Freeciv

Problem: The receiving side doesn't know the array's old size (since it already overwrote that).

• Solution 1: As a general rule, don't diff the elements of a varsize array.
  • Upside: The easy way out; least amount of work now; keeps both the generating and generated code simple.
  • Downside: Less efficient if the sub-arrays usually only change a little at a time.
• Solution 2: Transmit the newly added elements in full, but still in the same format as if they were diff'd.
  • Upside: We can diff the sub-arrays.
  • Downside: Very inefficient whenever new sub-arrays are added.
  • Downside: Adds more bloat to generated code (now three ways to transmit a field type).
• Solution 3: Make the receiving side remember the old size.
  • Upside: We can diff the existing sub-arrays and plainly transmit the new sub-arrays in full.
  • Upside: Opens the door for future optimizations based on being conscious of the size changing.
  • Downside: Requires code changes that aren't local, and generating variable names that don't collide with anything.

I'll probably go with solution 1 for now. None of the performance aspects actually matter until we actually get in this actual situation; this whole patch is already addressing a case that doesn't currently happen, and I don't want to write too much code that doesn't actually get executed/generated, for the sake of both testing and avoiding bloat.
Solution 3's potential for extra optimizations is all based on varsize arrays with complex elements. So if and when we find ourselves making a varsize array of vectors that we want to fully deep-diff, then it becomes reasonable to do that.