rustc docs: Re-exported types' implementations are lost

[camelid]

For example, ObligationCause is defined in rustc_middle, and its docs look like this:

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But it is also re-exported by rustc_trait_selection, where its inherent and trait implementations don't appear in the docs:

---

This regressed in 1.47.0 (look at /1.46.0/nightly-rustc vs /1.47.0/nightly-rustc to compare). That is also the release when the rustc_private warning stopped showing up in the docs, so I wonder if the two changes are connected somehow. Furthermore, I have never witnessed this bug outside of the rustc docs, which makes me even more suspicious that something about rustc_private or rustbuild is causing this.

[camelid]

cc @rust-lang/rustdoc

[apiraino]

Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.

@rustbot label -I-prioritize +P-medium

[fmease]

Triage: Still an issue.