Function from_raw_parts_elem_ty 

pub(crate) fn from_raw_parts_elem_ty<'tcx>(
    tcx: TyCtxt<'tcx>,
    caller: DefId,
    dest: Option<Local>,
) -> Option<Ty<'tcx>>

Element type of a from_raw_parts result: &[T]/*[T]/Vec<T> yield T; other types (including String) return None.