[rustdoc] Generate documentation for type definitions

[elichai]

Would it be possible to add traits and functions to type definitions? For example, it's really hard to figure out what functions exist on this type: https://docs.rs/blake2/0.10.4/blake2/type.Blake2b.html
It's a type definition of a generic struct where some of the impls on the generic struct depend on the traits the generic implements.
so having docs on the type definition will allow seeing what traits/functions end up implemented on that type

[fmease]

@rustbot label T-rustdoc C-enhancement

[jsha]

I think this is equivalent to a request for #[doc(inline)] to be implemented on type definitions. Is that right? https://doc.rust-lang.org/rustdoc/write-documentation/the-doc-attribute.html#inline-and-no_inline

[elichai]

I think this is equivalent to a request for #[doc(inline)] to be implemented on type definitions. Is that right? https://doc.rust-lang.org/rustdoc/write-documentation/the-doc-attribute.html#inline-and-no_inline

Hmm I'm not sure, because I'm also proposing that if the type definition selects a specific generic type then the impls will change accordingly (e.g. if there are impl<T: SomeTrait> for Type<T> and the typedef is type A = Type<B> where B doesn't imply SomeTrait, then there's no reason to show that impl in the docs of A)