should wait until it is stabilised.