rustdoc should be able to build documentation from compiled crates

[brson]

rustdocs are built from attributes which are (should be) stored in the crate metadata. rustdoc should be able to construct the crate documentation from the crate metadata with nearly the same fidelity as from the original source.

Update(2024, fmease): In today's terms, this essentially asks for the option¹ to build docs based on rustc_middle::ty instead of the HIR in the all cases, not just for inlined cross-crate re-exports and synthetic impls.

Footnotes

1. Some t-rustdoc members (incl. myself, @fmease) are even inclined to drop "HIR cleaning" entirely once the important A-cross-crate-reexports Area: Documentation that has been re-exported from a different crate issues have been addressed (which is non-trivial I have to add). In the long term, that move would greatly improve the correctness of rustdoc and make it easier to develop certain features. Namely anything that depends on more complex "type-based reasoning" (for which rustc_middle::ty is a lot better suited compared to the HIR). ↩

[msullivan]

Part of #5413, I suppose.

[emberian]

#8125

This is not yet implemented by rustdoc_ng.

[catamorphism]

Low, not 1.0

[alexcrichton]

Note that all of the infrastructure is in place for doing this, there's just a few more wires to put in place.

[flaper87]

Triage bump: Still an issue. Priorities and tags seem correct to me. (cc @steveklabnik to make sure he's notified when things change here)

[steveklabnik]

Triage: no changes here.