Instead of using ParamEnv::reveal_all, the ParamEnv should be tracked in the mono item - this is necessary to enable more complex forms of polymorphization in future.

something simple like trying layout_of(ty) on various types in the MIR body and ignoring generics in ty if it succeeds, should work, but the lack of a correct ParamEnv would make it fail later (especially if e.g. it's something like &T which only has a constant layout if T: Sized)

(source: eddyb in Zulip)