advanceBucketAlloc

fn advanceBucketAlloc(
    s: *SafeAllocator,
    old: *AllocFooter,
    new_start: u32,
    start_moved: bool,
    alignment: Alignment,
    new_len: usize,
    ra: usize,
) bool {
    assert(AllocFooter.allocAlign(alignment).check(new_start));
    const b: *Bucket = .of(s, @ptrCast(old));

    const old_is_extended = old.isExtended();
    const old_footer_len = AllocFooter.lenBucket(s, old_is_extended);
    const old_fill: u32 = @intCast(Bucket.fillAt(s, @ptrCast(old)) + old_footer_len);

    const new_is_extended = old_is_extended or start_moved or
        AllocFooter.requiresExtended(new_len, alignment);
    const new_footer_len = AllocFooter.lenBucket(s, new_is_extended);
    const new_fill: u32 = @intCast(new_start + AllocFooter.allocOffset(new_len) + new_footer_len);

    assert(old_fill <= new_fill);
    if (new_fill > s.bucketSize()) {
        return false;
    }

    if (old_fill == new_fill or @cmpxchgStrong(
        Bucket.Fill,
        &b.fill,
        .{ .last_is_extended = old_is_extended, .at = @intCast(old_fill) },
        .{ .last_is_extended = new_is_extended, .at = @intCast(new_fill) },
        .monotonic,
        .monotonic,
    ) != null) {
        return false;
    }

    _ = AllocFooter.populate(
        @alignCast(b.bytes(s)[new_start..new_fill]),
        new_len,
        alignment,
        ra,
        new_is_extended,
        old.data.prev_extended,
        .{ .bucket_prev = old.bucketPrev(b, s) },
        s,
    );
    b.alloc_count.fenceAcqRel();
    return true;
}