-Znext-solver Remove the forced ambiguity hack from search graph

[ShoyuVanilla]

As discussed in rust-lang/trait-system-refactor-initiative#257

r? lcnr

[lcnr]

integrated this in #133502, want to check whether this causes any significant hangs, so going to mark this as blocked for now

@rustbot blocked

[lcnr]

This PR removes a performance optimization which seems like it is no longer necessary, as we no longer have any affected tests.

This performance regression was originally added during the stabilization of -Znext-solver=coherence in #128828 to avoid hangs in fuchsia. The minimization of that hang no longer does, even after removing this optimization.

The optimization does incorrectly force goals to be ambiguous, breaking binius_field see rust-lang/trait-system-refactor-initiative#257. Given that undesirable behavior and the fact that it doesn't seem necessary anymore, I propose removing this optimization, accepting the fact that it may theoretically cause yet unknown hangs in coherence.

In this case we do cause hangs, we can then find a different solution and potentially temporarily revert this change again. I did not immediately see any concerning regressions in the crater run with -Znext-solver=globally with this change, but think we should spend some time looking over the crater run during the FCP here. See https://rust-lang.zulipchat.com/#narrow/channel/364551-t-types.2Ftrait-system-refactor/topic/help.20looking.20for.20hangs.20on.20crater/with/572360464

@rfcbot fcp merge types

[rust-rfcbot]

Team member @lcnr has proposed to merge this. The next step is review by the rest of the tagged team members:

• @BoxyUwU
• @jackh726
• @lcnr
• @oli-obk
• @spastorino

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

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[rust-rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rust-rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

[lcnr]

@bors r+ rollup=never

[rust-bors]

📋 This PR cannot be approved because it currently has the following label: S-blocked.

[lcnr]

@bors r+

[rust-bors]

📌 Commit 9e63845 has been approved by lcnr

It is now in the queue for this repository.