> Things may have changed since, but AFAIK the C++ implementation would always allocate on the heap for nested messages

This is no longer the case if you use arenas: https://developers.google.com/protocol-buffers/docs/referenc...

> and perhaps even for optional scalars

This has never been the case, except for string fields where std::string forces us to allocate.

Ideally we will eventually use std::string_view for string accessors instead of std::string, so that even string data can be allocated on an arena instead of the heap.