transmutability: Stall new solver when inference variables remain

[lapla-cogito]

fixes #141400, fixes #148809

The new trait solver's consider_builtin_transmute_candidate() was passing types with inference variables directly to rustc_transmute(), which attempts layout computation and ICEs.

The old solver already guards against this in assemble_candidates_for_transmutability() by setting candidates.ambiguous = true. Add the equivalent guard in the new solver by returning forced_ambiguity when the predicate contains non-region inference variables.

[rustbot]

r? @lcnr

rustbot has assigned @lcnr.
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

[lcnr]

It seems wrong to return an error here. It should not be impossible to transmute. We should instead stall somehow here. There should probably just be an Answer::Maybe or sth to deal with cases where there are still inference variable to get the trait solver to try again later

[rust-bors]

☔ The latest upstream changes (presumably #151003) made this pull request unmergeable. Please resolve the merge conflicts.

[reddevilmidzy]

@lapla-cogito any updates on this? thanks.

[lapla-cogito]

I will update this patch in a few days.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[lapla-cogito]

There should probably just be an Answer::Maybe or sth to deal with cases where there are still inference variable to get the trait solver to try again later

Since adding Answer::Maybe would ultimately result in the trait solver converting it to Certainty::AMBIGUOUS, I determined that this would only increase indirect references. That's why I implemented the changes as they are now (another reason being that the old solver already handles this conversion in a similar way, see the updated PR description).

@rustbot ready

[lcnr]

can you move this check into ecx.is_transmutable? That should be able to just return Certainty::AMBIGUOUS which is then properly handled already

we want to handle structurally_normalize_const to return something ambiguous or to take something that references an infer var and the returned assume then does not as the infer var wasn't used.

[lapla-cogito]

Thanks, done.

[reddevilmidzy]

issue resolved in #154991