Consider new public/private declaration format to account for restricted visibility

[TedDriggs]

With restricted visibility stabilizing in 1.18 (see rust-lang/rust#32409), it would be good for derive_builder to support declaring restricted visibility for the generated structs, setters, and fields.

Proposal

#[derive(Builder)]
#[builder(vis = "pub(crate)")]
pub(crate) struct Foo {
    bar: u8
}

This would also be available in the setter, field, and build_fn sub-options. We could keep the current #[builder(public)] option for compatibility, but I think it probably makes sense to deprecate it since the method above is more future-proof (and easier to validate that it only appears once).

[colin-kiegel]

Sounds reasonable

• What would be the new expression for #[builder(setter(private))]? This is needed for overrides. Would it be vis="" or vis="priv"? Both don't look particularly good IMO.

[colin-kiegel]

Alternatively we could introduce #[builder(pub(crate)] or #[builder(public(crate)]...

[TedDriggs]

I tried #[builder(pub(crate))], but that doesn't seem to work because the format uses pub(in mod), which won't get passed by syn as a valid meta-item. The private thing is interesting; don't have a good answer there.

[TedDriggs]

We haven't heard any requests for this; closing it.