48bit address space and 128bit of return value on systemv make pointer, size, 32bit of capacity-past-the-end attractive on x64. Specifically ptr, size as u64 with the extra capacity stored across the high 16 bits of each of them.
What do you do when you have a string that's longer than 2^32 that gets truncated to len=0? Instantly freeing the buffer might not be what the user wants, if they intend to immediately reuse it for an another very long string, for example.
I think that's a pretty bad case of premature optimization, especially because the first CPUs with 57 bit support are now hitting mainstream. Just use 3 words, it's not that much extra space.
Realloc/remap down to 4gb in that case sounds OK to me. > 4gb allocated from a structure which can't do any resizing seems moderately unlikely, but sure, I guess free is also correct in that case.
Two 64 bit values can be returned in registers on the systemv x64 abi, three get passed as a pointer to stack memory. It's an optimisation but I think it's a valid one.
57 bit address space has been coming any year now for maybe a decade, I'll worry about that when it happens.
