If I have a rule that takes a dataclass A as input and I send it dataclass B which is a subclass of A, will it come through to the rule as B or does the engine convert it to the superclass when it serializes it for caching?

Also wondering when 2.32 is slated to leave dev?

Shortly. It was just waiting on https://github.com/pantsbuild/scie-pants/pull/509 to drop

I believe this is relevant to your first question. An instance of B and an instance of A will be considered non-equal, even if they are equal on all superclass fields, for the purpose of memoization of rules. But this is in-memory, and there is no serialization as far as I can remember, so you'll get back whatever the rule emitted when it was memoized (I think - worth testing!) Note that on-disk caching is just for running Processes, and Process/ProcessResult are Rust structs that are populated from and to the corresponding Python equivalents. So subclassing them on the Python side won't have any effect.

Note that on-disk caching is just for running Processes,

I was confused by this recently too. I convinced myself on disk was "just" a process cache, but I'm not following what these other two types are:

enum CacheKeyType {
  PROCESS = 0;
  URL = 1;
  DEP_INFERENCE_REQUEST = 2;
}

https://github.com/pantsbuild/pants/blob/9b3c1562e2081d8e60433341ad8dc5585ec37323/src/rust/protos/protos/pants/cache.proto#L7

Ah yes, those are two special cases for native implementation of URL fetching and of dep inference. Those are also cached on disk without going through the Process mechanism