Rollup merge of #124624 - WaffleLapkin:old_unit, r=fmease

Use `tcx.types.unit` instead of `Ty::new_unit(tcx)`

I don't think there is any need for the function, given that we can just access the `.types`, similarly to all other primitives?

compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs

@@ -781,7 +781,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                             && self.fallback_has_occurred
                         {
                             let predicate = trait_predicate.map_bound(|trait_pred| {
-                                trait_pred.with_self_ty(self.tcx, Ty::new_unit(self.tcx))
+                                trait_pred.with_self_ty(self.tcx, tcx.types.unit)
                             });
                             let unit_obligation = obligation.with(tcx, predicate);
                             if self.predicate_may_hold(&unit_obligation) {

compiler/rustc_trait_selection/src/traits/object_safety.rs

@@ -519,7 +519,7 @@ fn virtual_call_violations_for_method<'tcx>(
 
             // e.g., `Rc<()>`
             let unit_receiver_ty =
-                receiver_for_self_ty(tcx, receiver_ty, Ty::new_unit(tcx), method.def_id);
+                receiver_for_self_ty(tcx, receiver_ty, tcx.types.unit, method.def_id);
 
             match abi_of_ty(unit_receiver_ty) {
                 Some(Abi::Scalar(..)) => (),