populate

    fn populate(
        memory: []align(@alignOf(AllocFooter)) u8,
        len: usize,
        alignment: Alignment,
        ra: usize,
        /// `true` for large allocations
        is_extended: bool,
        /// `false` for large allocations
        prev_extended: bool,
        container: Extended.Container,
        s: *SafeAllocator,
    ) *AllocFooter {
        const footer: *AllocFooter = @ptrCast(@alignCast(memory[allocOffset(len)..].ptr));

        if (!is_extended) {
            footer.data = .{
                .len = @fromBackingInt(@intCast(len)),
                .alignment = @intCast(@backingInt(alignment)),
                .prev_extended = prev_extended,
            };
            assert(!footer.isExtended());
        } else {
            footer.data = .{
                .len = @fromBackingInt(@intCast(Data.Len.extended_start + (@backingInt(alignment) >> 2))),
                .alignment = @truncate(@backingInt(alignment)),
                .prev_extended = prev_extended,
            };
            assert(footer.isExtended());
            footer.extended().* = .{
                .len = len,
                .container = container,
            };
        }

        captureStackTrace(footer.allocTrace(s), ra);
        footer.checksum = footer.actualChecksum(s) ^ s.canary;
        footer.modify.setNone();

        return footer;
    }