render_some should have an overload that takes a user-provided output buffer

[pdimov]

E.g.

void render_some( core::string_view in, char* out, std::size_t n, std::size_t& bytes_read, std::size_t& bytes_written );
void finish( char* out, std::size_t n, std::size_t& bytes_written );

or maybe core::span<char> out.

[vinniefalco]

I'm fine with individual pointer and size, keeps things simple

[vinniefalco]

Or this https://github.com/CPPAlliance/buffers/blob/469dd479111d4fe53be3c6784642908520361eed/include/boost/buffers/source.hpp#L34