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Restore `pred_known_to_hold_modulo_regions` As requested by `@lcnr` in https://github.com/rust-lang/rust/issues/123275#issuecomment-2031885563 this PR restores `pred_known_to_hold_modulo_regions` to fix that "unexpected unsized tail" beta regression. This also adds the reduced repro from https://github.com/rust-lang/rust/issues/123275#issuecomment-2041222851 as a sub-optimal test is better than no test at all, and it'll also cover #108721. It still ICEs on master, even though https://github.com/phlip9/rustc-warp-ice doesn't on nightly anymore, since https://github.com/rust-lang/rust/pull/122493. Fixes #123275. r? `@compiler-errors` but feel free to close if you'd rather have a better test instead cc `@wesleywiser` who had signed up to do the revert Will need a backport if we go with this PR: `@rustbot` label +beta-nominated