Why not return a WrittenBuffer<'a>? This can also be used to specify the methodset allowed or required plus any further type transitions out of WrittenBuffer, for example into a new CompressedBuffer, or similar patterns.

That's fair, but you would have to make sure that write_buffer is the only function that can create a WrittenBuffer<'a>. And then the second function could take a WrittenBuffer<'a> as an unused arg

I mean there's many ways to skin a cat!