New lints: Breaking changes related to union types

[obi1kenobi]

Now that we've added union types to the schema we use for writing lints, we have a lot of union-related lints to implement!

Major lints:

• pub union removed
  • make sure that converting a union to a struct or enum (ImplOwner type in our query schema) by the same name isn't flagged -- it's sometimes possible to change up types like this without a major breaking change
  • see the struct_missing lint's implementation, which has to do the same thing for the same reason
• pub union's pub field removed / renamed (no longer present under its previous name, by whatever cause)
• pub union is no longer repr(C): Replace per-type repr(C) lints with a single lint for all of struct/enum/union #678
• pub union became doc(hidden) and is no longer public API (related to New lints: item that was previously public API is now #[doc(hidden)] i.e. no longer public API #578): rule: add union_now_doc_hidden #679
• pub union's field became doc(hidden) and is no longer public API (also related to New lints: item that was previously public API is now #[doc(hidden)] i.e. no longer public API #578): Add union pub field now doc hidden lint #844
  • analogous lint for structs: https://github.com/obi1kenobi/cargo-semver-checks/blob/main/src/lints/struct_pub_field_now_doc_hidden.ron

Minor lints:

• pub union #[must_use] added: Adds union_must_use_added lint #845
  • analogous lint for structs: https://github.com/obi1kenobi/cargo-semver-checks/blob/main/src/lints/struct_must_use_added.ron
  • remember to add the new lint to the README sections on configuring #[must_use] lints:
    • https://github.com/obi1kenobi/cargo-semver-checks?tab=readme-ov-file#common-configurations-make-must_use-lints-warn-only
    • https://github.com/obi1kenobi/cargo-semver-checks?tab=readme-ov-file#common-configurations-disable-must_use-lints-entirely

[pksunkara]

pub union is no longer repr(C)

@obi1kenobi Can I rename the existing lints? I am wondering if we can combine this to existing similar ones.

[obi1kenobi]

You could combine it with the enum and the struct versions of that lint, and use the ImplOwner interface plus the __typename property to express them in one lint.

If you're interested in doing that, go for it. I don't have a strong preference between adding a separate lint or replacing them with one lint that works for struct / enum / union all in one.

[obi1kenobi]

Nice job on this issue, everyone! We're two lints away from closing it completely! 🚀

[obi1kenobi]

All done here! 🚀