An allocator that fully trusts the size provided to `free` is hugely unsafe when the size turns out to be incorrect. As an extreme example, the allocator may kn

An allocator that fully trusts the size provided to `free` is hugely unsafe when the size turns out to be incorrect. As an extreme example, the allocator may know that every 4-byte allocation is put into the arena that is aligned to 0x1000-byte block, so it may jump straight into the arena header which won't be there if the size was incorrect. So a real-life allocator is likely to do some checking even when the size is provided, but that checking can be much more simplified in that case.