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This ensures that `ParamEnv::and` preserves the original `caller_bounds` when we have a value containing fresh tys/consts. This ensures that when we cache a `SelectionCandidate`, the cache key (a `ParamEnvAnd`) contains all of the information that influenced the computation of our result (e.g. we may end up choosing a `ParamCandidate`)
For more information about how rustc works, see the rustc dev guide.