Unify StorageKey and CacheKey

[vlopes11]

Description

Currently, StorageKey and CacheKey are equivalent types. They historically were different, but as the implementation evolved, they converged to have the same. However, StorageKey implements serialization, whereas CacheKey doesn't.

This limits our implementation as the conversion between the types may force us to take the key as owned for simple operations such as get, when a reference should suffice

sovereign-sdk/module-system/sov-state/src/scratchpad.rs

Line 187 in 0137568

let key = key.as_cache_key();

Solution

Deprecate CacheKey and replace its usage by StorageKey. This will allow us to pass references downstream and make the map API more powerful, as the types are just borrowed references and can be passed downstream without any conversion. This will unlock a simplification requested on #427

Alternatives

We could unsafely cast one type to the other, but we have no clear gain to maintain the distinction and would create a potential undefined behavior if the layout of any of the types ever change. This would create a maintenance burden to cross-check both types whenever they are updated.

A working alternative suggested by @preston-evans98 is to use yoke to cast one type to the other. This will allow a zero-cost solution and will preserve both types, in case we want to maintain the distinction. The CacheKey is currently used for memory-only representation, and StorageKey is used for serialization. It might be desirable to maintain this domain separation if we want to have a layer that don't interact with I/O.

[bkolad]

We could even have a single type StoargeSlot for keys and values.

[citizen-stig]

As discussed on today's daily:

• Keep Key and Value types for semantic
• Rename CacheKey and CacheValue to SlotKey and SlotValue
• Remove StorageKey and StorageValue in favour of SlotKey(previously CacheKey) and SlotValue (previously CacheValue)