A common extension of idl types is the generation of a hash function (for use in unordered containers).

We currently have free functions in the codebase which use both all fields, as well as limited subsets, so a code reducing impl should include support for both.

An example syntax might look like:

For all fields:

hash: all

For a subset:

hash: [a,c, b]

I'm on the fence whether we should expose these as:

• std::hash specializations
• dependent types as T::Hash
• adl lookup.

We probably also want to support a facility for attaching a c++ function to a type as the hash function (to bootstrap user defined types)