newAllocEntry

fn newAllocEntry(s: *SafeAllocator, t: *Thread, ra: usize) error{OutOfMemory}!*Allocs.Entry {
    if (t.free_entry) |ent| {
        @branchHint(.likely);
        t.free_entry = ent.toFree();
        return ent;
    }

    if (s.allocs_entry_count - t.allocs_next != 0) {
        @branchHint(.likely);
        const ent = &t.allocs_first.?.entries(s)[t.allocs_next];
        t.allocs_next += 1;
        return ent;
    }

    const new_segment: *Allocs = @ptrCast(@alignCast(s.backing.rawAlloc(
        (1 + s.allocs_entry_count) * @sizeOf(usize),
        .of(usize),
        ra,
    ) orelse return error.OutOfMemory));
    new_segment.next = t.allocs_first;
    t.allocs_first = new_segment;
    t.allocs_next = 1;
    return &new_segment.entries(s)[0];
}