Function iter_fields 

pub fn iter_fields<'tcx>(
    ty: Ty<'tcx>,
    tcx: TyCtxt<'tcx>,
    typing_env: TypingEnv<'tcx>,
    f: impl FnMut(Option<VariantIdx>, FieldIdx, Ty<'tcx>),
)

Invokes f on all direct fields of ty.