Function constness

fn constness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Constness

Checks whether an item is considered to be const. If it is a constructor, anonymous const, const block, const item or associated const, 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.