mirth-lab rustdoc-diff (check 6): rustdoc consistency; findings 33-39

[zmaril]

Check 6 of docs/checks.md, rustdoc consistency. The new subcommand is mirth-lab rustdoc-diff.

What it checks

For each standalone UI test, it runs rustdoc (nightly-2026-10-06, the same commit as the campaign rustc) next to rustc --emit=metadata. rustdoc runs four ways: HTML, HTML with private items, JSON, and JSON with private items.

• ice: rustdoc panics, on any test.
• rejects: rustdoc reports an error on a test rustc accepts.
• json: an id reachable from the root is in neither index nor paths. These are the same rules src/tools/jsondoclint applies.
• reexport: a root pub use is missing from the root module.
• auto-trait: for each synthesized auto-trait impl (Send, Sync, Unpin, UnwindSafe, RefUnwindSafe), a probe appended to the test requires the trait under exactly the bounds rustdoc shows. rustc must accept the probe, or reject it with E0277 for a negative impl (#162274).

Full sweep

• Tests: 18,624. rustc accepts 7,473 and rejects 10,969; 207 were skipped.
• Probes: 4,562 auto-trait probes in 577 tests.
• Results: 19 tests with findings, giving 7 findings (33–39). Facts and reduced tests are under docs/hunt/.

#	finding
33	rustdoc panics on use {{}};, which rustc accepts. A regression on nightly-2026-09-26; the range contains #161349.
34	rustdoc skips rustc's post-parse option adjustments. -Ccodegen-units is ignored, and -Zassumptions-on-binders with -Znext-solver=no causes an ICE.
35	rustdoc rejects an associated-const binding rustc accepts. The cause is the FIXME(generic_const_items) in clean_hir_term.
36	rustdoc panics on an associated-const binding through a supertrait.
37	rustdoc fails with E0080 on a type alias whose array length does not evaluate.
38	rustdoc ICEs on delayed bugs whose real error comes from a step it skips. One case is stable: a static too large for the target.
39	rustdoc JSON: an impl in a function body names a body-local type, and its id dangles even with private items.

These are labelled and not reported:

• rustdoc lints denied by a test, and tiny recursion limits.
• Dangling ids for stripped items, which are open upstream (#113674 family).
• The #155728 fn_delegation ICE.
• The unsafe_binder JSON FIXME.

No re-export was missing, and no auto-trait impl was contradicted by its probe.

Also adds rustdoc-diff to rustc/big-run.sh and documents how to run it.

The finding numbers 33–39 may collide with other check branches opened at the same time. Renumber at merge if needed.

🤖 Generated with Claude Code

https://claude.ai/code/session_01QXiEXbESemwqMLYKaWLDbT