Function resolve_self_field_origin 

pub fn resolve_self_field_origin<'tcx>(
    tcx: TyCtxt<'tcx>,
    def_id: DefId,
    local: usize,
    fields: &[usize],
) -> Option<FieldOrigin>

If local (typically 1 = self) with fields in def_id’s body corresponds to a struct field, return its identity.