New lints: item that was previously public API is now #[doc(hidden)] i.e. no longer public API

[obi1kenobi]

For example, uncommenting the #[doc(hidden)] line is a breaking change since the struct is no longer considered public API, even though it is public. This is because some non-public API may need to be accessible from outside its crate, for example in macro-heavy crates like pyo3.

// #[doc(hidden)]
pub struct Example;

We need lints for this kind of situation for:

• structs: implement struct_now_doc_hidden lint #587
• enums: implement enum_now_doc_hidden lint #593
• unions: rule: add union_now_doc_hidden #679
• functions: implement function_now_doc_hidden lint #594
• traits: implement trait_now_doc_hidden lint #595
• statics: Add lint pub_static_now_doc_hidden #712
• module-level constants: Added lint public_module_level_constant_now_doc_hidden #690, Rename new lint to pub_module_level_const_now_doc_hidden. #703
• ImplOwner (struct/enum/union) inherent associated functions / methods: Inherent method now doc hidden #821
• ImplOwner associated constants: Added inherent_associated_const_now_doc_hidden #820
• struct fields (Add lint for pub field becoming doc(hidden) #649)
• union fields (Add union pub field now doc hidden lint #844)
• enum variants #596
• enum tuple variant fields: Added lint enum_tuple_variant_field_now_doc_hidden #685
• enum struct variant fields: Added lint enum_struct_variant_field_now_doc_hidden #687
• trait associated functions / methods: add trait_method_now_doc_hidden lint #684
• trait associated types: Added lint trait_associated_type_now_doc_hidden #776
• trait associated constants: Added lint trait_associated_const_now_doc_hidden #779
• macros (declarative, proc macro, etc.), tracked in detail by New lints: Breaking changes in macros #946

Remaining work item

We should figure out what are the appropriate semantics of marking an impl block #[doc(hidden)] with respect to the block's contents. Are the contents considered not public API anymore? Is #[doc(hidden)] simply ignored on that impl block?

The action items here are:

• Figure out how rustdoc itself treats items defined inside a #[doc(hidden)] impl block. Does it think they are hidden, or no?
• Adjust all affected lints accordingly (e.g. Added inherent_associated_const_now_doc_hidden #820, Inherent method now doc hidden #821)

[obi1kenobi]

Four lints to go here, and we can close this issue! 🚀