rustdoc: `rustc_serialize::Decodable` is missing some implementors

[camelid]

The docs for rustc_serialize::Decodable are not showing implementors from downstream crates (e.g. the impls in rustc_metadata for CrateNum and DefIndex). My understanding was that implementors in downstream crates that are compiled into the same docs should be displayed on the upstream trait.

Not sure if this is related to #76736 or #78615.

[camelid]

#78615 seems to be fixed now, but this issue is not. Probably it's related to #76736 though.