Avoid synthetic auto-trait ICE with next solver in rustdoc

[TaKO8Ki]

Fixes #156487

AutoTraitFinder still uses old-solver-only selection APIs. When rustdoc runs with -Znext-solver=globally, synthetic auto-trait impl generation reaches that path with the new solver enabled and ICEs in trait selection.

[rustbot]

r? @camelid

rustbot has assigned @camelid.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: rustdoc
• rustdoc expanded to 9 candidates
• Random selection from GuillaumeGomez, camelid, fmease, lolbinarycat, notriddle

[camelid]

Could you explain a bit more about the purpose of this PR? It seems like it just turns a rustdoc ICE into silently producing incorrect results with -Znext-solver, which doesn't seem good to me. Is it to test the other parts of rustdoc with -Znext-solver? Apologies if I'm missing some context.

View changes since this review

[TaKO8Ki]

@camelid

Sorry, I opened this PR while I was still debugging and before I had completed the changes. I think it is better for rustdoc to return a normal error with fatal as an example for now, e.g.

rustdoc does not generate auto-trait impls for display with -Znext-solver=globally

rather than causing an ICE.

What do you think?