"Slice" is, by now, a fairly established term in PL design which implies a tuple of (start, end) or (start, length), so it specifically excludes prefixed length, null termination etc - because experience has shown that slices are the only sane choice.

What I meant by optimization is not treating them same as other structs, but e.g. guaranteeing pass-by-register like other primitive types, spelled out explicitly in the ABI. The choice of length field type would be size_t, obviously.