trait solver: Track canonical response universe assumptions - #159987
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Canonical responses can return query-created placeholders through opaque type constraints. Record empty assumptions when recreating those universes in the caller so eager placeholder handling does not ICE.
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Fixes #159889
Canonical responses recreate universes for placeholders created while evaluating a query. Region constraints are handled inside the query, but opaque type constraints can still carry those placeholders back to the caller. Without an assumptions entry, eager placeholder handling hits the
unwrap()inget_placeholder_assumptions.Record empty assumptions when recreating response universes under
-Zassumptions-on-binders. idk if empty assumptions is the final shape irl, but imo keeping this next tocreate_next_universeis the right local fix. It restores the invariant where the universe enters the caller and matches bookkeeping from other binder entry paths. fyi the issue repro now reports E0277 instead of ICEing.