factor 'is this type allowed as union field on stable' into separate function

Author: RalfJung

compiler/rustc_passes/src/stability.rs

@@ -772,13 +772,21 @@ impl<'tcx> Visitor<'tcx> for Checker<'tcx> {
                 let ty = self.tcx.type_of(item.def_id);
                 let ty::Adt(adt_def, substs) = ty.kind() else { bug!() };
 
+                #[allow(rustc::usage_of_qualified_ty)] // `Ty` is the wrong type here, we really want `ty::Ty`.
+                fn allowed_union_field<'tcx>(
+                    tcx: TyCtxt<'tcx>,
+                    param_env: ty::ParamEnv<'tcx>,
+                    ty: ty::Ty<'tcx>,
+                ) -> bool {
+                    ty.ty_adt_def().map_or(false, |adt_def| adt_def.is_manually_drop())
+                        || ty.is_copy_modulo_regions(tcx.at(DUMMY_SP), param_env)
+                }
+
                 // Non-`Copy` fields are unstable, except for `ManuallyDrop`.
                 let param_env = self.tcx.param_env(item.def_id);
                 for field in &adt_def.non_enum_variant().fields {
                     let field_ty = field.ty(self.tcx, substs);
-                    if !field_ty.ty_adt_def().map_or(false, |adt_def| adt_def.is_manually_drop())
-                        && !field_ty.is_copy_modulo_regions(self.tcx.at(DUMMY_SP), param_env)
-                    {
+                    if !allowed_union_field(self.tcx, param_env, field_ty) {
                         if field_ty.needs_drop(self.tcx, param_env) {
                             // Avoid duplicate error: This will error later anyway because fields
                             // that need drop are not allowed.