some design questions:

which types?

List and Vector are good targets -- the other custom types provided here are small enough to not need caching given the complexity it would entail (lots of wrapping etc)

An interesting one is containers with the SimpleSerialize proc macro...

One option is an attribute macro to change the struct definition in-place

Another option is to modify the derive macro so that it has a helper attribute and then it is on the caller to include the cache as needed (using the attribute to signal to the derive macro it should be used when computing the hash tree root).

I personally lean towards this last option at the moment as it is explicit and gives the caller the most flexibility on how to add caching to their custom types.

mutability model?

interior or exterior mutability? the cleanest thing would be to encapsulate all of the mutable caching behind an immutable type

relevant data to consider:

• support concurrency across relays in the RelayMux mev-rs#27