Function field_name_from_arg 

pub(crate) fn field_name_from_arg<'tcx>(
    tcx: TyCtxt<'tcx>,
    adt_def_id: DefId,
    arg: &PropertyArg<'tcx>,
) -> Option<String>

The field name targeted by an Allocated/Owning-style property argument, whether it was parsed as a bare identifier or as a resolved place.