[tialaramex]

> But I don't see the need to know the size of the memory block that fulfilled your request.

If we don't know, we can't use anything except what we asked for. For some allocations that's not important, but e.g. Vec can just upgrade the capacity to match, buffering mechanisms often may just as well use the entire available buffer not just the part they specifically asked for.

> E.g. you request 200 bytes, a block of 256 bytes is actually allocated. Now you request with some other code 30 bytes. If the allocator had told you at allocation time "here are your 200 bytes, but just so you know, there's another 56" it could not hand the unused part of the 256 bytes to the next allocation.

You're assuming that the allocator is able to remember that these extra 56 bytes are available. That may or may not be practical, it's a shame if we have to otherwise throw the 56 bytes away.