fn constness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Constness
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Checks whether an item is considered to be const. If it is a constructor, it is const. If it is a trait impl/function, return if it has a const modifier. If it is an intrinsic, report whether said intrinsic has a rustc_const_{un,}stable attribute. Otherwise, return Constness::NotConst.