BucketHeader

        const BucketHeader = struct {
            allocated_count: SlotIndex,
            freed_count: SlotIndex,
            prev: ?*BucketHeader,
            next: ?*BucketHeader,
            canary: usize = config.canary,

            fn fromPage(page_addr: usize, slot_count: usize) *BucketHeader {
                const unaligned = page_addr +% page_size -% bucketSize(slot_count);
                return @ptrFromInt(unaligned & ~(@as(usize, @alignOf(BucketHeader)) - 1));
            }

            fn usedBits(bucket: *BucketHeader, index: usize) *usize {
                const ptr: [*]u8 = @ptrCast(bucket);
                const bits: [*]usize = @ptrCast(@alignCast(ptr + @sizeOf(BucketHeader)));
                return &bits[index];
            }

            fn requestedSizes(bucket: *BucketHeader, slot_count: usize) []LargestSizeClassInt {
                if (!config.safety) @compileError("requested size is only stored when safety is enabled");
                const start_ptr = @as([*]u8, @ptrCast(bucket)) + bucketRequestedSizesStart(slot_count);
                const sizes = @as([*]LargestSizeClassInt, @ptrCast(@alignCast(start_ptr)));
                return sizes[0..slot_count];
            }

            fn log2PtrAligns(bucket: *BucketHeader, slot_count: usize) []mem.Alignment {
                if (!config.safety) @compileError("requested size is only stored when safety is enabled");
                const aligns_ptr = @as([*]u8, @ptrCast(bucket)) + bucketAlignsStart(slot_count);
                return @ptrCast(aligns_ptr[0..slot_count]);
            }

            fn stackTracePtr(
                bucket: *BucketHeader,
                slot_count: usize,
                slot_index: SlotIndex,
                trace_kind: TraceKind,
            ) *[stack_n]usize {
                const start_ptr = @as([*]u8, @ptrCast(bucket)) + bucketStackFramesStart(slot_count);
                const addr = start_ptr + one_trace_size * traces_per_slot * slot_index +
                    @backingInt(trace_kind) * @as(usize, one_trace_size);
                return @ptrCast(@alignCast(addr));
            }

            fn captureStackTrace(
                bucket: *BucketHeader,
                ret_addr: usize,
                slot_count: usize,
                slot_index: SlotIndex,
                trace_kind: TraceKind,
            ) void {
                // Initialize them to 0. When determining the count we must look
                // for non zero addresses.
                const stack_addresses = bucket.stackTracePtr(slot_count, slot_index, trace_kind);
                collectStackTrace(ret_addr, stack_addresses);
            }
        }