add documentation for pointers to zero bit types

[winksaville]

I don't have a clear understanding of what the semantics of u0 are, hence I'm openning this issue so that they can be discussed. This is an offshoot of #1554, #1549, #1530 and maybe others.

[andrewrk]

Are you talking specifically about u0 or about 0-bit types in general?

[andrewrk]

• mention that void, u0, enum with 1 tag, struct with all 0 bit fields, are all 0 bit types
• document semantics of pointers to 0 bit types
• example ?*void
• example comparing 0 bit pointers
• example u0 in packed struct

[winksaville]

@andrewrk, sorry but you've changed the title in such a way that, to me you, are assuming the current semantics for u0 are correct, which I feel they may not be. It is my opinion that the semantics of u0 are more closely related to the semantics of u1..uN and have only incidental releationships with other types.

In anycase, I feel we should agree on the semantics of u0, u1, ... uN, whatever they may turn out to be, before we can answer your issue "add documentation for pointers to zero bit types".

If you agree, can you open another issue with a title something like "semantics of ux" or ...

As a starting point I've created the repo zig-ux-semantics with some tests for a subset of the possible semantics.

[andrewrk]

@winksaville you are welcome to open a new issue with proposed, altered semantics.

[winksaville]

txs, I'll do that. The proposal will be pretty simple, a u0 should have the same semantics as u1..u128. But I'll wait until I flesh out zig-ux-semantics to find out what works and what doesn't.