Fix item visibility conversion for rustc

[xFrednet]

With nightly-2022-06-30 rustc's internal visibility representation changed to a simple enum that holds three vales. See https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.Visibility.html

This is an interesting and perhaps a nicer representation that the one in linter_api. For the sync I only temporarily fixed the conversion for rustc since it was already not working and every situation.

This issue has two tasks:

1. decide an API representation for visibility (See also Rust Reference)
2. fix the conversion in linter_driver_rustc

[DevinR528]

This is a perfect example of what we are trying for. We need to make sure this change can be made without a semver break to the linter_api crate. Obviously it's still early enough that we are fine breaking compatibility but we should keep examples like this in mind.

[xFrednet]

Exactly, that's why I've also not fixed this directly but created an issue. Choosing an representation for Visibility is a thing that will affect us for a long time. This is something I would also like to discuss in the design repo. (I just wanted to create the PR first (which failed now -.-))

[xFrednet]

I'm creating an issue for this 🙃

[jhpratt]

Imo having structs with everything private is probably the simplest way to be forward compatible. Methods can exist to expose what's deemed necessary to construct and use values.

[xFrednet]

I've created rust-marker/design#22

Imo having structs with everything private is probably the simplest way to be forward compatible. Methods can exist to expose what's deemed necessary to construct and use values.

I think that enums marked with non_exhaustive can be equivalently effective depending on the context. Using a struct for the visibility is something I haven't considered. Would you mind proposing that in the linked issue? 🙃

[jhpratt]

I think that enums marked with non_exhaustive can be equivalently effective depending on the context.

Not necessarily, as the variants themselves are still always public. You might want to change the top-level structure, which would no longer be permissible. Leaving a comment there as requested momentarily.

[xFrednet]

Also true! I think in some instance it's unavoidable to use enums. When I've used them, I always wrap a single struct, which can be updated and expanded later. But that's not totally equivalent.