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

[Predrag Gruevski]

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: #587
• enums: #593
• unions: #679
• functions: #594
• traits: #595
• statics: #712
• module-level constants: #690, #703
• ImplOwner (struct/enum/union) inherent associated functions / methods: #821
• ImplOwner associated constants: #820
• struct fields (#649)
• #596
• enum tuple variant fields: #685
• enum struct variant fields: #687
• trait associated functions / methods: #684
• trait associated types: #776
• trait associated constants: #779

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. #820, #821)

[Predrag Gruevski]

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