Back in March I opened PR #153497 which among other things uncommented a debug assertion that was accidentally commented out in refactoring PR #53816 all the way back in 2018 (that's 8 years ago!), see file `src/librustc/ty/sty.rs` in the patch set. CI was happy but then after I rebased one month later in April, CI suddenly failed since rustc@stage1 could no longer build rustc@stage2 as it now triggered the assertion when trying to compile `itertools` ([raw logs](https://productionresultssa16.blob.core.windows.net/actions-results/288185b9-b16b-4634-82c3-0e8da79f1253/workflow-job-run-3a1881ae-1dbd-5372-95ad-d8b853bdd404/logs/job/job-logs.txt?rsct=text%2Fplain&se=2026-05-29T19%3A30%3A11Z&sig=PArXiWU38ybc9Ofibtl72IlrXgtDDVUTSKPVCM%2FGELk%3D&ske=2026-05-29T20%3A59%3A45Z&skoid=ca7593d4-ee42-46cd-af88-8b886a2f84eb&sks=b&skt=2026-05-29T16%3A59%3A45Z&sktid=398a6654-997b-47e9-b12b-9515b896b4de&skv=2025-11-05&sp=r&spr=https&sr=b&st=2026-05-29T19%3A20%3A06Z&sv=2025-11-05)). It would be pretty crazy if this assertion never(?) triggered for 8+ years only to start triggering one month after me opening my PR, so I don't exactly know what's going on. Honestly, I don't even know for sure if this assertion is correct in the first place, somebody please enlighten me. Otherwise this needs a bisection. For now, I'm keeping it commented out but I'll at least add a FIXME. Side note: By now I do know of the unimplemented but accepted [MCP 433 | Introduce ty::WhereClause to align chalk and rustc dyn representation](https://github.com/rust-lang/compiler-team/issues/433) from 2021 which would considerably change the representation & API of existential entities and would probably lead to this function getting completely replaced by `Binder` methods but of course it's been 5 years and in any case I think it's worth investigating what's going wrong here even if all of this ends up getting rewritten. To reproduce the ICE: Uncomment the `debug_assert!(!self_ty.has_escaping_bound_vars());` in `ExistentialTraitRef::with_self_ty` in `compiler/rustc_type_ir/src/predicate.rs`. Then `./x build library --stage 2`.