What is the test for a state to enforce its own federal rights?

14 "[F]ederal demands upon state courts go no further than to require that substantive rights granted litigants under federal law shall not be abridged by the states.... [p] Just as clear, however, is the premise that as to matters which in no way affect the substantive 'federal rights' of the parties, state law will be observed in state courts." (Carlson v. Pacific Far East Lines, supra, 29 Cal.App.3d at p. 888, 105 Cal.Rptr. 885.)