refactor: use SciMLBase.get_save_idxs_and_saved_subsystem

lib/OrdinaryDiffEqCore/src/solve.jl

@@ -264,29 +264,7 @@ function DiffEqBase.__init(
     end
 
     ### Algorithm-specific defaults ###
-    if save_idxs === nothing
-        saved_subsystem = nothing
-    else
-        if !(save_idxs isa AbstractArray) || symbolic_type(save_idxs) != NotSymbolic()
-            _save_idxs = [save_idxs]
-        else
-            _save_idxs = save_idxs
-        end
-        saved_subsystem = SciMLBase.SavedSubsystem(prob, parameter_values(prob), _save_idxs)
-        if saved_subsystem !== nothing
-            _save_idxs = SciMLBase.get_saved_state_idxs(saved_subsystem)
-            if isempty(_save_idxs)
-                # no states to save
-                save_idxs = Int[]
-            elseif !(save_idxs isa AbstractArray) || symbolic_type(save_idxs) != NotSymbolic()
-                # only a single state to save, and save it as a scalar timeseries instead of
-                # single-element array
-                save_idxs = only(_save_idxs)
-            else
-                save_idxs = _save_idxs
-            end
-        end
-    end
+    save_idxs, saved_subsystem = SciMLBase.get_save_idxs_and_saved_subsystem(prob, save_idxs)
 
     if save_idxs === nothing
         ksEltype = Vector{rateType}