rustc: remove deprecated private_in_public lint

[kaffarell]

The private_in_public rustc lint is now deprecated and throws a warning on each cargo check. According to the rfc ¹ this lint is being substituted by three other ones: private_interfaces, private_bounds and unnameable_types.
unnameable_types is not yet stabilized ², so it will throw a warning, but we will be ready when it is :)

Footnotes

1. https://rust-lang.github.io/rfcs/2145-type-privacy.html ↩

2. https://github.com/rust-lang/rust/pull/120144 ↩

[mladedav]

We could add #![allow(unknown_lints)] for the time being so that there are no warnings now and we can keep the lint that is yet to be stabilised.

Ideally with a TODO comment to get rid of it once the last lint is stabilized.

[kaffarell]

We could... but I am not sure—this could lead to us forgetting about it :/
@hawkw what do you think?

[hawkw]

This has already been fixed on master in #2814, so I've backported that commit to v0.1.x. Thanks!

Regarding adding #[allow(unknown_lints)], I think it would be a good idea to change our CI builds to pass --allow unknown_lints when running lint checks (e.g., here:

tracing/.github/workflows/CI.yml

Line 69 in 908cc43

args: --all --examples --tests --benches -- -D warnings

). That way, contributors still get warnings from local runs of cargo check/cargo clippy when adding attributes referencing lists that don't exist as part of a change, but CI won't fail when a lint is removed/deprecated. That seems like a good compromise between not breaking CI on new Rust versions and warning contributors that a lint isn't valid. What do you think?

[kaffarell]

Yes, I agree!