Auto merge of #46083 - petrochenkov:morepriv, r=nikomatsakis
Type privacy polishing
Various preparations before implementing https://github.com/rust-lang/rfcs/pull/2145 containing final minor breaking changes (mostly for unstable code or code using `allow(private_in_public)`).
(Continuation of https://github.com/rust-lang/rust/pull/42125, https://github.com/rust-lang/rust/pull/44633 and https://github.com/rust-lang/rust/pull/41332.)