Function prep_buff_size 

pub fn prep_buff_size(buf: &mut LuaBuffer, sz: usize) -> Result<(), LuaError>

Ensure at least sz free bytes are available in buf.

C: static char *prepbuffsize(luaL_Buffer *B, size_t sz, int boxidx) C: LUALIB_API char *luaL_prepbuffsize(luaL_Buffer *B, size_t sz)