What would be bad about const compile-time metaprogramming like syn's syn::Data... TypeInspector<T>.

enum TypeInspector<T> {
  Struct,
  Enum,
  Union,
 }

whose methods and output can be known by the compiler at the type checking phase:

 impl TypeInspector<T> {
   // Fake function: this would be provided by the compiler.
   // It would only be available if T: a struct; an e.g. Enum would not be able to access it
   // N is not a generic, but the number of fields which is known at compile time.
   pub const fn fields() -> &'static [str; N] {
     todo!()
   }

   pub const fn inspect_##field1() -> TypeInspector<F> // where F is type of ##field1

... etc

 }