I'm currently searching for and evaluating libraries that will let me share as much shader code with the CPU (regular functions, structs and HLSL "built-in"s) as possible.

HLSL2021 now has templates and stuff, so I'm wondering how close your type declarations, etc. are to HLSL2021 internals

For example, now matrices are defined in terms of a template in HLSL, so apparently you can do this in HLSL2021 to implement a chainRule utility function:
microsoft/hlsl-specs#24 (comment)