The public set is expanded with trait items, impls and their items, foreign items, exported macros, variant fields, i.e. all the missing parts. Now it's a subset of the exported set.
This is needed for https://github.com/rust-lang/rust/pull/29083 because stability annotation pass uses the public set and all things listed above need to be annotated.
Rustdoc can now be migrated to the public set as well, I guess.
Exported set is now slightly more correct with regard to exported items in blocks - 1) blocks in foreign items are considered and 2) publicity is not inherited from the block's parent - if a function is public it doesn't mean structures defined in its body are public.
r? @alexcrichton or maybe someone else
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