This has false negatives when the last allocation had an adjusted_index. In such case we won't be able to determine what the last allocation was because the alignForward operation done in alloc is not reversible.
pub fn isLastAllocation(self: *FixedBufferAllocator, buf: []u8) bool
pub fn isLastAllocation(self: *FixedBufferAllocator, buf: []u8) bool {
return buf.ptr + buf.len == self.buffer.ptr + self.end_index;
}